[is the universe a void with bodies?
Helpful Assistant The universe is not simply a void with bodies. It is a vast, complex system that includes various celestial bodies, such as stars, planets, galaxies, and black holes, as well as other forms of matter and energy. The universe also contains vast regions of empty space, known as the cosmic voids, which are areas with very low densities of galaxies and matter.
In addition to visible matter, the universe is also composed of dark matter and dark energy. Dark matter is a form of matter that does not emit or absorb light, making it difficult to detect directly. It is believed to make up about 27% of the universe's total mass-energy content. Dark energy, on the other hand, is a mysterious form of energy that is thought to be responsible for the accelerated expansion of the universe. It is estimated to make up about 68% of the universe's total mass-energy content.
In summary, the universe is a complex system containing various celestial bodies, matter, and energy, including dark matter and dark energy, as well as vast regions of empty space.](https://chat.forefront.ai/)
[how many are the atoms of the universe?
Helpful Assistant It is difficult to determine the exact number of atoms in the universe, as it is constantly expanding and our observations are limited to a small portion of it. However, scientists have estimated the number of atoms in the observable universe to be around 10^80, which is a very large number. This estimate is based on measurements of the cosmic microwave background radiation, which is the afterglow of the Big Bang, and observations of the large-scale structure of the universe. It is important to note that this estimate only includes the atoms that make up ordinary matter, which is only a small fraction of the total matter in the universe. The majority of the matter in the universe is thought to be dark matter, which does not interact with light and is therefore difficult to detect.](https://chat.forefront.ai/)
Μια σύντομη ιστορία του διαστήματος What shape is the universe? | Astronomy.com
+Where_Is_the_Edge_of_the_Universe +diasthma_istoria +Brief_History_of_the_Universe +Gaia_Universe +limits +Unobservable_Universe_Will_Be_Able_To_See +flat +shape +expanding +forever
A timeless question: How old is the universe? | Popular Science A Short History of the Universe The origins of the universe facts and information A Brief History of the Universe – Explaining Science The Origin of the Universe, Earth, and Life | Science and Creationism: A View from the National Academy of Sciences, Second Edition | The National Academies Press
The Universe is all of time and space and its contents. The Universe includes planets, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles, and all matter and energy. The observable universe is about in diameter at the present time. The size of the whole Universe is not known and may be infinite. Observations and the development of physical theories have led to inferences about the composition and evolution of the Universe.
+The_Big_Bang_and_the_Big_Crunch
The universe is a huge wide-open space that holds everything from the smallest particle to the biggest galaxy. No one knows just how big the Universe is. Astronomers try to measure it all the time. They use a special instrument called a spectroscope to tell whether an object is moving away from Earth or toward Earth. Based on the information from this instrument, scientists have learned that the universe is still growing outward in every direction.
Scientists believe that about 13.7 billion years ago, a powerful explosion called the Big Bang happened. This powerful explosion set the universe into motion and this motion continues today. Scientists are not yet sure if the motion will stop, change direction, or keep going forever.
A light-year is the distance light travels in one year. It is 9.5 trillion (9,500,000,000,000) kilometers. The size of a galaxy may be as little as a thousand light-years across or as much as a million light-years across.
A galaxy contains stars, gas, and dust which are held together as a group by gravity. There may be millions, or even billions, of stars in one galaxy. There are billions of galaxies in the universe.
Galaxies are labeled according to their shape. Some galaxies are called "spiral", because they look like giant pinwheels in the sky. The galaxy we live in, the Milky Way, is a spiral galaxy. Some galaxies are called "elliptical", because they look like flat balls. A galaxy may be called "irregular" if it doesn't really have a shape. A new type of galaxy was discovered recently, called a "starburst" galaxy. In this type of galaxy, new stars just seem to 'burst out' very quickly.
There are over 100 billion stars in the Milky Way galaxy. If you tried to count them one by one, it would take you over 3000 years!
The Milky Way is over 100,000 light-years wide. It is called a spiral galaxy because it has long arms which spin around like a giant pinwheel. Our Sun is a star in one of the arms. When you look up at the night sky, most of the stars you see are in one of the Milky Way arms.
Before we had telescopes, people could not see many of the stars very clearly. They blurred together in a white streak across the sky. A myth by the ancient Greeks said this white streak was a "river of milk". The ancient Romans called it the Via Galactica, or "road made of milk". This is how our galaxy became known as the Milky Way.
Stars change over time. It may take millions to billions of years for a star to live out its life. That is a very, very long time!
A star is a big ball of gas which gives off both heat and light. So where do stars come from? What happens to them as they grow older?
A galaxy contains clouds of dust and gas, as well as stars. It is in the clouds of dust and gas that stars are born. As more and more of the gas (which is mostly hydrogen) is pulled together by gravity into a cloud, the cloud starts to spin. The gas atoms start to bump into each other faster and faster. This creates heat energy. The cloud gets hotter and hotter. Finally, it gets so hot within the cloud that something called "nuclear fusion" happens. The cloud begins to glow. The glowing cloud of gas is now known as a protostar. The protostar continues to grow. Once it stops growing, it is known as a main sequence star. A main sequence star can shine for millions of years or more. The amount of time it lives is determined by how big it is.
In medium size stars, after the nuclear fusion has used up all the fuel it has, gravity will pull the remaining material closer together. The star will shrink. In fact, it may get to be only a few hundred kilometers wide! The star is then called a "white dwarf". It can stay like this for a long time. Eventually, it will stop producing any light at all. It is then called a "black dwarf" and it will stay that way forever.
In large size stars, nuclear fusion will continue until iron is formed. In stars, iron acts like an energy sponge. It soaks up the star's energy. This energy is eventually released in a big explosion called a supernova. The little bit of matter that used to be at the center of the star before the supernova will then be either a neutron star or a black hole. Which object it becomes depends on the size of the original star. A star that is 1.5 to 4 times larger than our Sun will become a neutron star. Stars that are even bigger than that will become black holes.
Quasars give off more energy than 100 normal galaxies combined.
Quasars are farther away from Earth than any other known object in the universe. Because they are so far away from us, it takes billions of years for the light they give off to reach Earth. The light stays the same, it just has to travel a long time to get to us. When we look at a quasar, it is like we are looking back in time. The light we see today is what the quasar looked like billions of years ago. Some scientists think that when they study quasars they are studying the beginning of the universe.
Quasars give off huge amounts of energy. They can be a trillion times brighter than the Sun! Astronomers think that quasars are located in galaxies which have black holes at their centers. The black holes may provide quasars with their energy. Quasars are so bright that they drown out the light from all other stars in the same galaxy. The word quasar is short for quasi-stellar radio source. Quasars give off radio waves, X-rays, gamma-rays, ultraviolet rays, and visible light. Most of them are larger than our solar system.
Most scientists believe that there is a black hole in the center of our very own Milky Way.
Black holes were once massive stars that used up all their fuel. As they died out, they collapsed inward due to the pull of their own gravity. The gravity of a black hole is so powerful that not even light can escape its pull! Once any matter falls into a black hole, it disappears from the visible universe.
It is very hard to see a black hole. Any object that gets too close to a black hole will be pulled inside it. We only know they are there because of the effects they have on other objects that are near them. Any object, whether some dust, or a star, or anything, that gets too close to a black hole will be pulled inside it. As the objects fall toward the black hole, they heat up and get very hot. Scientists can use special instruments to detect the heat the objects give off. That is how we know the black hole must be there.
Dark matter was once called "missing matter". It was called this because scientists looking at the sky could not find it.
Matter is anything that takes up space and has mass. We are used to matter which we will call visible matter. Visible matter can be seen because it gives off light or reflects light given off by another object. Dark matter cannot be seen. It does not give off light or reflect light.
Image of matter clumping in the early universe as predicted by computer simulation
Scientists believe that over ninety-percent of the matter in the universe is dark matter. They also believe that by studying dark matter they will gain new information about the universe. Some of the information they hope to discover is the size, shape and future of the universe. Scientists also hope to learn about how galaxies formed by studying dark matter.
Scientists cannot see dark matter, so they have a special way of studying it. Scientists study dark matter by looking at how it affects visible matter. Scientists use computers and satellites to study dark matter. The Hubble Space Telescope has taken pictures that have helped scientists discover where dark matter can be found.
Wow! The universe is getting bigger every day!
Cosmology is a branch of science which studies the evolution of our universe. A scientist who studies cosmology is called a cosmologist. Cosmologists want to know what the universe was like billions of years ago. They want to understand how it is today. They also want to find out what the universe will be like billions of years in the future.
To understand the universe, cosmologists first needed to know our place in it. About 400 years ago, most people believed that Earth was at the center of the universe. They thought the Moon, the Sun, and the other planets all traveled around Earth. Nicholas Copernicus, Galileo Galilei, and Isaac Newton helped us to learn the truth. We now know that the Moon travels around Earth. Earth and the other planets in our solar system travel around the Sun.
About 100 years ago, astronomer Edwin Hubble made observations that showed the universe was getting bigger and bigger! This led most cosmologists to believe in a theory called the Big Bang. The Big Bang theory says the universe began with a huge explosion ten to twenty billion years ago. Cosmologists think the universe did not exist before the Big Bang. Some cosmologists think the universe will keep getting bigger forever. Other cosmologists think the Universe will someday start getting smaller. They think it will shrink until it no longer exists. No one knows yet which opinion is correct.
COBE satellite data cosmic radiation everywhere in the Universe as a result of the Big Bang
Cosmologists use the data gathered by telescopes and astronomy satellites to help them understand the universe. They also use computers to model their ideas. Each new discovery helps them to understand more about the past, present, and future of our universe.
PHYSICS: Doppler Shift
The universe is a vast expanse of space which contains all of the matter and energy in existence. The universe contains all of the galaxies, stars, and planets. The exact size of the universe is unknown. Scientists believe the universe is still expanding outward. They believe this outward expansion is the result of a violent, powerful explosion that occurred about 13.7 billion years ago. This explosion is known as the Big Bang. By looking at an object's electromagnetic spectrum, scientists can determine if an object is moving away from Earth or towards Earth. When distant objects, such as quasars, are viewed from Earth, their spectrum is shifted towards red. Whenever there is a shift in a spectrum, it is called a Doppler Shift. If the shift is toward red, the light given off by the object is in longer wavelengths. When an object moves away from Earth, the light that it is giving off is seen in longer wavelengths. When an object moves toward Earth, the light that it is giving off is seen in shorter wavelengths. This causes a shift in the object's spectrum towards violet. The amount of shift in an object's spectrum is determined by how fast the object is moving. All of the distant galaxies have tremendous red shifts. Based on these data, scientists believe the universe is still expanding outward.
Guess what? Only three galaxies outside of the Milky Way can be seen by the unaided eye here on Earth. People in the Northern Hemisphere can see the Andromeda Nebula while people in the Southern Hemisphere can see the Large and Small Magellanic Clouds.
Our Sun is a star in the Milky Way Galaxy. If you were looking down on the Milky Way, it would look like a large pinwheel rotating in space. Our Galaxy is a spiral galaxy that formed approximately 14 billion years ago. Contained in the Milky Way are stars, clouds of dust and gas called nebulae, planets, and asteroids. Stars, dust, and gas fan out from the center of the Galaxy in long spiraling arms. The Milky Way is approximately 100,000 light-years in diameter. Our solar system is 26,000 light-years from the center of the Galaxy. All objects in the Galaxy revolve around the Galaxy's center. It takes 250 million years for our Sun to pull us through one revolution around the center of the Milky Way. The stars we see over our head every night are also members of the Milky Way family.
It is interesting to note that astronomers capitalize the "G" in galaxy when talking about our Milky Way!
Wow! A light-year is 9.5 trillion kilometers and is the distance light travels in one year. Galaxies range from a few thousand to a million light-years in diameter.
A galaxy is a cluster (σμήνος) of stars, dust, and gas which is held together by gravity. Galaxies are scattered throughout the universe and they vary greatly in size. A galaxy may be alone or it may be in a large group of galaxies called a "supercluster". Galaxies are classified by scientists according to their shape and appearance. An irregular galaxy has an undefined shape and is full of young stars, dust, and gas. A spiral galaxy is shaped like a disk. The disk tends to resemble a pinwheel with arms which spiral outward as it rotates. Spiral galaxies tend to contain more middle-aged stars along with clouds of gas and dust. The next galaxy classification is an elliptical galaxy. The elliptical galaxies contain older stars and very little gas and dust. Elliptical galaxies vary in their shape from round to flattened, elongated spheres.
M100, a typical spiral galaxy
By studying the Doppler shift of different galaxies, scientists have concluded that all of the galaxies are moving away from each other. Galaxies that are the farthest from Earth appear to be traveling the fastest (relative to the Earth) of any galaxies.
A starburst galaxy has an exceptionally high star birth rate. The Hubble Space Telescope's high resolution has allowed astronomers to see dense star clusters, dust lanes with tiny regions of dense gas, and filaments of glowing gas in the core of a starburst galaxy.
Wow! Quasars give off more energy than 100 normal galaxies combined.
Many astronomers believe that quasars are the most distant objects yet detected in the universe. Quasars give off enormous amounts of energy - they can be a trillion times brighter than the Sun! Quasars are believed to produce their energy from massive black holes in the center of the galaxies in which the quasars are located. Because quasars are so bright, they drown out {make imperceptible; To cover, obscure, or overwhelm by being louder or more intense than} the light from all the other stars in the same galaxy.
Quasar at the heart of a galaxy A Quasar
Despite their brightness, due to their great distance from Earth, no quasars can be seen with an unaided eye. Energy from quasars takes billions of years to reach the Earth's atmosphere. For this reason, the study of quasars can provide astronomers with information about the early stages of the universe.
The word quasar is short for "quasi-stellar radio source". This name, which means star-like emitters of radio waves, was given in the 1960s when quasars were first detected. The name is retained today, even though astronomers now know most quasars are faint radio emitters. In addition to radio waves and visible light, quasars also emit ultraviolet rays, infrared waves, X-rays, and gamma-rays. Most quasars are larger than our solar system. A quasar is approximately 1 kiloparsec in width.
Wow! Stars evolve, or change, over time. It may take millions of years or it may take billions of years for a star to complete its life cycle.
A star is a brilliantly glowing sphere of hot gas whose energy is produced by an internal nuclear fusion process. Stars are contained in galaxies. A galaxy contains not only stars, but clouds of gas and dust. These clouds are called nebulae, and it is in a nebula where stars are born. In the nebula is hydrogen gas which is pulled together by gravity and starts to spin faster. Over millions of years, more hydrogen gas is pulled into the spinning cloud. The collisions which occur between the hydrogen atoms starts to heat the gas in the cloud. Once the temperature reaches 15,000,000 degrees Celsius, nuclear fusion takes place in the center, or core, of the cloud. The tremendous heat given off by the nuclear fusion process causes the gas to glow creating a protostar. This is the first step in the evolution of a star. The glowing protostar continues to accumulate mass. The amount of mass it can accumulate is determined by the amount of matter available in the nebula. Once its mass is stabilized, the star is known as a main sequence star. The new star will continue to glow for millions or even billions of years. As it glows, hydrogen is converted into helium in the core by nuclear fusion. The core starts to become unstable and it starts to contract. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and starts to glow red. The star has now reached the red giant phase. It is red because it is cooler than the protostar phase and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths the star will take.
The Pleiades (M45) The Cat's Eye Planetary Nebula
As a red giant, the hydrogen gas in the outer shell continues to burn as the temperature in the core continues to rise. At 200,000,000 degrees Celsius, the helium atoms fuse to form carbon atoms in the core. The last of the hydrogen gas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula. When the last of the helium atoms in the core are fused into carbon atoms, the medium size star begins to die. Gravity causes the last of the star's matter to collapse inward and compact. This is the white dwarf stage which is extremely dense. White dwarfs shine with a white hot light but once all of their energy is gone, they die. The star has now reached the black dwarf phase.
Once massive stars reach the red giant phase, the core temperature continues to increase as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull together the carbon atoms in the core until the temperature reaches 600,000,000 degrees Celsius. At this temperature, carbon atoms form heavy elements such as oxygen and nitrogen. The fusion and production of heavy elements continues until iron starts to form. At this point, fusion stops and the iron atoms start to absorb energy. This energy is eventually released in a powerful explosion called a supernova. A supernova can light the sky up for weeks. The temperature in a supernova can reach 1,000,000,000 degrees Celsius. This high temperature can lead to the production of new elements which may appear in the new nebula that results after the supernova explosion. The core of a massive star that is 1.5 to 4 times as massive as our Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves. If the radio waves appear to be emitted in pulses (due to the star's spin), these neutron stars are called pulsars. The core of a massive star that has 10 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It has now become a black hole which readily swallows any matter and energy that comes too near it. Some black holes have companion stars whose gases they pull off. As the gases are pulled down into the black hole, they heat up and give off energy in the form of X-rays. Black holes are detected by the X-rays which are given off as matter falls down into the hole.
Guess what? Some scientists believe that there is a black hole here in our very own Milky Way.
Black holes are extremely compact space objects that were once massive stars which collapsed inward due to the force of their own gravity. Consequently, black holes are very dense. If it were not for the effect that black holes have on the objects around them, we would be unable to detect them. A black hole has a powerful gravitational field which traps everything that goes near it. Scientists now theorize that some galaxies have huge black holes in their centers which release tremendous amounts of energy that powers the spectacular energetic events that go on within the galaxy. The fuel for the black hole, scientists believe, may be the trapped gas, stars, and dust that are pulled into the hole. Gas that is pulled into a black hole swirls down into the hole much like a whirlpool. By using a spectroscope, the Hubble Space Telescope has the ability to clock the speed of this gas as it swirls around the entrance to the hole. The speed with which the gas swirls is considered the black hole's signature. By knowing the speed of the gas, the mass of the black hole can be calculated. A black hole in the center of the M87 galaxy in the constellation Virgo, which is 50 million light-years away, has been calculated to have a mass equal to that of 3 billion Suns! An even more effective way of studying black holes is through the use of X-ray observations. X-rays have the capacity to penetrate through gas and dust much better than optical light. With the data delivered to us by X-ray observations and the Hubble Space Telescope, scientists now believe that the presence of black holes explains a lot of the powerful cosmological events which occur in the universe.
I can't believe it!
Dark matter was initially called "missing matter" because astronomers could not find it by observing the universe in any part of the electromagnetic spectrum.
There is no current problem of greater importance to cosmology than that of dark matter. Dark matter is composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly. We know that dark matter exists because of the effect it has on objects that we can observe directly.
Scientists study dark matter by looking at the effects it has on visible objects. Scientists believe that dark matter may account for the unexplained motions of stars within galaxies. Computers play an important role in the search for dark matter data. They allow scientists to create models which predict galaxy behavior. Satellites are also being used to gather dark matter data. In 1997, a Hubble Space Telescope image (seen on the right) revealed light from a distant galaxy cluster being bent by another cluster in the foreground of the image. Based on the way the light was bent, scientists estimated the mass of the foreground cluster to be 250 times greater than the visible matter in the cluster. Scientists believe that dark matter in the cluster accounts for the unexplained mass.
HSt image of a gravitational lens created by a galaxy cluster Gravitational Lens Created by Galaxy Cluster Reveals Presence of Dark Matter
Scientists have produced many theories about what exactly dark matter may be. Some believe that it may be normal objects such as cold gasses, dark galaxies, or massive compact halo objects (called MACHOs, they would include black holes and brown dwarfs). Other scientists believe that dark matter may be composed of strange particles which were created in the very early universe. Such particles may include axions, weakly interacting massive particles (called WIMPs), or neutrinos.
Understanding dark matter is important to understanding the size, shape and future of the universe. The amount of dark matter in the universe will determine if the universe is open (continues to expand), closed (expands to a point and then collapses) or flat (expands and then stops when it reaches equilibrium). Understanding dark matter will also aid in definitively explaining the formation and evolution of galaxies and clusters. As a galaxy spins it should be torn apart. This does not happen, so something is holding the galaxy together. The something is gravity; the amount of gravity required to do this, however, is enormous and could not be generated by the visible matter in the galaxy.
Imagine that!
Dark energy is an even bigger concern to scientists than dark matter. Dark matter makes up about 23% of our universe today, while dark energy makes up 72%!
In 1998, by observing distant supernova explosions, scientists came to the conclusion that the universe is expanding at a faster and faster rate. This was very surprising! Before that, it was believed that the universe would be either expanding at a constant rate or expanding at a slower and slower rate (due to gravity).
So what causes the change in the expansion rate? The answer seems to be what scientists have called dark energy. At present, dark energy is a complete mystery. In fact, that is how it got its name - dark energy refers to the fact that some kind of something must fill the vast reaches of the universe in order to be able to make space accelerate in its expansion. But what exactly this something is is completely unknown to us.
Several ideas have been pursued to explain or account for dark energy. One idea is related to the work of Albert Einstein who thought that space itself might contain energy. Another idea defines a new, fifth state of matter called quintessence (to go along with the usual four states of solid, liquid, gas, and plasma). Neither of these ideas has proven to work very well to explain what astronomers see happening in the universe. So it looks like it is up to future physicists (maybe you) to solve this important problem.
Wow! Recent observations have shown the universe to be about 13.7 billion years old!
Like early astronomers from around the world, the ancient Greeks struggled to understand the universe. Thales, often called the father of Greek science and mathematics, asked questions about the universe that were not based on the actions of gods or demons. It is said that Thales provided the bridge between the world of myth and the world of reason. He used the astronomical records of the Babylonians and Egyptians to accurately predict a solar eclipse in the sixth century BC. Thales believed the Earth was flat and floated on water like a log.
Aristotle, who lived from 384 to 322 BC, believed the Earth was round. He thought Earth was the center of the universe and that the Sun, Moon, planets, and all the fixed stars revolved around it. Aristotle's ideas were widely accepted by the Greeks of his time. The exception, a century later, was Aristarchus, one of the earliest believers in a heliocentric or sun-centered universe. In the 100s BC, Hipparchus, the most important Greek astronomer of his time, calculated the comparative brightness of as many as 1,000 different stars. He also calculated the Moon's distance from the Earth.
The first astronomer to make truly scientific maps of the heavens, Claudius Ptolemaeus (better known as Ptolemy of Alexandria), came along 300 years later. Like most astronomers before him, he believed the Sun, Moon, and other planets circled the Earth. He thought that each space body moved in a small circle (an epicycle) that was itself orbiting Earth. This explained why planets sometimes appeared to travel backward in the sky. The Earth-centered view of the universe was widely accepted for about 1500 years. It was not seriously challenged until 1543, when the Polish monk Nicolaus Copernicus suggested that the Sun was at the center of the universe. Because the Church taught that the Earth was central, Copernicus' theory was regarded as heresy. Perhaps this is why he did not want it published until after his death. Copernicus' published theory, On the Revolution of the Celestial Spheres, met with great hostility from the Church. The two events most responsible for eventual acceptance of Copernicus' views were Tycho Brahe's precise observations of the sky and Galileo's use of the telescope.
One night in 1572, Danish astronomer Tycho Brahe saw what he thought was a brilliant new star in the constellation Cassiopeia. (We now know he was observing a supernova.) In 1604, a second supernova was observed. These discoveries caused scientists to seriously question Ptolemy's theory that all stars were contained in an outermost sphere of the universe that never changed.
In 1609, Italian scientist Galileo Galilei heard about the invention of a spyglass. He made one for himself and turned it on the heavens. One of his first discoveries was of four moons circling the planet Jupiter. Galileo's telescope revealed a miniature version of Copernicus' solar system, with the moons moving around the planet in simple, circular orbits. Galileo's discoveries forever changed the face of astronomy.
The beginnings of modern science can be attributed to Galileo and to the British genius Isaac Newton. Newton was born in the same year that Galileo died. Isaac Newton took known facts and used mathematics to explain them. He developed mathematical laws that explained how objects move on Earth as well as in space. Newton explained the movement of orbiting planets as the result of motion along a straight line combined with the gravitational pull of the Sun. His laws are all based on the idea that nothing is naturally at rest. He reasoned that all heavenly bodies are constantly moving, with no limits on space and time.
In 1917, Albert Einstein proposed a description of the universe based on his Theory of General Relativity. Einstein's theory inspired many other scientists, including Willem de Sitter in Holland and Alexandr Friedmann in Russia. In fact, much of today's cosmology is based on Freedman's solutions to the mathematical equations included in Einstein's Theory. Friedmann built on the General Relativity equations to develop models that helped explain the evolution of the universe.
image taken by Edwin Hubble in 1923 which showed fuzzy patches were actually whole other galaxies
A major breakthrough in our understanding of the universe took place in the 1920's thanks to American astronomer Edwin Hubble. For centuries, astronomers believed that the Milky Way made up the entire universe. Hubble was among the first to show that the fuzzy patches in the sky seen through telescopes were other galaxies, not distant parts of the Milky Way. By looking at the spectra of these galaxies, he concluded they were speeding away from us - that the universe was expanding!
Georges Lemaitre, a Belgian astrophysicist and Catholic priest, came to be known as the "Father of the Big Bang". Lemaitre proposed that the universe began as a single primordial atom of energy, something hot and dense that exploded, causing space to expand outward. In the late 1940s, George Gamow, a Russian-American physicist, conceived of the Big Bang theory as we know it today. He and his colleagues proposed that if a big bang had occurred, it would have left an afterglow, traces of background radiation that would still be present.
artist's conception of big bang explosion
In 1965, physicists Arno Penzias and Robert Wilson started to search for faint radio signals (actually microwaves) from the outskirts of the Milky Way Galaxy. While conducting their investigations, they actually found the afterglow predicted by Gamow. It was important evidence that the universe began with a hot big bang. More recently, NASA's COBE satellite measured this radiation in great detail. All of the measurements were consistent with the Big Bang theory.
In 1979, particle physicist Alan Guth performed calculations that led to the idea of "cosmic inflation", a brief period of rapid expansion in the early universe. Inflation solves many problems with the simple, original Big Bang. It explains why the universe is so big and so smooth, why at least four different forces act in it today, and where the large amounts of matter that make up the universe came from.
The idea that the universe had a specific beginning did not appeal to all scientists. The Steady State Theory, developed in 1948, concluded the universe had no beginning and no end. This theory describes an expanding universe that stays in perfect balance like a pool kept full to overflowing by a trickle from a faucet. The "faucet" of the universe would be the continuous creation of matter from energy. Arguments against the Steady State Theory include the discovery of background cosmic heat radiation. The fact that the amount of helium observed in the universe exactly fits what was predicted by the Big Bang and the discovery that galaxies were more crowded together in the past further discredit the Steady State theory.
diagram showing how in the Steady State Theory, new matter must be created to keep the same amount of matter in the same volume of space as the universe expands
Today cosmologists are concerned with the ultimate fate of the universe. Will it expand forever, expand to a certain size and stop, or will it stop and begin to collapse? Data suggesting that the universe is expanding at an accelerating rate were published in 1998. For more than ten years astronomers studied the expansion of the universe by measuring the redshift and brightness of distant supernovae. By 1998, enough information had been gathered to lead scientists to the startling discovery that the expansion of the universe is not slowing but accelerating. The supernova data combined with information from other cosmological studies strongly suggest that the universe is filled with an unidentified form of energy (currently being called "dark energy" since we know nothing about it) that is causing the expansion of the universe to accelerate. If these observations and analyses turn out to be correct, the universe would be expected to continue to expand forever.
The expanding Universe, the dynamic state of the extragalactic realm, is the discovery of which has transformed 20th-century cosmology. The development of general relativity and its application to cosmology by Albert Einstein, Wilhelm de Sitter, and other theoreticians, along with the detection of extragalactic redshift (a shift to the longer wavelengths of light from galaxies beyond the Milky Way) by Vesto Slipher, led to the realization in the 1920s that all galaxies are receding. Edwin Hubble correlated these observations in mathematical form to provide evidence that the universe is expanding. The discovery of the 2.7 K background radiation in 1965 by Arno A. Penzias and Robert W. Wilson is regarded as convincing evidence that the universe originated approximately 15 billion years ago in a very dense and hot state referred to as the big bang.
Observations so far have not succeeded in determining whether the universe is open (of infinite extent in space) or closed (of finite extent) and whether the universe in the future will continue to expand indefinitely or will eventually collapse back into an extremely dense, congested state.
Excerpt from the Encyclopedia Britannica without permission.
[How do you reconcile the universe not having an expansion center with observations that galaxies are moving away from us?
You can see this for yourself by looking at the surface of a balloon speckled with 'galaxies'. As you inflate the balloon, every spot moves away from every other spot and if you were on one of these you would see yourself at the center of the expansion. But in fact, the center of the expansion is at the center of the balloon, not on the surface of the balloon. An analogous effect happens in general relativity when considering the universe produced by a Big Bang.
Copyright 1997 Dr. Sten Odenwald Return to Special & General Relativity Questions and Answers page. ](https://einstein.stanford.edu/content/relativity/q2262.html)
By what method is the size (or volume) of the universe at the moment of the big bang (or shortly thereafter) determined? Related to this, how is the size of a black hole determined?
Asked by: Todd Andresen
Answer
Actually, we really don't know the absolute size of the universe right now, but the 'Big Bang' is /defined/ as the point when the universe had zero volume. The way it works is that we know, from experiments that the universe is expanding. We've even managed to measure the expansion rate moderately well. So we look for solutions of Einstein's equations of general relativity that have an expanding, homogeneous and isotropic universe. To solve this we need to know the expansion rate today (which we know) and the size today (which we don't really know). However, we can solve for the /ratio/ of the size at some past (or future) time to the size today. If you solve the equation for the time in the past when the ratio was zero, you call that the time of the 'Big Bang' and you've just computed the age of the universe.
So the basic answer is that we can't really measure the absolute size of the universe (much of which may even be outside our current horizon) but we can solve for the size relative to the size today.
As for black holes, their size is determined in a completely different way. By the 'size' of a black hole we typically mean the Schwarzschild radius (or 'event horizon' as people like to say). This radius is determined solely by the mass of the black hole and is given by:
R_s = 2mG_N/c^2
We can then measure the size of the black hole by measuring it's gravitational attraction with other objects near it (like stars) -- and hence its mass. Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley
[how big is the universe? is it finite or infinite? ChatGPT
The size of the universe is not well defined, as it appears to be infinite in extent, at least as far as we can observe. The observable universe refers to the part of the universe that we can see from our vantage point on Earth, limited by the speed of light and the age of the universe. This region is estimated to be about 93 billion light-years in diameter, based on measurements of the cosmic microwave background radiation and other astronomical observations.
However, the observable universe is just a small part of the entire universe, which is believed to be much larger, and perhaps even infinite in size. Current models of the universe suggest that it is spatially flat, meaning that it has no overall curvature, and is infinite in extent. This is consistent with measurements of the cosmic microwave background radiation, which show that the geometry of the universe is consistent with a flat, infinite universe.
It is also possible that the universe could be finite in size but still have no boundary, similar to the way in which the surface of a sphere is finite but has no edges or boundaries. This is a topic of ongoing research and debate in cosmology, and the true nature of the size and shape of the universe is still not fully understood.](https://chat.openai.com/?model=text-davinci-002-render-sha)
[what it means for the universe to be infinite? ChatGPT
When we say that the universe is infinite, we mean that there is no limit to its size or extent. In other words, it goes on forever and has no boundaries or edges. This means that no matter how far we travel in any direction, we would never reach the "edge" of the universe or encounter a boundary beyond which there is nothing.
The concept of an infinite universe can be difficult to grasp, as it is not something that we can directly observe or experience. However, current models of the universe based on observations and theoretical predictions suggest that it is indeed infinite in extent, at least on large scales.
One consequence of an infinite universe is that there must be an infinite number of stars, galaxies, and other structures within it. This is because if the universe were finite in size, there would be a limit to the number of objects that could exist within it, and eventually we would encounter empty space beyond which there is nothing.
The idea of an infinite universe also raises interesting philosophical and theological questions, such as the nature of existence and the possibility of multiple copies of ourselves or other intelligent life existing elsewhere in the vast expanse of space.](https://chat.openai.com/?model=text-davinci-002-render-sha)
Created Κυριακή 27 Σεπτεμβρίου 2020
Brief History of the Universe
NASA - Brief History of the Universe
Our universe began in a tremendous explosion known as the Big Bang about 13.7 billion years ago (left side of strip). Observations by NASA's Cosmic Background Explorer and Wilkinson Anisotropy Microwave Probe revealed microwave light from this very early epoch, about 400,000 years after the Big Bang, providing strong evidence that our universe did blast into existence. Results from the Cosmic Background Explorer were honored with the 2006 Nobel Prize for Physics.
A period of darkness ensued, until about a few hundred million years later, when the first objects flooded the universe with light. This first light is believed to have been captured in data from NASA's Spitzer Space Telescope. The light detected by Spitzer would have originated as visible and ultraviolet light, then stretched, or redshifted, to lower-energy infrared wavelengths during its long voyage to reach us across expanding space. The light detected by the Cosmic Background Explorer and the Wilkinson Anisotropy Microwave Probe from our very young universe traveled farther to reach us, and stretched to even lower-energy microwave wavelengths.
Astronomers do not know if the very first objects were either stars or quasars. The first stars, called Population III stars (our star is a Population I star), were much bigger and brighter than any in our nearby universe, with masses about 1,000 times that of our sun. These stars first grouped together into mini-galaxies. By about a few billion years after the Big Bang, the mini-galaxies had merged to form mature galaxies, including spiral galaxies like our own Milky Way. The first quasars ultimately became the centers of powerful galaxies that are more common in the distant universe.
NASA's Hubble Space Telescope has captured stunning pictures of earlier galaxies, as far back as ten billion light-years away.
Image credit: NASA/JPL-Caltech
Created Κυριακή 27 Σεπτεμβρίου 2020
Μια σύντομη ιστορία του διαστήματος
ΕΛΛΗΝΙΚΗ ΕΤΑΙΡΙΑ ΣΠΟΥΔΩΝ ΦΥΣΙΚΗΣ ΥΨΗΛΩΝ ΕΝΕΡΓΕΙΩΝ
ΕΛΛΗΝΙΚΗ ΟΜΑΔΑ ΕΚΛΑΪΚΕΥΣΗΣ OUTREACH GROUP
Μία σύντομη ιστορία του Διαστήματος
Peter Kalmus
Στην αριστερή φωτογραφία φαίνεται το πεδίο γύρω από το Σουπερνόβα SN1987A και στη δεξιά φωτογραφία φαίνεται το ίδιο πεδίο δύο εβδομάδες μετά την εμφάνιση του Σουπερνόβα. Η διαφορά στην ποιότητα της εικόνας είναι αποτέλεσμα της γήινης ατμόσφαιρας, που ήταν πιο σταθερή στην πρώτη περίπτωση. Και οι δύο φωτογραφίες προέρχονται από υλικό του Αγγλο-Αυστραλιανού Τηλεσκοπίου.
Στις 23 Φεβρουαρίου του 1987 μία έκρηξη δισεκατομμύρια δισεκατομμυρίων δισεκατομμύρια φορές δυνατότερη από την έκρηξη μιας υδρογονοβόμβας ανιχνεύτηκε στη Γη. 'Ηταν το Σουπερνόβα 1987Α, το πρώτο άστρο που εξερράγη και έγινε αντιληπτό με γυμνό μάτι, μετά από εκείνο που παρατηρήθηκε από τον Κέπλερ το 1604. Το άστρο, 170.000 έτη φωτός μακριά, στο Μεγάλο Νεφέλωμα του Μαγγελάνου, ξέμεινε από πυρηνικά καύσιμα, κατέρρευσε από την επιρροή της ίδιας του της βαρύτητας και, σε λίγα δευτερόλεπτα, απελευθέρωσε εκατό φορές περισσότερη ενέργεια από όση έχει απελευθερώσει ο 'Ηλιος μας σε ολόκληρη τη ζωή του.
Παρ' όλα αυτά, πριν ακόμα ένας Καναδός αστρονόμος σε ένα βουνό της Χιλής παρατηρήσει πρώτος το φως του Σουπερνόβα 1987Α, αγγελιοφόροι-φαντάσματα που ονομάζονται νετρίνα καταγράφηκαν σε δύο τεράστιους υπόγειους ανιχνευτές, στις Ηνωμένες Πολιτείες και στην Ιαπωνία. Αυτοί οι ανιχνευτές, που αποτελούνται από μερικές χιλιάδες τόνους πολύ καθαρού νερού, εξοπλισμένοι με φωτοπολλαπλασιαστές και ηλεκτρονικά συστήματα, είχαν κατασκευαστεί για έναν εντελώς διαφορετικό σκοπό. Είχαν σχεδιαστεί για να ελέγξουν κατά πόσον τα πρωτόνια είναι σταθερά και αν είναι δυνατό να υφίστανται μια πολύ αργή ραδιενεργή διάσπαση. Ακόμα και σήμερα δεν έχουν παρατηρηθεί τέτοιες διασπάσεις πρωτονίων, αλλά η ανίχνευση των νετρίνων που προέρχονταν από το Σουπερνόβα έδωσε σημαντικές πληροφορίες και γι' αυτά τα σωματίδια και για την κατάρρευση των άστρων, και απέδειξε θεαματικά την αλληλεπίδραση της αστρονομίας με τη σωματιδιακή φυσική.
Βεβαίως, η μεγαλύτερη έκρηξη απ' όλες ήταν η Μεγάλη 'Εκρηξη (Big Bang) - η δημιουργία του σύμπαντος, περίπου 12 δισεκατομμύρια χρόνια πριν. Το πρώιμο σύμπαν ήταν απίστευτο - μια πυκνή αρχέγονη σούπα από στοιχειώδη σωματίδια, που συγκρούονταν συνεχώς μεταξύ τους σε καταπληκτικά μεγάλες ενέργειες - ένα εκτυφλωτικό σόου από πυροτεχνήματα. Πραγματικά, το σημερινό σύμπαν με όλη την ομορφιά και την ποικιλομορφία του είναι απλώς ο καπνός που απέμεινε από τη γιορτή των πυροτεχνημάτων.
Η σύγχρονη φυσική των σωματιδίων μας επιτρέπει, με έναν τρόπο, να αναδημιουργήσουμε κάποιες από τις συνθήκες που επικρατούσαν στο πρώιμο σύμπαν. Οι αναγνώστες που έχουν περάσει πολλές ώρες μελετώντας την ιστορία των λίγων τελευταίων χιλιάδων ετών, ίσως μείνουν ευχαριστημένοι βλέποντας την ιστορία του σύμπαντος σε ένα μάλλον απλό διάγραμμα. Σ' αυτό έχει σχεδιαστεί η θερμοκρασία του σύμπαντος σε βαθμούς Κέλβιν στον δεξιό άξονα-y ως προς το χρόνο σε δευτερόλεπτα, που έχει σχεδιαστεί στον κάτω άξονα-x. Και οι δύο άξονες είναι λογαριθμικοί. Στον αριστερό άξονα-y είναι σχεδιασμένη η μέση ενέργεια ανά σωματίδιο, που είναι ανάλογη της θερμοκρασίας. Η πυκνότητα ενέργειας-μάζας του σύμπαντος είναι σχεδιασμένη στον πάνω άξονα-x, σε μονάδες πυκνότητας μάζας ισοδύναμης με την πυκνότητα του γήινου νερού.
Οι επιταχυντές LEP του CERN και Tevatron του Fermilab έχουν ενέργειες γύρω στα 100 GeV ανά στοιχειώδες συστατικό (κουάρκ ή λεπτόνιο), και τέτοιας τάξης μεγέθους ενέργειες ήταν οι επικρατούσες όταν το σύμπαν είχε θερμοκρασία 1015 βαθμούς Κέλβιν, περίπου 10-11 δευτερόλεπτα μετά το Big Bang. Τα σωματίδια που αποτελούσαν το σύμπαν -ακόμα και τα νετρίνα- ήταν σε σχεδόν τέλεια ισορροπία μεταξύ τους. Η εξαΰλωση και η δημιουργία σωματιδίων ήταν ισορροπημένες. Το σύμπαν τότε, όπως και τώρα, περιείχε σημαντικά περισσότερα φωτόνια από κουάρκ, και οι ενέργειες ανά κουάρκ ή λεπτόνιο ήταν πολύ μεγαλύτερες από τις αντίστοιχες μάζες ηρεμίας, έτσι το σύμπαν περιγράφεται με ακρίβεια ως "κυριαρχούμενο από ακτινοβολία". Καθώς το σύμπαν διαστελλόταν, το μήκος κύματος της ακτινοβολίας απλώθηκε τόσο ώστε τα φωτόνια απέκτησαν χαμηλότερες ενέργειες. Επίσης, η συγκέντρωση στοιχειωδών σωματιδίων μειώθηκε, κι έτσι το σύμπαν "κρύωσε". Καθώς ακολουθούμε αυτή τη θερμική ιστορία, συμβαίνουν κάποια αξιοσημείωτα γεγονότα, που οδηγούν στο σημερινό κόσμο μας.
Γύρω στα 10^-6 δευτερόλεπτα η μέση ενέργεια είχε πέσει σε λίγα γίγα-ηλεκτρονιοβόλτ, και τα κουάρκ μπορούσαν να ενωθούν δημιουργώντας αδρόνια, και λίγο αργότερα σταθερά πρωτόνια και (σχετικά σταθερά) νετρόνια. Περίπου στο ένα δευτερόλεπτο, παρ' όλο που η πυκνότητα ήταν ακόμα μερικές εκατοντάδες χιλιάδες φορές μεγαλύτερη από την πυκνότητα του νερού, οι συγκρούσεις νετρίνων άρχισαν να γίνονται σπάνιες - δεν μπορούσαν πλέον να είναι σε θερμική ισορροπία με άλλα σωματίδια και ουσιαστικά αποσυζεύκτηκαν για πάντα από την υπόλοιπη ύλη και ακτινοβολία. Μετά από λίγα ακόμη δευτερόλεπτα, με την ενέργεια να έχει πέσει κάτω από το επίπεδο του μεγα-ηλεκτρονιοβόλτ, δεν μπορούσαν πια να δημιουργηθούν ηλεκτρόνια και ποζιτρόνια, κι έτσι εξαϋλώθηκαν, αφήνοντας έναν ικανό αριθμό ηλεκτρονίων για να αντισταθμίσει το φορτίο των πρωτονίων που υπήρχαν.
Κάποια από τα πρωτόνια και τα νετρόνια συνενώθηκαν σε δευτερόνια, και μετά σε σωματίδια άλφα, πριν η πυκνότητα και ο ρυθμός συγκρούσεων γίνει πολύ χαμηλός. Τότε όσα νετρόνια είχαν μείνει ασύζευκτα διασπάστηκαν τις επόμενες ώρες. Οι μετρήσεις των ποσοτήτων δευτερίου και ηλίου που μπορούν να γίνουν σήμερα αποτελούν έναν κρίσιμο έλεγχο των συνθηκών που επικρατούσαν τότε, και επομένως έναν σημαντικό έλεγχο του ίδιου του μοντέλου της Μεγάλης 'Εκρηξης. Μικροσκοπικές τροχιές ισοτόπων λιθίου θα μπορούσαν να έχουν δημιουργηθεί, αλλά η απουσία σταθερών πυρήνων μάζας 5 και 8 απέτρεψαν τη δημιουργία περισσότερων πυρήνων. Η σύνθεση πυρήνων άρχισε να συμβαίνει περίπου στα 3 λεπτά.
Μετά από περίπου 300.000 χρόνια η θερμοκρασία είχε πέσει γύρω στους 104 βαθμούς Κέλβιν και η μέση ενέργεια γύρω στο 1 eV, κάτω από το δυναμικό ιονισμού των ατόμων. Ουδέτερα άτομα υδρογόνου και ηλίου δημιουργήθηκαν. Τα φωτόνια δεν εμποδίζονταν πια από τις συχνές αλληλεπιδράσεις με την ύλη (συζεύκτηκαν με φορτισμένα σωματίδια), και το σύμπαν, που μέχρι τότε ήταν αδιαφανές, έγινε διαφανές. Η επικρατούσα μορφή ενέργειας μετά απ' αυτό ήταν η μάζα (που περιείχε και σκοτεινή ύλη, η φύση της οποίας δεν έχει ακόμα εξακριβωθεί), ενώ στην προηγούμενη φάση ήταν η ακτινοβολία. Εντούτοις, οι θερμοκρασίες και οι ενέργειες που φαίνονται στο διάγραμμα, ακόμα και στην εποχή που επικρατούσε η ύλη, αντιπροσωπεύουν την εποχή της κυριαρχίας της ακτινοβολίας. Με τη διαστολή του σύμπαντος, αυτή η ακτινοβολία έχει τώρα κρυώσει στους 2,7 βαθμούς Κέλβιν, και λέγεται κοσμική ακτινοβολία υποβάθρου.
Η βαρύτητα, που εμφανιζόταν σε κυματώσεις πυκνότητας που έχουν ανιχνευθεί σήμερα ώς μικρές ανισοτροπίες της κοσμικής ακτινοβολίας υποβάθρου, ανάγκασε την ύλη να σχηματίσει μορφώματα, που αργότερα έγιναν άστρα και γαλαξίες. 'Ετσι τα αντικείμενα της αστρονομίας τελικά κάνουν την παρουσία τους στην κάτω δεξιά γωνία του διαγράμματος. Τα πρώτα άστρα συντέθηκαν μόνο από υδρογόνο και ήλιο. Διαδικασίες σύντηξης και άλλες πυρηνικές αντιδράσεις στους πυρήνες των άστρων δημιούργησαν όλα τα υπόλοιπα στοιχεία.
Τα πιο μεγάλα σε μάζα άστρα είχαν μικρότερη διάρκεια ζωής, και κάποια εκρήγνυνταν σαν σουπερνόβα, μολύνοντας έτσι τη γειτονιά τους με αυτά τα χημικά, και συμβάλλοντας στη δημιουργία του μείγματος των στοιχείων από τα οποία τα πιο νέα άστρα, μεταξύ των οποίων και ο 'Ηλιος μας και το ηλιακό του σύστημα, μπόρεσαν να δημιουργηθούν. Κάθε άτομο άνθρακα – o βαρύτερος πυρήνας που υπάρχει πάνω στη Γη και στα σώματά μας – δημιουργήθηκαν στον πυρήνα κάποιο άστρου που τώρα έχει εκραγεί. Είμαστε όλοι φτιαγμένοι από αστρικό υλικό!
Μέχρι πρόσφατα, οι κοσμολογικές μετρήσεις ήταν συνεπείς με το μοντέλο της Μεγάλης Έκρηξης και της ακόλουθης διαστολής του σύμπαντος, που αντιτίθεται στην έλξη της βαρύτητας. Ανάλογα με την πυκνότητα μάζας-ενέργειας του σύμπαντος, αυτό θα μπορούσε να οδηγήσει σε συνεχή διαστολή ή στην τελική συστολή: τη Μεγάλη Συμπύκνωση. Οι περισσότερες κοσμολογικές μετρήσεις δεν ήταν ιδιαίτερα ακριβείς. Λίγα χρόνια πριν, κάποιες μετρήσεις έδειξαν ότι η ηλικία του σύμπαντος ήταν λίγο μικρότερη από την ηλικία κάποιων αστεριών, αλλά, εξαιτίας των σφαλμάτων παρατήρησης, ένας παράγοντας μικρότερος από δύο δεν προκάλεσε υπερβολική ανησυχία.
Παρ' όλα αυτά οι παρατηρήσεις βελτιώνονται συνέχεια. Τα αποτελέσματα του τελευταίου έτους από δύο συνεργασίες υποδηλώνουν ότι η διαστολή του σύμπαντος μπορεί στην πραγματικότητα να επιταχύνεται: πολύ μακρινά σουπερνόβα φαίνονται πιο αμυδρά από το αναμενόμενο, δίνοντας μία ένδειξη ότι μπορεί να είναι πιο μακριά από ό,τι δείχνει η ερυθρή τους μετατόπιση. Αυτή τη στιγμή γίνονται προσεκτικοί έλεγχοι στις αναλύσεις των σουπερνόβα, ενώ κι άλλα μακρινά σουπερνόβα βρίσκονται τώρα στο βεληνεκές των υπαρχόντων τηλεσκοπίων.
Οι παρατηρήσεις μπορούν να εξηγηθούν αν επικαλεστούμε την κοσμολογική σταθερά του Αϊνστάιν - ένα είδος κοσμικής απώθησης της αρνητικής πίεσης κενού, την οποία αργότερα ο Αϊνστάιν θεώρησε ως το "μεγαλύτερό του λάθος". Αποκλίσεις, όπως η "πεμπτουσία" -μία πέμπτη δύναμη που μεταβάλλεται με το χρόνο-, κερδίζουν ολοένα μεγαλύτερη προσοχή. Η πυκνότητα ενέργειας που σχετίζεται με μία κοσμολογική σταθερά θα μπορούσε να έχει επηρεάσει τις πρώιμες δομές του σύμπαντος και έτσι να έχει προκύψει η γωνιακή ανισοτροπία της κοσμικής ακτινοβολίας υποβάθρου - οι ρυτίδες του σύμπαντος.
Οι διακυμάνσεις της κοσμικής ακτινοβολίας υποβάθρου υπολογίζονται με ικανοποιητική ακρίβεια σε μικρότερες γωνιακές κλίμακες, και η ανάλυσή τους δείχνει μεγαλύτερη συνέπεια με τη συμπεριφορά των σουπερνόβα και με άλλες παρατηρήσεις εάν επιτραπεί μία πρόσθετη κοσμική δύναμη απώθησης. Η πυκνότητα ενέργειας που συνδέεται με την κοσμολογική σταθερά φαίνεται ότι είναι μεγαλύτερη από τη σταθερά μάζας. Νέα αποτελέσματα ανισοτροπίας της κοσμικής ακτινοβολίας υποβάθρου που προκύπτουν από το πρόγραμμα Boomerang που βρίσκεται πάνω σε ένα αερόστατο πρόκειται να ανακοινωθούν σύντομα, και θα ακολουθήσουν στα επόμενα χρόνια οι αποστολές MAP της NASA και Planck της ESA (European Space Agency). Η εποχή της κοσμολογίας ακριβείας φαίνεται ότι μόλις αρχίζει, και η συμβίωσή της με τη σωματιδιακή φυσική θα οδηγήσει σε πιο συναρπαστική επιστήμη το νέο αιώνα. Peter Calmus , Περιοδικό CERN Courier, Vol 40(2), Μαρτιος 2000, http://www.cerncourier.com Μετάφραση: Ε. Συμεωνίδου, elenasym@yahoo.com
Ο Peter Kalmus ασχολείται με πειράματα σωματιδιακής φυσικής στις Ηνωμένες Πολιτείες για πάνω από 40 χρόνια. Μοιράστηκε, με τον John Dowell, το Μετάλλιο Rutherford του Ινστιτούτου Φυσικής το 1988, για τη συμβολή του στην ανακάλυψη των σωματιδίων W και Z στο CERN το 1983. Είναι αντιπρόεδρος του Ινστιτούτου Φυσικής και πρόεδρος της Επιτροπής Σωματιδίων και Πεδίων του IUPAP (International Union of Pure and Applied Physics). Από τότε που αποσύρθηκε τυπικά το 1998, έχει αφιερώσει πολύ από το χρόνο του προς την κατεύθυνση της ευρύτερης κατανόησης της φυσικής επιστήμης συνεισφέροντας σε οργανισμούς όπως το Βασιλικό Ινστιτούτο και η Βρετανική 'Ενωση για την πρόοδο της Επιστήμης και δίνοντας δημόσιες ομιλίες. Μια γραπτή έκδοση της ομιλίας του δημοσιεύτηκε στο Physics Education (1999 34(2)).
...πίσω στον κατάλογο των άρθρων
Created Πέμπτη 11 Μαΐου 2023
[Why We Still Don't Know How Fast the Universe is Expanding Behind the astronomical dispute that’s splitting apart the cosmos. By Corey S. PowellJan 20, 2019 2:00 AM Observatory - Fermilab Observational data has led astronomers to differing conclusions about how fast the cosmos is expanding. (Credit: Reidar Hahn/Fermilab)
Back in the 1980s, astronomers were caught up in a debate so huge, you could drive a universe through it. The point of contention was a number called the Hubble constant, which describes the rate at which the cosmos is expanding and, by extension, how much time has passed since the Big Bang: the slower the expansion rate, the older the universe.
On one side was Allan Sandage, the towering successor to Edwin Hubble at Mount Wilson Observatory, who calculated the age of the universe was roughly 20 billion years. On the other side was a group of apostates whose observations showed it was only about half that old. Tempers ran hot.
The Hubble constant is measured in arcane units (kilometers per second per megaparsec), so for brevity, the researchers would use just the number itself. “We measure the Hubble constant to be 100,” one of the upstarts would announce in a talk. “It’s 50,” Sandage would roar in response, ridiculing his colleagues for their flawed measurements. For me, a young reporter covering the field at the time, it was a revelatory display of scientific passion. The entire history of the universe was at stake!
Three decades later, a revived debate on the Hubble constant has our understanding of the universe hanging in the balance all over again. At least the old shouting matches are long settled. Vastly improved data from the Hubble Space Telescope and the Planck satellite showed that Sandage and his rivals were both wrong; the true age of the universe lies in between, at 13.8 billion years. With this new clarity, though, a new conflict has popped into view.
Measurements of the Hubble constant made by directly observing stars yield a cosmic expansion rate of 73. Measurements made by studying the distant microwave glow of the early universe give a value of about 68. Although the gap is small, it has proven persistent. Resolving the earlier debate led to the startling realization that we live in an accelerating universe being pushed apart by a mysterious “dark energy.” The implications of this latest debate could be equally momentous.
Adam Riess at the Space Telescope Science Institute, who shared the 2011 Nobel Prize in physics for the discovery of dark energy, is alternately amazed and amused by the latest turn of events. “Who would have thought we’d be measuring the Hubble constant and seeing differences at just the 10 percent level and saying, ‘Hmm, there’s something wrong here?’ ” he asks. “But here we are.”
Three-Steps-to-Hubble-Constant - NASA etc The parallax effect changes the apparent position of an object depending on the point of observation — it’s why your fingers can jump around if you look at them with just your left eye, then your right eye. By measuring exactly how much stars jump between Earth’s position in June and its position in December, astronomers can deduce their rough distances. (Credits: NASA, ESA, A. Feild/STScI, A. Riess/STScI/JHU)
Climbing the Ladder
Riess and his team specialize in the star-oriented measurements of the Hubble constant — what’s known as the distance ladder approach. The researchers begin with relatively nearby objects whose distance they can measure accurately, then use those objects to calibrate distances to things farther away, and so on, like climbing the rungs of a ladder. Once you know true distances to cosmic objects, you can combine that information with measurements of their motion to derive the expansion rate of the universe.
The distance ladder is how Edwin Hubble discovered the universe was expanding at all, back in 1929, but using it to deduce the rate of the expansion proved problematic. The huge challenge was calibrating the first rung on the ladder, to a type of star called a Cepheid variable. These pulsate in a rhythm that depends on their intrinsic luminosity. If you measure a Cepheid’s period of pulsations, you’ll know how bright the star really is. Compare that with how bright it appears, you can figure out how far it is — and away you go.
Unfortunately, that Cepheid trick gives only relative distances. To get actual distances, you need to know the exact locations of a few nearby Cepheid stars to use as references for all the rest. “This is what plagued people 20 or 25 years ago. They had so much difficulty calibrating anything in an absolute sense,” Riess says. He has spent much of his career trying to rid cosmology of that plague.
In Sandage’s day, the Hubble constant was uncertain by about 50 percent. By 2001, data from Hubble pushed the error down to 10 percent. In his most recent paper, Riess claims an accuracy of 2.2 percent — “and it’s possible to push closer to 1 percent. That’s my goal over the next few years, and that’s looking feasible,” he says. As a result, he’s confident when he says the value of the Hubble constant is 73.
Echoes of Expansion
This all sounds great until you talk to the equally persuasive scientists who work with data from the Planck satellite. From 2009 to 2013, Planck made exacting observations of the cosmic microwave background, the relic radiation from the Big Bang. Whereas the distance-ladder folks begin nearby, Planck researchers go deep, studying radiation from the very edge of the visible universe and examining it for patterns that reveal the expansion history of the universe.
It’s a cunning approach, liberated from the uncertainties of local cosmic measurements. And here the disagreements begin.
“With the Planck data, we get a Hubble constant of 68 or 67,” says astrophysicist Jo Dunkley of Princeton University. The discrepancy with the distance ladder results became glaring enough that Dunkley wanted to make sure the Planck researchers weren’t fooling themselves in any way.
The task ended up falling to one of Dunkley’s former graduate students, Graeme Addison, now a cosmologist at Johns Hopkins University. Addison began exploring yet another way to reckon the expansion of the universe, this time by looking at vibrations from the Big Bang that left an enormous pattern of ripples etched into the distribution of galaxies across the universe. That pattern is visible in the latest large-scale surveys. The size of the cosmic ripples, when combined with other information about the Big Bang, yields a measurement of the Hubble constant.
“It’s a third opinion on what’s going on,” Addison says. That opinion aligns with Planck’s value of 67, “which suggests pretty strongly that you can’t blame this discrepancy all on the Planck data.” Expanding Universe - Kelly/Discover We know the universe is expanding, but the conflicting numbers for the rate could mean greater mysteries, and surprises, are in the future. (Credit: Roen Kelly/Discover)
What the Heck is Going On?
Compared with old-school brawlers like Sandage, today’s researchers are a genial and cautious bunch. They begin, sensibly, by considering human error: the possibility that somebody messed up in collecting or analyzing the data. But that’s looking increasingly unlikely.
“If you’d asked me three years ago, I’d have said, ‘The distance ladder is pretty complicated, and there’s astrophysics that needs to be understood.’ But my opinion on that has changed because of the work, mainly from Riess and his collaborators,” Addison says. “They’ve revisited the steps of the distance ladder, done statistical tests, and none of that analysis has shifted the Hubble constant anything near the amount you need to reconcile with Planck.”
What’s most troubling — and exciting — is that so many lines of evidence converge on two inconsistent answers. The persistent gap is forcing cosmologists to consider that both measurements might be correct. Perhaps the universe has a split identity: Maybe the early universe studied by Planck, and the late universe studied by the Hubble telescope, really were inconsistent, due to some undiscovered aspect of how the universe works.
Riess ticks off a range of possibilities, any one of which would qualify as a major discovery. Space itself could have a slight curvature. There could be an unknown type of neutrino, a ghostly type of particle that rarely interacts with matter. Dark matter and dark energy could have “funny funky” properties.
Dunkley suspects that the problems with current scientific understanding may go even deeper than the kinds of relatively minor adjustments that Riess describes. “There’s no single extension of the standard model of cosmology that can explain the Planck data and the local measurements and this vast suite of other data,” she says. “We’ve also got big issues to figure out, like why is the universe accelerating, and why did it begin expanding in the first place? I would not be surprised if we’ve got a major upheaval coming.”
The current debate may seem smaller than the one that came before, but it could prove large enough to deliver a whole new universe.](https://www.discovermagazine.com/the-sciences/why-we-still-dont-know-how-fast-the-universe-is-expanding)
Created Τετάρτη 10 Μαΐου 2023
[give evidence indicating the flatness of the universe ChatGPT
The flatness of the universe refers to the idea that the universe has zero curvature, which is one of the key predictions of the inflationary big bang model. There are several lines of evidence that suggest that the universe is indeed flat:
Cosmic Microwave Background Radiation: The cosmic microwave background radiation (CMB) is the oldest light in the universe, which was emitted about 380,000 years after the Big Bang. The CMB contains small temperature fluctuations that are directly related to the geometry of the universe. Measurements of these fluctuations by the Planck satellite and other experiments have shown that the universe is flat to within a few percent.
Large Scale Structure: The distribution of galaxies in the universe is also related to the geometry of the universe. The observed large scale structure of the universe, including galaxy clusters and superclusters, suggests that the universe is flat.
Supernova Data: Observations of distant supernovae have shown that the expansion of the universe is accelerating, which suggests the presence of dark energy. The properties of dark energy, as inferred from these observations, are consistent with a flat universe.
Baryon Acoustic Oscillations: Baryon acoustic oscillations (BAOs) are subtle ripples in the distribution of matter in the universe, which were imprinted in the early universe. Observations of BAOs in the distribution of galaxies provide further evidence that the universe is flat.
Taken together, these lines of evidence suggest that the universe is flat with a high degree of confidence.](https://chat.openai.com/?model=text-davinci-002-render-sha)
[what are the implications for our understanding of gravity and relativity of the flatness of the universe ChatGPT
The flatness of the universe has significant implications for our understanding of gravity and relativity.
First, the flatness of the universe is consistent with Einstein's theory of general relativity, which describes gravity as the curvature of spacetime. In a flat universe, the curvature of spacetime is zero, which means that the geometry of the universe is consistent with the predictions of general relativity.
Second, the flatness of the universe also implies that the total energy of the universe is precisely zero. According to the laws of physics, energy cannot be created or destroyed, so the total energy of the universe must always be conserved. In a flat universe, the total energy is balanced between the positive energy of matter and radiation and the negative energy of gravity. This balance is required for the universe to be flat, and it provides important constraints on theories of the early universe, such as inflation.
Finally, the flatness of the universe also has implications for the fate of the universe. If the universe were not flat, the geometry of the universe would change over time, and the universe would either collapse or expand indefinitely. However, in a flat universe, the geometry remains constant over time, and the universe will continue to expand forever, albeit at a decreasing rate due to the effects of dark energy.
Overall, the flatness of the universe is a key prediction of modern cosmology, and it provides important constraints on theories of gravity and the early universe.](https://chat.openai.com/?model=text-davinci-002-render-sha)
Created Πέμπτη 11 Μαΐου 2023
[Does the Universe go on Forever? Is outer space finite or infinite? Explore this age-old question. By Korey HaynesMar 2, 2020 11:12 PM universe history timeline - NASA This illustration summarizes the almost 14-billion-year-long history of our universe. (Credit: NASA/JPL-CalTech)
First, it’s still possible the universe is finite. All we know for sure (mostly for sure) is that it’s bigger than we can observe, essentially because the farthest edges of the universe we can see don’t look like edges. The observable universe is still huge, but it has limits. That’s because we know the universe isn’t infinitely old — we know the Big Bang occurred some 13.8 billion years ago.
That means that light has had “only” 13.8 billion years to travel. That’s a lot of time, but the universe is big enough that scientists are pretty sure that there’s space outside our observable bubble, and that the universe just isn’t old enough yet for that light to have reached us.
(In a fun twist, the universe has expanded since the Big Bang, so the objects that we see emitting light 13.8 billion years ago are now some 46 billion light-years away.)
A few generations of telescopes — COBE, WMAP, and Planck — sought to understand the shape of the universe. Was it curved like a ball, or flat like a piece of paper? If it were curved, and if we could measure the curve, we could work out the size of the universe.
But all three telescopes say the universe is flat, which tells us nothing about the size of the universe.](https://www.discovermagazine.com/the-sciences/does-the-universe-go-on-forever)
Created Κυριακή 27 Σεπτεμβρίου 2020
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time. -- T. S. Eliot
Chapter 1: The Question of Life, the Universe, and Everything
Where do we come from? Where, when, and how, did it all start? What's the point of it all (if any)? These questions, simple but fundamental, have endured since the onset of human consciousness and continue to occupy philosophers and thinkers of all persuasions. Way back in the dark ages the popular belief was that the Earth was in the centre of the Universe <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/ptolemy.htm>, and that it was flat <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/flat_earth.gif> (with waterfalls and dragons around the edges) and for all we knew the moon might have been made of green cheese. We know better today. During the Renaissance, the combined efforts of Tycho Brahe, Copernicus, Kepler, and Galileo <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/galileo.htm> helped to knock the Earth (and with it man) from centerstage <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/sisteam.gif> to the more humble place of a planet orbiting around the sun. This paradigm shift, however, was not without pain. The revolutionary change in thinking that it accompanied caused intellectuals to be at odds with the powers that be (princes, kings, and of course the all powerful Catholic Church). Some, like for example Giordano Bruno, got burned at the stake for their aberrations of thought, while others, like Martin Luther, succeeded to cause enough upheaval to change the existing order permanently. I should add here that Luther, a highly religious man, opposed the heliocentric world view. His main quarrel was with the worldly appetites that in his view had corrupted the Catholic Church. He did most likely, however, benefit in his efforts from the intellectual foment of the times.
To confine our attention to terrestrial matters would be to limit the human spirit. Stephen Hawking
When thinking about science, sometimes deceptively simple thought experiments can be quite illuminating. With regard to the Universe, Olbers' Paradox <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/olbers.htm> (or why it should not be dark at night) is one of these.
Over the centuries, whenever new observations and facts suggested that the accepted view of the world was in need of revision, conflicts arose. Either because the new ideas sat uneasy with the establishment, or because spiritual leaders felt that they had to defend religious dogma. These conflicts are far from over (even in our relatively enlightened times), and are likely to persist for a long time to come. Think for example of the recent (8/12/99) decision to remove all mention of evolution from the state school curriculum in Kansas, or of the fatwa issued against Salman Rushdie by Ayatollah Khomeini in February of
<https://geol105.sitehost.iu.edu/images/gaia_chapter_1/vmoonflg.mpg>, we are making great strides to catalog and unearth the mysteries of the human genetic code, and it it is going to get ever more exiting from here on out. In the end the facts will not be hostage to ideology, but rather will speak for themselves. We will boldly go where no man has gone before, and the journey has just begun.
ThreeShips.jpg (4200 bytes)
Some say the world will end in fire, some say ice. From what I've tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice.
-- Robert Frost
Although the main objective of this class is to learn about how the Earth works, it is beneficial to understand how the Earth fits into the "cosmic" scheme of things, and thus we will start with an overview of our current thought about the origin of the Universe and the Solar System.
Something mysteriously formed, Born before heaven and earth. In the silence and the void, Standing alone and unchanging, Ever present and in motion. Perhaps it is the mother of tenthousand things. I do not know its name. Call it Tao. For lack of a better word, I call it great.
– From the Tao Te Ching by Lao Tsu, an early Chinese philosopher of the 6th century B.C.
Big Bang-c.jpg (23947 bytes)
Big Bang animation by Leonard Wikberg III of Science Data <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/bigbang.mpg>
If we look at creation myths over a broad range of cultures, there is the recurring theme that initially all was dark, that light appeared, and that after that things got gradually organized into stars, planets, oceans, land, plants, beasts, etc. (useful references would for example be Joseph Campbell's book "Primitive Mythology", and Sir James George Frazer's "Golden Bough"). In essence the universe (paraphrased as the "tenthousand things" by Lao Tsu), appeared magically out of nowhere. In the King James version of the bible, for example, "And God said, Let there be light : and there was light". Apache and Navaho creation myths state that "In the beginning nothing was here where the world now stands: no earth - nothing but Darkness, Water, and Cyclone". In the Icelandic Eddas, Yamir, the first man was born from the yawning void of the beginning, and he was then cut up to form everything there is in the world today. A survey of the mythologies of many cultures across the world yields comparable creation myths. That the scientific theory, popularly known as the "Big Bang", seems to bear similarity to these myths has encouraged some to claim the theory as proof of divine origin for the universe. As yet, however, we are only at the beginning of a long intellectual journey (the book "A Brief History of Time", by Stephen Hawkin makes for interesting reading in this context). So far the scientists have been, and will be for a long time, busy trying to describe what the universe is. The question of why there is a universe, why it bothers to exist, whether it needs creative input, is an entirely different matter. Perhaps some day we will indeed have enough knowledge and understanding to produce a unified theory of the universe and everything in it. If it is indeed compelling it will over time not only be understood by a few specialists, but all of us will be able to participate in the essential discussion of why we and the universe exist. To arrive at this point will be the ultimate triumph of human reason. May be then it will be our turn to say "Let there be light".
Let us now outline the current view of the history of our universe (there might be others, universes that is) (useful readings on this topic can be found in "Life in the Universe", a special edition of Scientific American from 1994, as well as in the January 2001 issue of Scientific American <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/sciam_peebles.html>; direct link to Scientific American <http://www.sciam.com/2001/0101issue/0101peebles.html>)
This view of a universe that was initially very hot and dense, and then cooled and expanded, is in agreement with the observational evidence we have been able accumulate to date (it's popular name is the Big Bang <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/big_bang_was_not_a_fireworks_dis.htm> theory). There are many details to be answered still, but the theory and its supporting evidence represent one of the great achievements of 20th-century science. We have come a long way from the waterfalls and dragons. Among the questions of interest are for example whether the universe will keep expanding infinitely, or whether it has enough mass to eventually collapse back into itself (the "Big Crunch"). If we take for example all the observable mass (stars, planets, galaxies etc.) it would seem that it should expand indefinitely, but if there is enough "invisible" dark matter <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/dark_matter_in_the_universe.htm> hidden in interstellar gas clouds (sometimes referred to as the "missing mass") it might eventually slow its expansion and collapse. Another complex of questions concerns to earliest history of the Big Bang and how it might have influenced the world we experience today. For example, at stage 3. (see above) particles and antiparticles would have been constantly annihilated and created (by collision), and if there had been a perfect balance between matter and antimatter the universe would be very different. Instead of consisting of matter and energy, it would consist of pure energy (electromagnetic radiation). A little bit more antimatter than matter would have lead to an anti-universe (they may be out there). Scientist estimate that there was initially a 1 part in 10^10 excess of matter over antimatter, and it is this tiny excess that constitutes the mass of our universe. A tiny difference, but one of the key conditions that determined the future development of the universe. All the rest (the 10^10 excess) of our initial universe was transformed into radiation/heat, and spread out evenly throughout the universe. Because the universe has expanded so tremendously this "background" heat has dropped to very low temperature levels (2.726 Kelvin's) and is the cause for the so called thermal cosmic background radiation. This radiation had been predicted from theoretical considerations, and was found to actually exist in the 1960's. It is one piece of physical evidence for the Big Bang.
Read about a new theory, the "Big Splat" <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/big_splat.htm> that has been proposed in April 2001.
Another piece of evidence is the measurable fact that the universe is indeed expanding. That the universe is expanding was discovered by Edwin Hubble <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/edwin_hubble.htm>. The first clue that something like expansion was happening came from the observation of the spectral redshift. Hot bodies such as stars and galaxies send out a continuous light spectrum. This spectrum is a composite of the specific wavelengths emitted by the various chemical elements in the periodic table, and because the latter are not uniform in abundance, there are maxima and minima in the spectrum that can be attributed to emission and absorbtion of light by chemical elements <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/visible_light_spectral_emission_.htm>. This phenomenon can be conveniently studied in the lab and is also used to determine the chemical composition of materials. When we combine a spectrometer with a telescope we can then determine the composition of a star or of any other light source out there. The redshift means that emission and absorbtion maxima within the spectrum of a distant astronomical object seem to shift towards longer wave length <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/spec_figures2.gif>, meaning towards red (red light = long waves; blue light = short waves). Hubble discovered that the farther a galaxy is away, the more pronounced the redshift becomes. To give a comparison to your own life experience, remember waiting next to a railroad track as the train comes, passes, and moves away. As the train approaches its horn makes a high pitched sound, and as it moves away the pitch of the sound drops very noticeably. What happened is that as the train approached the sound waves were "compressed" (shorter wave length = higher frequency/pitch) and as it moved away the sound waves were "stretched" (longer wave length = lower frequency/pitch). This phenomenon is called the Doppler Effect, and can also be used to explain the redshift (lower frequency light) of galaxies that move away from us. The faster they move away, the more the spectral maxima and minima shift to lower wave lengths and "redder" light. Hubble's Law states that the velocity of a receding galaxy equals its distance multiplied by a constant (the Hubble Constant). This is a very important number for astronomers. The main problem with its precise determination is to get good measurements of the distance of galaxies. The other unknown, the amount of the redshift is easy enough to measure with a spectrometer. Judging from our current determinations of the Hubble constant, some of the most distant galaxies we can observe may be moving away from us at close to 90% of the speed of light. The Hubble Constant is critical for calculations of the age of the universe <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/age_of_universe1.htm>, although there is an alternative method <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/age_of_universe2.htm> to do so. As of late, both methods seem to agree that the universe is approximately 12-14 billion years old.
PHYSICS: fundamental constants of the universe | anthropic principle
Minor imbalances between matter and antimatter, so important for the outcome of the Big Bang, could be due to random fluctuations or inhomogeneities during stage 2. (see above), and could have other side effects as well. Scientists wonder for example whether the values of fundamental constants of the universe were set at random during this early stage. Under fundamental constants we understand quantities that are fundamental to the laws we know, but which we can not account for from basic principles. They usually show up as coefficients in the laws of physics, such a the gravitational constant, the fine structure constant <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/fine_structure1.htm>. Planck's number, Bolzmann's constant, the mass ratio between electron and proton, the mass of subatomic particles, the speed of light, the number p, and the base of natural logarithms. A slight change in any of these constants <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/fine_structure2.htm> could conceivably have prevented life as we know it from ever existing. That these number seem so finely adjusted that they permit life in our universe can suggest to some that there was a divine purpose in "creating" the universe as it is, but on the other hand it may simply be an expression of the "anthropic principle".
The_"anthropic principle"_ is a theory according to which due to aforesaid inhomogeneities, there may be either many different universes or many different regions of a single universe, that each have their own set of constants and possibly even their own set of laws of science. In most of these universes (or regions), conditions would not be right for the development of complicated organisms (such as we find living on earth), and only in universes that are pretty much like ours would intelligent beings develop and inquire why the universe is the way we see it. According to the "anthropic principle" there is a simple reason why we are here to inquire into these matters: If the universe (ours) had been any different, we simply would not be here to ask the question! Now, all this may seem like a cop-out, or at least an elaborate display of circular reasoning, but there is a sizeable number of cosmologists (including Stephen Hawking) that presently favor it as the most sensible approach to the dilemma.
Just so that we don't feel too complacent, most recently acquired data even suggest that the universal expansion is accelerating rather than slowing down. Some mysterious, repulsive "dark energy" seems to fuel the acceleration, overpowering the tendency of the expansion to decelerate. What this "dark energy" is, is everyone's guess at the moment <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/quintessence.htm>.
The beginning of the universe Is the mother of all things. Knowing the mother, one also knows the sons. Knowing the sons, yet remaining in touch with the mother, Brings freedom from the fear of death.
Lao TzU, The Tao Te Ching
Galaxies formed at stage 7 in our history of the universe. They are vast islands of stars, gas, and dust that populate the universe by the billions. Galactic size and structure range from subtle ellipticals to grand pinwheel spirals with the mass of at least 100 billion stars. Instead of randomly scattered throughout the universe, galaxies tend to be clumped together in clusters. Nonetheless, detailed surveys have revealed that galaxies are quite evenly distributed through the universe. Our own galaxy is known as the Milky Way <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/milky_way.htm>, because we see it as a broad ribbon of abundant stars in the summer sky (a possible view from far away <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/edge[1].jpg>, and a view from a vantage point far above <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/milkyway[1].jpg>).
galaxy_distribution.tif (337994 bytes)
Going with our earlier Big Bang scenario, the universe was for all intents and purposes a fairly homogeneous entity in the beginning. How then, was it possible that matter separated out into galaxies, stars, and planets as we see them today? If it had been perfectly homogeneous at the beginning, how did that happen? To reconcile the apparent uniformity of the early universe with the lumpy distribution of galaxies in the present universe is a challenge that scientists are currently working on. If there were for example slight irregularities in the density of the early universe, the denser regions would have had more of a gravitational pull upon further expansion and may have been able to collect matter into gas clouds and galaxies. Because these density irregularities also imply slight temperature variations, they should be reflected in the cosmic background radiation <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/cmbfluctuations.htm>. Astronomers known now that the latter shows slight irregularities, and various theories for galaxy formation also predict that there should be small scale, but systematic (not random) fluctuations of the cosmic background radiation associated with galaxy formation. To test these theories will requires very sophisticated measurements. Several promising Experiments <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/embers_of_universe.htm> to that end have been undertaken recently <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/microwave_polarization.htmor> will be undertaken in the near future.
Our most sophisticated and powerful telescope, the Hubble Space Telescope <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/render.gif>, has pushed the limits of space observation to a distance of about 11 billion light years. That means that light that reaches the telescope today had to travel 11 billion years to get there. That means a telescope is also a way to observe the universe as it was in the past, and at 11 billion years we are actually getting close to the point where theory predicts that the first galaxies appeared (possibly as close as 90% of the way back to the Big Bang). A picture taken during a long exposure survey (10 days, to capture even very faint light sources) shows numerous galaxies <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/distant_galaxies.htm> even that far away.
Astronomers think that so called globular clusters <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/globular_clusters.htm> (although much smaller than typical galaxies) may be the most ancient accumulations of stars, possibly being some type of primordial galaxies. Thus, they study them to learn more about the life history of stars. The Andromeda Galaxy <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/andromeda_galaxy.htm>, M31, is the nearest major galaxy in our vicinity, and located 2.3 million light years away. As a whole, galaxies are amazing and beautiful. The next page contains a collection of images <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/images_of_various_galaxies.htm> with brief comment. Han Solo says.... <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/strastuf.wav>
Useful web Links: Scientific American; <http://www.sciam.com/> NASA <http://www.nasa.gov/;Space.com> <http://www.space.com/(all> three sites can be searched for items mentioned above); National Geographic <http://www.nationalgeographic.org/>
Once we come to accept that galaxies formed as a result of slight irregularities in the matter distribution (density) of the early universe, and grew and accreted {συσσωρεύω} due to gravitational forces. Gravity pulled the denser regions together into a cosmic tapestry of voids and matter concentrations (clouds of gas initially), and it is only a small step to assume that even in these there were irregularities that caused further gravitational differentiation into denser and less dense regions. Lets assume now that we have one of these gas clouds, possibly many times the size of the present solar system, and that due to the intense forces of the Big Bang, these gas could have maintained at least some degree of turbulence and rotational momentum.
*What follows here is a brief outline of the current theory of how scientists think that stars and also our solar system evolved from that point forward. *
dust) cloud, some areas start contracting under gravity. As these grow denser over time, they exert more gravitational pull, pull in more matter, etc. Once started, this process leads to a gravitational collapse. Even if there are no significant original variations in matter distribution, a triggering disturbance might come from the shock wave sent out by a nearby supernova.
It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. Because of the gravitational contraction, the shrinking gas cloud starts to rotate faster and faster (just like a figure skater that pulls in his/her arms during a pirouette).
the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star. The gas is rotating <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/ssanim.avi>, and the centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off.
hydrogen and helium) to condense into tiny particles in the "accretion disk".
particles (gravitational contraction at a small scale), and this goes on until the particles get to the size of boulders or small asteroids.
nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to lunar size in the inner solar system, and one to fifteen times the Earth's size in the outer solar system. The accretion of these "planetesimals" is believed to have taken taken a few hundred thousand to about twenty million years, with the outermost taking the longest to form.
about this time (step 6.), about 1 million years after the nebula cooled, the star's interior would have reached pressures and temperatures high enough to allow fusion of hydrogen and deuterium into helium. The star heats up, and once it is hot enough, radiation and gases will emanate from it (the so called solar wind) and gradually sweep away the gases left in the solar nebula <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/protostar.jpg> and the interplanetary space. If a protoplanet was large enough (and far enough from the sun to escape excessive heating), its gravity would pull in the nebular gas, and it would become a gas giant. If it was smaller, it depended whether is was far away from the sun, or relatively close. If it was close, the heat and solar wind would prevent accumulation of gases, and vaporize most of the gases accreted earlier. As a result we would get a body made mostly of heavier elements (like the earth or mars). If it was far away we would get a mixture of solids and condensed gases, but it would tend to be small because of lesser density in the outer gas disk (we would have a comet).
that is orbited by solid, protoplanetary bodies and gas giants. As time passes the planets may loose further volatiles (gases) as they equilibrate with the heat put out by the central star.
have a solar system like ours, with planets in stable orbits. These planets may undergo further differentiation, and their surfaces may be heavily modified by collisions with other bodies (asteroids, comets).
In its basic outlines, this "solar nebula" theory has been around for decades, but in recent years observations made with the Hubble Space Telescope have finally provided observations that show that the theory was in essence correct (it's cool when a plan comes together). Pictures taken from the Eagle Nebula (M16) show huge pillar-like *dust clouds in which new stars are developing <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/stellar_maternity_ward.htm>. Other pictures from the Orion Nebula (M42), show new stars that begin to shine through the covers of a protoplanetary disk <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/orion_proplyds.htm> (proplyds). Scientists have also developed a variety of methods to develop evidence for the existence of planets around other stars than our sun, and have in recent years compiled a growing list of extrasolar planets <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/extrasolar_planets.htm>.
Much what we know about the processes that occur within stars has come from the study of nuclear physics, mostly in connection with research into nuclear weapons. As pointed out earlier, hydrogen and helium were the chemical elements that emerged from the Big Bang. Learning about nuclear physics, scientists realized that all the more complex elements, such as carbon, oxygen, etc. can be produced by fusing together these light elements under the right pressures and temperatures.
The nuclear fusion "food chain
At the extreme temperatures inside a star (15*10^6 Kelvin's), hydrogen atoms (protons) are stripped of their electrons, and undergo frequent collisions (density is very high). Although the hydrogen nuclei repel each other (same electric charge), under the pressures inside the core of a star they will go through a series of nuclear reactions and fuse into helium nuclei <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/a_sunnukreact.mpg> (2 protons, 2 neutrons). In the process 2 positrons, 2 neutrinos and energy will be emitted as well (in the animation protons are red, neutrons are yellow, neutrinos are green, and positrons are blue). After a long period of hydrogen fusion, more and more helium accumulates in the core, and the star eventually changes.
Red Giants and White Dwarfs: <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/white-dwarf.jpg> Because matter is getting more densely packed, the stars core shrinks and also heats up. The increased radiation pressure causes the outer layers of the star to expand (about 50 times for a star the size of the sun) and it transforms into what is known as a red giant. Because the central furnace gets ever hotter and denser, nuclear reactions that would have been impossible before can now become reality. Helium, previously the end product of the nuclear fire, now becomes fuel. Two heliums lead to a short-lived form of beryllium, and the latter forms a carbon nucleus when it collides (in time) with yet another helium nucleus. Add another helium nucleus to carbon -- and we get oxygen. The outcome of this process depends on the initial size of the star. If its the size of the sun, it will swell up into a cool but luminous red giant <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/red_giants.htm>, become unstable, and cast off its outer layers, form a gas cloud around itself (a planetary nebula <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/planetary_nebula.htm>). It and may also release some of the heavier elements synthesized within. Once the fuel runs out completely, the core will contract as much as it can and the remainder will be a so called white dwarf <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/white_dwarf.htm>.
_Supernovas, Neutron Stars, and Black Holes: _Stars more massive than the sun have a more interesting fusion history. They are rarer, and also more short lived. To support the stars outer layers, the pressures and temperatures in the core have to be higher, and as a result much more energy is produced (and emitted). A star 20 times as heavy as the sun will burn 20,000 times a bright, and goes through its hydrogen reserves fairly rapidly. While the sun may need 10 billion years to get to the red giant stage, it may take that star just 10 million years (1000 times faster). Under the more intense conditions inside this star we can generate neon and magnesium by fusing carbon, fuse oxygen to make silicon and sulfur, and keep fusing our way up to iron. Once the "iron stage" has been reached, no further fusion occurs. While so far every fusion step provided more energy for the operation of the star, to make even heavier elements now requires energy input. Their production will consume energy rather than creating it.
Once the core of a star has converted to iron, there is no more radiation pressure to counterbalance gravity. The core collapses (very rapidly, probably within seconds), and depending on mass it will either become a neutron star or a black hole <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/what_is_a_black_hole.htm>. The core is as dense as the nucleus of an atom (or denser), and as the outer shell collapses onto it, it rebounds with tremendous force. A shock wave travels through the outer layers of the star, and the star explodes <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/supernova.mpg> with an intense flash of light <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/supernova.jpg>. The star surface may shine for weeks with the intensity of a billion suns (a supernova <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/supernova_pictures.htm>) while its outer surface expands at a rate of several 1000 km per second. During this short time span the star may emit as much energy as throughout its previous existence.
The conditions in the outgoing shockwave are also conducive to the formation of elements heavier than iron (e.g. neutron bombardment of iron produces gold, gold is transformed into lead, neutron bombardment of lead leads to all the other elements up to uranium). Because supernova explosions are the last step in a long history of nucleosynthesis <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/nucleosynthesis.htm>, and because they are comparatively rare, elements heavier than iron are of small cosmic abundance. Some newly released simulations suggest that a collision of neutron stars <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/neutron_star_nucleosynthesis.htm> could also provide the conditions necessary for nucleosynthesis of elements heavier than iron.
composition-universe.jpg (43694 bytes) Nuclear fusion processes and the relative abundance of elements in the universe. Fusion from helium skipped over Li, Be, and B, to carbon and created all the elements up to iron. Note that abundances are shown on a logarithmic scale. Hydrogen and helium are about 8 to 9 orders of magnitude (100 million to a billion times) more abundant than all others. Thus, even today the universe is primarily composed of hydrogen and helium
How do we know so much about the chemical composition of the universe?
The answer is spectroscopy
<https://geol105.sitehost.iu.edu/images/gaia_chapter_1/visible_light_spectral_emission_.htm>.
Just as we can look at the spectrum of a single star or galaxy for
spectral lines and chemical signatures, we also can conduct a systematic
census of chemical composition. This was expressed more than hundred
years ago one of the pioneers of astronomical spectroscopy in the
following way:
One important object of this original spectroscopic investigation of the light of the stars and other celestial bodies, namely to discover whether the same chemical elements as those of our earth are present throughout the universe, was most satisfactorily settled in the affirmative; a common chemistry, it was shown, exists throughout the universe. - Sir William Huggins
Because the supernovas, unlike white dwarfs and red giants, propel large quantities of their more evolved fusion products into the surrounding space, they are very important contributors of elements heavier than helium to interstellar space. The "ashes" of nuclear burning and phantastic cataclysms are the materials we ourselves are made of (carbon, oxygen, etc.). Because first generation universes and stars (about 1 billion years after the Big Bang) would essentially have consisted of hydrogen and helium, most of the stars and galaxies we observe today must be younger than that, because they show spectra that reveal the entire periodic table. As our table above shows, the composition of the universe is mostly hydrogen and helium (more than 99%), with hydrogen being dominant. Thus, more than 10 billion years of nuclear fusion have converted only a small proportion of the initial material into heavier elements.
Because stars are in a way "factories" for heavier elements, scientists can look at the composition of stars (determined via spectroscopy), in order to figure out their relative age of formation within a galaxy <https://geol105.sitehost.iu.edu/images/gaia_chapter_1/star_evolution.htm>.
Chapter 2 <https://geol105.sitehost.iu.edu/1425chap2.htm>
Created Τετάρτη 06 Οκτωβρίου 2021
What are the limits of the known universe?
Right now, in a 13.8 billion year old Universe, our current visibility limit is 46 billion light-years. Our future visibility limit is approximately 33% greater: 61 billion light-years. Mar 5, 2019
How Much Of The Unobservable Universe Will We Someday Be Able To See?
How much of the universe can we not see? Only five percent of the universe is visible. What is the rest made up of? The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms.
Dark Matter and Dark Energy | National Geographic
Can we see 90% of our universe? But before that ultraviolet light can get out of one of those galaxies, it gets reabsorbed by gas inside the galaxy itself. We never see it. ... As it happens, 90% of the matter in the Universe is in a form that emits no light, but affects other matter through gravity.Mar 24, 2010
Found: 90% of the distant Universe | Discover Magazine
What is the maximum age of the visible universe? In 2013, Planck measured the age of the universe at 13.82 billion years. Both of these fall within the lower limit of 11 billion years independently derived from the globular clusters, and both have smaller uncertainties than that number.Jun 7, 2017
How Old is the Universe? | Space
Does the universe have an end? The end result is unknown; a simple estimation would have all the matter and space-time in the universe collapse into a dimensionless singularity back into how the universe started with the Big Bang, but at these scales unknown quantum effects need to be considered (see Quantum gravity).
Ultimate fate of the universe - Wikipedia
What is outside the universe? Image result The universe, being all there is, is infinitely big and has no edge, so there's no outside to even talk about. ... The current width of the observable universe is about 90 billion light-years. And presumably, beyond that boundary, there's a bunch of other random stars and galaxies.May 2, 2021
Is there anything beyond the universe? | Space
Created Πέμπτη 11 Μαΐου 2023
What shape is the universe? | Astronomy.com
[explain the shape of the universe (saddle shaped (negative curvature), spherical (positive curvature) or flat (no curvature).) ChatGPT
The shape of the universe is still a subject of ongoing scientific debate and research. However, current observational evidence suggests that the overall geometry of the universe is flat with no detectable curvature.
This means that if you were to draw a triangle with three points on a very large scale, the sum of its angles would add up to exactly 180 degrees, as it does in flat Euclidean geometry. This is consistent with observations of the cosmic microwave background radiation, which is believed to be the residual heat left over from the Big Bang.
However, it's important to note that while the universe as a whole appears to be flat, there may be regions where the curvature is different. These could be caused by variations in the distribution of matter and energy in the universe or by the presence of cosmic structures like galaxy clusters or superclusters.
Additionally, while the overall curvature of the universe is believed to be flat, it's important to understand that this doesn't mean that the universe is empty or lacking in structure. There are still vast and complex cosmic structures, including galaxies, clusters of galaxies, and superclusters, that exist within this seemingly flat space. User explain saddle shaped (negative curvature), spherical (positive curvature) or flat (no curvature). ChatGPT
To understand the concepts of saddle-shaped, spherical, and flat geometry, it's helpful to think about how the geometry of a two-dimensional surface can be characterized by its curvature.
Positive curvature: A surface with positive curvature curves outward, like a sphere. If you draw a triangle on a positively curved surface, the sum of its angles will be greater than 180 degrees.
Negative curvature: A surface with negative curvature curves inward, like a saddle. If you draw a triangle on a negatively curved surface, the sum of its angles will be less than 180 degrees.
Flat curvature: A surface with zero curvature is flat, like a piece of paper. If you draw a triangle on a flat surface, the sum of its angles will be exactly 180 degrees.
These concepts can be extended to the geometry of three-dimensional space, which is what we are interested in when we talk about the shape of the universe. A saddle-shaped universe would have a negative curvature, meaning that space would curve inward in all directions. A spherical universe would have a positive curvature, meaning that space would curve outward in all directions, like the surface of a sphere. A flat universe would have zero curvature, meaning that space would be flat, like a sheet of paper.
While it's important to note that the overall curvature of the universe appears to be flat, as I explained in my previous answer, there may be regions where the curvature is different and could take on these other shapes.](https://chat.openai.com/?model=text-davinci-002-render-sha)
Created Δευτέρα 02 Δεκεμβρίου 2019
The Big Bang and the Big Crunch - The Physics of the Universe
Created Τετάρτη 06 Οκτωβρίου 2021
How Much Of The Unobservable Universe Will We Someday Be Able To See?
Mar 5, 2019,02:00am EST|93.126 views
How Much Of The Unobservable Universe Will We Someday Be Able To See?
Starts With A Bang <https://www.forbes.com/sites/startswithabang/> Ethan Siegel <https://www.forbes.com/sites/ethansiegel/Senior> Contributor Starts With A Bang <https://www.forbes.com/sites/startswithabang/Contributor> GroupOpinions expressed by Forbes Contributors are their own. Science <https://www.forbes.com/science> The Universe is out there, waiting for you to discover it.
Our deepest galaxy surveys can reveal objects tens of billions of light years away, but there are... [+]more galaxies within the observable Universe we still have yet to reveal. Most excitingly, there are parts of the Universe that are not yet visible today that will someday become observable to us. Sloan Digital Sky Survey (SDSS)
Even though it's been billions of years since the Big Bang, there's a cosmic limit to how far we can observe the objects that occupy our Universe. The Universe has been expanding all this time, but that expansion rate is both finite and well-measured. If we were to calculate how far a photon emitted at the instant the Big Bang occurred could have traveled by today, we come up with the upper limit to how far we can see in any direction: 46 billion light-years.
That's the size of our observable Universe, which contains an estimated two trillion galaxies in various stages of evolutionary development. But beyond that, there ought to be much more Universe beyond the limits of what we can presently see: the unobservable Universe. Thanks to our best measurements of the part we can see, we're finally figuring out what lies beyond, and how much of it we'll someday be able to perceive and explore.
On a logarithmic scale, we can illustrate the entire Universe, going all the way back to the Big... [+]Bang. Although we cannot observe farther than this cosmic horizon which is presently a distance of 46.1 billion light-years away, there will be more Universe to reveal itself to us in the future. The observable Universe contains 2 trillion galaxies today, but as time goes on, more Universe will become observable to us. Wikipedia user Pablo Carlos Budassi
The Big Bang tells us that at some point in the distant past, the Universe was hotter, denser, and expanding much more rapidly than it is today. The stars and galaxies we see throughout the Universe in all directions only exist as they do because the Universe has expanded and cooled, allowing gravitation to pull matter into clumps. Over billions of years, gravitational growth has fueled generations of stars and the formation of galaxies, leading to the Universe we see today.
Everywhere we look, in all directions, we see a Universe that tells us the same cosmic story. But part of that story is the fact that the farther away we look, the farther we're looking back in time. The Universe hasn't been around, forming stars and growing galaxies, forever. According to the Big Bang and the observations that support it, the Universe had a beginning.
The full suite of what's present today in the Universe owes its origins to the hot Big Bang. More... [+]fundamentally, the Universe we have today can only come about because of the properties of spacetime and the laws of physics. Although the Universe is expanding, the total amount of Universe we can observe is increasing, too. NASA / GSFC
In the early stages after the Big Bang, the Universe was filled with a variety of ingredients, and it began with an incredibly rapid initial expansion rate. These two factors — the initial expansion rate and the gravitational effects of everything in the Universe — are the two head-to-head players in the ultimate cosmic race.
On the one hand, the expansion works to push everything apart, stretching the fabric of space and driving the galaxies and the large-scale structure of the Universe apart. But on the other hand, gravitation attracts all forms of matter and energy, working to pull the Universe back together. Normal matter, dark matter, dark energy, radiation, neutrinos, black holes, gravitational waves and more all play a role in the expanding Universe.
The relative importance of different energy components in the Universe at various times in the past.... [+]Note that when dark energy reaches a number near 100% in the future, the energy density of the Universe (and, therefore, the expansion rate) will remain constant arbitrarily far ahead in time. Owing to dark energy, distant galaxies are already speeding up in their apparent recession speed from us, and have been since the dark energy density was half of the total matter density, 6 billion years ago. E. Siegel
The expansion rate began large, but has been decreasing as the Universe expands. There's a simple reason for this: as the Universe expands, its volume increases, and therefore the energy density goes down. As the density drops, so does the expansion rate. Light that was once too far away from us to be seen can now catch up to us.
This fact carries with it a huge implication for the Universe: over time, galaxies that were once too distant to be revealed to us will spontaneously come into view. It may have been 13.8 billion years since the Big Bang occurred, but with the expansion of the Universe, there are objects as far away as 46.1 billion light-years whose light is just reaching us.
An illustration of how redshifts work in the expanding Universe. As a galaxy gets more and more... [+]distant, it must travel a greater distance and for a greater time through the expanding Universe. In a dark-energy dominated Universe, this means that individual galaxies will appear to speed up in their recession from us, but that there will be distant galaxies whose light is just reaching us for the first time today. Larry McNish of RASC Calgary Center, via http://calgary.rasc.ca/redshift.htm
All told, if we were to add up all the galaxies that exist within this volume of space, we'd find there are a whopping two trillion of them within our observable Universe. As enormous as this number is, it's still finite, and our observations don't reveal an edge in space in any direction we look.
The amount of time that's passed since the Big Bang, the speed of light, and the ingredients in our Universe determine the limit of what's observable. Any farther than that, and even something moving at the speed of light since the moment of the hot Big Bang will not have had sufficient time to reach us.
But all of this will change in time. As the years and aeons tick by, light that was unable to reach us will finally catch up to our eyes, revealing more of the Universe than we've ever seen before.
You might think that if we waited for an arbitrarily long amount of time, we'd be able to see an arbitrarily far distance, and that there would be no limit to how much of the Universe would become visible.
But in a Universe with dark energy, that simply isn't the case. As the Universe ages, the expansion rate doesn't drop to lower and lower values, approaching zero. Instead, there remains a finite and important amount of energy intrinsic to the fabric of space itself. As time goes on in a Universe with dark energy, the more distant objects will appear to recede from our perspective faster and faster. Although there's still more Universe out there to discover, there's a limit to how much of it will ever become observable to us.
The different possible fates of the Universe, with our actual, accelerating fate shown at the right.... [+]After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. We can only look to the past to infer dark energy's presence and properties, which require at least one constant, but its implications are larger for the future. NASA & ESA
Based on the expansion rate, the amount of dark energy we have, and the present cosmological parameters of the Universe, we can calculate what we call the future visibility limit <http://adsabs.harvard.edu/abs/2005ApJ...624..463G>: the maximum distance we'll ever be able to observe. Right now, in a 13.8 billion year old Universe, our current visibility limit is 46 billion light-years. Our future visibility limit is approximately 33% greater: 61 billion light-years. There are galaxies out there, right now, whose light is on the way to our eyes, but has not had the opportunity to reach us yet.
If we were to add up all the galaxies in the parts of the Universe that we'll someday see but cannot yet access today, we might be shocked to learn that there are more yet-to-be-revealed galaxies than there are galaxies in the visible Universe. There are an additional 2.7 trillion galaxies waiting to show us their light, on top of the 2 trillion we can already access.
The observable Universe might be 46 billion light years in all directions from our point of view,... [+]but there's certainly more unobservable Universe, perhaps even an infinite amount, just like ours, beyond that. Over time, we'll be able to see a bit, but not a lot, more of it. Frédéric MICHEL and Andrew Z. Colvin, annotated by E. Siegel
Compared to what the future holds for us, we're presently only seeing 43% of the galaxies that we'll someday be able to observe. Beyond our observable Universe lies the unobservable Universe, which ought to look just like the part we can see. The way we know that is through observations of the cosmic microwave background and the large-scale structure of the Universe.
If the Universe were finite in size, had an edge to it, or its properties began to change as we looked to greater distances, our measurements of these phenomena would reveal it. The observed spatial flatness of the Universe tells us that it is neither positively nor negatively curved to a precision of 99.6%, meaning that if it curves back on itself, the unobservable Universe is at least 250 times as large as the presently visible part.
The magnitudes of the hot and cold spots, as well as their scales, indicate the curvature of the... [+]Universe. To the best of our capabilities, we measure it to be perfectly flat. Baryon acoustic oscillations and the CMB, together, provide the best methods of constraining this, down to a combined precision of 0.4%. Smoot Cosmology Group / LBL
We will never be able to see anything close to those extraordinary distances. The future visibility limit will take us to distances that are presently 61 billion light-years away, but no farther. It will reveal slightly more than twice the volume of the Universe we can observe today. The unobservable Universe, on the other hand, must be at least 23 trillion light years in diameter, and contain a volume of space that's over 15 million times as large as the volume we can observe.
The simulated large-scale structure of the Universe shows intricate patterns of clustering that... [+]never repeat. But from our perspective, we can only see a finite volume of the Universe, which appears uniform on the largest scales. V. Springel et al., MPA Garching, and the Millenium Simulation
At the same time that we ponder the Universe beyond our observational limits, however, it's worth remembering how little of that Universe we can actually access or visit. All that we're looking forward to viewing is based on light that was already emitted many billions of years ago: close to the Big Bang in time. As it stands today, even if we left right now at the speed of light, we wouldn't be able to reach nearly all of the galaxies throughout space.
Dark energy is causing the Universe to not only expand, but for distant galaxies to speed up in their apparent recession from us. Although there are a total of 4.7 trillion galaxies that we will someday be able to observe out to a distance of 61 billion light-years, the limit of what we can reach today is much more modest.
The observable (yellow, containing 2 trillion galaxies) and reachable (magenta, containing 66... [+]billion galaxies) portions of the Universe, which are what they are thanks to the expansion of space and the energy components of the Universe. Beyond the yellow circle is an even larger (imaginary) one containing 4.7 trillion galaxies, the maximum portion of the Universe that will be accessible to us in the far future. E. Siegel, based on work by Wikimedia Commons users Azcolvin 429 and Frédéric MICHEL
Only those galaxies within approximately 15 billion light-years, or a quarter of the radius at the future visibility limit, can be reached today, which equates to about 66 billion galaxies only. This is only 1.4% of the total number of galaxies that will ever become visible to us. In other words, in the future, we will have a total of 4.7 trillion galaxies to view. Most of them will only ever appear to us as they were in the very distant past, and most of them will never get to see us as we are today. Of all those galaxies we'll someday see, 4.634 trillion of them are already forever unreachable, even at the speed of light.
You might notice an interesting occurrence: the future visibility limit is exactly equal to the reachable limit (of 15 billion light-years) added to the current visibility limit (of 46 billion light-years). This no coincidence; the light that will ultimately reach us is right at that reachable limit today, after journeying 46 billion light-years since the Big Bang. Someday far in the future, it will arrive at our eyes. With each moment that passes, we come ever closer to our ultimate cosmic viewpoint, as the light from the last galactic holdouts continues on its inevitable journey towards us in the expanding Universe.
I am a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges. I have won numerous awards for science writing…
Read More
I am a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges. I have won numerous awards for science writing since 2008 for my blog, Starts With A Bang <http://www.forbes.com/sites/startswithabang/>, including the award for best science blog by the Institute of Physics. My two books, Treknology: The Science of Star Trek from Tricorders to Warp Drive <http://amzn.to/2eW5nhO>, Beyond the Galaxy: How humanity looked beyond our Milky Way and discovered the entire Universe <http://www.worldscientific.com/worldscibooks/10.1142/9547#t=aboutBook>, are available for purchase at Amazon <http://amzn.to/1SQHXsU>. Follow me on Twitter @startswithabang <http://twitter.com/startswithabang>.
Created Πέμπτη 11 Μαΐου 2023
[Where Is the Edge of the Universe? We may never know. By Eric BetzNov 6, 2020 10:45 PM
This story originally appeared in the December issue of Discover magazine as "Is the Universe Infinite?" Support our science journalism by becoming a subscriber.
When Galileo Galilei pointed his first telescope to the heavens in 1610, he discovered “congeries of innumerable stars” hidden in the band of light called the Milky Way. Our cosmos grew exponentially that day. Roughly three centuries later, the cosmic bounds exploded once again when astronomers built telescopes big enough to show the Milky Way is just one of many “island universes.” Soon they learned the universe was expanding {το σύμπαν διαστέλλεται}, too, with galaxies retreating from each other at ever-accelerating speeds.
Since then, ever-larger telescopes have shown the observable universe spans an incomprehensible 92 billion light-years {8.69612 x 10^24 kilometers} across and contains perhaps 2 trillion galaxies. And yet, astronomers are still left wondering how much more universe is out there, beyond what they observe.
“The universe has always been slightly larger than what we can see,” says Virginia Trimble of the University of California, Irvine, an astronomer and expert in the field’s history.
Building bigger telescopes won’t help extend the cosmos anymore. “Telescopes only observe the observable. You can’t see back in time further than the age of the universe,” explains Nobel Prize-winning cosmologist John Mather of NASA’s Goddard Space Flight Center, who’s also chief scientist for the James Webb Space Telescope. “So we are totally limited. We’ve already seen as far as you could possibly imagine.” At the edge, we see the leftover glow from the Big Bang — the so-called cosmic microwave background radiation (CMB). But this isn’t some magical edge of the universe. Our cosmos keeps going. We just may never know how far.
In recent decades, cosmologists have tried to solve this mystery by first determining the universe’s shape, like the ancient Greek mathematician Eratosthenes calculating Earth’s size using simple trigonometry. In theory, our universe can have one of three possible shapes, each one dependent on the curvature of space itself: saddle shaped (negative curvature), spherical (positive curvature) or flat (no curvature).
Few have championed a saddle-shaped universe, but a spherical cosmos makes sense to us earthlings. Earth is round, as are the sun and planets. A spherical universe would let you sail into the cosmos in any direction and end up back where you started, like Ferdinand Magellan’s crew circumnavigating the globe. Einstein called this model a “finite yet unbounded universe.”
But starting in the late 1980s, a series of orbiting observatories built to study the CMB made increasingly precise measurements showing that space has no curvature at all. It’s flat to the limits of what astronomers can measure — if it is a sphere, it’s a sphere so huge that even our entire observable universe doesn’t register any curvature.
“The universe is flat like an [endless] sheet of paper,” says Mather. “According to this, you could continue infinitely far in any direction and the universe would be just the same, more or less.” You’d never come to an edge of this flat universe; you’d only find more and more galaxies.
That’s all well and good with most astronomers. A flat universe agrees with both observation and theory, so the idea now sits at the heart of modern cosmology.
The problem is that, unlike a spherical universe, a flat one can be infinite — or not. And there’s no real way to tell the difference. “What could you look for to see whether there’s an infinite universe?” Trimble says. “Nobody quite knows.”
So instead, astronomers hope an answer can come from theory — a model that could offer indirect proof one way or the other. For example, the Standard Model of physics predicted the existence of numerous particles, like the Higgs Boson, years before they were actually discovered. Yet physicists still presumed those particles were real.
“If you have a good description of everything you’ve observed so far and it predicts something is true, then you expect it is,” Trimble says. “That’s how most scientists think about how science works.”](https://www.discovermagazine.com/the-sciences/where-is-the-edge-of-the-universe)
![https://geol105.sitehost.iu.edu/images/gaia_chapter_1/edge[1].jpg](https://geol105.sitehost.iu.edu/images/gaia_chapter_1/edge[1].jpg){kind=link}
![https://geol105.sitehost.iu.edu/images/gaia_chapter_1/milkyway[1].jpg](https://geol105.sitehost.iu.edu/images/gaia_chapter_1/milkyway[1].jpg){kind=link}
