Misconceptions about the Big Bang

Does the expansion of the universe confuse you? You are not alone. Even astronomers sometimes get it wrong

 
 
By: Charles H. Leinweaver and Tamra M. Davis. The article was first published in the old version of the Hidan website on 18.6.2005/14.12.2006/XNUMX, and was re-uploaded following Hawking's lecture at the Hebrew University last Thursday (XNUMX/XNUMX/XNUMX) whose translation was published at the same time this evening
 
 The expansion of the universe is perhaps the most important fact we have ever discovered about our beginnings. If the universe had not expanded you would not be reading this article and humans would not exist. Even cold molecular bodies, such as living organisms or Earth-like planets, would not have formed unless the universe began to expand from a fiery big bang, cooling in the process. The formation of all the structures in the universe: galaxies, stars, planets and even Scientific American articles - all depend on the expansion of the universe.
In July 2005, it will be 40 years since a group of scientists announced the discovery of clear evidence for the expansion of the universe from a hotter and denser primordial state. They found the cool twilight light left over from the Big Bang: the cosmic microwave background radiation. Since that discovery, the expansion and cooling of the universe have been the mainstays of cosmology just as Darwin's evolution is the mainstay of biology. Similar to Darwinian evolution, the expansion of the universe provides the context in which simple structures are created and evolve over time into complex structures. Without evolution and expansion neither modern biology nor modern cosmology has much meaning.
The expansion of the universe is similar to Darwinian evolution in another, very interesting way: most scientists think they understand it, but not many of them agree on its true meaning. Even today, 150 years after the publication of the book "Origin of Species", biologists are still debating the mechanisms and consequences of Darwin's theory (though not about its truth), while a significant part of the general public is still in a pre-Darwinian state. Similarly, 75 years after the expansion of the universe was first described, many still misunderstand it. A senior cosmologist who played an important role in interpreting the meaning of the cosmic background radiation - James Peebles of Princeton University - wrote in 1993: "The dimensions and richness of this picture [the Big Bang model] are not yet fully understood, as I think it should be... even among some of the contributors the most fascinating to the flow of ideas.”
Well-known physicists, authors of textbooks in astronomy and important popular science writers have written about the expansion of the universe things that are wrong, misleading or open to misinterpretation. Since expansion is the basis of the Big Bang model, such misunderstandings are of great importance. Expansion is a simple and enticing idea, but what exactly does it mean to say that the universe is expanding? Where does it spread? Is the earth spreading too? And as if the existing confusion wasn't enough, it now turns out that the expansion of the universe is accelerating, and this process has consequences that really and truly embrace the power of imagination.
What is proliferation anyway?
When some ordinary and familiar body expands, for example a sprained ankle or the Roman Empire or a bomb, it becomes larger by expanding into the space around it. Anklets, empires and bombs, they all have centers and there are fringes. Beyond the fringes, there is a place to undress. The universe, apparently, has no edges, no center and no outside, so how can it even spread?
As a good analogy, imagine that you are ants living on the surface of an inflatable balloon. Your world is two-dimensional; The only directions you know are left, right, forward and backward. You have no idea what the words "up" and "down" mean. One day one ant discovers that the road she has to travel to milk her aphids takes longer than it did before: one day she covered the road in five minutes, the next day in six minutes, the next day - in seven. And the amount of time needed to go to other familiar places also got longer. That ant is sure that it is not going slower and that the aphids are going around to them randomly in groups, and not crawling in an organized manner from it.
This is the important point: the distances to the aphids became longer, even though the aphids themselves did not migrate anywhere. They remained as they were, at rest in relation to the rubber sheet of the balloon, yet the distances to and between them are increasing. When you notice these facts you can conclude that the ground under your feet is spreading. This is very strange, because you have already surrounded your entire world and have not found any fringes or "outsides" into which the world can spread.
The expansion of our universe is very similar to the inflation of a balloon. The distances to distant galaxies are getting longer and longer. Astronomers casually talk about "retreating" or "moving away" distant galaxies, but galaxies do not move in space away from us. They are not fragments of a huge cosmic bomb. Instead, the space between the galaxies and us is expanding. The different galaxies move randomly around within their clusters, but the galaxy clusters are actually at rest. The term "at rest" can be defined very precisely. The cosmic microwave background radiation fills the universe and defines a universal frame of reference, like the balloon sheet in the analogy, in relation to which movements can be measured.
But let's not overstretch the balloon parable. From our point of view outside the balloon, the expansion of the convex two-dimensional rubber sheet is only possible because it is contained in three-dimensional space. In this third dimension the balloon has a center, and its surface spreads out into the surrounding air as it inflates. From this we can conclude that the expansion of our three-dimensional space requires the presence of a fourth dimension. But in Einstein's theory of general relativity, the cornerstone of modern cosmology, space is dynamic. It is able to expand, contract and curve even though it is not contained within a space with more dimensions.
In this sense, the universe stands on its own. It doesn't need a center to spread out from, or an empty space outside of it (whatever that is) to spread into. When it spreads, it does not take over a space that was previously free around it. Some modern theories, like string theory, do talk about extra dimensions, but our expanding XNUMXD universe doesn't need those extra dimensions to expand into.
A perpetual cosmic traffic jam
In our universe, like on the surface of an inflating balloon, all things move away from each other. Thus, the Big Bang was not an explosion within space; It was more like the explosion of space. It did not take place in a certain place and spread outward from there, into some imaginary space that existed even before. It happened in all places at once.
If we imagine that the clock moves backwards in time, every given region of the universe will shrink, and all the galaxies will become closer to each other, until they collide together in a cosmic traffic jam - the Big Bang. From this analogy to a traffic jam, it can be understood as if it is a local problem that can be bypassed if you listen to the traffic center on the radio. But the Big Bang was a traffic jam that there is no way around. Imagine the surface of the earth shrinking, with all the roads and roads, but the cars keep their size. In the end there will be bumper-to-bumper crowding on every road and road. No traffic center will be useful against such a traffic jam: congestion occurs everywhere.
Similarly, the Big Bang happened anywhere and everywhere - in the room where you are reading this article, at a point slightly to the left of the star Alpha Centauri, of all places. It was not like a bomb that explodes at a certain point, which we recognize as the focus of the explosion. Returning to the balloon analogy, there is no particular place on the surface of the balloon that is the focus of expansion.
This presence of the Big Bang at every site remains the same regardless of questions such as what is the size of the universe, or even whether its size is finite or infinite. Cosmologists sometimes say that the universe was once the size of a grapefruit, but what they really mean is that the part of the universe that we can see now - the observable universe - was the size of a grapefruit.
Observers living in the Andromeda Galaxy, or beyond, have their own observable universes, different from ours but somewhat overlapping. Andromedas can see galaxies we can't just because they are a little closer to them, and the opposite is also true. Their observable universe was also once the size of a grapefruit. Therefore, we can imagine the universe at its beginning as a pile of partially overlapping grapefruits, extending to infinity in all directions. Similarly, the idea that the Big Bang was "small" is misleading. The space as a whole was apparently infinite. Shrink infinite space to an arbitrary degree, and it will still be infinite.
Moving away at a speed greater than the speed of light
Another set of misconceptions concerns the quantitative description of the spread. The rate at which the distances between galaxies increase follows a distinct law, which the American astronomer Edwin Hubble discovered in 1929: the speed at which a galaxy moves away from us (v) is directly proportional to its distance from us (d), that is, v = Hd. The coefficient H is called Hubble's constant, and it quantifies the speed of expansion of space - not only around us, but around every observer in the universe.
Some people get confused because not all galaxies obey Hubble's law. Andromeda, the large galaxy closest to us than any other, actually moves towards us, not away from us. These exceptions are due to the fact that Hubble's law describes the average behavior of galaxies. Galaxies can also have modest local velocities, when they move back and forth and exert a gravitational pull on each other - and this is what happens between the Milky Way and Andromeda. More distant galaxies also have small local velocities, but from our vantage point (high values ​​of d), these random velocities are marginal compared to the high receding velocities (v). Therefore, with respect to such galaxies, Hubble's law holds with great accuracy.
Please note: according to Hubble's law, the universe does not expand at a single speed. Some galaxies are moving away from us at a rate of 1,000 km per second, others (which are twice as far away from us) at a rate of 2,000 km per second, and so on. In fact, Hubble's law predicts that galaxies that are beyond a certain distance, known as the Hubble distance, move away from us at a speed faster than the speed of light. According to the measured value of Hubble's constant, this distance is about 14 billion light years.
Does this prediction about galaxies whose speed exceeds the speed of light show that Hubble's law is wrong? Didn't Einstein's special theory of relativity state that nothing can move faster than the speed of light? This question has confused generations of students. The answer is that special relativity only applies to "normal" speeds - to movements in space. The speed to which Hubble's law refers is the speed of receding, the cause of which is the expansion of space itself, and not movement within space. This is an effect of the general theory of relativity, which is not subject to the caveat of the special theory of relativity. A speed of distance that exceeds the speed of light does not violate the special theory of relativity. It is still true that nothing can travel faster than a beam of light.
Stretching and cooling down
The initial observation that the universe is expanding developed between 1910 and 1930. Atoms emit and absorb light at specific wavelengths, as measured in laboratory experiments. These emission and absorption patterns are also seen in light coming from distant galaxies, but these patterns are shifted to higher wavelengths. The astronomers say that the light of the galaxies is "redshifted". The explanation is quite simple: when space expands, light waves stretch. If space doubled in size during the waves' journey, their wavelengths doubled and their energy dropped to half its original value.
This process can be described in terms of temperature. The photons emitted by a body have something in common - temperature - a certain distribution of energy that reflects the degree of heat of the body. As the photons traverse the expanding space, they lose energy and their temperature drops. In this way the universe cools as it expands, just as the compressed air in a diver's tank cools when it is released and allowed to expand. For example, the temperature of the cosmic microwave background radiation today stands at about 3 degrees Kelvin, although the process that released this radiation occurred at a temperature of approximately 3,000 degrees Kelvin. During the time that has passed since this radiation was emitted, the size of the universe has increased 1,000 times, and therefore the temperature of the photons has decreased by this rate. Astronomers observing the gas in distant galaxies directly measured the radiation temperature in the distant past, and these measurements confirmed that over time the universe is cooling.
There is a lot of misunderstanding about the relationship between redshift and speed. Many confuse the redshift that originates from the expansion of the universe, and the more everyday redshift that originates from the Doppler effect. The well-known Doppler effect causes sound waves to lengthen if the source of the sound moves away from us - for example, the getting lower and lower horn of a train locomotive. This principle also applies to light waves, which lengthen when the light source moves in space away from us.
This is similar, but not identical, to what happens to the pines of distant galaxies. The cosmological redshift is not a normal Doppler shift. Astronomers often talk about the two as if there is no difference between them and unnecessarily confuse their students. The Doppler redshift and the cosmological redshift are described using two different formulas. The first originates from the theory of special relativity, which does not take into account the expansion of space, while the second originates from the theory of general relativity, which does take it into account. The two formulas are almost identical when it comes to nearby galaxies, but increasingly diverge when it comes to distant galaxies.
According to the usual Doppler formula, bodies whose speed in space is close to the speed of light show a redshift close to infinity. The wavelengths emitted from them become so long that it is no longer possible to distinguish them. If this statement applied to the galaxies, the most distant visible bodies in the sky would have to move away at speeds that fall only slightly below the speed of light, but the cosmological redshift formula leads to a completely different conclusion. In the current standard model of cosmology, galaxies whose redshift is approximately 1.5 - that is, whose wavelengths of light are 1.5 times higher than the reference value measured in the laboratory - move away from us at the speed of light. The astronomers observed about 1000 galaxies with redshifts above 1.5. In other words, they saw about 1000 bodies moving away from us at a speed higher than the speed of light; And accordingly, we are moving away from these galaxies at a speed greater than the speed of light. The cosmic microwave background radiation has traveled even greater distances, and its redshift is about 1,000. When the hot plasma of the young universe emitted the radiation we see now, it was moving away from us at about 50 times the speed of light.
run to stay in place
The talk about galaxies whose speed is faster than the speed of light sounds like mysticism, but this is actually possible due to changes in the rate of expansion. Imagine a beam of light whose distance from us exceeds the distance of Hubble, 14 billion light years, and it is trying to make its way to us. It is moving towards us at the speed of light, relative to its local space, but its local space is moving away from us at a speed greater than the speed of light. Although the beam of light moves towards us at all possible speed, it cannot catch up with the stretching of space. This is similar to a child trying to go up an escalator against the direction of its movement. Photons in the Hubble distance remind of the Red Queen who told Alice that in Mirror Land you have to run as fast as you can just to stay in the same place.
From this it may be concluded that light beyond the distance of mourning will never reach us, and therefore its source will forever remain unobserved. But the grieving distance is not constant, because the grieving constant, on which it depends, changes over time. In more detail, the value of the constant is directly proportional to the rate of increase in the distance between two galaxies, divided by this distance. (For the purpose of the calculation, any pair of galaxies can be used.) In models of the universe that fit the observational data, the denominator grows faster than the numerator, and therefore the Hubble constant is getting smaller. Because of this, the mourning distance is increasing. In the process, light moving towards us, which was initially just beyond the Hubble distance, and moved away from us (due to the expansion of space), may later find itself within the Hubble distance. From now on, these photons are in the region of space whose distance from us is slower than the speed of light, and therefore they can reach us.
But the galaxy from which the photons came may be continuing to move away at a speed greater than the speed of light. That is why we can observe the halo of galaxies that have always moved away from us - and will always move away from us - at a speed higher than the speed of light. In other words, the Hubble distance is not constant, and does not indicate the limit of the observable universe.
If so, what marks the limit of the observable universe? Again, this is a subject of great confusion. If space had not expanded, the most distant body we could see would be about 14 billion light years away from us, because that is the distance that light could travel in the 14 billion years since the Big Bang. But since the universe is expanding, the space the photon has already traversed continues to expand behind it during its journey. Thus, the current distance to the farthest object we can see is about three times that, about 46 billion light years.
When it was recently discovered that the expansion of the universe is accelerating, the situation became even more interesting. Before that, cosmologists thought that we live in a universe that is slowing down, so more and more galaxies will join our field of vision. But in a universe that is accelerating, we are surrounded by a boundary beyond which events occur that we cannot see forever - a cosmic event horizon. In order for the light of galaxies moving away at a speed higher than the speed of light to be able to reach us, the Hubble constant has to increase; But in an accelerated universe, it stops growing. Distant events can send beams of light towards us, but this light remains trapped beyond the Hubble distance due to the acceleration of expansion.
The accelerating universe, then, is similar to a black hole in that it has an event horizon - a language beyond which we cannot see. The current distance to our cosmic event horizon is 16 billion light years, which is well within our field of view. Light that emanated from galaxies that are now beyond the event horizon will never be able to reach us; The distance which now stands at 16 billion light years will spread at too high a speed. We will still be able to see events that occurred in these galaxies before they crossed the horizon, but later events will be hidden from our view forever.
Is Brooklyn spreading?
In the movie "My Affair with Annie", the boy representing the young Woody Allen explains to his doctor and mother why he is not interested in preparing lessons. "The universe is expanding... the universe is everything, and if it is expanding, then someday it will fall apart, and that will be the end of everything!" But his mother knows what is right and what is wrong: "You are here in Brooklyn. Brooklyn is not spreading!”
And justice with her. Brooklyn is not spreading. Many think that as the universe expands, all things expand as well. But this is not true. The expansion itself - that is, expansion at a constant speed, which is neither accelerated nor decelerated - does not generate any force. The wavelengths of the photons stretch with the universe because photons, unlike atoms or cities, are not coherent objects whose size is determined by a compromise between forces. While changing the rate of expansion adds a new force to the mix, even this new force does not cause objects to expand or contract.
For example, if gravity became stronger, our spine would be compressed until the electrons in the vertebrae would reach a new equilibrium, a little closer together. We would get lower, but we wouldn't keep shrinking. Similarly, if we lived in a universe governed by gravity, as most cosmologists thought until a few years ago, the expansion would slow down, exerting a slight compression on the bodies in the universe, and they would reach a smaller equilibrium size. But when they reached it, they would not continue to contract.
But in practice, the expansion of our universe is accelerated, and this exerts an outward light force on the bodies in it. Due to this, bound bodies become slightly larger than they were in a non-accelerated universe, because the equilibrium between the forces is achieved at a slightly higher size. On the Earth's surface, the outward acceleration - from the center of the Earth onward - is a tiny fraction (10-30) of the normal inward acceleration of gravity. If this acceleration is constant, it does not cause the earth to expand; Instead our world simply settles for a slightly larger size of static equilibrium.
This analysis changes if the acceleration is not constant, as some cosmologists estimate. If the acceleration itself increases, it will eventually be so strong that it will disintegrate all the structures - the "great rupture". But this rupture will not occur because of the expansion or acceleration per se, but because of the acceleration of the acceleration.
The Big Bang model is based on observations of the expansion, the cosmic background radiation, the chemical composition of the universe and the accumulations of matter in it. Like all scientific ideas, it is possible that in the future this model will give way to another. But until then it fits the data we have better than any other proposed model. As new and precise measurements accumulate, allowing cosmologists to better understand expansion and acceleration, they can ask deeper questions about the earliest times and largest scales of the universe. What caused the spread? Many cosmologists attribute it to a process called inflation, which was a type of accelerated expansion. But this answer must be only partial, since knowledge gives that the universe must be in expansion, in order for it to be accelerated. And what about the largest scales, beyond what we can see? Are different parts of the universe expanding at different rates, so that our universe is just one inflated bubble in a much larger multiverse? no one knows Despite the many questions that are still pending, the observations are becoming more and more accurate, and now they suggest that the universe will continue to expand forever. But we hope that the confusion surrounding this expansion will actually shrink.

 

Overview / Cosmic Confusion The expansion of the universe is one of the most fundamental ideas in modern science, but it is misunderstood by many.
The key to preventing misunderstandings is not to take the term "Big Bang" literally. The big bang is not a bomb that exploded in the center of the universe and sent chunks of matter out into space that existed from the beginning. Instead it was an explosion of space itself, occurring anywhere and everywhere, just as the expansion of the surface of a balloon occurs at each and every point of the balloon's surface.
This difference between expansion within space and expansion of space may not seem important, but it has far-reaching implications for the size of the universe, the rate at which galaxies are moving away from each other, the types of observations astronomers can make, and the nature of the accelerated expansion likely occurring even now. .
If we are careful with our language, the Big Bang model can say very little about the Big Bang itself. He describes what happened next.

What exploded in the big bang?
False: The big bang was similar to a bomb that exploded in a certain place, in a space that was empty before.
According to this view, the universe was created when matter exploded out of some particular place. The pressure was highest in the center, and lowest in the surrounding space; This pressure difference pushed the material outward.
True: what exploded was space itself.
The space we are in expands by itself. There was no center for this explosion; It happened everywhere. The density and pressure were equal everywhere, so there were no pressure differences of the kind that would drive a normal explosion.

Can galaxies move away faster than the speed of light?
Not true: Of course not. Einstein's special theory of relativity forbids it.
Imagine a region of space that has several galaxies. The galaxies move away from us - the farther away a galaxy is, the higher its speed [yellow arrows]. If the speed of light is the upper limit, the speed of galaxies must eventually stabilize at it.
Correct: Of course it is. Special relativity does not apply to receding velocity.
In an expanding universe, the speed of receding increases steadily with distance. Beyond a certain distance, called the Hubble distance, it exceeds the speed of light. This is not a violation of the theory of relativity, because the reason for the speed of distance is not movement within space, but the expansion of space.

A tedious hypothesis
Whenever Scientific American publishes an article on cosmology, several readers write to us and claim that the galaxies are not really moving away from us, that the expansion of space is nothing but an illusion. According to them, the redshifts of the galaxies are not caused except by the "fatigue" of the light during its long journey. It is possible that some unknown process causes the light to lose energy spontaneously, and this results in its grounding as it moves through space.
Some scientists first proposed this idea 75 years ago, and like any good model, it makes predictions that can be put to the test. But like any bad model, its predictions don't match the observations. For example, when a star explodes as a supernova, it brightens and then dims - a process that lasts about two weeks, in the type of supernovae that astronomers use to map space. During these two weeks, the supernova emits a stream of photons. The tired light hypothesis predicts that these photons will lose energy on their way, but observers will always see a stream that lasts for two weeks.
But in an expanding universe, not only do individual photons become more stretched (and therefore lose energy), but the entire photon stream also stretches. That is why photons need more than two weeks to reach the earth. Recent observations confirm this effect. A supernova in a galaxy whose redshift is 0.5 apparently lasts three weeks; In a galaxy whose offset is 1, it apparently lasts four weeks.
The tired light hypothesis also contradicts observations of the spectrum of the cosmic microwave background radiation, and the surface brightness of distant galaxies.

Is it possible to see galaxies receding faster than the speed of light?
Not true: Of course not. The light of such galaxies will never reach us.
A galaxy whose distance exceeds the Hubble distance is moving away from us at a speed greater than the speed of light. It emits a photon [yellow zigzag line]. As space expands, the photon is dragged backwards like a fish trying to swim against the current. The photon will never reach us.
Correct: Of course it is, because the rate of expansion changes over time.
Initially, the photon does not have the power to reach us. But the mourning distance is not fixed; It is growing, and may grow until it contains the photon. Once this happens, the photon will approach us and eventually reach us.

Why is there a cosmic redshift?
Incorrect: because the receding galaxies move in space and show us a Doppler shift.
According to the Doppler effect, the movement of a galaxy away from us stretches the light waves and they become redder. This wavelength of light remains the same throughout its journey through space. The observer seeing the light measures its Doppler shift and calculates the speed of the galaxy.
True: Galaxies hardly move in space, so they emit light in more or less the same wavelengths, in all directions [above]. Wavelengths get longer during the journey because space expands. Therefore the light gradually dims [in the middle and below]. The amount of redshift is different from what would be obtained from the Doppler effect.

What is the size of the observable universe?
Incorrect: The age of the universe is 14 billion years, therefore the radius of its observable part is 14 billion light years.
See the most distant galaxy we can observe - a galaxy that started emitting photons shortly after the Big Bang, and they are only now starting to reach us. A light year is the distance photons travel in one year. Therefore a photon from this galaxy traveled 14 billion light years.
Correct: Because the universe is expanding, the radius of its observable part exceeds 14 billion light years.
As the photon moves, the space it crosses expands. By the time the photon reaches us, the total distance to the galaxy from which it came is actually three times greater than the simple calculation based on the duration of the journey.

Do bodies within the universe also expand?
Incorrect: Yes. Expansion enlarges the universe and everything in it.
See galaxies in a cluster. As the universe grows, the galaxies grow and the entire cluster grows. The edge of the cluster [yellow frame] moves outward.
The expansion of the universe is one of the most fundamental ideas in modern science, but many people misunderstand it. The key to preventing misunderstandings is not to take the term "big bang" literally. The big bang is not a bomb that exploded in the center of the universe and sent chunks of matter out into space that existed from the beginning. Rather, it was an explosion of space itself, occurring everywhere, just as the expansion of the surface of a balloon occurs at each and every point of the balloon's surface. This difference between expansion within space and expansion of space may not seem important, but it has far-reaching implications for The size of the universe, the rate at which the galaxies are moving away from each other, the types of observations that astronomers can make, and the nature of the accelerated expansion that is apparently occurring, even now. He describes what happened next. Incorrect: The big bang was like a bomb that exploded in a certain place, in a space that was empty before. According to this view, the universe was created when matter exploded out of some certain place. The pressure was highest in the center, and lowest in the surrounding space; This pressure difference pushed the material outwards. That's right: what exploded was the space itself. The space we are in expands on its own. There was no center for this explosion; It happened everywhere. The density and pressure were equal in all places, so there were no pressure differences of the kind that would push a normal explosion. Incorrect: Of course not. Einstein's special theory of relativity forbids this. Imagine a region in space that has several galaxies. The galaxies move away from us - the farther away a galaxy is, the higher its speed [yellow arrows]. If the speed of light is the upper limit, the speed of the galaxies must eventually stabilize at it. Correct: Of course it is. Special relativity does not apply to the speed of receding. In an expanding universe, the receding speed increases steadily with distance. Beyond a certain distance, called the Hubble distance, it exceeds the speed of light. This is not a violation of the theory of relativity, because the reason for the speed of receding is not movement within space, but the expansion of space. Whenever Scientific American publishes an article on cosmology, several readers write to us and claim that the galaxies are not really moving away from us, because the expansion of space is nothing but an illusion. According to them, the redshifts of the galaxies are not caused except by the "fatigue" of the light during its long journey. It's possible that some unknown process causes light to spontaneously lose energy, causing it to ground as it moves through space. Several scientists first proposed this idea 75 years ago, and like any good model, it makes predictions that can be put to the test. But like any bad model, its predictions don't match the observations. For example, when a star explodes as a supernova, it brightens and then dims - a process that lasts about two weeks, in the type of supernovae that astronomers use to map space. During these two weeks, the supernova emits a stream of photons. The tired light hypothesis predicts that these photons will lose energy on their way, but observers will always see a stream that lasts for two weeks. But in an expanding universe, not only do individual photons become more tense (and therefore lose energy), but the entire stream of photons also stretches. That is why photons need more than two weeks to reach the earth. Recent observations confirm this effect. A supernova in a galaxy whose redshift is 0.5 apparently lasts three weeks; In a galaxy whose offset is 1, it apparently lasts four weeks. The tired light hypothesis also contradicts observations of the spectrum of the cosmic background radiation in microwaves, and the surface brightness of distant galaxies. Incorrect: of course not. The light of such galaxies will never reach us. A galaxy whose distance exceeds the Hubble distance is moving away from us at a speed higher than the speed of light. It emits a photon [yellow zigzag line]. As space expands, the photon is dragged backwards like a fish trying to swim against the current. The photon will never reach us. True: Of course it will, because the rate of propagation changes over time. Initially, the photon does not have the power to reach us. But the mourning distance is not fixed; It is growing, and may grow until it contains the photon. Once this happens, the photon will come closer to us and eventually reach us. Incorrect: because the receding galaxies move in space and present a Doppler shift to us. According to the Doppler effect, the movement of a galaxy away from us stretches the light waves and they become redder. This wavelength of light remains the same throughout its journey through space. The observer who sees the light measures its Doppler shift and calculates the speed of the galaxy. That's right: galaxies hardly move in space, so they emit light at more or less the same wavelengths, in all directions [above]. Wavelengths get longer during the journey because space expands. Therefore the light gradually dims [in the middle and below]. The amount of redshift is different from that which would be obtained from the Doppler effect. Incorrect: the age of the universe is 14 billion years, therefore the radius of its observable part is 14 billion light years. Look at the most distant galaxy we can observe - a galaxy that started emitting photons shortly after the big bang , and they are starting to reach us only now. A light year is the distance photons travel in one year. Therefore, a photon from this galaxy traveled a path of 14 billion light years. Correct: since the universe is expanding, the radius of its observable part exceeds 14 billion light years. As the photon moves, the space it crosses is expanding. By the time the photon reaches us, the total distance to the galaxy from which it came is actually three times greater than the simple calculation based on the duration of the journey. Incorrect: Yes. Expansion enlarges the universe and everything in it. See galaxies in a cluster. As the universe grows, the galaxies grow and the entire cluster grows. The edge of the cluster [yellow frame] moves outward. Correct: no.
Neighboring galaxies are initially pulled in different directions, but eventually their mutual gravity overcomes the expansion. A cluster is formed, and is fixed in a state of equilibrium.

About the authors
Charles H. Lineweaver and Tamra M. Davis are astronomers at the Mount Stromlow Observatory near Canberra, Australia. They deal with a wide range of questions, from cosmology to life in the universe. In the early 90s, while Leinweaver was at the University of California at Berkeley, he was included in the COBE satellite team, which discovered fluctuations in the cosmic microwave background radiation. He has degrees not only in astrophysics but also in history and English literature. In the past he played soccer in a semi-professional setting, and is the father of two young soccer stars, Colleen and Deirdre. Davis is a member of the Supernova and Accelerator Detector team - a space telescope that is now in the planning stages. She represents Australia in the sport called ultimate frisbee and participated in two world championships.

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