In the history of knowledge of the world around us, a general direction is clearly traced - the gradual recognition of the inexhaustibility of nature, its infinity in all respects. The universe is infinite in space and time, and if we discard I. Newton's ideas about the "first push", then this kind of worldview can be considered quite materialistic. The Newtonian universe claimed that space is the repository of all celestial bodies, with the movement and mass of which it is in no way connected; The Universe is always the same, i.e., stationary, although the death and birth of worlds is constantly taking place in it.
It would seem that the sky of Newtonian cosmology promised to be cloudless. However, the opposite was soon to be seen. AT during XIX in. three contradictions were discovered, which were formulated in the form of three paradoxes, called cosmological ones. They seemed to undermine the notion of the infinity of the universe.
photometric paradox. If the Universe is infinite and the stars are evenly distributed in it, then in any direction we should see some kind of star. In this case, the background of the sky would be dazzlingly bright, like the Sun.
gravitational paradox. If the Universe is infinite and the stars uniformly occupy its space, then the gravitational force at each of its points should be infinitely large, and therefore, the relative accelerations would also be infinitely large. space bodies which, as you know, is not.
thermodynamic paradox. According to the second law of thermodynamics, everything physical processes in the Universe ultimately come down to the release of heat, which is irreversibly dissipated in world space. Sooner or later all bodies will cool down to a temperature absolute zero, the movement will stop and come forever" heat death". The universe had a beginning, and its inevitable end awaits.
First quarter of the 20th century passed in anxious anticipation of the denouement. Of course, no one wanted to deny the infinity of the Universe, but, on the other hand, no one managed to eliminate the cosmological paradoxes of the stationary Universe. Only the genius of Albert Einstein brought a new stream to the cosmological disputes.
Newtonian classical physics, as already mentioned, considered space as a receptacle of bodies. According to Newton, there could not have been any interaction between bodies and space.
In 1916, A. Einstein published the basics general theory relativity. One of its main ideas is that material bodies, especially large mass, noticeably bend the space. Because of this, for example, a beam of light passing near the Sun changes its original direction.
Let us now imagine that in the entire part of the Universe we observe, matter is evenly "smeared" in space and the same laws operate at any point in it. At a certain average density of cosmic matter, the selected limited part of the Universe will not only bend space, but
even close it "on itself". The Universe (more precisely, its selected part) will turn into a closed world resembling an ordinary sphere. But only it will be a four-dimensional sphere, or a hypersphere, which we, three-dimensional beings, are not able to imagine. However, thinking by analogy, we can easily understand some of the properties of the hypersphere. It, like an ordinary sphere, has a finite volume containing a finite mass of matter. If you fly in the world space all the time in one direction, then after a certain number of billions of years you can get to the starting point.
The idea of the possibility of the Universe being closed was first expressed by A. Einstein. In 1922 Soviet mathematician A. A. Friedman proved that Einstein's "closed Universe" cannot possibly be static. In any case, its space either expands or contracts with all its contents.
In 1929, the American astronomer E. Hubble discovered a remarkable pattern: the lines in the spectra of the vast majority of galaxies are shifted to the red end, and the shift of bodies is the greater, the farther away the galaxy is from us. This interesting phenomenon is called redshift. Explaining the redshift by the Doppler effect, i.e., by changing the wavelength of light due to the movement of the source, scientists came to the conclusion that the distance between our and other galaxies is continuously increasing. Of course, galaxies do not scatter in all directions from our Galaxy, which does not occupy any special position in the Metagalaxy, but there is a mutual removal of all galaxies. This means that an observer located in any galaxy could, like us, detect a redshift, it would seem to him that all galaxies are moving away from it. Thus, the Metagalaxy is non-stationary. The discovery of the expansion of the Metagalaxy indicates that the Metagalaxy in the past was not the same as it is now, and it will be different in the future, i.e. the Metagalaxy is evolving.
The receding velocities of galaxies are determined from the redshift. In many galaxies they are very large, commensurate with the speed of light. The highest speeds, sometimes exceeding
250 thousand km / s, some quasars, which are considered the most distant objects of the Metagalaxy from us, have.
The law according to which the redshift (and hence the speed of removal of galaxies) increases in proportion to the distance from galaxies (Hubble's law) can be written as: v - Hr, where v is the radial velocity of the galaxy; r - distance to it; H is the Hubble constant. By modern estimates, the value of H lies within:
Consequently, the observed rate of expansion of the Metagalaxy is such that the galaxies separated by distance 1 Mpc (3 10 19 km), move away from each other at a speed of 50 to 100 km/s. If the galaxy's receding rate is known, then the distance to distant galaxies can be calculated.
So, we live in an expanding Metagalaxy. This phenomenon has its own characteristics. The expansion of the Metagalaxy manifests itself only at the level of clusters and superclusters of galaxies, that is, systems whose elements are galaxies. Another feature of the expansion of the Metagalaxy is that there is no center from which galaxies scatter.
The expansion of the Metagalaxy is the most grandiose natural phenomenon known at present. Its correct interpretation is of exceptionally great ideological significance. It is no coincidence that the fundamental difference between the philosophical views of scientists was sharply manifested in the explanation of the cause of this phenomenon. Some of them, identifying the Metagalaxy with the entire Universe, are trying to prove that the expansion of the Metagalaxy confirms the religious about the supernatural, divine origin Universe. However, the universe knows natural processes, which may have caused the observed expansion in the past. In all likelihood, these are explosions. Their scale strikes us already when studying certain types galaxies. One can imagine that the expansion of the Metagalaxy
also began with a phenomenon resembling a colossal explosion of matter with enormous temperature and density.
Since the Universe is expanding, it is natural to think that it used to be smaller and that at one time all space was compressed into a superdense material point. It was the moment of the so-called singularity, which cannot be described by the equations of modern physics. For unknown reasons, a process similar to an explosion took place, and since then the Universe has begun to "expand". The processes occurring in this case are explained by the theory of the hot Universe.
In 1965, American scientists A. Penzias and R. Wilson found experimental proof the stay of the Universe in a superdense and hot state, i.e., relic radiation. It turned out that outer space is filled with electromagnetic waves, which are the messengers of that ancient era development of the Universe, when there were no stars, galaxies, nebulae. Relic radiation permeates all space, all galaxies, it participates in the expansion of the Metagalaxy. Relic electromagnetic radiation is in the radio range with wavelengths from 0.06 cm to 60 cm. The energy distribution is similar to the spectrum of a completely black body with a temperature of 2.7 K. Energy density relic radiation equal to 4 10 -13 erg / cm 3, the maximum radiation falls on 1.1 mm. In this case, the radiation itself has the character of a certain background, because it fills the entire space and is completely isotropic. It is a witness to the initial state of the universe.
It is very important that although this discovery was made by accident while studying cosmic radio interference, the existence of the CMB was predicted by theorists. One of the first to predict this radiation was D. Gamow, developing a theory of the origin chemical elements that appeared in the first minutes after big bang. Prediction of the existence of relic radiation and its detection in outer space- another convincing example of the cognizability of the world and its laws.
In all developed dynamical cosmological models, the idea of the expansion of the Universe from some superdense and superhot state, called singular, is affirmed. The American astrophysicist D. Gamow came to the concept of the Big Bang and the hot Universe in the early stages of its evolution. Problem Analysis initial stage evolution of the universe was made possible by new ideas about the nature of the vacuum. The cosmological solution obtained by W. de Sitter for vacuum (r ~ e Ht) showed that the exponential expansion is unstable: it cannot continue indefinitely. After a relatively short period of time, the exponential expansion stops, a phase transition occurs in vacuum, during which the vacuum energy passes into ordinary matter and kinetic energy expansion of the universe. The Big Bang was 15-20 billion years ago.
According to the standard model of the hot Universe, superdense matter began to expand and gradually cool after the Big Bang. As expansion took place phase transitions, which resulted in physical forces interactions of material bodies. At experimental values of such basic physical parameters, as density and temperature (p ~ 10 96 kg/m 3 and T ~ 10 32 K), at the initial stage of the expansion of the Universe, the difference between elementary particles and four types physical interactions practically absent. It begins to manifest itself when the temperature decreases and the differentiation of matter begins.
In this way, modern ideas about the history of the emergence of our Metagalaxy are based on five important experimental observations:
1. Research spectral lines stars shows that the Metagalaxy, on average, has a single chemical composition. Hydrogen and helium predominate.
2. In the spectra of elements of distant galaxies, a systematic shift of the red part of the spectrum is detected. Value
This shift increases as the galaxies move away from the observer.
3. Measurements of radio waves coming from space in the centimeter and millimeter ranges indicate that outer space is uniformly and isotropically filled with weak radio emission. The spectral characteristic of this so-called background radiation corresponds to the radiation of a completely black body at a temperature of about 2.7 degrees Kelvin.
4. According to astronomical observations, the large-scale distribution of galaxies corresponds to a constant mass density, which, according to modern estimates, is at least 0.3 baryons per cubic meter.
5. Process analysis radioactive decay in meteorites shows that some of these components must have originated between 14 and 24 billion years ago.
Just a hundred years ago, scientists discovered that our Universe is rapidly increasing in size.
A hundred years ago, ideas about the universe were based on Newtonian mechanics and Euclidean geometry. Even a few scientists, such as Lobachevsky and Gauss, who admitted (only as a hypothesis!) The physical reality of non-Euclidean geometry, considered outer space to be eternal and unchanging.
In 1870, the English mathematician William Clifford came to a very deep idea that space can be curved, and not uniformly in different points, and that its curvature may change over time. He even admitted that such changes are somehow connected with the motion of matter. Both of these ideas later formed the basis of the general theory of relativity many years later. Clifford himself did not live to see this - he died of tuberculosis at the age of 34, 11 days before the birth of Albert Einstein.
Redshift
The first information about the expansion of the Universe was provided by astrospectrography. In 1886, the English astronomer William Huggins noticed that the wavelengths of starlight were slightly shifted compared to the terrestrial spectra of the same elements. Based on the formula for the optical version of the Doppler effect, derived in 1848 French physicist Armand Fizeau, one can calculate the value of the radial velocity of a star. Such observations make it possible to track the movement of a space object.
A hundred years ago, ideas about the universe were based on Newtonian mechanics and Euclidean geometry. Even a few scientists, such as Lobachevsky and Gauss, who admitted (only as a hypothesis!) the physical reality of non-Euclidean geometry, considered outer space to be eternal and unchanging. Due to the expansion of the universe, it is not easy to judge the distance to distant galaxies. The light that reached 13 billion years later from the galaxy A1689-zD1, 3.35 billion light years away (A), "reddens" and weakens as it overcomes expanding space, and the galaxy itself moves away (B). It will carry information about the distance in redshift (13 billion light years), in angular size (3.5 billion light years), in intensity (263 billion light years), while the real distance is 30 billion light years. years.
A quarter of a century later, Westo Slifer, an employee of the Flagstaff Observatory in Arizona, used this opportunity in a new way, who since 1912 studied the spectra of spiral nebulae with a 24-inch telescope with a good spectrograph. To obtain a high-quality image, the same photographic plate was exposed for several nights, so the project moved slowly. From September to December 1913, Slifer studied the Andromeda nebula and, using the Doppler-Fizo formula, came to the conclusion that it was approaching the Earth by 300 km every second.
In 1917 he published data on the radial velocities of 25 nebulae, which showed a significant asymmetry in their directions. Only four nebulae were approaching the Sun, the rest were running away (and some very quickly).
Slipher did not seek fame or publicize his results. Therefore, they became known in astronomical circles only when the famous British astrophysicist Arthur Eddington paid attention to them.
In 1924, he published a monograph on the theory of relativity, which included a list of the radial velocities of 41 nebulae found by Slifer. The same four blueshift nebulae were present there, while the other 37 had their spectral lines redshifted. Their radial velocities varied in the range of 150-1800 km/s and, on average, were 25 times higher than the velocities of the Milky Way stars known at that time. This suggested that the nebulae are involved in other movements than the "classical" luminaries.
space islands
In the early 1920s, most astronomers believed that the spiral nebulae were located on the periphery of the Milky Way, and beyond it there was nothing but empty dark space. True, back in the 18th century, some scientists saw giant stars in nebulae. star clusters(Immanuel Kant called them island universes). However, this hypothesis was not popular, since it was impossible to reliably determine the distances to the nebulae.
This problem was solved by Edwin Hubble, who worked on a 100-inch reflecting telescope at California's Mount Wilson Observatory. In 1923-1924, he discovered that the Andromeda Nebula consists of many luminous objects, among which are variable stars the Cepheid family. Then it was already known that the period of change in their apparent brightness is related to the absolute luminosity, and therefore Cepheids are suitable for calibrating cosmic distances. With their help, Hubble estimated the distance to Andromeda at 285,000 parsecs (according to modern data, it is 800,000 parsecs). The diameter of the Milky Way was then believed to be approximately equal to 100,000 parsecs (in fact, it is three times smaller). It followed from this that Andromeda and the Milky Way must be considered independent star clusters. Soon Hubble identified two more independent galaxies, which finally confirmed the hypothesis of "island universes".
In fairness, it should be noted that two years before Hubble, the distance to Andromeda was calculated by the Estonian astronomer Ernst Opik, whose result - 450,000 parsecs - was closer to the correct one. However, he used a number of theoretical considerations that were not as convincing as Hubble's direct observations.
By 1926, Hubble had made a statistical analysis of observations of four hundred "extra-galactic nebulae" (he used this term for a long time, avoiding calling them galaxies) and proposed a formula to relate the distance to a nebula to its apparent brightness. Despite the huge errors of this method, new data confirmed that the nebulae are more or less evenly distributed in space and are located far beyond the boundaries of the Milky Way. Now there was no longer any doubt that space is not limited to our Galaxy and its nearest neighbors.
Space fashion designers
Eddington became interested in Slipher's results even before the final clarification of the nature of spiral nebulae. By this time, a cosmological model already existed, in in a certain sense predicting the effect Slipher discovered. Eddington thought a lot about it and, of course, did not miss the opportunity to give the observations of the Arizona astronomer a cosmological sound.
Modern theoretical cosmology began in 1917 with two revolutionary papers presenting models of the universe based on general relativity. One of them was written by Einstein himself, the other by the Dutch astronomer Willem de Sitter.
Hubble laws
Edwin Hubble empirically found an approximate proportionality between redshifts and galactic distances, which he turned into a proportionality between velocities and distances using the Doppler-Fizeau formula. So we are dealing with two different patterns here.
Hubble didn't know how they were related to each other, but what does today's science say?
As Lemaitre showed, the linear correlation between cosmological (caused by the expansion of the Universe) redshifts and distances is by no means absolute. In practice, it is well observed only for offsets less than 0.1. So the Hubble empirical law is not exact, but approximate, and the Doppler-Fizo formula is valid only for small shifts of the spectrum.
But theoretical law, relating the radial velocity of distant objects with the distance to them (with a proportionality factor in the form of the Hubble parameter V=Hd), is valid for any redshifts. However, the speed V that appears in it is not at all the speed of physical signals or real bodies in physical space. This is the rate of increase in distances between galaxies and galaxy clusters, which is due to the expansion of the universe. We would be able to measure it only if we were able to stop the expansion of the Universe, instantly stretch measuring tapes between galaxies, read the distances between them and divide them into time intervals between measurements. Naturally, the laws of physics do not allow this. Therefore, cosmologists prefer to use the Hubble parameter H in another formula, where the scale factor of the Universe appears, which just describes the degree of its expansion into various space ages(because this parameter changes over time, its current value is denoted by H0). The universe is now expanding at an accelerating rate, so the value of the Hubble parameter is increasing.
By measuring cosmological redshifts, we obtain information about the degree of expansion of space. The light of the galaxy, which came to us with a cosmological redshift z, left it when all cosmological distances were 1+z times smaller than in our era. Get about this galaxy additional information, such as its present distance or its rate of receding from the Milky Way, is only possible with the help of a specific cosmological model. For example, in the Einstein-de Sitter model, a galaxy with z = 5 is moving away from us at a speed equal to 1.1 s (the speed of light). But if you make a common mistake and simply equalize V / c and z, then this speed will be five times the speed of light. The discrepancy, as we see, is serious.
The dependence of the speed of distant objects on the redshift according to SRT, GR (depends on the model and time, the curve shows the present time and the current model). At small displacements, the dependence is linear.
Einstein, in the spirit of the time, believed that the Universe as a whole is static (he tried to make it also infinite in space, but could not find the correct boundary conditions for his equations). As a result, he built a model of a closed universe, the space of which has a constant positive curvature (and therefore it has a constant finite radius). Time in this universe, on the contrary, flows in a Newtonian manner, in the same direction and at the same speed. The space-time of this model is curved due to the spatial component, while the temporal one is not deformed in any way. The static nature of this world provides a special "insert" in the main equation that prevents gravitational collapse and thus acts as an omnipresent antigravitational field. Its intensity is proportional to a special constant, which Einstein called the universal constant (now called the cosmological constant).
Lemaitre's cosmological model describing the expansion of the Universe was far ahead of its time. Lemaitre's universe begins with the Big Bang, after which the expansion first slows down and then starts to accelerate.
Einstein's model made it possible to calculate the size of the universe, total matter and even the value of the cosmological constant. For this, only the average density of cosmic matter is needed, which, in principle, can be determined from observations. It is no coincidence that this model was admired by Eddington and used in practice by Hubble. However, it is ruined by instability, which Einstein simply did not notice: at the slightest deviation of the radius from the equilibrium value, the Einstein world either expands or undergoes a gravitational collapse. Therefore, to real universe this model has nothing to do with it.
empty world
De Sitter also built, as he himself believed, a static world of constant curvature, but not positive, but negative. Einstein's cosmological constant is present in it, but matter is completely absent. When test particles of arbitrarily small mass are introduced, they scatter and go to infinity. In addition, time at the periphery of the de Sitter universe flows more slowly than at its center. Because of this, from great distances light waves come with a redshift, even if their source is stationary relative to the observer. So in the 1920s, Eddington and other astronomers wondered if de Sitter's model had anything to do with the reality reflected in Slifer's observations.
These suspicions were confirmed, albeit in a different way. The static nature of the de Sitter universe turned out to be imaginary, since it was associated with an unfortunate choice of coordinate system. After correcting this error, the de Sitter space turned out to be flat, Euclidean, but non-static. Thanks to the anti-gravitational cosmological constant, it expands while maintaining zero curvature. Due to this expansion, the wavelengths of photons increase, which entails the shift of spectral lines predicted by de Sitter. It is worth noting that this is how the cosmological redshift of distant galaxies is explained today.
From statistics to dynamics
The history of openly non-static cosmological theories begins with two papers Soviet physicist Alexander Fridman, published in German magazine Zeitschrift fur Physik in 1922 and 1924. Friedman calculated models of the universes with time-varying positive and negative curvature, which became the golden fund of theoretical cosmology. However, these works were hardly noticed by contemporaries (Einstein at first even considered Friedman's first article to be mathematically erroneous). Friedman himself believed that astronomy did not yet have an arsenal of observations to decide which of the cosmological models is more consistent with reality, and therefore limited himself to pure mathematics. Perhaps he would have acted differently if he had read Slipher's results, but this did not happen.
Georges Lemaitre, the greatest cosmologist of the first half of the 20th century, thought differently. At home, in Belgium, he defended his dissertation in mathematics, and then in the mid-1920s studied astronomy - at Cambridge under Eddington and at the Harvard Observatory with Harlow Shapley (during a stay in the USA, where he prepared a second dissertation at MIT, he met Slipher and Hubble). Back in 1925, Lemaitre was the first to show that the static nature of the de Sitter model is imaginary. Upon returning to his homeland as a professor at the University of Louvain, Lemaitre built the first model of an expanding universe with a clear astronomical justification. Without exaggeration, this work has become a revolutionary breakthrough in space science.
universal revolution
In his model, Lemaitre retained the cosmological constant with the Einstein numerical value. Therefore, his universe begins in a static state, but over time, due to fluctuations, enters the path of constant expansion with increasing speed. At this stage, it retains a positive curvature, which decreases as the radius increases. Lemaitre included in his universe not only matter, but also electromagnetic radiation. Neither Einstein nor de Sitter, whose work Lemaitre knew, nor Friedmann, about whom he knew nothing at the time, did this.
Related coordinates
In cosmological calculations, it is convenient to use the accompanying coordinate systems, which expand in unison with the expansion of the universe. In the idealized model, where galaxies and galaxy clusters do not participate in any proper motions, their comoving coordinates do not change. But the distance between two objects in this moment time is equal to their constant distance in comoving coordinates multiplied by the magnitude of the scale factor for that instant. This situation can be easily illustrated on an inflatable globe: the latitude and longitude of each point does not change, and the distance between any pair of points increases with increasing radius.
The use of comoving coordinates helps to understand the profound differences between the cosmology of an expanding universe, special relativity, and Newtonian physics. Thus, in Newtonian mechanics, all motions are relative, and absolute immobility has no physical sense. On the contrary, in cosmology the immobility in comoving coordinates is absolute and in principle can be confirmed by observations. The special theory of relativity describes the processes in space-time, from which it is possible, using the Lorentz transformations an infinite number ways to isolate spatial and temporal components. Cosmological space-time, on the contrary, naturally breaks up into a curved expanding space and a single space time. In this case, the speed of the recession of distant galaxies can many times exceed the speed of light.
Lemaitre, back in the US, suggested that the redshifts of distant galaxies arise due to the expansion of space, which "stretches" light waves. Now he has proved it mathematically. He also demonstrated that small (much smaller units) redshifts are proportional to the distances to the light source, and the proportionality factor depends only on time and carries information about the current rate of expansion of the Universe. Since it followed from the Doppler-Fizeau formula that the radial velocity of a galaxy is proportional to its redshift, Lemaitre concluded that this velocity is also proportional to its distance. After analyzing the speeds and distances of 42 galaxies from the Hubble list and taking into account the intragalactic speed of the Sun, he established the values of the proportionality coefficients.
Unseen work
Lemaitre published his work in 1927 on French in the little-read journal "Annals of the Brussels scientific society". It is believed that this was the main reason for which she initially went almost unnoticed (even by his teacher Eddington). True, in the fall of that year, Lemaitre was able to discuss his findings with Einstein and learned from him about Friedmann's results. The creator of general relativity had no technical objections, but he resolutely did not believe in the physical reality of Lemaître's model (just as he had not accepted Friedmann's conclusions earlier).
Hubble charts
Meanwhile, in the late 1920s, Hubble and Humason discovered a linear correlation between the distances of up to 24 galaxies and their radial velocities calculated (mostly by Slifer) from redshifts. Hubble concluded from this that the radial velocity of a galaxy is directly proportional to its distance. The coefficient of this proportionality is now designated H0 and is called the Hubble parameter (according to the latest data, it is slightly higher than 70 (km / s) / megaparsec).
Hubble paper with graph linear dependence between galactic velocities and distances was published in early 1929. A year earlier, a young American mathematician, Howard Robertson, followed Lemaitre in deriving this dependence from a model of an expanding universe, which Hubble may have known. However, this model was not mentioned directly or indirectly in his famous article. Later, Hubble expressed doubts that the velocities appearing in his formula actually describe the movements of galaxies in outer space, but he always refrained from their specific interpretation. He saw the meaning of his discovery in demonstrating the proportionality of galactic distances and redshifts, leaving the rest to theoreticians. Therefore, with all due respect to Hubble, there is no reason to consider him the discoverer of the expansion of the Universe.
And yet it is expanding!
Nevertheless, Hubble paved the way for the recognition of the expansion of the universe and the Lemaitre model. Already in 1930, such masters of cosmology as Eddington and de Sitter paid tribute to her; a little later, scientists noticed and appreciated the work of Friedman. In 1931, at the suggestion of Eddington, Lemaitre translated into English his article (with small cuts) for the Monthly Newsletter of the Royal Astronomical Society. In the same year, Einstein agreed with Lemaitre's conclusions, and a year later, together with de Sitter, he built a model of an expanding universe with flat space and curved time. This model, due to its simplicity for a long time was very popular among cosmologists.
In the same 1931, Lemaitre published a brief (and without any mathematics) description of yet another model of the universe that combined cosmology and quantum mechanics. In this model initial moment there is an explosion of the primary atom (Lemaitre also called it a quantum), which gave rise to both space and time. Since gravity slows down the expansion of the newborn Universe, its speed decreases - it is possible that almost to zero. Lemaitre later introduced a cosmological constant into his model, which caused the universe to go into a steady state of accelerating expansion over time. So he anticipated both the idea of the Big Bang and modern cosmological models taking into account the presence of dark energy. And in 1933, he identified the cosmological constant with the vacuum energy density, which no one had thought of before. It is simply amazing how much this scientist, certainly worthy of the title of the discoverer of the expansion of the Universe, was ahead of his time!
When we look at the distant Universe, we see galaxies everywhere - in all directions, for millions and even billions of light years. Since there are two trillion galaxies that we could observe, the sum of everything beyond them is bigger and cooler than our wildest imaginations. One of the most interesting facts is that all the galaxies we have ever observed obey (on average) the same rules: the farther they are from us, the faster they move away from us. This discovery, made by Edwin Hubble and his colleagues back in the 1920s, led us to a picture of an expanding universe. But what if it expands? Science knows, and now you will too.
At first glance, this question may seem reasonable. Because everything that expands usually consists of matter and exists in the space and time of the universe. But the Universe itself is space and time containing matter and energy in itself. When we say that "the universe is expanding," we mean the expansion of space itself, as a result of which individual galaxies and clusters of galaxies move away from each other. It would be easiest to imagine a ball of dough with raisins inside, which is baked in the oven, says Ethan Siegel.
Model of an expanding "bun" of the universe, in which relative distances increase as space expands
This dough is the fabric of space, and the raisins are related structures(like galaxies or clusters of galaxies). From the point of view of any raisin, all other raisins will move away from it, and the further they are, the faster. Only in the case of the universe of the oven and the air outside the dough does not exist, there is only dough (space) and raisins (substance).
Redshift is created not just by receding galaxies, but rather by the space between us.
How do we know that this space is expanding and not the galaxies are receding?
If you see objects moving away from you in all directions, there is only one reason that can explain this: the space between you and these objects is expanding. Also, one would assume that you are near the center of the explosion, and many objects are simply further away and removed faster, because they got more energy explosion. If this were the case, we could prove it in two ways:
- At greater distances and at higher speeds, there will be fewer galaxies, since over time they would spread out in space a lot.
- The ratio of redshift and distance will take on a particular shape at large distances, which will be different from the shape if the fabric of space was expanding.
When we look at great distances, we find that the density of galaxies farther in the Universe is higher than closer to us. This is consistent with the picture in which space is expanding, because looking further is the same as looking into the past, where there has been less expansion. We also find that distant galaxies have a redshift-to-distance ratio corresponding to the expansion of space, and not at all - if the galaxies were simply rapidly moving away from us. Science can answer this question in two ways. different ways, and both answers support the expansion of the universe.
Has the universe always expanded at the same rate?
We call it the Hubble constant, but it is only constant in space, not time. Universe in this moment expanding more slowly than in the past. When we talk about the expansion rate, we are talking about the speed per unit distance: about 70 km/s/Mpc today. (Mpc is megaparsec, approximately 3,260,000 light years). But the rate of expansion depends on the densities of all the different things in the universe, including matter and radiation. As the Universe expands, the matter and radiation in it become less dense, and as the density decreases, so does the rate of expansion. The universe has expanded faster in the past and has been slowing down since the Big Bang. The Hubble constant is a misnomer, it should be called the Hubble parameter.
The distant fates of the universe offer different possibilities, but if dark energy is indeed constant, as the data suggests, we will follow a red curve.
Will the universe expand forever or will it ever stop?
Several generations of astrophysicists and cosmologists have puzzled over this question, and it can only be answered by determining the rate of expansion of the Universe and all the types (and amounts) of energy present in it. We have already successfully measured how much ordinary matter, radiation, neutrinos, dark matter and dark energy, as well as the expansion rate of the universe. Based on the laws of physics and what happened in the past, it looks like the universe will expand forever. Although the probability of this is not 100%; if something like dark energy behaves differently in the future compared to the past and the present, all our conclusions will have to be reconsidered.
Do galaxies move faster than the speed of light? Isn't it forbidden?
From our point of view, the space between us and the remote point is expanding. The farther it is from us, the faster it seems to us that it is moving away. Even if the expansion rate were tiny, a distant object would one day cross the threshold of any velocity limit, because the expansion rate (speed per unit distance) would be multiplied many times over with sufficient distance. The OTO favors such a scenario. The law that nothing can move faster speed light only applies to the movement of an object through space, not to the expansion of space itself. In reality, the galaxies themselves move at only a few thousand kilometers per second, well below the 300,000 km/s limit set by the speed of light. It is the expansion of the universe that causes the recession and redshift, not the true movement of the galaxy.
There are approximately 2 trillion galaxies within the observable universe (yellow circle). Galaxies that are closer than a third of the way to this border, we will never be able to catch up due to the expansion of the universe. Only 3% of the volume of the Universe is open for development by human forces
The expansion of the universe is a necessary consequence of the fact that matter and energy fill space-time, which is subject to the laws of general relativity. As long as there is matter, there is gravitational attraction, so either gravity wins and everything contracts again, or gravity loses and wins the expansion. There is no center of expansion and there is nothing outside of space that expands; it is the very fabric of the universe that is expanding. What is most interesting, even if we left the Earth at the speed of light today, we would be able to visit only 3% of the galaxies in the observable universe; 97% of them are already out of our reach. The universe is complex.
The universe is not static. This was confirmed by the studies of astronomer Edwin Hubble back in 1929, that is, almost 90 years ago. He was led to this idea by observations of the movement of galaxies. Another discovery of astrophysicists at the end of the twentieth century was the calculation of the expansion of the Universe with acceleration.
What is the expansion of the universe called?
Some are surprised to hear what scientists call the expansion of the universe. This name is associated with the majority of the economy, and with negative expectations.
Inflation is the process of expansion of the Universe immediately after its appearance, and with a sharp acceleration. Translated from English, "inflation" - "pump up", "inflate".
New doubts about the existence of dark energy as a factor in the theory of inflation of the Universe are used by opponents of the theory of expansion.
Then scientists proposed a map of black holes. The initial data differs from those obtained at a later stage:
- Sixty thousand black holes with the distance between the most distant more than eleven million light years - data four years ago.
- One hundred and eighty thousand black hole galaxies thirteen million light-years away. Data obtained by scientists, including Russian nuclear physicists, at the beginning of 2017.
This information, astrophysicists say, does not contradict classical model Universe.
The expansion rate of the universe is a challenge for cosmologists
The rate of expansion is indeed a challenge for cosmologists and astronomers. True, cosmologists no longer argue that the expansion rate of the Universe does not have a constant parameter, the discrepancies moved to another plane - when the expansion began to accelerate. Data on wandering in the spectrum of very distant supernova galaxies of the first type prove that the expansion is not a sudden onset process.
Scientists believe that the universe was shrinking for the first five billion years.
The first consequences of the Big Bang first provoked a powerful expansion, and then a contraction began. But dark energy still influenced the growth of the universe. And with acceleration.
American scientists have begun to create a map of the size of the universe for different eras to find out when the acceleration started. By observing supernova explosions, as well as the direction of concentration in ancient galaxies, cosmologists have noticed features of acceleration.
Why is the universe "accelerating"
Initially, it was assumed that in the compiled map, the acceleration values were not linear, but turned into a sinusoid. It was called the "wave of the universe."
The wave of the Universe says that the acceleration did not go with constant speed: it slowed down, then accelerated. And several times. Scientists believe that there were seven such processes in the 13.81 billion years after the Big Bang.
However, cosmologists cannot yet answer the question of what the acceleration-deceleration depends on. Assumptions boil down to the idea that the energy field from which dark energy originates is subject to the wave of the Universe. And, moving from one position to another, the Universe either expands the acceleration, or slows it down.
Despite the persuasiveness of the arguments, they still remain a theory so far. Astrophysicists hope that information from the Planck orbiting telescope will confirm the existence of a wave in the universe.
When dark energy was found
For the first time they started talking about it in the nineties due to supernova explosions. The nature of dark energy is unknown. Although Albert Einstein singled out the cosmic constant in his theory of relativity.
In 1916, a hundred years ago, the universe was still considered unchanging. But gravity intervened: the cosmic masses would invariably slam against each other if the universe were stationary. Einstein declares gravity due to the cosmic repulsive force.
Georges Lemaitre will substantiate this through physics. Vacuum contains energy. Due to its vibrations, leading to the appearance of particles and their further destruction, the energy acquires a repulsive force.
When Hubble proved the expansion of the universe, Einstein called it nonsense.
The influence of dark energy
The universe is moving apart at a constant speed. In 1998, the world was presented with data from an analysis of type 1 supernova explosions. It has been proven that the universe is growing faster and faster.
This happens because of an unknown substance, it was nicknamed "dark energy". It turns out that it occupies almost 70% of the space of the Universe. The essence, properties and nature of dark energy have not been studied, but its scientists are trying to find out if it existed in other galaxies.
In 2016, they calculated the exact expansion rate for the near future, but a discrepancy appeared: the Universe is expanding at a faster rate than astrophysicists had previously assumed. Among scientists, disputes broke out about the existence of dark energy and its influence on the rate of expansion of the limits of the universe.
The expansion of the universe occurs without dark energy
The theory of the independence of the expansion of the Universe from dark energy was put forward by scientists in early 2017. They explain the expansion as a change in the structure of the universe.
Scientists from the Budapest and Hawaiian universities came to the conclusion that the discrepancy between the calculations and the real expansion rate is associated with a change in the properties of space. No one took into account what happens to the model of the Universe during expansion.
Doubting the existence of dark energy, scientists explain: the largest concentrates of matter in the universe affect its expansion. In this case, the rest of the content is distributed evenly. However, the fact remains unaccounted for.
To demonstrate the validity of their assumptions, scientists proposed a model of a mini-universe. They presented it in the form of a set of bubbles and started calculating the growth parameters of each bubble at its own rate, depending on its mass.
This modeling of the universe has shown scientists that it can change without regard to energy. And if you "mix" dark energy, then the model will not change, scientists believe.
In general, the controversy is still ongoing. Supporters of dark energy say that it affects the expansion of the boundaries of the universe, opponents stand their ground, arguing that the concentration of matter matters.
The expansion rate of the universe now
Scientists are convinced that the Universe began to grow after the Big Bang. Then, almost fourteen billion years ago, it turned out that the expansion rate of the universe more speed Sveta. And she continues to grow.
Stephen Hawking and Leonard Mlodinov's book The Shortest History of Time notes that the rate of expansion of the boundaries of the universe cannot exceed 10% per billion years.
In the summer of 2016, Nobel Prize winner Adam Riess calculated the distance to pulsating Cepheids in galaxies close to each other to determine what the rate of expansion of the universe is. These data allowed us to calculate the speed. It turned out that galaxies at a distance of at least three million light years can move away at a speed of almost 73 km / s.
The result was amazing: orbiting telescopes, the same Planck, they talked about 69 km / s. Why such a difference was recorded, scientists are unable to answer: they do not know anything about the origin of dark matter, on which the theory of the expansion of the Universe is based.
dark radiation
Another factor in the "acceleration" of the universe was discovered by astronomers using Hubble. Dark radiation is believed to have appeared at the very beginning of the formation of the universe. Then there was more energy in it, not matter.
Dark radiation "helped" dark energy to expand the boundaries of the universe. Differences in determining the speed of acceleration were due to the unknown nature of this radiation, scientists say.
Further work by Hubble should make the observations more accurate.
Mysterious energy could destroy the universe
Scientists have been considering such a scenario for several decades, data from the Planck space observatory say that this is far from just speculation. They were published in 2013.
"Planck" measured the "echo" of the Big Bang, which appeared at the age of the Universe about 380 thousand years, the temperature was 2,700 degrees. And the temperature changed. "Planck" also determined the "composition" of the Universe:
- almost 5% are stars, cosmic dust, space gas, galaxies;
- almost 27% is the mass of dark matter;
- about 70% is dark energy.
Physicist Robert Caldwell suggested that dark energy has a power that can grow. And this energy will separate space-time. The galaxy will move away in the next twenty to fifty billion years, the scientist believes. This process will occur with the increasing expansion of the boundaries of the Universe. This will tear the Milky Way away from the star, and it will also disintegrate.
The cosmos has been measured out to be about sixty million years old. The sun will become a dwarf fading star, and the planets will separate from it. Then the earth will explode. In the next thirty minutes, space will tear the atoms apart. The final will be the destruction of the structure of space-time.
Where does the Milky Way go?
Jerusalem astronomers are convinced that the Milky Way has gained top speed, which is higher than the expansion rate of the Universe. Scientists explain this by the desire of the Milky Way to the "Great Attractor", which is considered the largest. So the Milky Way leaves the cosmic desert.
Scientists use different techniques measurement of the expansion rate of the universe, so no single result this setting.
material from the book "The Shortest History of Time" by Stephen Hawking and Leonard Mlodinov
Doppler effect
In the 1920s, when astronomers began to study the spectra of stars in other galaxies, something very interesting was discovered: they turned out to be the same characteristic sets of missing colors as stars in our own galaxy, but they were all shifted towards the red end of the spectrum. , and in the same proportion. To physicists, color or frequency shift is known as the Doppler effect.
We are all familiar with how this phenomenon affects sound. Listen to the sound of a car passing by. When it approaches, the sound of its engine or horn seems higher, and when the car has already passed by and began to move away, the sound decreases. A police car traveling towards us at a speed of one hundred kilometers per hour develops about a tenth of the speed of sound. The sound of his siren is a wave, alternating crests and troughs. Recall that the distance between the nearest crests (or troughs) is called the wavelength. The shorter the wavelength, the more vibrations reach our ear every second and the higher the tone, or frequency, of the sound.
The Doppler effect is caused by the fact that an approaching car, emitting each following ridge sound wave, will be closer and closer to us, and as a result, the distances between the ridges will be less than if the car was standing still. This means that the wavelengths of the waves coming to us become shorter, and their frequency becomes higher. Conversely, if the car moves away, the length of the waves we catch becomes longer and their frequency becomes lower. And the faster the car moves, the stronger the Doppler effect manifests itself, which allows it to be used to measure speed.
When the source emitting waves moves towards the observer, the wavelength decreases. On the contrary, when the source is removed, it increases. This is called the Doppler effect.
Light and radio waves behave in a similar way. The police use the Doppler effect to determine the speed of vehicles by measuring the wavelength of the radio signal reflected from them. Light is vibrations, or waves, electromagnetic field. Wavelength visible light extremely small - from forty to eighty millionths of a meter. human eye perceives light waves of different wavelengths as various colors, with the longest wavelengths corresponding to the red end of the spectrum, and the shortest - related to the blue end. Now imagine a source of light at a constant distance from us, such as a star, emitting light waves of a certain wavelength. The length of the recorded waves will be the same as that of the emitted ones. But suppose now that the light source began to move away from us. As in the case of sound, this will increase the wavelength of light, which means that the spectrum will shift towards the red end.
Universe expansion
Having proved the existence of other galaxies, Hubble in subsequent years was engaged in determining the distances to them and observing their spectra. At the time, many assumed that galaxies were moving randomly and expected that the number of blueshifted spectra would be about the same as the number of redshifted ones. Therefore, it was a complete surprise to discover that the spectra of most galaxies show a redshift - almost all star systems are moving away from us! Even more surprising was the fact discovered by Hubble and published in 1929: the magnitude of the redshift of galaxies is not random, but directly proportional to their distance from us. In other words, the farther away a galaxy is from us, the faster it is receding! It followed from this that the Universe cannot be static, unchanged in size, as previously thought. In fact, it is expanding: the distance between galaxies is constantly growing.
The realization that the universe is expanding has made a real revolution in the minds, one of the greatest in the twentieth century. When you look back, it may seem surprising that no one thought of this before. Newton and other great minds must have realized that a static universe would be unstable. Even if at some point it would be stationary, the mutual attraction of stars and galaxies would quickly lead to its compression. Even if the universe were expanding relatively slowly, gravity would eventually put an end to its expansion and cause it to contract. However, if the expansion rate of the universe is greater than some critical point, gravity will never be able to stop it and the universe will continue to expand forever.
Here you can see a distant resemblance to a rocket rising from the surface of the Earth. At a relatively low speed, gravity will eventually stop the rocket and it will begin to fall towards the Earth. On the other hand, if the speed of the rocket is higher than the critical one (more than 11.2 kilometers per second), gravity cannot hold it and it leaves the Earth forever.
In 1965, two American physicists, Arno Penzias and Robert Wilson of Bell Telephone Laboratories in New Jersey, were debugging a very sensitive microwave receiver. (Microwaves are radiation with a wavelength of about a centimeter.) Penzias and Wilson were worried that the receiver was picking up more noise than expected. They found bird droppings on the antenna and eliminated other potential causes of failure, but soon exhausted all possible sources of interference. The noise differed in that it was recorded around the clock throughout the year, regardless of the rotation of the Earth around its axis and its revolution around the Sun. Since the motion of the Earth directed the receiver into various sectors of space, Penzias and Wilson concluded that the noise was coming from outside solar system and even from outside the galaxy. It seemed to come in equal measure from all sides of the cosmos. We now know that wherever the receiver is directed, this noise remains constant, apart from negligible variations. So Penzias and Wilson stumbled upon a striking example that the universe is the same in all directions.
What is the origin of this cosmic background noise? Around the same time that Penzias and Wilson were investigating the mysterious noise in a receiver, two American physicists from Princeton University, Bob Dick and Jim Peebles, also became interested in microwaves. They studied the assumption of George (George) Gamow that in the early stages of development the Universe was very dense and white-hot. Dick and Peebles believed that if this was true, then we should be able to observe the glow of the early universe, since light from very distant regions of our world is only now reaching us. However, due to the expansion of the Universe, this light must be so strongly shifted to the red end of the spectrum that it will turn from visible radiation into microwave radiation. Dick and Peebles were just preparing to search for this radiation when Penzias and Wilson, hearing about their work, realized that they had already found it. For this discovery, Penzias and Wilson were awarded the Nobel Prize in 1978 (which seems somewhat unfair to Dick and Peebles, not to mention Gamow).
At first glance, the fact that the universe looks the same in any direction suggests that we occupy some special place in it. In particular, it might seem that since all the galaxies are moving away from us, then we must be in the center of the universe. There is, however, another explanation for this phenomenon: the universe can look the same in all directions from any other galaxy as well.
All galaxies are moving away from each other. This is reminiscent of the spreading of colored spots on the surface of an inflated balloon. As the size of the ball increases, the distances between any two spots also increase, but in this case, none of the spots can be considered the center of expansion. Moreover, if the radius of the balloon is constantly growing, then the further apart the spots on its surface are, the faster they will be removed during expansion. Let's say the radius of the balloon doubles every second. Then two spots, initially separated by a distance of one centimeter, in a second will be already at a distance of two centimeters from each other (if measured along the surface of the balloon), so that their relative speed will be one centimeter per second. On the other hand, a pair of spots that were separated by ten centimeters will, one second after the start of expansion, move apart by twenty centimeters, so that their relative speed will be ten centimeters per second. The speed at which any two galaxies move away from each other is proportional to the distance between them. Thus, the redshift of a galaxy should be directly proportional to its distance from us - this is the same dependence that Hubble later discovered. The Russian physicist and mathematician Alexander Fridman managed to propose a successful model in 1922 and anticipate the results of Hubble's observations, his work remained almost unknown in the West until a similar model was proposed in 1935. American physicist Howard Robertson and British mathematician Arthur Walker are already on the trail of the expansion of the universe discovered by Hubble.
As the universe expands, galaxies are moving away from each other. As time passes, the distance between distant stellar islands increases more than between nearby galaxies, just as it happens with spots on an inflated hot-air balloon. Therefore, to an observer from any galaxy, the rate of removal of another galaxy seems to be the greater, the farther it is located.
Three types of expansion of the universe
The first class of solutions (the one found by Friedman) assumes that the expansion of the universe is slow enough that the attraction between galaxies gradually slows it down and eventually stops it. After that, the galaxies begin to converge, and the Universe begins to shrink. According to the second class of solutions, the universe is expanding so rapidly that gravity will only slightly slow down the recession of galaxies, but will never be able to stop it. Finally, there is a third solution, according to which the universe is expanding just at such a rate as to avoid collapse. Over time, the speed of the expansion of galaxies becomes less and less, but never reaches zero.
An amazing feature of Friedman's first model is that in it the Universe is not infinite in space, but at the same time there are no boundaries anywhere in space. Gravity is so strong that space is curled up and closes on itself. This is somewhat similar to the surface of the Earth, which is also finite, but has no boundaries. If you move along the surface of the Earth in a certain direction, you will never come across an insurmountable barrier or edge of the world, but in the end you will return to where you started from. In Friedman's first model, space is arranged in exactly the same way, but in three dimensions, and not in two, as in the case of the Earth's surface. The idea that you can go around the universe and return to the starting point is good for science fiction, but it does not practical value, since, as can be proven, the universe will shrink into a point before the traveler returns to the beginning of his journey. The universe is so big that you need to move faster than light to finish your journey where you started, and such speeds are forbidden (by the theory of relativity). In Friedman's second model, space is also curved, but in a different way. And only in the third model is the large-scale geometry of the Universe flat (although space is curved in the vicinity of massive bodies).
Which of Friedman's models describes our Universe? Will the expansion of the Universe ever stop, and will it be replaced by contraction, or will the Universe expand forever?
It turned out that answering this question is more difficult than scientists initially thought. Its solution depends mainly on two things - the currently observed rate of expansion of the Universe and its current average density (the amount of matter per unit volume of space). The higher the current expansion rate, the greater the gravity, and hence the density of the matter, is required to stop the expansion. If the average density is above some critical value (determined by the rate of expansion), then the gravitational attraction of matter can stop the expansion of the universe and cause it to contract. This behavior of the Universe corresponds to the first Friedman model. If the average density is less than the critical value, then the gravitational attraction will not stop the expansion and the Universe will expand forever - as in the second Friedmann model. Finally, if the average density of the universe is exactly equal to the critical value, the expansion of the universe will slow down forever, getting closer to a static state, but never reaching it. This scenario corresponds to the third Friedman model.
So which model is correct? We can determine the current rate of expansion of the universe if we measure the rate at which other galaxies are moving away from us using the Doppler effect. This can be done very accurately. However, the distances to galaxies are not well known because we can only measure them indirectly. Therefore, we only know that the rate of expansion of the Universe is from 5 to 10% per billion years. Even more vague is our knowledge of the current average density of the universe. Thus, if we add up the masses of all the visible stars in our own and other galaxies, the sum is less than a hundredth of what is required to stop the expansion of the universe, even at the lowest estimate of the expansion rate.
But that's not all. Our and other galaxies must contain a large number of some kind of "dark matter" that we cannot directly observe, but whose existence we know due to its gravitational influence on the orbits of stars in galaxies. Perhaps the best evidence for the existence of dark matter comes from the orbits of stars at the periphery. spiral galaxies, similar Milky Way. These stars revolve around their galaxies too fast to be kept in orbit by the gravity of the galaxy's visible stars alone. In addition, most galaxies are part of clusters, and we can similarly infer the presence of dark matter between galaxies in these clusters by its effect on the motion of galaxies. In fact, the amount of dark matter in the Universe far exceeds the amount of ordinary matter. If we take into account all the dark matter, we get about a tenth of the mass that is needed to stop the expansion.
However, it is impossible to exclude the existence of other forms of matter, not yet known to us, distributed almost evenly throughout the Universe, which could increase its average density. For example, there are elementary particles, called neutrinos, which interact very weakly with matter and are extremely difficult to detect.
For the last few year s different groups researchers studied the smallest ripples in the microwave background that Penzias and Wilson discovered. The size of this ripple can serve as an indicator of the large-scale structure of the universe. Her character seems to indicate that the universe is still flat (as in Friedman's third model)! But since the total amount of ordinary and dark matter is not enough for this, physicists postulated the existence of another, not yet discovered, substance - dark energy.
And as if to further complicate the problem, recent observations have shown that the expansion of the universe is not slowing down, but accelerating. Contrary to all Friedman's models! This is very strange, since the presence of matter in space - high or low density - can only slow down the expansion. After all, gravity always acts as a force of attraction. The acceleration of cosmological expansion is like a bomb that collects rather than dissipates energy after the explosion. What force is responsible for the accelerating expansion of the cosmos? No one has a reliable answer to this question. However, Einstein may have been right after all when he introduced the cosmological constant (and the corresponding anti-gravity effect) into his equations.
The expansion of the universe could have been predicted at any time in the nineteenth or eighteenth century, and even at the end of the seventeenth century. However, the belief in a static universe was so strong that delusion held sway over minds until the early twentieth century. Even Einstein was so sure of the static nature of the universe that in 1915 he made a special correction to the general theory of relativity by artificially adding a special term to the equations, called the cosmological constant, which ensured the static nature of the universe.
The cosmological constant manifested itself as the action of some new force - "anti-gravity", which, unlike other forces, had no definite source, but was simply an inherent property inherent in the very fabric of space-time. Under the influence of this force, space-time showed an innate tendency to expand. By choosing the value of the cosmological constant, Einstein could vary the strength of this trend. With its help, he managed to exactly balance the mutual attraction of all existing matter and get a static universe as a result.
Einstein later rejected the idea of the cosmological constant, recognizing it as his "most big mistake". As we shall soon see, there are reasons today to believe that Einstein might, after all, have been right in introducing the cosmological constant. But what must have upset Einstein most of all was that he let his belief in a stationary universe override the conclusion that the universe must expand, predicted by his own theory. It seems that only one person saw this consequence of the general theory of relativity and took it seriously. While Einstein and other physicists were looking for ways to avoid a non-static universe, Russian physicist and the mathematician Alexander Friedman, on the other hand, insisted that it was expanding.
Friedman made two very simple assumptions: that it looks the same no matter in which direction we look, and that this statement is true no matter from which point in the universe we look. Based on these two ideas and solving the equations of general relativity, he proved that the universe cannot be static. Thus, in 1922, a few years before the discovery of Edwin Hubble, Friedman accurately predicted the expansion of the universe!
Centuries ago Christian church would recognize it as heretical, since church doctrine postulated that we occupy special place at the center of the universe. But today we accept Friedman's assumption for almost the opposite reason, a kind of modesty: we would find it completely surprising if the universe looked the same in all directions only to us, but not to other observers in the universe!
