Expanding universe. The Big Bang and the Expanding Universe

Where is the Universe expanding?
I think everyone has already heard that The universe is expanding, and often we imagine it as a huge ball filled with Galaxies and nebulae, which increases from some smaller state and the thought creeps in that at the beginning of time Universe In general, it was squeezed into a point.

Then the question arises, what is behind border , And where the universe is expanding ? But what border are we talking about?! Isn't it Universe not endless?! Still, let's try to figure this out.

Expansion of the Universe and the Hubble Sphere

Let's imagine that we are observing through a super-huge telescope, in which we can see anything in Universe . It is expanding and its galaxies are moving away from us. Moreover, the farther they are spatially relative to us, the faster the galaxies are moving away. Let's look further and further. And at some distance it turns out that all bodies are moving away relative to us at the speed of light. This creates a sphere called Hubble sphere . Now it's a little less 14 billion light years , and everything outside of it flies away relative to us faster than light. This would seem to contradict Theories of Relativity , because the speed cannot exceed light speed. But no, because here we are not talking about the speed of the objects themselves, but about the speed expansion of space . But this is completely different and it can be anything.
But we can look further. At some distance, objects move away so quickly that we will never see them at all. Photons emitted in our direction will simply never reach the Earth. They are like a person walking against the direction of an escalator. They will be carried back by the rapidly expanding space. The boundary where this happens is called Particle horizon . Now it's about 46.5 billion light years . This distance increases, because The universe is expanding . This is the border of the so-called Observable Universe . And we will never see everything beyond this border.
And here is the most interesting thing. What's behind it? Maybe this is the answer to the question?! It turns out everything is very prosaic. In fact, there is no border. And there the same galaxies, stars and planets extend for billions of billions of kilometers.

But how?! How does this happen?!

Center of Universe Expansion and Particle Horizon

Just Universe scatters quite cleverly. This happens at every point in space the same way. It’s as if we took a coordinate grid and increased its scale. This really makes it seem like all the Galaxies are moving away from us. But, if you move to another Galaxy, you will see the same picture. Now all objects will move away from it. That is, at every point in space it will seem that we are in expansion center . Although there is no center.
So if we find ourselves next to Particle horizon , neighboring Galaxies will not fly away from us faster speed Sveta. After all Particle horizon move with us and again it will be very far away. Accordingly, the boundaries will shift Observable Universe and we will see new Galaxies that were previously inaccessible to observation. And this operation can be done endlessly. You can move to the horizon of particles over and over again, but then it itself will shift, opening up new vistas. Universe . That is, we will never reach its borders, and it turns out that Universe and it's true infinite . Well, only the observable part of it has boundaries.
Something similar happens in Globe . It seems to us that the horizon is a border earth's surface, but it’s worth moving to that point and it turns out that there is no border. U Universe there is no limit beyond which there is no spacetime or something like that. It's just that here we come across infinity , which is unusual for us. But you can say this Universe has always been infinite and is stretching while continuing to remain infinite. She can do this because space does not have the smallest particle. It can stretch as long as desired. The Universe, for expansion, does not need boundaries and areas where to expand. So this simply doesn’t exist.

So wait a minute, what about Big Bang ?! Wasn’t everything that exists in space compressed into one tiny point?!

No! It was only compressed into a dot observable boundary of the universe . And as a whole, it never had boundaries. To understand this, let's imagine Universe billionths of a second after, when the observed part of it was the size of a basketball. Even then we can move to Particle horizon and everything visible Universe will move. We can do this as many times as we like and it turns out that Universe really infinite .
And we can do the same thing before. Thus, moving backward in time, we will find ourselves closer to Big Bang . But at the same time, each time we will discover that The universe is infinite in every period of time! Even at the moment of the Big Bang! And it turns out that it happened not at any specific point, but everywhere, at every point, which has no limit to the Cosmos.
However, this is just a theory. Yes, it is quite consistent and logical, but not without its shortcomings.

What state was the substance in at the moment? Big Bang ? What happened before it and why did it happen at all? So far, there are no clear answers to these questions. But the scientific world does not stand still, and perhaps we will even become eyewitnesses to the solution to these mysteries.

In the history of knowledge of the world around us, a general direction is clearly visible - 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 impulse,” then this kind of worldview can be considered completely materialistic. Newton's Universe argued that space is the container of all celestial bodies, with the movement and mass of which it is in no way connected; The Universe is always the same, that is, stationary, although the death and birth of worlds constantly occurs in it.

It would seem that the sky of Newtonian cosmology promised to be cloudless. However, very soon I had to be convinced of the opposite. During the 19th century. three contradictions were discovered, which were formulated in the form of three paradoxes, called cosmological. They seemed to undermine the idea of ​​the infinity of the universe.

Photometric paradox. If the Universe is infinite and the stars in it are evenly distributed, then in any direction we should see some star. In this case, the sky background would be dazzlingly bright, like the Sun.

Gravitational paradox. If the Universe is infinite and stars uniformly occupy its space, then the gravitational force at each point should be infinitely great, and therefore, the relative accelerations of cosmic bodies would be infinitely great, which, as is known, is not the case.

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 space. Sooner or later, all bodies will cool down to the temperature of absolute zero, movement will stop and “thermal death” will occur forever. The universe had a beginning and will inevitably end.

First quarter of the 20th century passed in languid anticipation of the denouement. Nobody, of course, 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 spirit to cosmological debates.

Newtonian classical physics, as already mentioned, considered space as a container of bodies. According to Newton, there could be no interaction between bodies and space.

In 1916, A. Einstein published the principles general theory relativity. One of its main ideas is that material bodies, especially large masses, noticeably bend space. Because of this, for example, a ray of light passing near the Sun changes its original direction.

Let us now imagine that throughout the entire part of the Universe we observe, matter is evenly “spread out” in space and the same laws apply 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

will even close it "on itself". The Universe (more precisely, a selected part of it) will turn into a closed world, reminiscent of an ordinary sphere. But only this 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 same direction all the time in cosmic space, then after a certain number of billions of years you can get to your starting point.

The idea of ​​the possibility of a closed Universe was first expressed by A. Einstein. In 1922, the Soviet mathematician A. A. Friedman proved that Einstein’s “closed Universe” could not 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 towards the red end, and the displacement of the bodies is greater the further the galaxy is from us. This interesting phenomenon called redshift. Having explained the red shift by the Doppler effect, i.e., a change in 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, the galaxies do not fly away 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 him. 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 will become different in the future, i.e. the Metagalaxy is evolving.

The receding speeds of galaxies are determined from the redshift. In many galaxies they are very large, comparable to the speed of light. The highest speeds, sometimes exceeding

some quasars, which are considered the most distant objects of the Metagalaxy from us, have a speed of 250 thousand km/s.

The law according to which the redshift (and therefore the speed of removal of galaxies) increases in proportion to the distance from the galaxies (Hubble's law) can be written as: v - Нr, where v is the radial velocity of the galaxy; r is the distance to it; H is the Hubble constant. By modern estimates, the value of H is within the limits:

Consequently, the observed rate of expansion of the Metagalaxy is such that galaxies separated by a distance of 1 Mpc (3 10 19 km) are moving away from each other at a speed of 50 to 100 km/s. If the speed at which the galaxy is moving away 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 the galaxies scatter.

The expansion of the Metagalaxy is the most ambitious currently known natural phenomenon. Its correct interpretation has extremely great worldview significance. It is no coincidence that in explaining the cause of this phenomenon, a radical difference in the philosophical views of scientists was sharply revealed. Some of them, identifying the Metagalaxy with the entire Universe, try to prove that the expansion of the Metagalaxy confirms the religious belief about the supernatural, divine origin of the Universe. However, there are known natural processes in the Universe that could have caused the observed expansion in the past. In all likelihood, these are explosions. Their scale amazes us even when studying individual types of galaxies. One can imagine that the expansion of the Metagalaxy

also began with a phenomenon reminiscent of 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 super-dense material point. This 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 occurred, and since then the Universe began 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 evidence that the Universe is in a superdense and hot state, i.e., cosmic microwave background radiation. It turned out that space filled with electromagnetic waves, which are messengers from that ancient era of the development of the Universe, when there were no stars, galaxies, or nebulae yet. CMB radiation permeates all space, all galaxies, it participates in the expansion of the Metagalaxy. CMB 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 an absolutely black body with a temperature of 2.7 K. The energy density of CMB radiation is 4 10 -13 erg/cm 3, the maximum radiation occurs at 1.1 mm. In this case, the radiation itself has the character of a certain background, because it fills all 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 chance while studying cosmic radio interference, the existence of the cosmic microwave background radiation was predicted by theorists. D. Gamow was one of the first to predict this radiation, developing the theory of origin chemical elements that arose in the first minutes after the Big Bang. Predicting the existence of cosmic microwave background radiation and detecting it in outer space is another convincing example of the knowability of the world and its laws.

All developed dynamic cosmological models affirm the idea of ​​the expansion of the Universe from some superdense and superhot state, called singular. American astrophysicist D. Gamow came to the concept of the Big Bang and the hot Universe in the early stages of its evolution. Analysis of the problems of the initial stage of the evolution of the Universe was made possible thanks to new ideas about the nature of vacuum. The cosmological solution obtained by W. de Sitter for vacuum (r ~ e Ht) showed that exponential expansion is unstable: it cannot continue indefinitely. After a relatively short period of time, the exponential expansion stops, a phase transition occurs in the vacuum, during which the vacuum energy transforms into ordinary substance and the kinetic energy of the expansion of the Universe. The Big Bang took place 15-20 billion years ago.

According to the standard model of a hot Universe, superdense matter after the Big Bang began to expand and gradually cool. As the expansion progressed, phase transitions occurred, as a result of which physical strength 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 initial stage expansion of the universe the difference between elementary particles and four types of physical interactions are practically absent. It begins to appear when the temperature decreases and differentiation of matter begins.

Thus, modern ideas about the history of the emergence of our Metagalaxy are based on five important experimental observations:

1. A study of the spectral lines of 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 in the red part of the spectrum is detected. Magnitude

This displacement 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 signature of this so-called background radiation corresponds to black body radiation at a temperature of about 2.7 degrees Kelvin.

4. By 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 arisen between 14 and 24 billion years ago.

The universe is expanding. But in a sense, the expansion has not yet been directly observed: theorists are building various models, allowing us to describe it, but we do not see how space objects in real time become further and further away.

The accuracy of observations needs to be greatly improved, and with current technology we will have to wait centuries, or at least decades, to accumulate data illustrating this process.

To build a model demonstrating the expansion of the Universe, we usually compare the expanding Universe with an inflating balloon. At the same time, we assume that the entire “observation area” is available to us entirely and in an instant. In fact, the more distant a galaxy we observe, the longer it takes for its light to reach the retina of our eyes. Consequently, at the moment of emission of this light, the galaxy seemed to be on the surface of a “less inflated” ball. The most distant galaxies we have observed are visible at a time when the “ball” was very small. Thus, due to the finite speed of light, we see a highly distorted picture of the world around us.

A special feature of this model of the expanding Universe is a kind of “look from the outside.” It’s as if we are looking from an “extra” dimension, and in addition we see everything at once, observing processes using a single “cosmic clock”, that is, we cover the entire Universe at once, receiving information at an infinite speed. This "view of God" is inaccessible to the ordinary observer.

We are on Earth, inside the Universe. Signals come to us at a finite speed - the speed of light. Therefore, we see distant objects as they were in the distant past. In astronomy, redshift is a shift of the spectrum towards the red. This phenomenon may be an expression of the Doppler effect, gravitational redshift, or combinations thereof. Both the cosmological redshift caused by the expansion of space in the Universe and the red (or violet) shift associated with the Doppler effect due to the proper motion of galaxies contribute to the shift of lines in galactic spectra.

Following the discovery of redshift in the spectra of distant galaxies, it was suggested that it was caused by something like "travel fatigue": some unknown process causes photons to lose energy as they move away from the light source and therefore "turn red."

But this hypothesis does not agree with observations. For example, when a star explodes as a supernova, it flares up and then dims. Type 1a supernovae, used to determine distances to galaxies, have a decay time of about two weeks. During this period of time, a certain number of photons are emitted. The "fatigue" hypothesis says that during the journey they will lose energy, but the observer will still see a stream of photons lasting two weeks. In an expanding space, not only the photons themselves are “stretched” (due to which they lose energy), but also their flow. Therefore, it takes more than two weeks for all of them to “get” to Earth.

There are two problems with distance in cosmology: everything is located very far from each other and moves quickly. While the light reaches the observer from the source, their distance will change greatly. At the same time, the distance to objects “right now” cannot be measured direct measurement, since this procedure takes a finite (and, generally speaking, quite large) time associated with signal propagation: we simply do not see distant objects as they are in this moment. This complicates everything, because, using everyday experience, we are accustomed to imagining everything “as it is now.” In cosmology, we can only calculate distances and speeds “right now” within the framework of a certain model, or obtain them in some “roundabout way”, but not using modern methods observations.

As the Universe expands, its observable region now has a radius of more than 14 billion light years. As light travels, the space it traverses expands. By the time it reaches us, the distance to the galaxy that emitted it becomes greater than simply calculated from the photon “travel” time (approximately the second).

Many people remember the events of yesterday better than the day before yesterday, but they don’t remember a week ago at all. But some memories of childhood and youth shine for them, as if it all happened yesterday. If we take a galaxy like ours, it turns out that up to a certain distance (and when looking at distant objects, we are looking into the past!) it will look smaller and smaller. But then - lo and behold! - the visible size will begin to increase. This is because the light from the observed galaxy was emitted when the Universe was young, when we were much closer. Accordingly, the angular distance to distant objects changes in the same bizarre way. The angle between light rays does not change as they propagate in a “flat” universe. Therefore, the angular distance to a space object depends only on how far it was at the moment of emission.

Proper distance is the physical distance between objects. It changes in accordance with the expansion of the Universe. The distance, which is usually mentioned in all articles and news, is equal to the path of light traveled from the source since the moment of emission. It is approximately equal to its own at relatively short distances, where during the propagation of the signal the Universe did not have time to expand noticeably. The accompanying coordinates are tied to a coordinate grid that expands along with the expansion of the Universe. The position of the objects relative to it remains unchanged, while the proper distances between them increase in accordance with the change in the scale factor. It is important that the angular distance is equal to the intrinsic distance at the moment of emission of radiation.

Until now, the horizon has risen as "the line where the earth meets the sky." As our understanding of the Universe improved, more and more “horizons” began to appear in the vocabulary of scientists, which were not possible to achieve (if only because the maximum possible speed in our world is limited by the speed of light). The particle horizon is an expanding sphere, the radius of which is determined by the distance to the most distant source, in principle observable at a given moment in time (we are talking about the own distance to the object at the moment of receiving the photon, and not at the moment of emission). Such a horizon cannot be defined as the speed of light multiplied by the time after the expansion began, since while the photon is traveling, the universe is expanding. But if we are talking about particles as galaxies that arose at some not too early moment in the evolution of the universe, then such a horizon will also be in accelerating models. It also exists in our Universe. The distance to the event horizon is the distance (currently) to the particle that our light signal sent right now can reach. We observe galaxies at redshift around 1.8. Light from such galaxies takes 10 billion years to reach us.

At the moment of emission, they were 5.7 billion light years away from us (their own distance at the time of emission). Now they are 16.1 billion light years away (their own distance at the moment), and the signal we sent to them will never reach them unless the dynamics of the Universe fundamentally changes in the future. Conversely, we will never see the events taking place in them now.

It turns out that the distance to the event horizon corresponds to the distance to such galaxies at the moment, but we see them now as they were in the distant past! In this sense, we will not see the event horizon, but we can say that its position corresponds to current situation galaxies observed by us at redshift 1.8. According to Hubble's law, the speed at which distant objects recede is directly proportional to their distances. Here we are talking about the rate of change of one’s own distance at the present moment.

The distance at which the receding speed equals the speed of light is called the “Hubble sphere.” There are sources that, both at the moment of emission and at the present moment, are outside its boundaries, that is, their escape velocity is higher than the speed of light both then and now.

In the current cosmological model (with a dark energy contribution of about 70%), all observed sources with a redshift greater than about 1.5 are currently moving away from us faster than the speed of light. That is, the relative speeds of points located at large distances from each other are not limited by the speed of light.

In a hypothetical stationary universe with a beginning in time, the particle horizon is a sphere expanding at the speed of light. If, 5 billion years after the “creation” of this world, an observer appears in one of the galaxies, for him this horizon of particles will turn out to be a sphere with a radius of 5 billion light years. In another billion years, its radius will be 6 billion light years, etc.

Let us imagine the first photon emitted at “time zero”. To its speed of movement, equal to the speed of light, is added the speed of expansion of space. During the existence of the Universe, this photon moved away from the place of its emission to a distance of 46 billion light years (it flew about 13.7 billion light years “on its own”, the rest due to the expansion of the Universe). Thus, without taking into account the expansion rate, it would have taken 46 billion years for it to cover such a distance. CMB originated when the Universe was 380 thousand years old. The accompanying redshift is 1089. Today, the proper distance to the source that emitted this radiation is almost 46 billion light years.

The observer can only see a finite part of his world. It is not possible for us to know what the Universe is like beyond the current horizon of particles. If space continues to expand at an accelerating rate, then even in the distant future it will be impossible to check what the Universe looks like beyond the horizon of particles. And our telescopes cannot “look” into the era when outer space was filled with plasma and did not contain free photons.

Based on material by Sergei Popov and Alexey Toporensky, prepared by Sergei RYABOSHAPKO, Samara

TO HOME

It is somewhat of an irony of nature that the most abundant form of energy in the Universe is also the most mysterious. After the stunning discovery of the accelerating expansion of the Universe, a consistent picture quickly emerged indicating that 2/3 of the cosmos is “made” of “dark energy” - some kind of gravitationally repulsive material. But is the evidence convincing enough to support these exotic new laws of nature? Maybe there are simpler astrophysical explanations for these results?

The prototype of this note was recently published in the popular science section of Habr, although under lock and key, so perhaps not everyone interested got it. In this version, quite significant additions have been made, which should be of interest to everyone.

The history of dark energy began in 1998, when two independent teams explored distant supernovae. in order to detect the rate at which the expansion of the Universe is slowing down. One of them, Supernova Cosmology Project, began work in 1988, and was led by Saul Perlmutter. Another, led by Brian Schmidt High-z Supernova Search Team, joined the research in 1994. The result shocked them: the Universe has been in an accelerated expansion mode for quite a long time.

Like detectives, cosmologists around the world were compiling a dossier on the accused responsible for the acceleration. Its special features: gravitationally repulsive, prevents the formation of galaxies (clustering of matter into galaxies), manifests itself in the stretching of space-time. The nickname of the accused is “dark energy.” Many theorists have suggested that the accused is a cosmological constant. It certainly corresponded to the scenario of accelerated expansion. But was there enough evidence to fully identify dark energy with the cosmological constant?

The existence of gravitational-repulsive dark energy would have dramatic consequences for fundamental physics. The most conservative assumption was that the Universe is filled with a homogeneous sea of ​​zero-point quantum energy, or a condensate of new particles whose mass is $((10)^(39))$ times less than an electron. Some researchers also suggested the need for changes to general relativity, in particular new long-range forces that weaken the effect of gravity. But even the most conservative proposals had serious shortcomings. For example, the zero-point energy density turned out to be 120 implausible orders of magnitude less than theoretical predictions. From the point of view of these extreme assumptions, it seemed more natural to look for a solution within the framework of traditional astrophysical concepts: intergalactic dust (the scattering of photons on it and the associated weakening of the photon flux) or the difference between new and old supernovae. This possibility has been supported by many cosmologists keeping watch in the night.

Observations of supernovae and their analysis carried out by S. Perlmutter, B. Schmidt and A. Riess made it clear that the decrease in their brightness with distance occurs noticeably faster than would be expected according to the cosmological models accepted at that time. More recently, this discovery was noted. This additional dimming means that a given redshift corresponds to some effective distance addition. But this, in turn, is possible only when the cosmological expansion occurs with acceleration, i.e. The speed at which the light source moves away from us does not decrease, but increases with time. The most important feature of the new experiments was that they made it possible not only to determine the very fact of accelerated expansion, but also to draw an important conclusion about the contribution of various components to the density of matter in the Universe.

Until recently, supernovae were the only direct evidence of accelerated expansion and the only convincing support for dark energy. Accurate measurements Cosmic Microwave Background (CMB) data including WMAP (Wilkinson Microwave Anisotropy Probe) data provided independent confirmation of the reality of dark energy. The same was confirmed by data from two more powerful projects: the large-scale distribution of galaxies in the Universe and the Sloan Digital Sky Survey (SDSS).

A combination of data from WMAP, SDSS and other sources found that the gravitational repulsion generated by dark energy is slowing the collapse of super-dense regions of matter in the Universe. The reality of dark energy immediately became significantly more acceptable.

Space expansion

Cosmic expansion was discovered by Edwin Hubble in the late 1920s and may be the most important feature of our Universe. Not only do astronomical bodies move under the influence of the gravitational interaction of their neighbors, but large-scale structures are even more stretched by cosmic expansion. A popular analogy is the movement of raisins in a very large cake in the oven. As the pie rises, the distance between any pair of raisins embedded in the pie increases. If we imagine that one particular highlight represents our galaxy, then we will find that all other highlights (galaxies) are moving away from us in all directions. Our Universe expanded from the hot, dense cosmic soup created by the Big Bang into the much cooler, thinner collection of galaxies and galaxy clusters we see today.

Light emitted by stars and gas in distant galaxies is similarly stretched, lengthening its wavelength as it travels to Earth. This shift in wavelength is given by the redshift $z=\left(\lambda_(obs)-\lambda_0\right)/\lambda_0$, where $\lambda_(obs)$ is the length of light on Earth and $\lambda_(0) $ is the wavelength of the emitted light. For example, the Lyman alpha transition in the hydrogen atom is characterized by a wavelength of $\lambda_0=121.6$ nanometers (when returning to the ground state). This transition can be detected in the radiation of distant galaxies. In particular, it was used to detect a record high redshift: a stunning z=10 with the Lyman alpha line at $\lambda_(obs)=1337.6$ nanometers. But redshift describes only the change in cosmic scale as light is emitted and absorbed, and does not provide direct information about the distance to the emitter or the age of the universe when the light was emitted. If we know both the distance to the object and the redshift, we can try to obtain important information about the dynamics of the expansion of the Universe.

Observations of supernovae have revealed some gravitational-repulsive substance that controls the acceleration of the Universe. This is not the first time that astronomers have encountered the problem of missing matter. The luminous masses of galaxies turned out to be significantly smaller than the gravitating masses. This difference was made up by dark matter - cold, non-relativistic matter, probably mostly composed of particles that interact weakly with atoms and light.

However, observations indicated that the total amount of matter in the Universe, including dark matter, is only 1/3 of the total energy. This has been confirmed by the study of millions of galaxies within the 2DF and SDSS projects. But general relativity predicts that there is a precise relationship between expansion and the energy content of the universe. We therefore know that the total energy density of all photons, atoms and dark matter must be added to some critical value, determined by the Hubble constant $H_(0)$: $((\rho)_(crit))=3H_(0 )^(2)/8\pi\cdot(G)$. The catch is that it doesn't, but that's a completely different story.

Mass, energy and space-time curvature are directly related in general relativity. One explanation, therefore, may be that the gap between the critical density and the observed matter density is filled by some energy density associated with the deformation of space at large scales and observable only at scales on the order of $c/((H)_(0)) \sim 4000\ Mpc$. Fortunately, the curvature of the Universe can be determined using precision ICF measurements. A relic, with origin 400,000 after the Big Bang, ICF is black body radiation, the source of which is the primordial plasma. When the Universe cooled below $3000\K$, the plasma became transparent to photons and they were able to freely propagate in space. Today, almost 15 billion years later, we observe a thermal reservoir of photons at a temperature of $2.726\K$, which represents the result of a redshift due to cosmic expansion.

A remarkable image of the ICF was obtained using the WMAP satellite, showing the slightest changes in the photon temperature of the “sky”. These variations, known as ICF anisotropy, reflect small variations in the density and motion of the early Universe. These variations, which arise at the $((10)^(-5))$ level, are the seeds of the large-scale structure (galaxies, clusters) that we observe today.

The coldest/hottest spots in the cosmic microwave background are due to photons that escaped from areas of the highest/least density gravitational potential. The dimensions of these regions are well determined by plasma physics. When we consider the full Universe, the apparent angular size of these anisotropies should be about $((0.5)^(0))$ if the Universe has enough curvature to fill the energy gap and twice the angular size in the absence of any curvature of space. The easiest way to visualize this geometric effect is to imagine a triangle with a fixed base and sides (just sides?), drawn on surfaces of varying curvature. For a saddle surface/sphere, the interior angles will be smaller/larger than the same triangle drawn on a flat surface (with Euclidean geometry).

Since 1999, a number of experiments have been carried out (TOCO, MAXIMA, BOOMERANG, WMAP), which have shown that the MCF spots have dimensions of the order of $((1)^(0))$. This means that the geometry of the Universe is flat. From the perspective of the missing energy problem, this means that something other than curvature must be responsible for filling the gap. To some cosmologists, this result looked like déjà vu. Inflation, the ICF's best theory for the origin of primordial fluctuations, suggests that the very early Universe experienced a period of accelerated expansion that was driven by a particle called an inflaton. The inflaton would stretch out any large-scale curvature, making the geometry of the universe flat or Euclidean. The evidence suggests the existence of a form of energy that prevents galaxy clustering, which is gravitationally repulsive, and which may be due to a particle other than the inflaton.

Cosmic harmony

CMB and supernova data have consistently confirmed that the source of cosmic acceleration is dark energy. But that was only the beginning. By combining precision ICF measurements from WMAP with radio, optical and X-ray sensing of large-scale matter distributions, astrophysicists have obtained further evidence of an accelerating rate of expansion of the Universe. It turned out that the gravitational potential holes of density and compaction in the Universe were stretched and smoothed over time, as if under the influence of repulsive gravity. This effect is known as the integral effect (Sachs-Wolfe (ISW)). It leads to a correlation between temperature anisotropy in the CMB and the large-scale structure of the Universe. Although the primordial plasma became transparent to photons as the Universe cooled, photons do not travel unimpeded. Space is riddled with irregularities that are strong at short distances (where matter clusters into stars, galaxies and nebulae) and gradually weaken over large length scales... During their flight, photons fall into and out of gravitational holes.

After cosmic rays were first detected (about 40 years ago), Sachs and Wolff showed that a time-varying potential should result in an energy shift in the ICF of photons passing through it. A photon gains energy when it falls into a gravitational hole and spends it when it gets out of it. If the potential became deeper during this process, then the photon as a whole would therefore lose energy. If the potential becomes shallower, the photon will gain energy.

In a Universe where the full critical density is formed only by atoms and dark matter, weak gravitational potentials at very large spatial scales (which correspond to gentle waves of matter density) evolve too slowly to leave noticeable traces in ICF photons. Denser regions simply absorb surrounding matter at the same rate at which cosmic expansion lengthens the waves, leaving the potential unchanged. However, with the faster expansion of the Universe due to dark energy, matter accretion cannot compete with stretching. Effectively, the gravitational collapse is slowed down by repulsive dark matter. Consequently, the gravitational potential tends to flatten and photons gain energy when passing through these areas. Likewise, photons lose energy when passing through regions of low density. (Not trivial!)

Negative pressure

The greatest mystery of cosmic acceleration is not that it implies that 2/3 of the substance filling the Universe is not visible to us, but that it imposes the existence of matter with gravitational repulsion. To consider this strange property of dark energy, it is useful to introduce the quantity $w=((p)_(dark))/((\rho )_(dark))$. This expression resembles the equation of state of a gas. In general relativity, the rate of change of cosmic expansion is proportional to $-\left(((\rho )_(total))+3((p)_(total)) \right)$. For accelerated expansion this value must be positive. Since $((\rho )_(total))$ is positive, and the average pressure of ordinary and dark matter is negligible (because it is cold and non-relativistic), we arrive at the requirement $3w\times ((\rho )_(dark ))+((\rho )_(total))

Why does pressure affect the expansion of the Universe? Einstein showed that matter and energy bend space-time. Therefore, for a hot gas, the kinetic energy of its atoms contributes to their gravitational forces, as measured by measuring the acceleration of distant bodies. However, the forces required to contain or isolate the gas work against this excess pressure. The universe on the other hand is neither isolated nor limited. The expansion of space filled with hot gas will effectively occur more slowly (due to self-gravity) than the expansion of a universe filled with cold gas. By the same logic, a medium with such negative pressure that $((\rho )_(total))+3p

Negative pressure is not like that a rare event. Water pressure in some tall trees becomes negative as nutrition rises through their vascular system. In a uniform electric or magnetic field, configurations with negative pressure can also be found. In these cases, the pressure is something like a stretched spring under tension caused by internal forces. At the microscopic level, the reservoir of Higgs bosons (hypothetical particles that generate particle mass in Standard Model) creates negative pressure when its thermal or kinetic excitations are small. Indeed, the inflaton can be considered as a heavy version of the Higgs boson. One proposed version of dark energy—quintessence—may even be a lighter version of the Higgs.

In principle, there is no lower limit to pressure in the Universe. Although strange things happen if $w$ drops to a value less than $-1.$ Isolated pieces of such material can have negative mass. …..But one thing is obvious. Such a strong negative pressure does not occur for normal particles and fields in general relativity. Numerous observations lead to a narrower range of dark energy parameters than those that follow from the above general reasoning.

A combination of predictions from various theoretical models and the best observations of the CMB, large-scale structures and supernovae leads to $$\Omega_(dark)= 0.728^(+0.015)_(-0.016)$$ $$w= -0.980\pm0.053 $ $

A Brief History of Dark Energy

Dark energy, or something similar to it, has appeared many times in the history of cosmology. Pandora's box was opened by Einstein, who introduced the gravitational field into his equations. Cosmic expansion had not yet been discovered and the equations correctly “suggested” that the Universe containing matter could not be static without the mathematical addition of the cosmological constant, which is usually denoted by $\Lambda$. The effect is equivalent to filling the Universe with a sea of ​​negative energy, in which stars and nebulae drift. The discovery of the extension eliminated the need for this ad hoc addition to the theory.

In subsequent decades, desperate theorists periodically introduced $\Lambda$ in an attempt to explain new astronomical phenomena. These returns were always short-lived and usually resulted in more plausible explanations for the data obtained. However, since the 60s, the idea began to emerge that the vacuum (zero) energy of all particles and fields should inevitably generate a term similar to $\Lambda$. In addition, there is reason to believe that the cosmological constant could naturally arise in the early stages of the evolution of the Universe.

In 1980, the theory of inflation was developed. In this theory, the early Universe experienced a period of accelerated exponential expansion. The expansion was due to negative pressure due to the new particle - . Inflaton proved to be very successful. He allowed a lot. These paradoxes include the problems of the horizon and the flatness of the Universe. The theory's predictions were in good agreement with various cosmological observations.

Dark energy and the future of the Universe

With the discovery of dark energy, ideas about what the distant future of our Universe might be like have changed dramatically. Before this discovery, the question of the future was clearly associated with the question of the curvature of three-dimensional space. If, as many previously believed, the curvature of space by 2/3 determined the current rate of expansion of the Universe, and there was no dark energy, then the Universe would expand without limit, gradually slowing down. Now it is clear that the future is determined by the properties of dark energy.

Since we know these properties poorly now, we cannot yet predict the future. One can only consider different variants. It is difficult to say what is happening in theories with new gravity, but other scenarios can be discussed now. If dark energy is constant over time, as is the case with vacuum energy, then the Universe will always experience accelerated expansion. Most galaxies will eventually move away from ours to an enormous distance, and our Galaxy, along with its few neighbors, will turn out to be an island in the void. If dark energy is quintessential, then in the distant future the accelerated expansion may stop and even be replaced by compression. In the latter case, the Universe will return to a state with hot and dense matter, a “Big Bang in reverse” will occur, back in time.

Energy budget of our Universe. It is worth paying attention to the fact that the share of familiar matter (planets, stars, the entire world around us) accounts for only 4 percent, the rest is made up of “dark” forms of energy.

An even more dramatic fate awaits the Universe if dark energy is a phantom, and such that its energy density increases without limit. The expansion of the Universe will become more and more rapid, it will accelerate so much that galaxies will be torn out of clusters, stars from galaxies, planets from solar system. It will come to the point that electrons will break away from atoms, and atomic nuclei will split into protons and neutrons. There will be, as they say, a big break.

Such a scenario, however, does not seem very likely. Most likely, the phantom's energy density will remain limited. But even then, the Universe may face an unusual future. The fact is that in many theories, phantom behavior - an increase in energy density over time - is accompanied by instabilities. In this case, the phantom field in the Universe will become highly inhomogeneous, its energy density in different parts The universe will be different, some parts will expand rapidly, and some may experience collapse. The fate of our Galaxy will depend on which region it falls into.

All this, however, relates to the future, distant even by cosmological standards. In the next 20 billion years, the Universe will remain almost the same as it is now. We have time to understand the properties of dark energy and thereby more definitely predict the future - and perhaps influence it.

When we look at the distant Universe, we see galaxies everywhere - in all directions, millions and even billions of light years away. 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 the picture of an expanding universe. But what if it expands? Science knows, and now you will know too.

At first glance, this question may seem like a common sense question. Because anything that expands is usually made of matter and exists in the space and time of the Universe. But the Universe itself is space and time containing matter and energy within itself. When we say that “the Universe is expanding,” we mean the expansion of space itself, causing individual galaxies and clusters of galaxies to move away from each other. The easiest way would be to imagine a ball of dough with raisins inside, which is baked in an oven, says Ethan Siegel.

An expanding "bun" model of the Universe, in which relative distances increase as space expands

This dough is the fabric of space, and the raisins are connected 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 away they are, the faster. Only in the case of the Universe, the oven and air outside the dough do not exist, there is only dough (space) and raisins (matter).

It's not just receding galaxies that create redshift, but rather the space between us

How do we know that this space is expanding and not galaxies moving away?

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. You could also assume that you are near the center of the explosion, and many objects are simply further away and moving away faster because they received more energy from the explosion. If this were the case, we could prove it in two ways:

• At greater distances and high speeds there will be fewer galaxies because over time they would spread out greatly in space
• The relationship between redshift and distance will take on a specific shape at greater distances, which will be different from the shape if the fabric of space were expanding

When we look at great distances, we find that further out in the Universe the density of galaxies is higher than those closer to us. This is consistent with a picture in which space is expanding, because looking further is the same as looking into the past, where less expansion occurred. We also find that distant galaxies have a redshift-to-distance ratio consistent with 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 constant only in space, not in time. The universe is currently expanding more slowly than in the past. When we talk about expansion speed, we are talking about speed per unit distance: about 70 km/s/Mpc today. (Mpc is a 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 drops, 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 fate of the universe offers different possibilities, but if dark energy is truly constant as the data suggests, we will follow the 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 there is, as well as the rate of expansion of the Universe. Based on the laws of physics and what has happened in the past, it appears that 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 present, all our conclusions will have to be reconsidered.

Do galaxies move faster than the speed of light? Isn't this prohibited?

From our point of view, the space between us and the distant point is expanding. The further it is from us, the faster it seems to us that it is moving away. Even if the rate of expansion were tiny, a distant object would one day cross the threshold of any speed limit, because the rate of expansion (speed per unit distance) would multiply many times over with sufficient distance. OTO approves of this scenario. The law that nothing can travel faster than the speed of light applies only to the movement of an object through space, not to the expansion of space itself. In reality, the galaxies themselves move at speeds of only a few thousand kilometers per second, far 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 motion of the galaxy.

There are approximately 2 trillion galaxies within the observable universe (yellow circle). We will never be able to catch up with galaxies that are closer than a third of the way to this boundary due to the expansion of the Universe. Only 3% of the volume of the Universe is open to human exploration.

The expansion of the Universe is a necessary consequence of the fact that matter and energy fill space-time, which obeys the laws of general relativity. As long as there is matter, there is also gravitational attraction, so either gravity wins and everything contracts again, or gravity loses and expansion wins. There is no center of expansion and there is nothing outside the space that is expanding; it is the very fabric of the Universe that is expanding. What's most interesting is that even if we left Earth at the speed of light today, we would only be able to visit 3% of the galaxies in the observable Universe; 97% of them are already out of our reach. The universe is complex.