Accelerated Expansion of the Universe: Some Popular Words

Some irony of nature is that the most abundant form of energy in the Universe is the most mysterious. After the stunning discovery of the accelerated expansion of the Universe, a coordinated picture quickly appeared, indicating that 2/3 of the cosmos was “made” of “dark energy” - some sort of gravitationally repulsive material. But is the evidence convincing enough confirming the new exotic laws of nature? Maybe there are more simple astrophysical explanations for these results?

Prehistory of the Nobel Prize

The history of dark energy began in 1998, when two independent teams explored remote supernovae in order to detect the rate of slowing expansion of the universe.
One of them, the Supernova Cosmology Project , began work in 1988, and was led by Saul Perlmutter. The other, led by Brian Schmidt of the High-z Supernova Search Team , joined the research in 1994.

The result shocked them: The universe has long been in the mode of accelerated expansion.
As detectives, cosmologists from all over the world collected dossiers on the accused responsible for acceleration. His special features: gravitationally repulsive, prevents the formation of galaxies (clustering of matter in galaxies), manifests itself in the stretching of space-time. The name of the accused is “dark energy”. Many theorists assumed that the accused is a cosmological constant. It certainly corresponded to the accelerated expansion scenario. But was there enough evidence to fully identify dark energy with a cosmological constant?
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The existence of gravitational-repulsive dark energy should 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 times less electron. Some researchers also suggested the need to change the general theory of relativity, in particular, new long-range forces that weaken the effect of gravity. But even in the most conservative proposals there were serious flaws. For example, the energy density of zero-point oscillations 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 seek a solution within the framework of traditional astrophysical concepts: intergalactic dust (photon scattering on it and the associated weakening of the photon flux) or the difference between new and old supernovae. This possibility was supported by many cosmologists awake at night.

Supernova observations and their analysis conducted by S. Perlmutter, B. Schmidt and A. Riess made it clear that their brightness decreases with distance noticeably faster than one would expect, according to cosmological models adopted at that time. More recently, this discovery was awarded the Nobel Prize in Physics . Such additional fading means that a certain effective distance additive corresponds to this redshift. But this, in turn, is possible only when the cosmological expansion occurs with acceleration, i.e. the speed of removal of the light source from us does not decrease, but increases with time. The most important feature of the new experiments was that they allowed not only to determine the very fact of accelerated expansion, but also to make an important conclusion about the contribution to the density of matter in the Universe of various components.
Until recently, supernovae were the only direct evidence of accelerated expansion and the only convincing pillar of dark energy. Accurate measurements of the cosmic microwave background, including WMAP (Wilkinson Microwave Anisotropy Probe) data provided independent confirmation of the reality of dark energy. The same was confirmed by the data of two more powerful projects: the large-scale distribution of galaxies in the Universe and Sloan Digital Sky Survey (SDSS).



Sloan Digital Sky Survey (SDSS) is a large-scale research project for images and spectra of stars and galaxies using a 2.5-meter wide-angle telescope at the Apache Point Observatory, New Mexico.

WMAP (Wilkinson Microwave Anisotropy Probe) - NASA spacecraft, designed to study the microwave background radiation generated by the Big Bang at the time of the origin of the universe.

The combination of WMAP, SDSS, and other data sources found that gravitational repulsion generated by dark energy slows down the collapse of superdense 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 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 located in the oven. When the cake approaches, the distance between any pair of raisins immersed in the cake increases. If we imagine that one particular highlight represents our galaxy, we will find that all the other highlights (galaxies) are moving away from us in all directions. Our Universe was expanding from a hot dense cosmic soup created in the process of the Big Bang to a much colder and more sparse collection of galaxies and clusters of galaxies, which we are seeing today.



The farther away from the Earth one or another galaxy is, the higher is the speed of its removal from us and, accordingly, the more strongly they are shifted to the red end of its spectrum line.




The light emitted by stars and gas in distant galaxies, stretches in a similar way, extending its wavelength during its journey to Earth. This shift in wavelength is given by the redshift. where - the length of light on Earth and - wavelength of light emitted. For example, the layman alpha transition in a hydrogen atom is characterized by a wavelength 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 redshift: a staggering z = 10 with a Lyman alpha line at nanometers. But the redshift describes only the change in the scale of the cosmos when emitting and absorbing light and does not give direct information about the distance to the radiator 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 found some gravitational-repulsive substance that controls the acceleration of the universe. Astronomers are not the first time faced with the problem of missing matter. The luminous masses of galaxies turned out to be significantly less than the gravitating masses. This difference was made up by dark matter — cold nonrelativistic matter, mostly, probably consisting of particles interacting 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 was confirmed by the study of millions of galaxies in the framework of 2DF and SDSS projects. But the general theory of relativity predicts that there is an exact connection 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 supplemented to some critical value, determined by the Hubble constant : . The catch is that which is not, but that is another story.

A brief history of dark energy

Dark energy, or something similar to it, appeared many times in the history of cosmology. Pandora’s Box was discovered by Einstein, who introduced the cosmological constant into his gravitational field equations. The cosmic expansion at that time was not yet open and the equations correctly “suggested” that the Universe containing matter cannot be static without a mathematical complement — a cosmological constant, which is usually denoted . The effect is equivalent to filling the universe with a sea of ​​negative energy, in which stars and nebulae drift. The discovery of expansion eliminated the need for this ad hoc theory addition.
In the following decades, desperate theorists periodically introduced in an attempt to explain new astronomical phenomena. These returns were always short-lived and usually resulted in more plausible explanations of the data obtained. However, from the 1960s, the idea that the vacuum (zero) energy of all particles and fields should inevitably generate a term like . In addition, there is reason to believe that the cosmological constant could naturally occur 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 . Inflaton was very successful. He resolved many paradoxes in the Big Bang model . These paradoxes include problems of the horizon and flatness of the universe. The predictions of the theory were in good agreement with various cosmological observations.

Dark energy and the future of the universe

With the discovery of dark energy, the ideas about how the distant future of our Universe can be changed. Prior to this discovery, the question of the future was clearly associated with the question of the curvature of three-dimensional space. If, as many had previously believed, the curvature of space by 2/3 would determine the current rate of expansion of the Universe, and dark energy was absent, the Universe would expand indefinitely, slowly slowing down. Now it is clear that the future is determined by the properties of dark energy.

Since we know these properties badly now, we cannot yet predict the future. You can only consider different options. It is difficult to say what happens in the theories with the new gravity, but other scenarios can be discussed now. If dark energy is constant in time, as in the case of 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, together with its few neighbors, will turn out to be an island in the void. If dark energy is the quintessence, then in the distant future, 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 opposite” will occur, back in time.




The energy budget of our universe. It is worth paying attention to the fact that the share of the usual substance (planets, stars, the whole world around us) accounts for only 4 percent, all the rest are “dark” forms of energy.

An even more dramatic fate awaits the Universe, if dark energy is a phantom, moreover, such that its energy density increases indefinitely. The expansion of the universe will be more and more rapid, it will accelerate so much that galaxies will be torn out of clusters, stars from galaxies, planets from the solar system. It comes to the fact that the electrons break away from the atoms, and the atomic nuclei are divided into protons and neutrons. There will be, as they say, a big gap.
Such a scenario, however, does not seem very likely. Most likely, the phantom energy density will remain limited. But even then the Universe can expect an unusual future. The fact is that in many theories phantom behavior — the increase in energy density with time — is accompanied by instabilities in the phantom field . In this case, the phantom field in the Universe will become strongly inhomogeneous, its energy density in different parts of the Universe will be different, some parts will expand rapidly, and some, perhaps, will experience collapse. The fate of our galaxy will depend on the area in which it falls. All this, however, refers 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 maybe, influence it.

A few words about yourself

Currently, I am professionally engaged in cosmology, the science that studies the largest of the existing objects - the entire Universe. At the same time, I am a long-time (and regular) reader of the beloved Habr, who never ceases to amaze with wonderful articles in all areas of IT technology. However, being a representative of cosmological science, I was very surprised and upset that there is no such site and community in cosmological, fairly modern and booming science.

We wanted to fill this niche, and create a site about modern cosmology - ModCos . For a number of reasons, not all of our plans came out, but what happened did not seem bad, and perhaps even useful.
Nota bene: Without being and not resorting to the help of third-party web developers, the site was written by us from scratch, and was our first pancake.