Dark energy. Dark matter and dark energy What is dark energy in the universe
There are three options for explaining the essence of dark energy:
To date (2017), all known reliable observational data do not contradict the first hypothesis, so it is accepted in cosmology as standard. The final choice between the two options requires very long and high-precision measurements of the rate of expansion of the Universe in order to understand how this rate changes over time. The expansion rate of the Universe is described by the cosmological equation of state. Resolving the equation of state for dark energy is one of the most pressing problems in modern observational cosmology.
According to observational data from the Planck space observatory published in March 2013, the total mass-energy of the observable Universe consists of 95.1% dark energy (68.3%) and dark matter (26.8%).
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Based on observations of type Ia supernovae carried out in the late 1990s, it was concluded that the expansion of the Universe is accelerating with time. These observations were then supported by other sources: measurements of cosmic microwave background radiation, gravitational lensing, and Big Bang nucleosynthesis. All obtained data fits well into the lambda-CDM model.
The cosmological constant has a negative pressure equal to its energy density. The reasons why the cosmological constant has a negative pressure follow from classical thermodynamics. The amount of energy contained in a “vacuum box” of volume V (\displaystyle V), equals ρ V (\displaystyle \rho V), Where ρ (\displaystyle \rho )- energy density of the cosmological constant. Increasing the volume of the “box” ( d V (\displaystyle dV) positively) leads to an increase in its internal energy, which means that it performs negative work. Since the work done by a change in volume d V (\displaystyle dV), equals p d V (\displaystyle pdV), Where p (\displaystyle p)- pressure, then p (\displaystyle p)- negatively and, in fact, p = − ρ (\displaystyle p=-\rho )(coefficient c 2 (\displaystyle c^(2)), connecting mass and energy, is equal to 1).
The most important unsolved problem of modern physics is that most quantum field theories, based on the energy of the quantum vacuum, predict a huge value of the cosmological constant - many orders of magnitude greater than what is admissible according to cosmological concepts. The usual formula of quantum field theory for the summation of vacuum zero-point oscillations of the field (with a cutoff at the wavenumber of vibrational modes corresponding to the Planck length) gives a huge vacuum energy density. This value, therefore, must be compensated by some action that is almost equal (but not exactly equal) in magnitude, but has the opposite sign. Some theories of supersymmetry (SATHISH) require that the cosmological constant be exactly zero, which also does not help resolve the problem. This is the essence of the “cosmological constant problem,” the most difficult “fine-tuning” problem in modern physics: not a single way has been found to derive from particle physics the extremely small value of the cosmological constant defined in cosmology. Some physicists, including Steven Weinberg, believe the so-called. The “anthropic principle” is the best explanation for the observed delicate energy balance of the quantum vacuum.
Despite these problems, the cosmological constant is in many ways the most parsimonious solution to the problem of an accelerating universe. A single numerical value explains many observations. Therefore, the current generally accepted cosmological model (lambda-CDM model) includes the cosmological constant as an essential element.
Quintessence
An alternative approach was proposed in 1987 by the German theoretical physicist Christoph Wetterich. Wetterich proceeded from the assumption that dark energy is a kind of particle-like excitation of a certain dynamic scalar field called “quintessence”. The difference from the cosmological constant is that the density of quintessence can vary in space and time. In order for the quintessence to not be able to “assemble” and form large-scale structures following the example of ordinary matter (stars, etc.), it must be very light, that is, have a large Compton wavelength.
No evidence of the existence of quintessence has yet been discovered, but such existence cannot be ruled out. The quintessence hypothesis predicts a slightly slower acceleration of the Universe compared to the cosmological constant hypothesis. Some scientists believe that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variations of fundamental constants in space or time. The existence of scalar fields is predicted by the standard model and string theory, but it poses a problem similar to the cosmological constant case: renormalization theory predicts that scalar fields should acquire significant mass.
The problem of cosmic coincidence raises the question of why the acceleration of the Universe began at a certain point in time. If the acceleration in the Universe began before this moment, stars and galaxies simply would not have time to form, and life would have no chance of arising, at least in the form we know. Supporters of the “anthropic principle” consider this fact to be the best argument in favor of their constructions. However, many quintessence models include so-called “tracking behavior”, which solves this problem. In these models, the quintessence field has a density that adjusts to the radiation density (without reaching it) until the moment of development of the Big Bang, when an equilibrium of matter and radiation develops. After this point, the quintessence begins to behave like the sought-after “dark energy” and eventually dominates the Universe. This development naturally sets dark energy levels low.
Manifestation of unknown properties of gravity
There is a hypothesis that there is no dark energy at all, and the accelerated expansion of the Universe is explained by the unknown properties of gravitational forces, which begin to manifest themselves at distances of the order of the size of the visible part of the Universe.
Consequences for the fate of the universe
It is estimated that the accelerating expansion of the Universe began approximately 5 billion years ago. It is assumed that before this, the expansion was slowed down due to the gravitational action of dark matter and baryonic matter. The density of baryonic matter in the expanding Universe is decreasing faster than the density of dark energy. Eventually, dark energy begins to dominate. For example, when the volume of the Universe doubles, the density of baryonic matter is halved, and the density of dark energy remains almost unchanged (or exactly unchanged - in the version with a cosmological constant).
If the accelerating expansion of the Universe continues indefinitely, then as a result, galaxies outside our Supercluster of Galaxies will sooner or later go beyond the event horizon and become invisible to us, since their relative speed will exceed the speed of light. This is not a violation of the special theory of relativity. In fact, it is impossible to even define "relative velocity" in curved spacetime. Relative speed makes sense and can be determined only in flat space-time, or on a sufficiently small (tending to zero) section of curved space-time. Any form of communication beyond the event horizon becomes impossible, and all contact between objects is lost.
This article was written by Vladimir Gorunovich for this site and the Wikiknowledge site, placed on this site for the purpose of protecting information, and then corrected.
Dark energy(eng. dark energy) - a hypothetical form of energy, the existence of which is assumed by some cosmological models (Accelerated expansion of the Universe).
Within these models, there are two options for explaining the essence of dark energy:- dark energy is a cosmological constant - a constant energy density that uniformly fills the space of the Universe (in other words, non-zero energy and vacuum pressure are postulated);
- dark energy is a kind of quintessence - a dynamic field, the energy density of which can change in space and time.
It is assumed that dark energy should also make up a significant part of the so-called hidden mass of the Universe.
1 Dark energy and cosmological models
2 Dark energy and the "expansion of the universe"
3 Dark energy and fundamental interactions
4 Dark energy and the law of conservation of energy
5 Dark energy and field theory
6 Dark energy - summary1. Dark energy and cosmological models
The conclusion about the presence of acceleration in the expansion of the Universe assumed (by the Big Bang hypothesis) was made on the basis of observations of supernovae carried out in the late 1990s. Then they added to the justification: the so-called cosmic microwave background radiation, gravitational lensing, nucleosynthesis of the hypothetical Big Bang. The obtained data are consistent with the lambda-CDM model.
In astronomy, distances that cannot be directly measured (distances to other galaxies) are determined using Hubble's law and redshift. But Hubble's law requires the introduction of a Hubble parameter equal to the ratio of a certain known distance to the redshift value. In astronomy, the distance to a Type Ia supernova can be determined from its luminosity using the “standard candle” method, using the fact that all exploding Type Ia supernovae at the same distance should have almost the same observed brightness. By comparing the observed brightness of supernovae in different galaxies, the distances to these galaxies can be determined.
In the late 1990s, for distant galaxies with type Ia supernovae, it was found that supernovae have a brightness lower than that which they should have based on the distance determined by Hubble's law. It turned out that the distance to these galaxies, calculated using the “standard candles” method (for supernovae Ia), turned out to be greater than the distance calculated using Hubble’s law based on the previously established value of the Hubble parameter. From which it was concluded that the Universe is expanding at an accelerating rate. Based on these observations, the existence of an unknown form of negative pressure energy called "dark energy" was postulated.
But one more conclusion can be drawn: Hubble's law does not work or is not accurate, and do not introduce a hypothetical acceleration of the fictitious expansion of the Universe. As for the date of the beginning of the accelerated expansion of the Universe (approximately 5 billion years ago), it has the same relation to reality as the age of the Universe assumed by the Big Bang hypothesis (13.75 billion years).
Cosmologists did not want to deal with their mistakes and transferred everything to physics. Of course, physics will deal with this fairy tale, but physics has enough other mathematical fairy tales awaiting investigation.
2. Dark energy and the “expansion of the Universe”
The expansion of the Universe has not been experimentally proven. No one has measured the distances to distant galaxies and shown that they are increasing over time. The red shift in the spectra of distant galaxies can be explained without resorting to the Doppler effect and the Big Bang hypothesis.
And since the very fact of the expansion of the Universe has not been proven, then we cannot talk about accelerating the non-existent expansion of the Universe. Consequently, cosmological models of the “Accelerated Expansion of the Universe” are just unproven hypotheses and the existence of dark energy arising from them is just an assumption of mathematical models, the accuracy of which has not been proven in physics and raises reasonable doubts.In addition, the Big Bang hypothesis is now rejected by physics:
- The Big Bang hypothesis ignores some of the laws of nature and therefore cannot be considered a theory,
- The Big Bang hypothesis introduces forms of energy, matter and elementary particles that do not exist in nature,
- the Big Bang hypothesis does not take into account the real properties of elementary particles,
- Big Bang hypothesis manipulates physical forces
3. Dark energy and fundamental interactions
The presence of the following two types of fundamental interactions in nature has been experimentally established:
- electromagnetic interactions,
- gravitational interactions.
- electromagnetic energy,
- gravitational energy.
Thus, dark energy, as a certain isolated type of energy, contradicts the fundamental interactions existing in nature.
4. Dark energy and the law of conservation of energy
Energy cannot arise from nothing - i.e. from a vacuum, created by nothing and disappeared into nothing. The law of conservation of energy is a fundamental law of nature. All forms of energy known to science obey this law. If dark energy does exist in nature, it must also obey the law of conservation of energy. The introduction of its own law of nature for dark energy goes beyond the boundaries of physics - physics studies only nature and its laws, and the world of fairy tales is not physics.
Consequently, processes of transformation of “dark” energy into other types of energy, as well as reverse transformations, must take place in nature. All that physics has managed to encounter so far are reactions similar to such processes with the participation of neutrinos in the microcosm. Since neutrinos interact extremely weakly with other elementary particles and in more than 99% of cases pass unnoticed through sensors, the illusion of energy disappearance is created (during the emission of neutrinos, for example, during the decay of a neutron) and similarly the illusion of energy appearing out of nothing (during the reaction of neutrino absorption). Physics has learned to recognize these events and has established that the law of conservation of energy works here too. Physics has not established any other “losses” or “gains” of energy.
Thus, if dark energy really exists in nature, it should obey the law of conservation of energy and discontinuous losses and appearances of known forms of energy should be observed in nature. From the absence of the latter in nature, it follows that dark energy as a separate form of energy does not exist in nature. In nature, processes with weakly interacting elementary particles (for example, neutrinos and their excited states) can be observed, creating the illusion of such events. But it will be a known form of energy.
Well, if any model ignores the laws of nature, then this means that before us is a mathematical fairy tale.
5. Dark energy and field theory
According to the field theory of elementary particles, any form of energy in nature must consist of or be created by elementary particles existing in nature. This form of energy can be transferred by elementary particles in a real state in accordance with the laws of nature, including the law of conservation of energy. Well, since all elementary particles consist of an electromagnetic field, this form of energy will be an electromagnetic form of energy (or its derivative - a form resulting from electromagnetic energy or created by electromagnetic energy).
Thus, dark energy either does not exist in nature or can be reduced to an electromagnetic (or gravitational) form of energy - this can be neutrino energy, emitted in gigantic quantities by stars (see the article Red shift and the Mystery of solar neutrinos).6. Dark energy - the result
Dark energy as a separate form of energy:
- contradicts the fundamental interactions existing in nature,
- is not observed during transformations of energy of different forms,
- does not have behind it any fields that actually exist in nature.
Therefore, dark energy as a separate form of energy cannot exist in nature. In nature, there are “invisible” forms of electromagnetic energy - this is the energy carried by neutrinos, emitted in gigantic quantities by stars. But in order to fill the Universe with neutrinos, 13.75 billion years is clearly not enough, and in general, it is better to say goodbye to the fairy tale about the big bang - which contradicts the laws of nature.
Vladimir Gorunovich
Everything we see around us (stars and galaxies) is no more than 4-5% of the total mass in the Universe!
According to modern cosmological theories, our Universe consists of only 5% of ordinary, so-called baryonic matter, which forms all observable objects; 25% dark matter detected due to gravity; and dark energy, making up as much as 70% of the total.
The terms dark energy and dark matter are not entirely successful and represent a literal, but not semantic, translation from English.
In a physical sense, these terms only imply that these substances do not interact with photons, and they could just as easily be called invisible or transparent matter and energy.
Many modern scientists are convinced that research aimed at studying dark energy and matter will likely help answer the global question: what awaits our Universe in the future?
Clumps the size of a galaxy
Dark matter is a substance consisting, most likely, of new particles, still unknown in terrestrial conditions, and possessing properties inherent in ordinary matter itself. For example, it is also capable, like ordinary substances, of gathering into clumps and participating in gravitational interactions. But the size of these so-called clumps can exceed an entire galaxy or even a cluster of galaxies.
Approaches and methods for studying dark matter particles
At the moment, scientists around the world are trying in every possible way to discover or artificially obtain particles of dark matter under terrestrial conditions, using specially designed ultra-technological equipment and many different research methods, but so far all their efforts have not been crowned with success.
One method involves conducting experiments at high-energy accelerators, commonly known as colliders. Scientists, believing that dark matter particles are 100-1000 times heavier than a proton, assume that they will have to be generated in the collision of ordinary particles accelerated to high energies through a collider. The essence of another method is to register dark matter particles found all around us. The main difficulty in registering these particles is that they exhibit very weak interaction with ordinary particles, which are inherently transparent to them. And yet, dark matter particles very rarely collide with atomic nuclei, and there is some hope of registering this phenomenon sooner or later.
There are other approaches and methods for studying dark matter particles, and only time will tell which one will be the first to succeed, but in any case, the discovery of these new particles will be a major scientific achievement.
Substance with anti-gravity
Dark energy is an even more unusual substance than dark matter. It does not have the ability to gather into clumps, as a result of which it is evenly distributed throughout the entire Universe. But its most unusual property at the moment is antigravity.
The nature of dark matter and black holes
Thanks to modern astronomical methods, it is possible to determine the rate of expansion of the Universe at the present time and simulate the process of its change earlier in time. As a result of this, information was obtained that at the moment, as well as in the recent past, our Universe is expanding, and the pace of this process is constantly increasing. That is why the hypothesis about the antigravity of dark energy arose, since ordinary gravitational attraction would have a slowing effect on the process of “galaxy recession”, restraining the expansion rate of the Universe. This phenomenon does not contradict the general theory of relativity, but dark energy must have negative pressure - a property that no currently known substance has.
Candidates for the role of "Dark Energy"
The mass of the galaxies in the Abel 2744 cluster is less than 5 percent of its total mass. This gas is so hot that it only glows in X-rays (red in this image). The distribution of invisible dark matter (which makes up about 75 percent of the cluster's mass) is colored blue.
One of the supposed candidates for the role of dark energy is vacuum, the energy density of which remains unchanged during the expansion of the Universe and thereby confirms the negative pressure of the vacuum. Another putative candidate is the “quintessence” - a previously unknown ultra-weak field that supposedly passes through the entire Universe. There are also other possible candidates, but not one of them has so far contributed to obtaining an exact answer to the question: what is dark energy? But it is already clear that dark energy is something completely supernatural, remaining the main mystery of fundamental physics of the 21st century.
Refers to the “Theory of the Universe”Dark matter and dark energy in the Universe
V. A. Rubakov,
Institute of Nuclear Research RAS, Moscow, Russia1. Introduction
Natural science is now at the beginning of a new, extremely interesting stage in its development. It is remarkable primarily because the science of the microcosm - the physics of elementary particles - and the science of the Universe - cosmology - are becoming a single science about the fundamental properties of the world around us. Using different methods, they answer the same questions: what kind of matter is the Universe filled with today? What has been its evolution in the past? What processes occurred between elementary particles in the early Universe that ultimately led to its present state? If relatively recently the discussion of this kind of questions stopped at the level of hypotheses, today there are numerous experimental and observational data that make it possible to obtain quantitative (!) answers to these questions. This is another feature of the current stage: cosmology has become an exact science over the past 10–15 years. Already today, observational cosmology data are highly accurate; Even more information about the modern and early Universe will be obtained in the coming years.
Recent cosmological data require a radical addition to modern ideas about the structure of matter and the fundamental interactions of elementary particles. Today we know everything or almost everything about the “building blocks” of which ordinary matter is composed - atoms, atomic nuclei, protons and neutrons that make up the nuclei - and how these “building blocks” interact with each other at distances up to 1 /1000 size of the atomic nucleus (Fig. 1). This knowledge was obtained as a result of many years of experimental research, mainly at accelerators, and theoretical understanding of these experiments. Cosmological data indicate the existence of new types of particles that have not yet been discovered under terrestrial conditions and constitute “dark matter” in the Universe. Most likely, we are talking about a whole layer of new phenomena in the physics of the microworld, and it is quite possible that this layer of phenomena will be discovered in earthly laboratories in the near future.
An even more surprising result of observational cosmology was the indication of the existence of a completely new form of matter - “dark energy.”
What are the properties of dark matter and dark energy? What cosmological data indicate their existence? What does it mean from the point of view of the physics of the microworld? What are the prospects for studying dark matter and dark energy in terrestrial conditions? The lecture offered to your attention is devoted to these questions.
2. Expanding Universe
There are a number of facts that speak about the properties of the Universe today and in the relatively recent past.
Universe as a whole homogeneous: All areas in the Universe look the same. Of course, this does not apply to small areas: there are areas where there are many stars - these are galaxies; there are areas where there are many galaxies - these are clusters of galaxies; There are also areas where there are few galaxies - these are giant voids. But regions 300 million light years or larger all look the same. This is clearly evidenced by astronomical observations, which resulted in a “map” of the Universe to distances of about 10 billion light years from us. It must be said that this “map” serves as a source of extremely valuable information about the modern Universe, since it allows us to determine at a quantitative level exactly how matter is distributed in the Universe.
On rice. 2 a fragment of this map is shown, covering a relatively small volume of the Universe. It can be seen that there are quite large structures in the Universe, but in general the galaxies are “scattered” uniformly in it.
Universe expanding: Galaxies are moving away from each other. Space stretches in all directions, and the farther this or that galaxy is from us, the faster it moves away from us. Today the rate of this expansion is small: all distances will double in about 15 billion years, but previously the rate of expansion was much greater. The density of matter in the Universe decreases over time, and in the future the Universe will become more and more rarefied. On the contrary, the Universe used to be much denser than it is now. The expansion of the Universe is directly evidenced by the “reddening” of light emitted by distant galaxies or bright stars: due to the general stretching of space, the wavelength of light increases as it flies towards us. It was this phenomenon that was discovered by E. Hubble in 1927 and served as observational evidence of the expansion of the Universe, predicted three years earlier by Alexander Friedman.
It is remarkable that modern observational data make it possible to measure not only the rate of expansion of the Universe at the present time, but to trace the rate of its expansion in the past. We will talk about the results of these measurements and the far-reaching conclusions that follow from them. Here we will say the following: the very fact of the expansion of the Universe, together with the theory of gravity - the general theory of relativity - indicates that in the past the Universe was extremely dense and expanded extremely quickly. If we trace the evolution of the Universe back into the past using the known laws of physics, we will come to the conclusion that this evolution began with the Big Bang; at this point, the matter in the universe was so dense and the gravitational interaction so strong that the known laws of physics did not apply. 14 billion years have passed since then, this is the age of the modern Universe.
The Universe is “warm”: it contains electromagnetic radiation characterized by a temperature of T = 2.725 degrees Kelvin (relict photons, today representing radio waves). Of course, this temperature today is low (lower than the temperature of liquid helium), but this was far from the case in the past. As the Universe expands, it cools, so that in the early stages of its evolution, the temperature, as well as the density of matter, was much higher than it is today. In the past, the Universe was hot, dense and rapidly expanding.
The photograph shown on rice. 3 , led to several important and unexpected conclusions. Firstly, it made it possible to establish that our three-dimensional space is Euclidean with a good degree of accuracy: the sum of the angles of a triangle in it is equal to 180 degrees, even for triangles with sides whose lengths are comparable to the size of the visible part of the Universe, i.e. comparable to 14 billion light years years. Generally speaking, the general theory of relativity allows that space may not be Euclidean, but curved; observational data indicate that this is not the case (at least for our region of the Universe). The method for measuring the “sum of triangle angles” on cosmological distance scales is as follows. It is possible to reliably calculate the characteristic spatial size of regions where the temperature differs from the average: at the moment of the plasma-gas transition, this size is determined by the age of the Universe, i.e., it is proportional to 300 thousand light years. The observed angular size of these regions depends on the geometry of three-dimensional space, which makes it possible to establish that this geometry is Euclidean.
In the case of Euclidean geometry of three-dimensional space, the general theory of relativity unambiguously connects the expansion rate of the Universe with the total density of all forms of energy and, just as in Newton’s theory of gravitation, the speed of the Earth’s revolution around the Sun is determined by the mass of the Sun. The measured expansion rate corresponds to the total energy density in the modern Universe
In terms of mass density (since energy I is related to mass by the relation E = mс 2 ) this number is
If the energy in the Universe were entirely determined by the rest energy of ordinary matter, then on average there would be 5 protons per cubic meter in the Universe. We will see, however, that there is much less ordinary matter in the Universe.
Secondly, from the photograph rice. 3 it is possible to establish what it was magnitude(amplitude) inhomogeneities temperature and density in the early Universe - it was 10 –4 –10 –5 from the average values. It was from these density inhomogeneities that galaxies and clusters of galaxies arose: regions with higher densities attracted surrounding matter due to gravitational forces, became even denser and ultimately formed galaxies.
Since the initial density inhomogeneities are known, the galaxy formation process can be calculated and the result compared with the observed distribution of galaxies in the Universe. This calculation is consistent with observations only if we assume that in addition to ordinary matter, there is another type of matter in the Universe - dark matter, whose contribution to the total energy density today is about 25%.
Another stage in the evolution of the Universe corresponds to even earlier times, from 1 to 200 seconds (!) from the moment of the Big Bang, when the temperature of the Universe reached billions of degrees. At this time, thermonuclear reactions occurred in the Universe, similar to the reactions occurring in the center of the Sun or in a thermonuclear bomb. As a result of these reactions, some protons bonded with neutrons and formed light nuclei - the nuclei of helium, deuterium and lithium-7. The number of light nuclei formed can be calculated, while the only unknown parameter is the density of the number of protons in the Universe (the latter, of course, decreases due to the expansion of the Universe, but its values at different times are simply related to each other).
A comparison of this calculation with the observed amount of light elements in the Universe is given in rice. 4 : lines represent the results of theoretical calculations depending on a single parameter - the density of ordinary matter (baryons), and rectangles - observational data. It is remarkable that there is agreement for all three light nuclei (helium-4, deuterium and lithium-7); There is also agreement with the data on cosmic microwave background radiation (shown by a vertical stripe in Fig. 4, designated CMB - Cosmic Microwave Background). This agreement indicates that the general theory of relativity and the known laws of nuclear physics correctly describe the Universe at the age of 1–200 seconds, when the matter in it had a temperature of a billion degrees or higher. It is important for us that all these data lead to the conclusion that the mass density of ordinary matter in the modern Universe is
that is, ordinary matter contributes only 5% to the total energy density in the Universe.
4. Energy balance in the modern Universe
So, the share of ordinary matter (protons, atomic nuclei, electrons) in the total energy in the modern Universe is only 5%. In addition to ordinary matter, the Universe also contains relic neutrinos - about 300 neutrinos of all types per cubic centimeter. Their contribution to the total energy (mass) in the Universe is small, since the masses of neutrinos are small, and is certainly no more than 3%. The remaining 90–95% of the total energy in the Universe is “what is unknown.” Moreover, this “unknown what” consists of two factions - dark matter and dark energy and, as depicted in rice. 5 .
At the same time, the matter in stars is still 10 times less; ordinary matter is found mainly in clouds of gas.
5. Dark matter
Dark matter is akin to ordinary matter in the sense that it is capable of gathering into clumps (the size of, say, a galaxy or cluster of galaxies) and participates in gravitational interactions in the same way as ordinary matter. Most likely, it consists of new particles that have not yet been discovered under terrestrial conditions.
In addition to cosmological data, measurements of the gravitational field in galaxy clusters and in galaxies support the existence of dark matter. There are several ways to measure the gravitational field in galaxy clusters, one of which is gravitational lensing, illustrated in rice. 6 .
The gravitational field of the cluster bends the rays of light emitted by the galaxy located behind the cluster, i.e. the gravitational field acts like a lens. In this case, sometimes several images of this distant galaxy appear; on the left half of Fig. 6 they are blue. The bending of light depends on the distribution of mass in the cluster, regardless of which particles create that mass. The mass distribution restored in this way is shown on the right half of Fig. 6 in blue; it is clear that it is very different from the distribution of the luminous substance. The masses of galaxy clusters measured in this way are consistent with the fact that dark matter contributes about 25% to the total energy density in the Universe. Let us recall that this same number is obtained from comparing the theory of the formation of structures (galaxies, clusters) with observations.
Dark matter also exists in galaxies. This again follows from measurements of the gravitational field, now in galaxies and their environs. The stronger the gravitational field, the faster the stars and clouds of gas rotate around the galaxy, so measuring rotation rates depending on the distance to the center of the galaxy allows us to reconstruct the distribution of mass in it. This is illustrated in rice. 7 : as you move away from the center of the galaxy, the speed of revolution does not decrease, which indicates that in the galaxy, including far from its luminous part, there is non-luminous, dark matter. In our Galaxy in the vicinity of the Sun, the mass of dark matter is approximately equal to the mass of ordinary matter.
What are dark matter particles? It is clear that these particles should not decay into other, lighter particles, otherwise they would decay during the existence of the Universe. This fact itself indicates that in nature there is new, not open yet conservation law, which prevents these particles from decaying. The analogy here is with the law of conservation of electric charge: an electron is the lightest particle with an electric charge, and that is why it does not decay into lighter particles (for example, neutrinos and photons). Further, dark matter particles interact extremely weakly with our matter, otherwise they would have already been discovered in earthly experiments. Then the area of hypotheses begins. The most plausible (but far from the only!) hypothesis seems to be that dark matter particles are 100–1000 times heavier than a proton, and that their interaction with ordinary matter is comparable in intensity to the interaction of neutrinos. It is within the framework of this hypothesis that the modern density of dark matter finds a simple explanation: dark matter particles were intensively born and annihilated in the very early Universe at ultra-high temperatures (about 10-15 degrees), and some of them have survived to this day. Given the specified parameters of these particles, their current number in the Universe turns out to be exactly what is needed.
Can we expect the discovery of dark matter particles in the near future under terrestrial conditions? Since today we do not know the nature of these particles, it is impossible to answer this question completely unambiguously. However, the outlook seems very optimistic.
There are several ways to search for dark matter particles. One of them is associated with experiments at future high-energy accelerators and colliders. If dark matter particles are indeed 100–1000 times heavier than a proton, then they will be born in collisions of ordinary particles accelerated at colliders to high energies (the energies achieved at existing colliders are not enough for this). The immediate prospects here are connected with the Large Hadron Collider (LHC), which is being built at the international center CERN near Geneva, which will produce colliding beams of protons with an energy of 7x7 Teraelectronvolts. It must be said that, according to today's popular hypotheses, dark matter particles are only one representative of a new family of elementary particles, so that along with the discovery of dark matter particles, one can hope for the discovery of a whole class of new particles and new interactions at accelerators. Cosmology suggests that the world of elementary particles is far from being exhausted by the “building blocks” known today!
Another way is to detect dark matter particles flying around us. There are by no means a small number of them: with a mass equal to 1000 times the mass of a proton, there should be 1000 of these particles here and now per cubic meter. The problem is that they interact extremely weakly with ordinary particles; the substance is transparent to them. However, dark matter particles occasionally collide with atomic nuclei, and these collisions can hopefully be detected. Search in this direction
Finally, another way is associated with recording the products of annihilation of dark matter particles among themselves. These particles should accumulate in the center of the Earth and in the center of the Sun (the matter is almost transparent to them, and they are able to fall into the Earth or the Sun). There they annihilate each other, and in the process other particles are formed, including neutrinos. These neutrinos pass freely through the thickness of the Earth or the Sun, and can be recorded by special installations - neutrino telescopes. One of these neutrino telescopes is located in the depths of Lake Baikal (NT-200, rice. 8 ), another (AMANDA) - deep in the ice at the South Pole.
As shown in rice. 9 , a neutrino coming, for example, from the center of the Sun, can with a low probability experience interaction in water, resulting in the formation of a charged particle (muon), the light from which is recorded. Since the interaction of neutrinos with matter is very weak, the probability of such an event is low, and a very large volume detector is required. Now the construction of a detector with a volume of 1 cubic kilometer has begun at the South Pole.
There are other approaches to searching for dark matter particles, for example, searching for the products of their annihilation in the central region of our Galaxy. Time will tell which of all these paths will lead to success first, but in any case, the discovery of these new particles and the study of their properties will be the most important scientific achievement. These particles will tell us about the properties of the Universe 10 -9 s (one billionth of a second!) after the Big Bang, when the temperature of the Universe was 10 15 degrees, and dark matter particles intensively interacted with cosmic plasma.
6. Dark energy
Dark energy is a much stranger substance than dark matter. To begin with, it does not gather in clumps, but is evenly “spread” throughout the Universe. There is as much of it in galaxies and galaxy clusters as outside them. The most unusual thing is that I, in a certain sense, do not experience dark energy antigravity. We have already said that modern astronomical methods can not only measure the current rate of expansion of the Universe, but also determine how it has changed over time. So, astronomical observations indicate that today (and in the recent past) the Universe is expanding at an accelerating rate: the rate of expansion is increasing with time. In this sense, we can talk about antigravity: ordinary gravitational attraction would slow down the retreat of galaxies, but in our Universe, it turns out that the opposite is true.
This picture, generally speaking, does not contradict the general theory of relativity, but for this, dark energy must have a special property - negative pressure. This sharply distinguishes it from ordinary forms of matter. It will not be an exaggeration to say that the nature of dark energy is the main mystery of fundamental physics of the 21st century.
One of the candidates for the role of dark energy is vacuum. The energy density of the vacuum does not change as the Universe expands, and this means negative vacuum pressure. Another candidate is a new super-weak field that permeates the entire Universe; the term “quintessence” is used for it. There are other candidates, but in any case, the dark energy self is something completely unusual.
Another way to explain the accelerated expansion of the Universe is to assume that the laws of gravity themselves change over cosmological distances and cosmological times. This hypothesis is far from harmless: attempts to generalize the general theory of relativity in this direction encounter serious difficulties.
Apparently, if such a generalization is possible at all, it will be associated with the idea of the existence of additional dimensions of space, in addition to the three dimensions that we perceive in everyday experience.
Unfortunately, there are currently no visible ways to directly experimentally study dark energy under terrestrial conditions. This, of course, does not mean that new brilliant ideas in this direction cannot appear in the future, but today hopes for clarifying the nature of dark energy and (or, more broadly, the reasons for the accelerated expansion of the Universe) are associated exclusively with astronomical observations and with obtaining new, more accurate cosmological data. We have to learn in detail exactly how the Universe expanded at a relatively late stage of its evolution, and this, hopefully, will allow us to make a choice between different hypotheses.
We are talking about observations of type 1a supernovae.
The change in energy and with a change in volume is determined by pressure, Δ E = -pΔ V. As the Universe expands, the energy of the vacuum increases along with the volume (the energy density is constant), which is possible only if the vacuum pressure is negative. Note that the opposite signs of pressure and energy and vacuum directly follow from Lorentz invariance.
7. Conclusion
As is often the case in science, spectacular advances in particle physics and cosmology have raised unexpected and fundamental questions. Today we do not know what constitutes the bulk of matter in the Universe. We can only guess what phenomena occur at ultra-short distances, and what processes took place in the Universe at the earliest stages of its evolution. It's great that many of these questions will be answered in the foreseeable future - within 10-15 years, and maybe even earlier. Our time is a time of a radical change in the view of nature, and the main discoveries are yet to come.
DISCUSSION
04/18/2005 09:32 | rykov I really liked Valery Anatolyevich Rubakov’s lecture. This is the first time I have heard a lecture based not on theory, but on observed data. It is known that there may be several theories that explain phenomena and even contradict each other. In addition, the data presented fit into the hypotheses about the nature of gravity and antigravity in the form of the charge and magnetic-mass structure of the “vacuum”. The excess charge of the “vacuum” is a source of Coulomb attraction between bodies of matter and at the same time a source of repulsive forces of the electric charge of the same name. This repulsion is observed in the form of the expansion of the Universe - at the beginning it was fast due to the high charge density, now it is slow due to the presence of approximately 2000 Coulombs/m^3. In hypothesis e, “dark” matter exists in the form of a magnetic-mass continuum as a source of masses of real particles and magnetic induction fluxes.
18.04.2005 15:12 | grechishkin 04/18/2005 16:40 | Markab The lecture surprised me. There is just a big problem with observational material. They took dark matter out of thin air to explain the lack of observed mass in galaxies, and then introduced dark energy to explain the observed expansion of the universe. The properties of dark matter were explained very logically: it does not enter into strong interactions (that is, it cannot combine into heavier elements), it is electrically neutral, it interacts very weakly with ordinary matter (like neutrinos it is therefore difficult to detect) and has a very large rest mass. The speaker probably needed a large rest mass to explain why this particle had not been discovered until now. There are simply no such accelerators yet. And if there were, they would certainly have found it. You need hidden mass - get it. The situation is like with ether in the old days.
Observational material indeed indicates that the galactic halo contains matter that is not detected by telescopes. The question "What could it be?" remains open for now, but why explain the problem of hidden mass through a family of new particles??
Regarding dark energy and. The expansion of the universe is an observed fact that has not yet been explained, but is not new either. To explain the expansion of the universe, the author requires dark energy. Mathematically, Einstein introduced the repulsion of matter in the form of the lambda term, but now physically we explain the lambda term by dark matter. One incomprehensible leads to another. In Newton’s philosophy, God was required to explain the stability of the planets’ orbits, since otherwise, due to gravity, the planets would have to fall into the Sun. Here dark energy is called God.
The balance of energy in the modern universe seems no less interesting. So, less than 10% is allocated to all matter, 25% of the energy is allocated to the particles invented by the speaker, and, well, everything else is dark energy. As they calculated: the universe is Euclidean -> the expansion rate is known -> we apply general relativity = we obtain the total energy of the Universe.
From what we received, we took energy away...04/18/2005 16:43 | Markab CONTINUATION
From what was obtained, the energy of the observed substance was taken away, and the remaining energy was divided between the repulsive force (dark energy) and the missing mass (dark matter).
Let's start with the Euclidean nature of the universe. The Euclidean nature of the Universe must be proven in several independent ways. The proposed method is unconvincing in that the moment of the plasma-gas transition of the Universe can be estimated at best with a factor of 2 in one direction or another. Therefore, will there be a Euclidean Universe if the cell size is taken to be 150 or 600 thousand light years? Most likely no. This means that general relativity cannot be used to estimate the total energy in the Universe.04/19/2005 19:58 | rykov In any outcome of Mark's counter-arguments, we observe an amazing coincidence between “dark” matter and the magnetic-mass continuum, between “dark” energy and the charge structure of the “physical vacuum”. Therefore, I consider a new word in cosmology as almost direct confirmation of the propagation of light and gravity in space. This is a very good coincidence.
19.04.2005 23:10 | Alex1998 It’s okay to feed people’s ears about “amazing coincidences.” Have you already forgotten how they poked your nose at you at ru.science? Not only will there be no coincidence with “dark” matter, but also with the school physics course.
Although your shot is, of course, rare in its unceremoniousness... And you’ve already managed to scold Maldacena and pat Ginsburg on the shoulder...
06/10/2005 15:15 | rykov Is this Lukyanov?
Read this: "Speed of Gravity"
http://www.inauka.ru/blogs/artic le54362/print.html
For your self-education. In general, the situation in physics is very strange. On this occasion:
1. Propagation of light (EMW) is impossible in a vacuum devoid of electrical charges. Physics says the opposite, contradicting the materiality of the Universe. This is perhaps the main flaw in physical theory.
2. The postulate of the constancy of the speed of light for the Universe leads to the following distortion of the materiality of our world: the need to introduce time dilation to explain the observed phenomena. Without this introduction of changes in the course of time, any interpretation of experimental data is impossible.
3. The curvature of space as a model of gravity and inertia also leads to the denial of the material basis of gravity. In this case, the universal value of the number pi in physics is violated, which is realized only in uncurved space.These are probably the main misconceptions in physics. Everything else can be perceived as the cost of growing understanding in the structure of the world. The whole complexity of the situation of idealism in physics is due to the fact that the results of observations and experiments “confirm” physical theories. The problem lies in the way of interpreting observations and experiments, which must be different in the case of the fallacy and truth of the theory. The essays make an attempt at correct interpretation in physics, contrasting interpretations from non-materialistic positions. Therefore, the second (sufficient) condition of any physical theory must be its materialistic validity. For example, all references to the possibility of transmitting physical interactions or transmitting so-called physical fields in emptiness are devoid of a material basis. The corresponding sections of theoretical physics must be corrected taking into account the materiality of the world.
04/20/2005 12:07 | Markab In addition to what has already been said, in the author’s discussions about dark matter, the report contains one more “dark place”.
1) From the observation results, see Fig. 7 of the report, it follows that the measured rotation speed of stars with distance from the galactic core turns out to be higher than the calculated one. In Fig. 7 they are designated “observations” and “without dark matter” (Unfortunately, the maximum of the “observation” curve is not shown; its ~logarithmic growth is visible). The author explains the observed “increased” speed by the presence of dark matter in our galaxy. In Fig. Figure 6 (right) provides an example of reconstructing the gravitational field from the observation of microlensing in Fig. 6(left). The resulting gravitational field is the total field, to which both the observed matter and dark matter contribute. From Fig. 6 (right) it follows that dark matter is distributed throughout the galaxy in the same way as ordinary matter - it is concentrated together with visible matter: in the galactic core, star clusters, stars and dark clouds.
2) From Fig. 5 it follows that dark matter is approximately 5 times larger than ordinary matter. That is, it is she who makes a decisive contribution to gravitational interaction. This matter must be in the Sun, and in the Earth, and in Jupiter, etc.
3) In the Solar System, the speed of planets with distance from the Sun does not increase, but decreases. Moreover, there is no local maximum in the velocities of planets with distance from the Sun. Why is it different in the Galaxy? Contradiction??
WHAT COULD THIS MEAN?
A) Dark matter in the author’s interpretation DOES NOT EXIST. In order to explain the “increased” speed of rotation of stars in the galaxy, one must look for ordinary matter, which can be hidden in molecular clouds, black holes, cooled neutron stars and white dwarfs.
B) Dark matter in the author's interpretation EXISTS. We don’t notice it because we are used to it. By the way, a good way to lose weight, better than any Herbalife: squeeze out the dark matter and become 5 times lighter!04/21/2005 13:42 | Markab Let us summarize the discussion about dark matter. Interpreting dark matter in the manner suggested by the speaker inevitably leads to a revision of the entire stellar evolution.
So, according to the author’s statements, dark matter is: a particle with a mass of 100-1000 rest masses of a proton, which has no electric charge, participates in gravitational interaction, and does not participate in strong interaction. It reacts weakly with ordinary matter, much like a neutrino. It obeys some kind of conservation law that prevents the decay of such a particle.
The mass of dark matter is approximately 5 times the mass of ordinary matter. (According to the report). Dark matter is concentrated in the same centers as ordinary matter - the nuclei of galaxies, star clusters, stars, nebulae, etc. (According to the report).
ASTROPHYSICAL CONSEQUENCES (introduction of dark matter)
1) On stars, the conditions of radiative equilibrium with gravity are met. Radiation is released as a result of nuclear reactions of the star's matter. Dark matter located in a star gravitationally compresses it, but does not take part in nuclear reactions. Therefore, the hypothetical introduction of dark matter into a star, provided its mass is conserved, leads to the fact that the amount of matter capable of participating in nuclear reactions decreases several times. This means that the lifespan of a star is reduced several times(!). Which is not true, at least in the example of our Sun, which happily exists for ~5 billion years and will exist for the same amount of time.
2) In the process of evolution, the proportion of dark matter on the star increases, since particles with a mass (100-1000 Mr) will not leave the star either by the stellar wind or by the ejection of the envelope. Moreover, due to its mass, dark matter will be concentrated in the core of the star. This means that at the end of stellar evolution, when the star turns into a white dwarf or neutron star, the vast majority of its mass must consist of dark matter! (Moreover, it is not known what statistics it (TM) obeys and what properties it has.) And this, in turn, should change the limit...04/21/2005 13:44 | Markab And this in turn should change the Chandrasekhar limit to white dwarfs and the Openheimer-Volkoff limit to neutron stars. However, no shift in mass of the Chandrasekhar white dwarf-neutron star limit is observed experimentally.
Both of these arguments once again convince us that there is simply no dark matter in Mr. Rubakov’s interpretation.04/21/2005 22:18 | Algen 04/27/2005 10:10 | Markab The process of matter condensation does not depend on the absolute speed of matter (the speed of rotation around the galactic nucleus), but on the relative one, i.e. the speed at which dark matter particles move relative to ordinary matter. As for the absolute value of the speed of 100-200 km/s, this value is not large. For example, the speed of matter moving around the nucleus in the vicinity of the Sun is about 250 km/s, which does not in any way interfere with the process of star formation.
04/20/2005 00:33 | voice Dear Mr. Rubakov! I read your lecture with interest, for which I am very grateful. I won’t go into details, because I’m an amateur.
Mr. Rubakov. I'm wondering about a question that I can't get a clear answer to. The point is this. Let's say there is a certain mass around which other masses revolve at a distance of millions of light years. Let’s assume a hypothetical case: a mass around which other masses revolve was swallowed up by a black hole over the course of a thousand years. Let's say roughly that the reason for the attraction of rotating bodies has disappeared/it is clear that this is not the case at all. This is not the point./ But bodies moving with acceleration will continue to move with the same accelerations for thousands of years. Until the disturbance of the gravitational field comes to them. It turns out that these thousands of years the masses interacted with the field? And it was the field that accelerated them? But if this is so, then according to the theory of short-range interaction it inevitably follows that accelerating bodies first interact with the gravitational field and are “repelled” from it. Therefore, the field has momentum and therefore mass. Which is automatically equal to the mass of the body accelerated by the field. But if so, then this means that in the Universe, in addition to the mass of the observed matter, there is the same exact hidden mass of the gravitational field. Moreover, the forces applied to this field are not applied to a point, but spread out to infinity. It is intuitively felt that this mass can be the reason for the expansion of the space of the Universe, since it clearly repels each other.
I won't fantasize. I would just like to know your opinion about these arguments, even if they are impartial. I am an amateur, for this reason devastating criticism of my reputation will not harm my reputation. In the absence of it.
Sincerely.
voice04/20/2005 09:03 | rykov Dear Voice! I am also an amateur and do not accept my answer to you as a replacement for the respected Valery Anatolyevich. It seems to me that if he responds, it will be to all the remarks at once. You can find my answer on the pages:
PROPAGATION OF LIGHT AND GRAVITY IN SPACE
http://www.inauka.ru/blogs/artic le41392.html
And
The key to understanding the Universe NEW! 12/27/2004
http://www.worldspace.narod.ru/r u/index.html04/21/2005 09:03 | rykov 04/21/2005 11:52 | voice 04/21/2005 22:16 | Algen Let's start with the fact that if the central mass is swallowed by a black hole, then nothing will happen to the gravitational field at a distance. It is what it was and will remain so.
However, your reasoning is correct. Really distant objects interact with the gravitational field and until signals about changes in the center of events reach them, they will move as before. Otherwise, a violation of causality would occur.
You make the correct conclusion that the gravitational field has energy and momentum. This is truly a physical field. However, the conclusion that this energy (mass) is “automatically” equal to something is unfounded and incorrect.
In general, the question of energy and the gravitational field is quite confusing. Experts have different opinions on it. That is, no one argues about the very fact of the presence of energy, but it is not entirely clear how to indicate where exactly this energy is localized. Penrose wrote about this quite well in her book “The King’s New Mind.” I recommend reading.I'm in the Universe7.files/f_line.gif">
Dear Algen! Let's continue with the fact that a black hole that has absorbed the central mass will change the characteristics of the newly emerged central mass. So the gravitational field, in my opinion, will undergo some changes over time. On the interaction of distant objects with the gravitational field. I did not mean that its mass is automatically equal to all stellar matter. I believed that the mass of stellar matter is automatically included in the mass of the gravitational field. Agree, this has a slightly different meaning. On the localization of energy and gravitational fields. In my opinion, talking about this is more than strange. The energy deposited by stellar matter into the gravitational field spreads out into infinity. Since it, nevertheless, “comes” from discrete bodies, it most likely experiences mutual repulsion, being one of the reasons for the expansion of the Universe. Of course, these are just hypotheses. But if we assume that this is so, then the interaction of these masses/energies can be described by Lobachevsky geometry. I wonder how the law of mutual universal repulsion, similar to our law of universal gravitation, can be written down in it?
Of course, I treat this statement as a hypothesis e.
Thank you for the information about Penrose's book. I'll look. If you have information on where and how to find it, I would be very grateful.06.05.2005 22:16 | Alex1998 15.05.2005 10:50 | Mikhail No dark matter, much less dark energy, exists in Nature - rather, it is darkness in the brains, which are trying with enviable persistence to “fasten” the universe to the existing absurd relativistic theories. Of course, Nature is full of many other types of radiation still unknown to science, including the main one - graviton. Grviton matter fills the entire Universe and makes up a significant fraction of its mass, but this matter itself does not have gravity (but creates it!). There is no antigravity in the Universe - Nature does not need it. The concept of antigravity is a fruit of thoughtlessness.
23.05.2005 06:30 | kpuser I draw the attention of the author and readers that the nature of dark matter, presented in the article as “the main mystery of fundamental physics of the 21st century,” is easily revealed within the framework of the neoclassical concept of physics, based on the description of the free movement of uncharged bodies by the generalized Lorentz equation. This equation presents two classical forces: the Newtonian inertial force of the body and the generalized Lorentz force, which takes into account the elastic interaction of the body with its own physical or force field.
Solving the equation indicates the magnetic nature of gravity and leads to two forms of the law of universal gravitation. One of them - the traditional Newtonian one - is applicable for local cosmic structures such as the Solar System, in which gravity is due to the mutual attraction of real or REAL masses of matter. Another shows that in large-scale cosmic structures such as galaxies and their clusters, antigravitational phenomena appear, caused by the mutual repulsion of IMAGINARY masses, in which the mass of force fields or DARK MATTER prevails. You can find out more about this on our website at: http://www.livejournal.com/commu I am in the Universe7.files/elementy">To Maxim Chicago
Could you, so to speak, “comply”: justify your “verdict” with appropriate arguments? What specifically in my work seems “anti-physics” to you? Or is this how you evaluate the generalized Lorentz equation, on which it was possible to construct an almost complete edifice of modern physics? Please explain.
K. Agafonov08.06.2005 16:40 | Che Copyright of Fornit website You've all probably heard this phrase: dark energy. But what is it, and why is it difficult to study? I'll start my story with history.
Let's say you have a candle. You know everything about it, including its brightness and distance to it. Like this:
If I move the candle twice the distance, its brightness should decrease by 4 times. If I move it three times the distance, its brightness should decrease by a factor of 9. If I move it a thousand times the distance, its brightness should decrease a million times relative to its original value.
But only in space, of course, there are no candles. But there is a special class of events, which, as far as we know, has an inherent brightness (with an accuracy of a few percent) throughout the Universe. This event is a Type Ia supernova. When our Sun, and most known stars in general, burn up all their fuel, they eventually turn into white dwarfs. Our Sun in this case will consist mainly of carbon and oxygen, but white dwarfs sometimes contain helium, neon and silicon. Here is one of them:
There is only one star in our solar system. Many systems have two or more stars. If one of them is a white dwarf, it can start stealing a lot of the others. In this case, it begins to grow. There is a critical limit to the mass that a white dwarf can hold before the atoms themselves begin to collapse. And when they collapse, it ends in an explosion so powerful that it is known as a Type Ia supernova. The following animation shows a simulated explosion. Notice how the remaining stars are ejected from the star system due to the powerful explosion:
By seeing these supernovae in different galaxies, we can measure their brightness, and knowing their inherent brightness, we can calculate their distance. We can also measure their redshift. This information is enough to understand how the Universe is expanding. You can imagine three possibilities for what the Universe might do after the Big Bang. In the beginning you have a huge amount of matter and energy expanding and flying away from each other, but gravity tries to bring it together. Here's what can happen:
There is so much matter and energy in the Universe, and as a result, gravitational attraction, that gravity wins and can reverse the explosion, causing the Universe to collapse into itself (closed Universe)
There is not enough matter and energy in the Universe to overcome the expansion, and the Universe continues to expand forever (open Universe)
There is just enough matter and energy in the Universe to resist expansion without causing it to collapse - only for the rate of expansion to drop to zero (a flat Universe).
Now, by looking at supernovae, we can see what they tell us about what is happening. And guess what? The Universe does not do any of the three things listed! For some time it seemed to correspond to the model of a flat Universe, but at some point the expansion rate stopped falling, and now it not only will not fall to zero, but will also become constant at 85% of its current value. Why? No one knows. But there must be some physics to it, and we gave it the name “dark energy” because if the Universe were filled with a new type of energy pushing it apart, this would lead to an acceleration of expansion. But this is a strange process, and it definitely continues, and we don’t yet know how to explain it correctly. That's what dark energy is!
