Scientists have taken an important step towards solving one of the main mysteries in the Universe - dark matter, which is believed to fill most of outer space. Specialists working on the project Dark Energy Survey , using a powerful telescope in the Andes were able to create a map, demonstrating the distribution of dark matter. on her large coils of dark matter are visible, strewn with galaxies and separated by free space.
Until now, scientists could only study dark matter by measuring the distortion of light from distant galaxies. As a result, experts want to measure dark energy- an even more mysterious force that is expanding the Universe at an ever-increasing speed.
Dark matterin astronomy and cosmology, as well as in theoretical physics, a hypothetical form of matter that does not emit or interact with electromagnetic radiation. This property of this form of matter makes its direct observation impossible.
The conclusion about the existence of dark matter was made on the basis of numerous, consistent with each other, but indirect signs of the behavior of astrophysical objects and the gravitational effects they create. Discovering the nature of dark matter will help solve the problem of hidden mass, which, in particular, lies in the anomalously high speed of rotation of the outer regions of galaxies.
The term became widespread after the work of Fritz Zwicky. Zwicky measured the radial velocities of eight galaxies in the Coma cluster (the constellation Coma Berenices) and found that for the cluster to be stable, one must assume that its total mass is tens of times greater than the mass of its constituent stars. Soon other astronomers came to the same conclusions for many other galaxies. Since the 1960s, when the rapid progress in observational astronomy began, the number of arguments in favor of the existence of dark matter has grown rapidly. At the same time, estimates of its parameters obtained from different sources and different methods are generally consistent with each other.
The presence of unknown matter in the Universe and its influence turned out to be a typical situation in the world of galaxies.
The motion in systems of double galaxies and in galaxy clusters was studied. It turned out that on these scales the proportion of dark matter is much higher than inside galaxies.
The stellar mass of elliptical galaxies, according to calculations, is insufficient to contain the hot gas entering the galaxy, if dark matter is not taken into account.
Estimating the mass of galaxy clusters that perform gravitational lensing gives results that include the contribution of dark matter and are close to those obtained by other methods.
A major contribution was made in the late 1960s and early 1970s by astronomer Vera Rubin of the Carnegie Institution, who was the first to make accurate and reliable calculations indicating the presence of dark matter. Along with a co-author (Kent Ford), Rubin announced at a conference of the American Astronomical Society in 1975 the discovery that most stars in spiral galaxies orbit at approximately the same angular velocity, leading to the idea that the mass density in galaxies is the same in those regions , where the majority of stars (bulge), and for those regions at the edge of the disk where there are few stars.
A study published in 2012 of the motions of more than 400 stars located at distances of up to 13,000 light-years from the Sun found no evidence of dark matter in the large volume of space around the Sun. According to theoretical predictions, the average amount of dark matter in the vicinity of the Sun should have been approximately 0.5 kg in the volume of the Earth. However, measurements gave a value of 0.00±0.06 kg of dark matter in this volume. This means that attempts to detect dark matter on Earth, for example through rare interactions of dark matter particles with “ordinary” matter, are unlikely to be successful.
According to observational data from the Planck space observatory published in March 2013, the total mass-energy of the observable Universe consists of 4.9% ordinary (baryonic) matter, 26.8% dark matter and 68.3% dark energy . Thus, the Universe consists of 95.1% dark matter and dark energy.
The most natural assumption seems to be that dark matter consists of ordinary, baryonic matter. , for some reason weakly interacting electromagnetically and therefore not detectable when studying, for example, emission and absorption lines.
However, theoretical models provide a large selection of possible candidates for the role of non-baryonic invisible matter - these are: light neutrinos, heavy neutrinos, axions, cosmions and supersymmetric particles such as photino, gravitino, higgsino, sneutrino, wine and zino.
There are alternative theories of dark matter and dark energy:
Matter from other dimensions (parallel Universes)
Some theories about extra dimensions take gravity as a unique type of interaction that can act on our space from extra dimensions. This assumption helps explain the relative weakness of the gravitational interaction compared to the other three main forces (electromagnetic, strong and weak). The effect of dark matter can be logically explained by the interaction of visible matter from our ordinary dimensions with massive matter from other (extra, invisible) dimensions through gravity. At the same time, other types of interactions, these dimensions and this matter in them cannot feel in any way, cannot interact with it. Matter in other dimensions (in fact, in a parallel Universe) can be formed into structures (galaxies, clusters of galaxies) in a way similar to our measurements or form their own, exotic structures, which in our measurements are felt like a gravitational halo around visible galaxies.
Topological defects of space
Dark matter may simply be primordial (Big Bang) defects in space and/or quantum field topology that may contain energy, thereby causing gravitational forces.
The question of the origin of the Universe, its past and future has worried people since time immemorial. Over the centuries, theories have arisen and been refuted, offering a picture of the world based on known data. Einstein's theory of relativity was a major shock to the scientific world. She also made a huge contribution to the understanding of the processes shaping the Universe. However, the theory of relativity could not claim to be the ultimate truth, not requiring any additions. Improved technologies have allowed astronomers to make previously unimaginable discoveries that required a new theoretical framework or a significant expansion of existing provisions. One such phenomenon is dark matter. But first things first.
Things from days gone by
To understand the term “dark matter,” let’s go back to the beginning of the last century. At that time, the dominant idea was that the Universe was a stationary structure. Meanwhile, the general theory of relativity (GTR) assumed that sooner or later it would lead to the “sticking together” of all objects in space into a single ball, the so-called gravitational collapse would occur. There are no repulsive forces between space objects. Mutual attraction is compensated by centrifugal forces, creating the constant movement of stars, planets and other bodies. In this way, the balance of the system is maintained.
In order to prevent the theoretical collapse of the Universe, Einstein introduced a cosmological constant - a value that brings the system to the necessary stationary state, but at the same time it is actually fictitious and has no obvious basis.
Expanding Universe
The calculations and discoveries of Friedman and Hubble showed that there was no need to violate the harmonious equations of general relativity using a new constant. It has been proven, and today almost no one doubts this fact, that the Universe is expanding, it once had a beginning, and there can be no talk of stationarity. Further development of cosmology led to the emergence of the big bang theory. The main confirmation of the new assumptions is the observed increase in the distance between galaxies over time. It was the measurement of the speed at which neighboring cosmic systems are moving away from each other that led to the formation of the hypothesis that dark matter and dark energy exist.
Data inconsistent with theory
Fritz Zwicky in 1931, and then Jan Oort in 1932 and in the 1960s, were engaged in calculating the mass of matter of galaxies in a distant cluster and its relationship with the speed of their removal from each other. Time after time, scientists came to the same conclusions: this amount of matter is not enough for the gravity it creates to hold together galaxies moving at such high speeds. Zwicky and Oort suggested that there is hidden mass, the dark matter of the Universe, which prevents cosmic objects from scattering in different directions.
However, the hypothesis received recognition from the scientific world only in the seventies, after the results of Vera Rubin’s work were announced.
She constructed rotation curves that clearly demonstrate the dependence of the speed of motion of the galactic matter on the distance that separates it from the center of the system. Contrary to theoretical assumptions, it turned out that the velocities of stars do not decrease as they move away from the galactic center, but increase. This behavior of the stars could only be explained by the presence of a halo in the galaxy, which is filled with dark matter. Astronomy was thus faced with a completely unexplored part of the universe.
Properties and composition
This is called dark because it cannot be seen by any existing means. Its presence is recognized by an indirect sign: dark matter creates a gravitational field, while not emitting electromagnetic waves at all.
The most important task facing scientists was to obtain an answer to the question of what this matter consists of. Astrophysicists tried to “fill” it with the usual baryonic matter (baryonic matter consists of more or less studied protons, neutrons and electrons). The dark halo of galaxies included compact weakly emitting stars of the type and huge planets close in mass to Jupiter. However, such assumptions did not stand up to scrutiny. Baryonic matter, familiar and familiar, thus cannot play a significant role in the hidden mass of galaxies.
Today, physics is engaged in the search for unknown components. Practical research by scientists is based on the theory of supersymmetry of the microworld, according to which for every known particle there is a supersymmetric pair. These are what make up dark matter. However, it has not yet been possible to obtain evidence of the existence of such particles; perhaps this is a matter of the near future.
Dark energy
The discovery of a new type of matter did not end the surprises that the Universe had prepared for scientists. In 1998, astrophysicists had another chance to compare theoretical data with facts. This year was marked by an explosion in a galaxy far from us.
Astronomers measured the distance to it and were extremely surprised by the data they received: the star flared up much further than it should have been according to the existing theory. It turned out that it is increasing over time: now it is much higher than it was 14 billion years ago, when the Big Bang supposedly happened.
As you know, in order to accelerate the movement of a body, it needs to transfer energy. The force that forces the Universe to expand faster has come to be called dark energy. This is no less mysterious part of space than dark matter. It is only known that it is characterized by a uniform distribution throughout the Universe, and its impact can be registered only at enormous cosmic distances.
And again the cosmological constant
Dark energy has shaken the big bang theory. Part of the scientific world is skeptical about the possibility of such a substance and the acceleration of expansion caused by it. Some astrophysicists are trying to revive Einstein’s forgotten cosmological constant, which can again go from being a major scientific mistake to a working hypothesis. Its presence in the equations creates antigravity, leading to acceleration of expansion. However, some of the presence implications are inconsistent with observational data.
Today, dark matter and dark energy, which make up most of the matter in the Universe, are mysteries for scientists. There is no clear answer to the question about their nature. Moreover, perhaps this is not the last secret that space keeps from us. Dark matter and energy may be the threshold of new discoveries that could revolutionize our understanding of the structure of the Universe.
A theoretical construct in physics called the Standard Model describes the interactions of all elementary particles known to science. But this is only 5% of the matter existing in the Universe, the remaining 95% is of a completely unknown nature. What is this hypothetical dark matter and how are scientists trying to detect it? Hayk Hakobyan, a MIPT student and employee of the Department of Physics and Astrophysics, talks about this as part of a special project.
The Standard Model of elementary particles, finally confirmed after the discovery of the Higgs boson, describes the fundamental interactions (electroweak and strong) of the ordinary particles we know: leptons, quarks and force carriers (bosons and gluons). However, it turns out that this whole huge complex theory describes only about 5-6% of all matter, while the rest does not fit into this model. Observations of the earliest moments of our Universe show us that approximately 95% of the matter that surrounds us is of a completely unknown nature. In other words, we indirectly see the presence of this hidden matter due to its gravitational influence, but we have not yet been able to capture it directly. This hidden mass phenomenon is codenamed “dark matter.”
Modern science, especially cosmology, works according to the deductive method of Sherlock Holmes
Now the main candidate from the WISP group is the axion, which arises in the theory of the strong interaction and has a very small mass. Such a particle is capable of turning into a photon-photon pair in high magnetic fields, which gives hints on how one might try to detect it. The ADMX experiment uses large chambers that create a magnetic field of 80,000 gauss (that's 100,000 times the Earth's magnetic field). In theory, such a field should stimulate the decay of an axion into a photon-photon pair, which detectors should catch. Despite numerous attempts, it has not yet been possible to detect WIMPs, axions or sterile neutrinos.
Thus, we have traveled through a huge number of different hypotheses seeking to explain the strange presence of the hidden mass, and, having rejected all the impossibilities with the help of observations, we have arrived at several possible hypotheses with which we can already work.
A negative result in science is also a result, since it gives restrictions on various parameters of particles, for example, it eliminates the range of possible masses. From year to year, more and more new observations and experiments in accelerators provide new, more stringent restrictions on the mass and other parameters of dark matter particles. Thus, by throwing out all the impossible options and narrowing the circle of searches, day by day we are becoming closer to understanding what 95% of the matter in our Universe consists of.
Michael Rampino, a professor of biology at New York University, said that the movement of the Earth through the galactic disk (our region in the Milky Way galaxy) could cause mass extinctions on Earth. This happened because our movement disrupted the orbits of comets in the outer solar system (known as the Oort cloud) and caused an increase in the heat of our planet's core.
Together with its planets, the Sun orbits the center of the Milky Way every 250 million years. During its journey, it weaves across the galactic disk every 30 million years. Rampino argues that Earth's passage through the disk coincides with comet impacts and mass extinctions on Earth, including the one that happened 65 million years ago when the dinosaurs went extinct. There is also a theory that just before the asteroid put an end to the giant lizards, their ranks were significantly thinned by volcanic eruptions.
The combination of unusual volcanic activity and an asteroid impact coincides with Earth's passage through the galactic disk: "As it passes through the disk, concentrations of dark matter disrupt the paths of comets that typically fly far from Earth in the outer solar system," says Rampino. "This means that comets that normally travel great distances from Earth are taking unusual paths to the point of colliding with the planet." Some believe Rampino's theory doesn't work because the dinosaurs became extinct because of an asteroid, not a comet. However, it is believed that 4% of the Oort cloud consists of asteroids, which is about eight billion.
In addition to this, Rampino believes that each Earth's passage through the galactic disk resulted in dark matter accumulating in the planet's core. As dark matter particles annihilate each other, they create intense heat, which can cause volcanic eruptions, sea level changes, mountain growth and other geological activity that seriously affects life on Earth.
The Milky Way could be a giant wormhole
We may be living in a giant tunnel that is a shortcut through the Universe. As predicted by Einstein's general theory of relativity, a wormhole is a region in which space and time are bent, creating a wormhole into a distant part of the universe. According to astrophysicists at the International School of Advanced Studies in Trieste, Italy, the dark matter in our galaxy may be distributed in such a way that it provides a stable wormhole. These scientists believe it's time to rethink the nature of dark matter; it may simply represent .
“If we combine the map of dark matter in the Milky Way with the latest Big Bang model,” says Professor Paulo Salucci, “and assume the existence of space-time tunnels, we find that our galaxy may well have one of these tunnels, and such a tunnel could be the size of an entire galaxy. Moreover, we can even go through this tunnel, since, according to our calculations, it will be navigable. Like the one we saw in the movie Interstellar.
Of course, this is just a theory. But scientists believe dark matter may be the key to creating and observing a wormhole. So far, no wormholes have been discovered in nature.
Discovery of Galaxy X
Galaxy X is also known as a dark matter galaxy, a largely invisible dwarf galaxy that may be responsible for strange ripples in cold hydrogen gas outside the Milky Way's disk. Galaxy X is thought to be a satellite galaxy of the Milky Way in a cluster of four Cepheid variables, pulsating stars that are used as markers to measure distances in space. We don't see the rest of this dwarf galaxy because it is made of dark matter, according to theory. However, the gravitational pull of this galaxy creates the ripples we see. Without the gravitational source of dark matter holding them together, the four Cepheids would likely fly apart.
“The discovery of Cepheid variables shows that our method for finding the locations of dark matter-dominated dwarf galaxies works,” says astronomer Sukanya Chakrabarty. “This could help us ultimately understand what dark matter is made of.” It also shows that Newton's theory of gravity can be used in the farthest reaches of the galaxy and there is no need to change our theory of gravity."
Decay of the Higgs boson into dark matter
Developed in the 1970s, particle physics is a set of theories that essentially predict all known subatomic particles in the universe and how they interact. With the confirmation of the Higgs boson (also known as the “God particle”) in 2012, the Standard Model was complete. Unfortunately, this model does not explain everything and says nothing about gravity and dark matter. The mass of the Higgs particle is also known to some scientists.
This prompted scientists from Chalmers University of Technology to propose a new model based on supersymmetry, which equips every known Standard Model particle with a heavier superpartner. According to the new theory, a small fraction of Higgs particles decay into a photon (a particle of light) and two gravitinos (hypothetical dark matter particles). , it will completely revolutionize our understanding of the fundamental building blocks of nature.
Dark matter on the Sun
Depending on the method used to analyze the Sun, the amount of elements heavier than hydrogen or helium will fluctuate by 20 to 30 percent. We can measure each of these elements by looking at the spectrum of light it emits, like a fingerprint, or by studying how it affects sound waves passing through the Sun. The mysterious difference in these two types of measurement of the elements of the Sun is called the problem of solar excess (or abundance).
We need to accurately measure these elements to understand the sun's chemical composition, as well as its density and temperature. In many ways, it will also help us understand the composition and behavior of other stars, as well as planets and galaxies.
For many years, scientists could not develop an acceptable solution. Then astrophysicist Aaron Vincent and his colleagues proposed the presence of dark matter in the Sun's core as a possible answer to the question. After testing many models, they came up with a theory that seemed to work. However, it included a special type of dark matter - "weakly interacting asymmetric dark matter", which could be either matter or antimatter at the same time.
Based on gravity measurements, scientists learned that the Sun is surrounded by a halo of dark matter. Asymmetric dark matter particles do not contain much antimatter, so they can survive contact with ordinary matter and accumulate in the core of the Sun. These particles can also absorb energy at the center of the Sun and then transport its heat to the outer edges, which could explain the problem of solar excess.
Dark matter may be macroscopic
Case Western Reserve scientists doubt we're looking for dark matter in the right places. Specifically, that dark matter may not consist of tiny exotic particles like WIMPs (weakly interacting massive particles), but of macroscopic objects that range from a few centimeters to the size of an asteroid. However, scientists limit their theory to what is already observed in space. Hence their belief that the Standard Model of particle physics will provide the answer. No new model needed.
The scientists called their dark matter objects "macros." They don't claim that WIMPs and axions don't exist, but they do admit that our search for dark matter may include other candidates. There are examples of matter that is neither ordinary nor exotic, but which fits the parameters of the Standard Model.
“The [scientific] community abandoned the idea that dark matter could be made of ordinary matter in the late 1980s,” says physics professor Glenn Starkman. “We wonder if it was wrong and if dark matter could be made of ordinary matter - quarks and electrons?”
Detecting dark matter using GPS
Two physicists have proposed using GPS satellites to search for dark matter, which scientists believe may not be particles in the conventional sense, but rather smudges in the fabric of space-time.
“Our research suggests that dark matter may be organized as a giant gas-like collection of topological defects, or energetic cracks,” says Andrei Derevyanko from the University of Nevada. “We propose to detect these defects, dark matter, using a network of sensitive atomic clocks. The idea is that when the clocks become out of sync, we will know that dark matter, a topological defect, has passed through that location. Essentially, we plan to use GPS satellites as the largest human-made dark matter detector."
Scientists are analyzing data from 30 GPS satellites and trying to test their theory with their help. If dark matter is indeed gaseous, the Earth will pass through it as it moves through the galaxy. Acting as wind, wisps of dark matter would be blown away by the Earth and its satellites, causing GPS clocks on the satellites and on the ground to lose sync every three minutes. Scientists will be able to monitor discrepancies down to one billionth of a second.
Dark matter can be powered by dark energy
According to one recent study, dark energy may feed off dark matter as they interact, which in turn slows the growth of galaxies and could ultimately leave the universe almost completely empty. It's possible that dark matter decays into dark energy, but we don't know that yet. The Planck spacecraft recently refined the physical composition of the Universe: 4.9% ordinary matter, 25.9% dark matter and 69.2% dark energy.
We don't see dark matter or dark energy. These terms are not even very well defined by the scientific community. They are more like shorthand that will remain until we understand what is really going on.
Dark matter attracts and dark energy repels. Dark matter is the frame or foundation upon which galaxies and their contents are built. Its gravitational pull is believed to hold stars together in galaxies. Gravity is stronger when objects are closer together and weaker when they are further apart.
On the other hand, dark energy refers to the force that causes the Universe to expand, sending galaxies flying away. As dark energy pushes away these objects, gravity weakens. This suggests that the expansion of space is accelerating rather than slowing down due to gravitational effects, as was once thought.
“Since the late 1990s, astronomers have become convinced that something is causing the expansion of our universe to accelerate,” says Professor David Wonds from the University of Portsmouth. - A simple explanation is that empty space - the vacuum - has an energy density that is a cosmological constant. However, there is growing evidence that this simple model cannot explain the full range of astronomical data to which scientists have access. In particular, the growth of cosmic structure, galaxies and galaxy clusters is occurring more slowly than expected."
Dark matter causes ripples in the galactic disk
If we look into space from Earth, we will see that the stars suddenly end 50,000 light years from the center of our galaxy. Therefore, this is the end of the galaxy. We won't see anything serious until we move 15,000 light years away from this boundary, the Monoceros Ring, the stars that lie above the plane of our galaxy. Some scientists believed that these stars were torn from another galaxy.
However, new analysis of data from the Sloan Digital Sky Survey has revealed that the Unicorn Ring is essentially a . This means that the Milky Way is at least 50% larger than we thought - and the diameter of our galaxy increases from 100,000–120,000 light years to 150,000–180,000 light years.
Looking from Earth, we do not see that they are connected due to gaps in the galactic disk. These ripples look like concentric circles that radiate from where the stone falls into the water. The wave rises and blocks the view of the ocean, leaving only higher waves visible. So, although our view was partially blocked by the shape of our galaxy, we saw the Unicorn Ring like the top of a tall wave.
This discovery changes our understanding of the structure of the Milky Way.
“We found that the Milky Way's disk is not just a disk of stars in one plane, it is corrugated,” says Heidi Newberg of the Rensselaer School of Science. - We see at least four depressions in the disk of the Milky Way. And since these four depressions are visible only from our point of view, we can assume that similar ripples exist throughout the entire disk of the Milky Way."
Scientists believe the ripples could be caused by a piece of dark matter or a dwarf galaxy cutting across the Milky Way. If this theory turns out to be correct, the Milky Way's concentric depressions will help scientists analyze the distribution of dark matter in our galaxy.
Gamma ray signature
Until recently, the only way scientists could detect dark matter was by observing its possible gravitational effects on other objects in space. However, scientists believe that gamma rays could be a direct indication that dark matter is hiding in our Universe. They may have already detected the first gamma-ray signature in Reticulum 2, a recently discovered dwarf galaxy near the Milky Way.
Gamma rays are a form of high-energy electromagnetic radiation emitted from the dense centers of galaxies. If dark matter is indeed composed of WIMPs, the dark matter particles could be the source of gamma rays produced by the mutual annihilation of WIMPs upon contact. However, gamma rays can also be emitted by other sources such as black holes and pulsars. If the analysis process can separate some sources from others, we will be able to obtain gamma rays from dark matter. But this is just a theory.
Scientists believe that most dwarf galaxies lack important sources of gamma rays; dark matter may account for 99%. That's why physicists at Carnegie Mellon, Brown and Cambridge universities are excited about gamma rays coming from Reticulum 2.
"Gravitational detection of dark matter can tell us very little about the behavior of dark matter particles," says Matthew Walker of Carnegie Mellon University. “We now have a non-gravitational detection that demonstrates that dark matter behaves like a particle, and this is extremely important.” Of course, the possibility remains that this gamma radiation came from other sources that have not yet been identified. However, near the Milky Way gives scientists the opportunity to further study this theory.
Based on materials from listverse.com
In the articles of the series we examined the structure of the visible Universe. We talked about its structure and the particles that form this structure. About nucleons, which play a major role, since it is from them that all visible matter consists. About photons, electrons, neutrinos, and also about the supporting actors involved in the universal play that unfolds 14 billion years after the Big Bang. It would seem that there is nothing more to talk about. But that's not true. The fact is that the substance we see is only a small part of what our world consists of. Everything else is something we know almost nothing about. This mysterious “something” is called dark matter.
If the shadows of objects did not depend on the size of these latter,
and if they had their own arbitrary growth, then perhaps
soon there would not be a single bright place left on the entire globe.
Kozma Prutkov
What will happen to our world?
After Edward Hubble's discovery of redshifts in the spectra of distant galaxies in 1929, it became clear that the Universe was expanding. One of the questions that arose in this regard was the following: how long will the expansion last and how will it end? The forces of gravitational attraction acting between individual parts of the Universe tend to slow down the retreat of these parts. What the braking will lead to depends on the total mass of the Universe. If it is large enough, gravitational forces will gradually stop the expansion and it will be replaced by compression. As a result, the Universe will eventually “collapse” again to the point from which it once began to expand. If the mass is less than a certain critical mass, then the expansion will continue forever. It is usually customary to talk not about mass, but about density, which is related to mass by a simple ratio, known from the school course: density is mass divided by volume.
The calculated value of the critical average density of the Universe is approximately 10 -29 grams per cubic centimeter, which corresponds to an average of five nucleons per cubic meter. It should be emphasized that we are talking about average density. The characteristic concentration of nucleons in water, earth and in you and me is about 10 30 per cubic meter. However, in the void that separates clusters of galaxies and occupies the lion's share of the volume of the Universe, the density is tens of orders of magnitude lower. The value of the nucleon concentration, averaged over the entire volume of the Universe, was measured tens and hundreds of times, carefully counting the number of stars and gas and dust clouds using different methods. The results of such measurements differ somewhat, but the qualitative conclusion is unchanged: the density of the Universe barely reaches a few percent of the critical value.
Therefore, until the 70s of the 20th century, the generally accepted forecast was the eternal expansion of our world, which should inevitably lead to the so-called heat death. Heat death is a state of a system when the substance in it is distributed evenly and its different parts have the same temperature. As a consequence, neither the transfer of energy from one part of the system to another, nor the redistribution of matter is possible. In such a system nothing happens and can never happen again. A clear analogy is water spilled on any surface. If the surface is uneven and there are even slight differences in elevation, water moves along it from higher to lower places and eventually collects in the lowlands, forming puddles. The movement stops. The only consolation left was that heat death would occur in tens and hundreds of billions of years. Consequently, you don’t have to think about this gloomy prospect for a very, very long time.
However, it gradually became clear that the true mass of the Universe is much greater than the visible mass contained in stars and gas and dust clouds and, most likely, is close to critical. Or perhaps exactly equal to it.
Evidence for dark matter
The first indication that something was wrong with the calculation of the mass of the Universe appeared in the mid-30s of the 20th century. Swiss astronomer Fritz Zwicky measured the speeds at which galaxies in the Coma cluster (one of the largest clusters known to us, it includes thousands of galaxies) move around a common center. The result was discouraging: the velocities of the galaxies turned out to be much greater than could be expected based on the observed total mass of the cluster. This meant that the true mass of the Coma cluster was much greater than the apparent mass. But the main amount of matter present in this region of the Universe remains, for some reason, invisible and inaccessible to direct observations, manifesting itself only gravitationally, that is, only as mass.
The presence of hidden mass in galaxy clusters is also evidenced by experiments on the so-called gravitational lensing. The explanation for this phenomenon follows from the theory of relativity. In accordance with it, any mass deforms space and, like a lens, distorts the rectilinear path of light rays. The distortion that galaxy clusters cause is so great that it is easy to notice. In particular, from the distortion of the image of the galaxy that lies behind the cluster, it is possible to calculate the distribution of matter in the lens cluster and thereby measure its total mass. And it turns out that it is always many times greater than the contribution of the visible matter of the cluster.
40 years after Zwicky’s work, in the 70s, American astronomer Vera Rubin studied the speed of rotation around the galactic center of matter located on the periphery of galaxies. In accordance with Kepler's laws (and they directly follow from the law of universal gravitation), when moving from the center of a galaxy to its periphery, the rotation speed of galactic objects should decrease in inverse proportion to the square root of the distance to the center. Measurements have shown that for many galaxies this speed remains almost constant at a very significant distance from the center. These results can be interpreted only in one way: the density of matter in such galaxies does not decrease when moving from the center, but remains almost unchanged. Since the density of visible matter (contained in stars and interstellar gas) rapidly falls towards the periphery of the galaxy, the missing density must be supplied by something that for some reason we cannot see. To quantitatively explain the observed dependences of the rotation rate on the distance to the center of galaxies, it is required that this invisible “something” be approximately 10 times larger than ordinary visible matter. This “something” was called “dark matter” (in English “ dark matter") and still remains the most intriguing mystery in astrophysics.
Another important piece of evidence for the presence of dark matter in our world comes from calculations simulating the process of galaxy formation that began about 300,000 years after the Big Bang. These calculations show that the forces of gravitational attraction that acted between the flying fragments of the matter generated during the explosion could not compensate for the kinetic energy of the expansion. The matter simply should not have gathered into the galaxies that we nevertheless observe in the modern era. This problem was called the galactic paradox, and for a long time it was considered a serious argument against the Big Bang theory. However, if we assume that particles of ordinary matter in the early Universe were mixed with particles of invisible dark matter, then everything falls into place in the calculations and the ends begin to meet - the formation of galaxies from stars, and then clusters of galaxies, becomes possible. At the same time, as calculations show, at first a huge number of dark matter particles accumulated in galaxies and only then, due to gravitational forces, elements of ordinary matter were collected on them, the total mass of which was only a few percent of the total mass of the Universe. It turns out that the familiar and seemingly studied in detail visible world, which we recently considered almost understood, is only a small addition to something that the Universe actually consists of. Planets, stars, galaxies and you and me are just a screen for a huge “something” about which we have not the slightest idea.
Photofact
The galaxy cluster (at the lower left of the circled area) creates a gravitational lens. It distorts the shape of objects located behind the lens - stretching their images in one direction. Based on the magnitude and direction of the stretch, an international group of astronomers from the Southern European Observatory, led by scientists from the Paris Institute of Astrophysics, constructed a mass distribution, which is shown in the bottom image. As you can see, the cluster contains much more mass than can be seen through a telescope.
Hunting dark, massive objects is a slow process, and the results don't look the most impressive in photographs. In 1995, the Hubble Telescope noticed that one of the stars in the Large Magellanic Cloud flashed brighter. This glow lasted for more than three months, but then the star returned to its natural state. And six years later, a barely luminous object appeared next to the star. It was a cold dwarf that, passing at a distance of 600 light years from the star, created a gravitational lens that amplified the light. Calculations have shown that the mass of this dwarf is only 5-10% of the mass of the Sun.
Finally, the general theory of relativity unambiguously connects the rate of expansion of the Universe with the average density of the matter contained in it. Assuming that the average curvature of space is zero, that is, the geometry of Euclid and not Lobachevsky operates in it (which has been reliably verified, for example, in experiments with cosmic microwave background radiation), this density should be equal to 10 -29 grams per cubic centimeter. The density of visible matter is approximately 20 times less. The missing 95% of the mass of the Universe is dark matter. Note that the density value measured from the expansion rate of the Universe is equal to the critical value. Two values, independently calculated in completely different ways, coincided! If in fact the density of the Universe is exactly equal to the critical density, this cannot be a coincidence, but is a consequence of some fundamental property of our world, which has yet to be understood and comprehended.
What is this?
What do we know today about dark matter, which makes up 95% of the mass of the Universe? Almost nothing. But we still know something. First of all, there is no doubt that dark matter exists - this is irrefutably evidenced by the facts given above. We also know for certain that dark matter exists in several forms. After the beginning of the 21st century, as a result of many years of observations in experiments SuperKamiokande(Japan) and SNO (Canada) it was established that neutrinos have mass, it became clear that from 0.3% to 3% of the 95% of the hidden mass lies in neutrinos that have long been familiar to us - even if their mass is extremely small, but their quantity is in The universe has about a billion times the number of nucleons: each cubic centimeter contains an average of 300 neutrinos. The remaining 92-95% consists of two parts - dark matter and dark energy. A small fraction of dark matter is ordinary baryonic matter, built from nucleons; the remainder is apparently accounted for by some unknown massive weakly interacting particles (the so-called cold dark matter). The energy balance in the modern Universe is presented in the table, and the story about its last three columns is below.
Baryonic dark matter
A small (4-5%) part of dark matter is ordinary matter that emits little or no radiation of its own and is therefore invisible. The existence of several classes of such objects can be considered experimentally confirmed. The most complex experiments, based on the same gravitational lensing, led to the discovery of so-called massive compact halo objects, that is, located on the periphery of galactic disks. This required monitoring millions of distant galaxies over several years. When a dark, massive body passes between an observer and a distant galaxy, its brightness briefly decreases (or increases as the dark body acts as a gravitational lens). As a result of painstaking searches, such events were identified. The nature of massive compact halo objects is not completely clear. Most likely, these are either cooled stars (brown dwarfs) or planet-like objects that are not associated with stars and travel around the galaxy on their own. Another representative of baryonic dark matter is hot gas recently discovered in galaxy clusters using X-ray astronomy methods, which does not glow in the visible range.
Nonbaryonic dark matter
The main candidates for nonbaryonic dark matter are the so-called WIMPs (short for English Weakly Interactive Massive Particles- weakly interacting massive particles). The peculiarity of WIMPs is that they show almost no interaction with ordinary matter. This is why they are the real invisible dark matter, and why they are extremely difficult to detect. The mass of WIMP must be at least tens of times greater than the mass of a proton. The search for WIMPs has been carried out in many experiments over the past 20-30 years, but despite all efforts, they have not yet been detected.
One idea is that if such particles exist, then the Earth, as it orbits the Sun with the Sun around the galactic center, should be flying through a rain of WIMPs. Despite the fact that WIMP is an extremely weakly interacting particle, it still has a very small probability of interacting with an ordinary atom. At the same time, in special installations - very complex and expensive - a signal can be recorded. The number of such signals should change throughout the year because, as the Earth moves in orbit around the Sun, it changes its speed and direction relative to the wind, which consists of WIMPs. The DAMA experimental group, working at Italy's Gran Sasso underground laboratory, reports observed year-to-year variations in signal count rates. However, other groups have not yet confirmed these results, and the question essentially remains open.
Another method of searching for WIMPs is based on the assumption that during billions of years of their existence, various astronomical objects (Earth, Sun, the center of our Galaxy) should capture WIMPs, which accumulate in the center of these objects, and, annihilating each other, give rise to a neutrino stream . Attempts to detect excess neutrino flux from the center of the Earth towards the Sun and the center of the Galaxy were made on underground and underwater neutrino detectors MACRO, LVD (Gran Sasso Laboratory), NT-200 (Lake Baikal, Russia), SuperKamiokande, AMANDA (Scott Station -Amundsen, South Pole), but have not yet led to a positive result.
Experiments to search for WIMPs are also actively carried out at particle accelerators. In accordance with Einstein's famous equation E=mс 2, energy is equivalent to mass. Therefore, by accelerating a particle (for example, a proton) to a very high energy and colliding it with another particle, one can expect the creation of pairs of other particles and antiparticles (including WIMPs), the total mass of which is equal to the total energy of the colliding particles. But accelerator experiments have not yet led to a positive result.
Dark energy
At the beginning of the last century, Albert Einstein, wanting to ensure independence of time for the cosmological model in the general theory of relativity, introduced the so-called cosmological constant into the equations of the theory, which he designated by the Greek letter “lambda” - Λ. This Λ was a purely formal constant, in which Einstein himself did not see any physical meaning. After the expansion of the Universe was discovered, the need for it disappeared. Einstein very much regretted his haste and called the cosmological constant Λ his biggest scientific mistake. However, decades later it turned out that the Hubble constant, which determines the rate of expansion of the Universe, changes with time, and its dependence on time can be explained by selecting the value of that very “erroneous” Einstein constant Λ, which contributes to the hidden density of the Universe. This part of the hidden mass came to be called “dark energy”.
Even less can be said about dark energy than about dark matter. First, it is evenly distributed throughout the Universe, unlike ordinary matter and other forms of dark matter. There is as much of it in galaxies and galaxy clusters as outside of them. Secondly, it has several very strange properties, which can only be understood by analyzing the equations of the theory of relativity and interpreting their solutions. For example, dark energy experiences antigravity: due to its presence, the rate of expansion of the Universe increases. Dark energy seems to push itself away, accelerating the scattering of ordinary matter collected in galaxies. Dark energy also has negative pressure, due to which a force arises in the substance that prevents it from stretching.
The main candidate for dark energy is vacuum. The vacuum energy density does not change as the Universe expands, which corresponds to negative pressure. Another candidate is a hypothetical super-weak field, called the quintessence. Hopes for clarifying the nature of dark energy are associated primarily with new astronomical observations. Progress in this direction will undoubtedly bring radically new knowledge to humanity, since in any case, dark energy must be a completely unusual substance, completely different from what physics has dealt with so far.
So, 95% of our world consists of something about which we know almost nothing. One can have different attitudes towards such a fact that is beyond any doubt. It can cause anxiety, which always accompanies a meeting with something unknown. Or disappointment, because such a long and complex path to constructing a physical theory that describes the properties of our world led to the statement: most of the Universe is hidden from us and unknown to us.
But most physicists are now feeling encouraged. Experience shows that all the riddles that nature posed to humanity were sooner or later resolved. Undoubtedly, the mystery of dark matter will also be resolved. And this will certainly bring completely new knowledge and concepts that we have no idea about yet. And perhaps we will meet new mysteries, which, in turn, will also be solved. But this will be a completely different story, which readers of “Chemistry and Life” will not be able to read until a few years later. Or maybe in a few decades.