Most of the matter that makes up the universe is reliably hidden from our eyes.

Composing in our head a visual representation of the structure of the galaxy, we probably see spirals of stars in front of us, rotating in a black cosmic void. With a very powerful telescope, we could actually see the individual stars that make up the arms of spiral galaxies, since they emit a sufficient amount of light and other waves. We could also "see" the dark regions inside galaxies - clouds of interstellar dust and gas that absorb, not emit light.

However, over the course of the 20th century, astrophysicists gradually came to the conclusion that visible and familiar images of galaxies contain no more than 10% of the matter actually contained in the Universe. Approximately 90% of the Universe consists of matter, the form of which remains a mystery to us, since we cannot observe it, and collectively, all this dark matter is called dark matter... (Sometimes they also talk about the missing mass, but this term cannot be called a good one, since in this terminology it would probably be better to call it redundant.) For the first time secret revelations of this kind in the distant 1933 were voiced by the Swiss astronomer Fritz Zwicky (Fritz Zwicky, 1898-1974). It was he who pointed out that the cluster of galaxies in the constellation Coma of Veronica appears to be held together by a much stronger gravitational field than would be assumed based on the apparent mass of matter contained in this galaxy cluster, which means most of the matter contained in this area of ​​the universe, remains invisible to us.

In the 1970s, Vera Rubin, a researcher at the Carnegie Institution (Washington), studied the dynamics of galaxies characterized by a high speed of rotation around their center, primarily, the behavior of matter at their periphery. According to all parameters, significant masses of the lightest interstellar gas, namely hydrogen, whose atoms should theoretically envelop the galaxy in a web of microscopic satellites, should have been thrown out to the periphery of rapidly rotating galaxies - according to the principle of a centrifuge. Consider, as an example, our solar system. Its bulk is concentrated in the center (on the Sun); the further the planet is removed from the center, the longer the period of its revolution around it. Jupiter, for example, takes eleven Earth years to complete a full annual revolution around the Sun, since it is in an orbit much more distant from the Sun and in one annual cycle makes not only a longer path, but also moves more slowly along it ( cm. Kepler's laws). Similarly, if all matter in a spiral galaxy were concentrated in its arms, where we observe visible stars, then atomized hydrogen atoms, obeying Kepler's third law, would move more and more slowly as they move away from the center of the galactic mass. Rubin, however, managed to experimentally find out that at any distance from the center of the galaxy, hydrogen moves at a constant speed. You might think that it is "glued" to a giant rotating sphere, consisting of some kind of invisible matter.

Now we know that dark matter is invisibly present not only within galaxies, but throughout the entire Universe, including intergalactic space. What we, however, have no idea about, is her nature. Some part of it may turn out to be ordinary celestial bodies that do not emit their own radiation, for example, massive planets like Jupiter. Their existence is confirmed by the results of observing the luminosity of stars in nearby galaxies, where sometimes "dips" are noted, which can be attributed to their partial eclipse during the passage of large planets on the path of rays on the way to us. In practice, the existence of interstellar eclipsing bodies that do not have their own radiation energy in the observed range can also be considered confirmed - they are called "massive compact halo objects".

However, the overwhelming majority of scientists agree that the mass of the invisible matter of the Universe is far from limited to the mass of ordinary celestial bodies and atomized matter hidden from us, but tend to add to it the total mass of still undiscovered types of elementary particles. They are usually called massive particles of weak interaction (MWPP). They do not manifest themselves in any way in interaction with light and other electromagnetic radiation. Their search today is a kind of renewal of the seemingly irrelevant search for "luminiferous ether" ( cm. Michelson-Morley experiment). The idea is that if our Galaxy is really clothed with a spherical MChSV envelope from all sides, the Earth, due to its motion, should constantly be under the influence of the "wind of hidden particles" that penetrate it in the same way as even in the most calm weather a car is blown by oncoming air currents. Sooner or later, one of the particles of such a "dark wind" will interact with one of the earth's atoms and excite the vibrations necessary for its registration by a supersensitive device in which it rests. Laboratories conducting such experiments are already reporting that the first hints have been obtained to confirm the real existence of a six-month half-period of fluctuations in the frequency of registration of signals about anomalous events of a similar series, and this is exactly what was to be expected, since the Earth has been moving in a circumsolar orbit towards the wind of hidden particles for half a year. and in the next six months the wind blows "after" and particles fly to the Earth less often.

MChSV are an example of what is commonly called cold dark matter because they are heavy and slow. It is assumed that they played an important role in the formation stage of galaxies in the early Universe. Some scientists also believe that at least some of the dark matter is in a state of fast, weakly interacting particles, such as neutrinos, which are an example hot dark matter. The main problem here is that before the formation of atoms, that is, for about the first 300,000 years after the big bang, the universe was in a protoplasmic state. Any nucleus of matter familiar to us disintegrated, without having time to form, under the most powerful energies of bombardment from the side of overheated particles of a hot, superdense, opaque plasma. After the Universe expanded to some degree of transparency of the space separating matter, light atomic nuclei finally began to form. But, alas, by this moment the Universe had already expanded so much that the forces of gravitational attraction could not to counteract the kinetic energy of the scattering of the fragments of the big bang, and all matter, in theory, should scatter, preventing the formation of stable galaxies that we observe. This was the so-called galactic paradox, questioning the very theory of the Big Bang.

However, if in the entire space of the volumetric big bang, ordinary matter was mixed with hidden particles of dark matter, after the explosion, dark matter, being mixed with explicit, could just serve as a restraining element. Due to the presence of a huge number of hidden heavy particles, it was the first to be pulled under the influence of the forces of gravitational attraction into the future nuclei of galaxies, which turned out to be stable due to the lack of interaction between the MHPM and the powerful centripetal energy radiation of the explosion. Thus, by the time the nuclei of atoms were formed, dark matter had time to form into galaxies and clusters of galaxies, and already on them, under the influence of the gravitational field, the liberated elements of ordinary matter began to gather. In this model, ordinary matter is drawn into clumps of dark matter like dry leaves being sucked into whirlpools on the dark surface of a fast-moving river. There is something to think about, isn't it? Not only us, but our entire galaxy, and the entire visible material world may turn out to be just foam on the surface of a strange universal game of hide and seek.

Vera Cooper Rubin, p. 1928

American astronomer. She was born in Philadelphia. She received her education and doctorate from Georgetown University (Washington, USA). Since 1954 he has been working at the Carnegie Institute, Washington, studying the structure of galaxies, primarily spiral ones, and, especially, the structure and movement of their arms. It was she who discovered that the speed of rotation of extended gas clouds in the arms of spiral galaxies does not decrease with distance from the center, but, on the contrary, increases, and this gives us the first convincing confirmation of the existence of dark matter in individual galaxies.

Dark matter Universe - no one saw it, measured it, no one knows what it is, but in existence dark matter the lion's share of physicists and astrophysicists insists. Because without the existence of dark matter, astrophysicists cannot explain many processes in the Universe.

That is, either there is dark matter, or our Universe is arranged completely differently and it is necessary to revise the physical theories. Naturally, it is more convenient for astronomers to accept that dark matter exists. Firstly, it is more convenient from the point of view of mathematics. Secondly, academics do not need to admit their miscalculations. But, I'm not talking about that ... 🙂

Are the rebels right who refute existence of dark matter- time will show. Personally, I am glad that research does not stand still, and physical theories have not turned into dogmas. Because I really want to see a breakthrough in the stagnation that has been observed in fundamental science for the last fifty years ... no subspace jumps, no time machine ... 🙂

And now, looking through the tape, I came across two independent messages at once on the topic of refuting the existence of dark matter.

Astronomers from St. Petersburg Nikolai and Elena Pit'ev analyzed data from 677 thousand measurements of the movements of bodies in the solar system over the past 100 years. These are measurement data both from the surface of the Earth and from spacecraft. The motion of the planets, their largest satellites and the trajectory of 301 asteroids were studied. According to the conclusions of the Petersburg astronomers, dark matter does not affect the motion of the studied bodies of the solar system. At least, this influence does not go beyond the measurement and calculation errors.
As far as I understand, such deviations must be if we compare the measured trajectories of these bodies with those trajectories that should have been for these bodies based only on their mass and speed, that is, without taking into account the influence of dark matter.
The article has not yet been officially published, but there are already preprints and it has been accepted for publication in Letters to Astronomical Journal.

The second work was done by astronomer Dr. Hongsheng Zhao of St Andrews University. He applied a modified MOND theory of gravity to the motion of our Milky Way galaxy with its satellites and galaxies. MOND was proposed in 1983 by Mordechai Milgrom of the Weizmann Institute and describes the behavior of gravity on a large scale differently than it should be according to the theories of Newton and Einstein. Until now, there has been no convincing evidence of its correctness.

According to Dr. Zhao's research, these two galaxies did not collide three billion years ago, as astronomers suggest, but much earlier - ten billion years ago. If the classical theories of Newton and Einstein were correct, then the galaxies already at that time would have merged into one supergalaxy, and not scattered to the sides after a collision.
Assuming that dark matter does not exist, then according to his research, it becomes clear why our galaxies collided and scattered again, scattering their "fragments" aside in the form of dwarf satellite galaxies. A huge mass of dark matter would glue our galaxies into one and prevent them from scattering.
By the way, classical theories cannot explain the oddities in the distribution of dwarf satellite galaxies around the Milky Way and Andromeda.

• Translation

An invisible civilization may exist right under your nose

Although we know that ordinary matter is responsible for only 1/20 of the energy of the Universe and 1/6 of the energy carried by matter (and everything else goes to the expense of dark energy), we consider ordinary matter to be a very important component. With the exception of cosmologists, almost all people concentrate on ordinary matter, although from an energetic point of view, it is not so important.

Ordinary matter is more dear to us, of course, because we are made of it - like the entire tangible world in which we live. But we are also interested in her because of the rich variety of her interactions. The interactions of ordinary matter include electromagnetic, weak and strong - they help matter to form complex dense systems. Not only stars, but also rocks, oceans, plants and animals exist thanks to the non-gravitational forces of nature responsible for interactions. Just as the reveler is more influenced by alcohol than the other components of beer, so ordinary matter, although it carries a small part of the energy density, affects itself and the environment much more noticeably than something just passing by.

The visible matter familiar to us can be considered as a privileged percentage - more precisely, 15% - of matter. In business and politics, 1% of people influence decisions and rules, and the remaining 99% of the population provide infrastructure and support - maintain buildings, keep cities running, deliver food. Likewise, ordinary matter affects almost everything that we observe, and dark matter, in its abundance and ubiquity, helps to create clusters and galaxies, ensures the formation of stars, but has little effect on our immediate environment.

The structures close to us are governed by ordinary matter. She is responsible for the movement of our bodies, for the energy sources that power our economy, for the computer screen or paper on which you read this, and for almost everything you can imagine. If something interacts in such a way that it can be measured, it is noteworthy because it can influence our environment.

Usually, dark matter doesn't have this interesting influence and structure. It is assumed that dark matter is the glue that holds galaxies and their clusters in amorphous clouds. But what if this is not the case, and it is only our bias - and ignorance, the root of bias - that causes our misconceptions?

In the Standard Model, there are six types of quarks, three types of charged leptons (including an electron), three types of neutrinos, particles responsible for all forces, and the newfound Higgs boson. What if the world of dark matter, maybe not so much, but also diverse? In this case, the interactions of dark matter will be negligible, but a small part of it will interact with forces that resemble the forces of ordinary matter. The rich and complex structure of particles and forces in the Standard Model is responsible for many interesting phenomena. If dark matter has an interacting component, it can also be influential.

If we were creatures consisting of dark matter, it would be wrong to believe that all particles of ordinary matter are the same. Perhaps people made of ordinary matter make the same mistake. Given the complexity of SM particle physics, which describes the simplest components of matter known to us, it seems strange to assume that all dark matter consists of only one kind of particles. Why not assume that some part of it is subject to its own interactions?

In this case, just as ordinary matter consists of different types of particles, and all these fundamental constituent parts interact through different combinations of charges, dark matter will also have different constituent parts - and at least one type of such particles will participate in non-gravitational interactions ... SM neutrinos are not affected by electrical force or strong interactions, unlike the six types of quarks.

Likewise, perhaps one type of dark matter particles interacts weakly or does not interact with anything other than through gravity, but some 5% of particles experience other interactions. Based on the study of ordinary matter, we can say that this option is more likely than the usual assumption about the presence of one weakly interacting particle.

It is a mistake for foreign public relations people to try to pile up the culture of another country and ignore the fact that there may be diversity in it that is obvious to their own country. Just as a good negotiator does not assume the predominance of one sector of society over another when trying to compare different cultures, so an unbiased scientist should not assume that dark matter is not as interesting as ordinary matter, and it lacks a variety of matter, similar to that what's in ours.

Popular science writer Corey Powell, reporting our research in Discover magazine, began by saying that he was a "light-matter chauvinist" - and that we are all too. He meant that we think that the matter we know is more important, and therefore more complex and interesting. Very similar views were overturned by the Copernican revolution. But most people insist that their point of view and belief in our importance correspond to the real world.

Many components of ordinary matter interact in different ways and affect the world in different ways. So maybe dark matter has different particles with different behaviors that affect the structure of the universe in a measurable way.

When I first started studying partially interacting dark matter, I was surprised that almost no one thought about the fact that the assumption that only ordinary matter exhibits a variety of types of particles and interactions is an arrogant delusion. Some physicists have tried to analyze models such as "specular dark matter", in which dark matter repeats everything that is inherent in ordinary. But such examples are exotic. Their consequences are difficult to combine with what we know.

Several physicists have studied more communication models for the interaction of dark matter. But they also assumed that all dark matter is the same and undergoes the same interactions. No one allowed the simple possibility that, although most of the dark matter does not interact with ordinary matter, a small fraction of it can do so.

One of the reasons for this is understandable. Most people believe that a new type of dark matter will not affect most of the observed phenomena if it is just a small fraction of dark matter. We have not yet even been able to observe the most important component of dark matter, and it seems premature to deal with its small component.

But if you remember that ordinary matter carries only 20% of the energy of the dark, while most of us notice only it, you can understand where this logic is wrong. Matter interacting through more powerful non-gravitational forces may be more interesting and more influential than most weakly interacting matter.

It is so with ordinary matter. It is overly powerful, despite its small number, as it contracts into dense discs from which stars, planets, Earth, and life can form. The charged component of dark matter - although there may not be that much of it - can also shrink and form discs, such as the visible disc in the Milky Way. It can even condense into objects that look like stars. In principle, such a structure can be observed, and, perhaps, it is even easier to do than ordinary cold dark matter scattered in a huge spherical halo.

Thinking in this way, the number of possibilities grows rapidly. After all, electromagnetism is just one of several non-gravitational interactions experienced by particles in the Standard Model. In addition to the force that binds electrons to nuclei, SM particles experience weak and strong nuclear interactions. In the world of ordinary matter, other interactions can also exist, but they are so weak at the energies available to us that no one has yet observed them. But even the presence of three non-gravitational interactions hints that non-gravitational interactions can also be present in the dark sector in addition to dark electromagnetism.

Perhaps, in addition to forces similar to electromagnetic ones, dark matter is also influenced by nuclear forces. It is possible that dark stars can form from dark matter, in which nuclear reactions take place, due to which structures are formed that behave in a way more similar to ordinary matter than the dark matter I have described so far. In this case, the dark disk may contain dark stars surrounded by dark planets consisting of dark atoms. Dark matter can have the same complexity as ordinary matter.

Partially interacting dark matter provides fertile ground for speculation and inspires us to consider possibilities that we would not otherwise have looked at. Writers and filmmakers may find all these additional forces and consequences lurking in the dark sector very tempting. They might even suggest the existence of a dark life that exists in parallel with ours. In this case, instead of the usual animated creatures fighting with other animated creatures, or, in rare cases, working with them, creatures of dark matter could march across the screen, which would drag the entire action onto themselves.

But it wouldn't be so interesting to watch. The problem is that filmmakers would have had a hard time filming a dark life that is invisible to us. Even if there were dark beings, we would not know about it. You may not know how cute a dark life could be - and you almost certainly won't.

While it is fun to speculate about the possibilities of dark life, it is much more difficult to figure out how to observe it - or at least detect its existence by circumstantial evidence. It is quite difficult to find life, consisting of the same components as we are, although the search for extrasolar planets is underway. But evidence for the existence of dark life, if it exists, will be even more elusive than evidence for the existence of ordinary life in distant worlds.

More recently, we have been able to observe gravitational waves emanating from huge black holes. We have practically no chance of detecting the gravity of a dark creature or an entire army of dark creatures, no matter how close they are to us.

Ideally, I would like to somehow communicate with this new sector. But if this new life is not influenced by the forces we are familiar with, this will not happen. Although we share gravity with them, such an influence from a single object or life form would be too weak to detect. Only very large objects, such as a disc in the plane of the Milky Way, can produce the observed effects.

Dark objects or dark life can exist very close to us - but if the total mass of dark matter is small, we will not know about it. Even with modern technology, or whatever technology we can imagine, only very specific capabilities can be tested. “Shadow life,” as exciting as it is, is unlikely to have tangible consequences and can be a seductive but unattainable opportunity. But the dark life is a very loose assumption. Scientists will have no problem creating it, but the Universe has many more obstacles for this. It is not clear which of the variants of chemical interactions are capable of supporting life, and we do not know what kind of environment is needed for those variants that are capable of doing so.

Nevertheless, in principle, dark life can exist, right under our noses. But without stronger interactions with the matter of our world, it can have fun, fight, be active or passive - and we will never know about it. Interestingly, however, when there are interactions in the dark world, whether or not life is involved, they can affect structure in a measurable way. And then we can learn much more about the dark world.

In the articles of the cycle, we examined the structure of the visible Universe. We talked about its structure and the particles that form this structure. About nucleons, which play the main role, since it is from them that all visible matter consists. About photons, electrons, neutrinos, and also about the minor actors involved in the universal performance that unfolds 14 billion years since the Big Bang. It would seem that there is nothing more to talk about. But this is not the case. The fact is that the substance we see is only a small part of what our world consists of. Everything else is something about which we know almost nothing. This mysterious "something" is called dark matter.

If the shadows of objects did not depend on the size of the latter,
but would have their own arbitrary growth, then, perhaps,
soon there would not be a single bright spot on the entire globe.

Kozma Prutkov

What will happen to our world?

After the discovery in 1929 by Edward Hubble of the redshift in the spectra of distant galaxies, 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 continue and how will it end? The forces of gravitational attraction, acting between separate parts of the Universe, tend to slow down the scattering of these parts. What deceleration will lead to depends on the total mass of the Universe. If it is large enough, the forces of gravity will gradually stop 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 about 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 the 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 separating clusters of galaxies and occupying the lion's share of the volume of the Universe, the density is tens of orders of magnitude lower. The value of the concentration of nucleons, averaged over the entire volume of the Universe, was measured tens and hundreds of times, carefully calculating the number of stars and gas and dust clouds by different methods. The results of such measurements are somewhat different, but the qualitative conclusion is unchanged: the value of the density of the Universe barely reaches a few percent of the critical value.

Therefore, up to the 70s of the XX century, it was generally accepted to predict 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 evenly distributed 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 over some surface. If the surface is uneven and there are even small differences in elevation, water moves along it from higher places to lower ones and eventually collects in lowlands, forming puddles. The movement stops. All that was left was to console myself with the fact that heat death would occur in tens and hundreds of billions of years. Consequently, for a very, very long time, one can not think about this gloomy prospect.

However, it gradually became clear that the true mass of the Universe is much larger than the visible mass contained in stars and gas and dust clouds and, most likely, is close to critical. And perhaps exactly equal to her.

Evidence for the existence of dark matter

The first indication that something was wrong with the calculation of the mass of the Universe appeared in the mid-1930s. The Swiss astronomer Fritz Zwicky measured the speed at which galaxies in the Coma cluster (and this is one of the largest clusters known to us, it includes thousands of galaxies) orbiting a common center. The result was discouraging: the speeds of the galaxies turned out to be much higher than one would expect based on the observed total mass of the cluster. This meant that the true mass of the Coma Cluster was much larger than the visible mass. But the main amount of matter present in this area of ​​the Universe remains, for some reason, invisible and inaccessible for direct observation, 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 of 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 causes a cluster of galaxies is so great that it's easy to spot. 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, the American astronomer Vera Rubin studied the speed of rotation around the galactic center of matter located at 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 the galaxy to its periphery, the speed of rotation 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 in only 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 provided by something that we, for some reason, cannot see. For a quantitative explanation of the observed dependences of the rotation rate on the distance to the center of galaxies, it is required that this invisible "something" be about 10 times greater than the usual visible matter. This "something" has received the name "dark matter" (in English " dark matter») And still remains the most intriguing mystery in astrophysics.

Another important evidence of the presence of dark matter in our world comes from calculations simulating the formation of galaxies, which began about 300 thousand years after the start of the Big Bang. These calculations show that the forces of gravitational attraction, which acted between the scattering fragments of the matter arising from the explosion, could not compensate for the kinetic energy of the scattering. The substance simply should not have collected in galaxies, which 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 in the calculations everything falls into place and ends begin to converge - the formation of galaxies from stars, and then clusters from galaxies becomes possible. At the same time, as calculations show, at first a huge number of dark matter particles were clustered 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 to the details of the visible world, which we recently considered almost understandable, is only a small addition to something that the universe actually consists of. Planets, stars, galaxies and even you and I are just a screen for a huge "something" about which we have no idea.

Photo fact

A cluster of galaxies (in the lower left part of the circled region) creates a gravitational lens. It distorts the shape of objects located behind the lens - stretching their images in one direction. In terms of the magnitude and direction of pulling, an international group of astronomers from the European Southern Observatory, led by scientists from the Paris Institute of Astrophysics, plotted the mass distribution, which is shown in the lower image. As you can see, much more mass is concentrated in the cluster than can be seen through a telescope.

Hunting for dark massive objects is not a quick business, and the result does not look very impressive in the photo. In 1995, the Hubble Telescope noticed that one of the stars of the Large Magellanic Cloud flared brighter. This glow lasted 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 600 light-years from the star, created a gravitational lens that amplifies 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 links the rate of expansion of the Universe with the average density of the matter contained in it. Under the assumption that the average curvature of space is zero, that is, the geometry of Euclid acts in it, and not Lobachevsky (which has been reliably verified, for example, in experiments with relic radiation), this density should be equal to 10 -29 grams per cubic centimeter. The density of the visible substance is about 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. The two values, independently calculated in completely different ways, are the same! If in reality the density of the Universe is exactly equal to the critical one, this cannot be an accidental coincidence, but is a consequence of some fundamental property of our world, which has yet to be understood and comprehended.

What is it?

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 presented above. We also know for sure that dark matter exists in several forms. After by the beginning of the XXI century, as a result of many years of observations in experiments SuperKamiokande(Japan) and SNO (Canada), it was found that neutrinos have mass, it became clear that from 0.3% to 3% of 95% of the hidden mass lies in neutrinos that have been familiar to us for a long time - even if their mass is extremely small, but the number is There are about a billion times the number of nucleons in the universe: each cubic centimeter contains, on average, 300 neutrinos. The remaining 92-95% consists of two parts - dark matter and dark energy. An insignificant fraction of dark matter is ordinary baryonic matter, built of nucleons; apparently, some unknown massive weakly interacting particles (the so-called cold dark matter) are responsible for the remainder. The energy balance in the modern Universe is presented in the table, and the story about its last three columns is shown below.

Baryonic dark matter

A small (4-5%) part of dark matter is an ordinary substance that does not emit or almost does not emit its own radiation and therefore is 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 the so-called massive compact haloobjects, that is, located on the periphery of galactic disks. This required tracking 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, since the dark body acts as a gravitational lens). As a result of painstaking searches, such events were identified. The nature of massive compact haloobjects 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 through the galaxy on their own. Another representative of baryonic dark matter is a hot gas recently discovered in galaxy clusters by X-ray astronomy, which does not glow in the visible range.

Non-baryonic dark matter

The main candidates for non-baryonic dark matter are the so-called WIMP (short for English Weakly Interactive Massive Particles- weakly interacting massive particles). The peculiarity of WIMP is that they hardly manifest themselves in any way in interaction with an ordinary substance. This is why they are the very real invisible dark matter, and this is why they are extremely difficult to detect. The WIMP mass should be at least tens of times greater than the proton mass. Searches for WIMPs have been conducted in many experiments over the past 20-30 years, but despite all efforts, they have not yet been found.

One of the ideas is that if such particles exist, then the Earth in its motion with the Sun in orbit around the center of the Galaxy should fly through the rain, consisting of WIMP. Despite the fact that WIMP is an extremely weakly interacting particle, it still has some very low 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, since, moving in an orbit around the Sun, the Earth changes its speed and direction of movement relative to the wind, consisting of WIMP. The DAMA Experimental Group at the Gran Sasso underground laboratory in Italy reports the observed annual variations in the signal count rate. However, other groups have not yet confirmed these results, and the question remains essentially open.

Another method of searching for WIMP is based on the assumption that during billions of years of their existence, various astronomical objects (the Earth, the Sun, the center of our Galaxy) should capture WIMPs that accumulate in the center of these objects, and, annihilating with each other, give rise to a neutrino flux ... Attempts to detect excess neutrino flux from the center of the Earth towards the Sun and the center of the Galaxy were undertaken using 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. According to Einstein's famous equation E = mc 2, energy is equivalent to mass. Therefore, accelerating a particle (for example, a proton) to a very high energy and colliding it with another particle, one can expect the production of pairs of other particles and antiparticles (including WIMP), 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, wishing to ensure the cosmological model in the general theory of relativity, independence from time, introduced into the equations of the theory the so-called cosmological constant, which he denoted 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 greatly 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 choosing the value of the same “erroneous” Einstein constant Λ, which contributes to the latent density of the Universe. This part of the latent 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 them. Secondly, it has several very strange properties, which can be understood only by analyzing the equations of the theory of relativity and interpreting their solutions. For example, dark energy experiences anti-gravity: due to its presence, the rate of expansion of the Universe increases. Dark energy, as it were, pushes itself apart, accelerating at the same time the scattering of ordinary matter collected in galaxies. And dark energy also has negative pressure, due to which a force arises in the substance that prevents it from stretching.

The main candidate for the role of dark energy is vacuum. The energy density of the vacuum does not change with the expansion of the Universe, which corresponds to negative pressure. Another candidate is a hypothetical superweak field called quintessence. Hopes for clarification of the nature of dark energy are associated primarily with new astronomical observations. Progress in this direction will undoubtedly bring radically new knowledge to mankind, since in any case, dark energy should be a completely unusual substance, completely different from what physics has dealt with until now.

So, our world is 95% composed of something about which we know almost nothing. It is possible to relate differently to such a fact that is not subject to any doubt. He can cause anxiety, which always accompanies meeting with something unknown. Or chagrin, because such a long and difficult path of building a physical theory describing the properties of our world has led to the statement: most of the Universe is hidden from us and is unknown to us.

But most physicists are now getting excited. Experience shows that all the riddles that nature posed to mankind were sooner or later solved. Undoubtedly, the mystery of dark matter will also be solved. And this will certainly bring completely new knowledge and concepts, which we have no idea about yet. And perhaps we will meet with new riddles, which, in turn, will also be solved. But it will be a completely different story, which readers of Chemistry and Life will be able to read not earlier than in a few years. And maybe in a few decades.

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Most of the Universe consists of “matter” that cannot be seen, possibly immaterial, and interacts with other things only through the force of gravity. Oh, yes, and physicists do not know what this matter is or why there is so much of it in the Universe - about four-fifths of its mass.

Scientists call it dark matter.

So where is this mysterious matter that makes up such a huge chunk of our universe, and when will scientists discover it?

How do we know this matter exists

The hypothesis of dark matter was first put forward by the Swiss astronomer Fritz Zwicky in the 1930s, when he realized that his measurements of the masses of galaxy clusters showed some of the mass in the Universe “missing”. Whatever makes galaxies heavier, it does not emit any light, nor interacts with anything other than through gravity.

Astronomer Vera Rubin, in the 1970s, discovered that the rotation of galaxies does not follow Newton's Law of Motion; stars in galaxies (in particular Andromeda) seemed to revolve around the center at the same speed, but those farther from the star move more slowly. As if something adds mass to the outer part of the galaxy that no one could see.

The rest of the evidence comes from gravitational lensing, which occurs when the gravity of a large object bends light waves around an object. According to general relativity, gravity bends space (like a sumo wrestler can deform the mat he is standing on), so that light rays bend around large objects, even though light itself is massless. Observations showed that there was not enough visible mass to bend the light as it did around individual galaxy clusters - in other words, the galaxies were more massive than they should be.

Then there is the relic radiation (CMB), the “echo” of the Big Bang and supernovae. “The CMB tells us that the universe is spatially flat,” said Jason Kumar, professor of physics at the University of Hawaii. “Spatially flat” means that if you draw two lines through the universe, they never intersect, even if the lines were billions of light-years across. In a steeply curved universe, these lines will meet at some point in space.

There is now a small debate among cosmologists and astronomers as to whether dark matter exists. It does not affect light, and it is not charged like electrons or protons. Until now, it eludes direct detection.

“It's a mystery,” said Kumar. There may be ways that scientists have tried to “see” dark matter - either through its interaction with ordinary matter, or through looking for particles that could become dark matter.

What dark matter is not

Many theories have come and gone as to what dark matter is. One of the first was logical enough: the question was hidden in massive astrophysical compact halo objects (MACHOs) such as neutron stars, black holes, brown dwarfs and rogue planets. They do not emit light (or they emit very little), so they are virtually invisible to telescopes.

However, exploring galaxies looking for small distortions in starlight produced by MACHO passing by - called microlensing - could not explain the amount of dark matter around galaxies, or even much of it. “MACHOs seem to be as excluded as ever,” said Dan Hooper, an associate researcher at the Fermi National Accelerator Laboratory in Illinois.

Dark matter does not appear to be a cloud of gas that cannot be seen through telescopes. Diffuse gas will absorb light from galaxies that are farther away, and at the top of that normal gas will re-emit radiation at longer wavelengths - there will be a huge emission of infrared light in the sky. Since this does not happen, we can rule it out.

What could it be

Weakly interacting massive particles (WIMPs) are some of the strongest contenders for the explanation of dark matter. Wimps are heavy particles - about 10 to 100 times heavier than the proton, which were created during the Big Bang and remain in small numbers today. These particles interact with normal matter through gravity and weak nuclear forces. The more massive WIMPs will move more slowly through space, and therefore may be candidates for “cold” dark matter, while the lighter ones will move faster and be candidates for “warm” dark matter.

One way to find them is through “direct detection,” such as the Large Underground Xenon (LUX) experiment, which is a container of liquid xenon in a South Dakota mine.

Another way to see wimps could be with a particle accelerator. Inside accelerators, atomic nuclei break at a speed close to the speed of light, and in the process this collision energy is converted into other particles, some of which are new to science. So far nothing has been found in particle accelerators that looks like putative dark matter.

Another possibility: axions. These subatomic particles could be detected indirectly by the kinds of radiation they emit, how they destroy or how they decay into other kinds of particles or appear in particle accelerators. However, there is no direct evidence for axions either.

Since the discovery of heavy, slow “cold” particles like wimps or axions has yet to produce results, some scientists are looking at the possibility of light, faster moving particles that they cause “warm” dark matter. There has been renewed interest in such a model of dark matter after scientists found evidence of an unknown particle using the Chandra X-ray Observatory, in the Perseus cluster, a group of galaxies about 250 million light-years from Earth. The known ions in this cluster produce certain lines of X-ray emission, and in 2014, scientists saw a new “line” that could correspond to an unknown light particle.

If dark matter particles are light, scientists will have a hard time finding them directly, said Tracy Slater, a physicist at MIT. She proposed new types of particles that can make up dark matter.

“Dark matter with a mass below about 1 GeV is really difficult to detect with standard direct detection experiments because they work by looking for unexplained recoils of atomic nuclei ... but when dark matter is much lighter than an atomic nucleus, the recoil energy is very small,” Tracy said Slater.

Much research has been done in the search for dark matter, and if current methods fail, new ones will be carried out. Using “liquid” liquid helium, semiconductors and even breaking chemical bonds in crystals are some of the new ideas for detecting dark matter.