Calculations by scientists have shown that 95% of the Universe consists of matter that has not yet been explored by people: 70% is dark energy, and 25% is dark matter. It is assumed that the first represents a certain field with non-zero energy, but the second consists of particles that can be detected and studied.
But it’s not for nothing that this substance is called hidden mass - its search lasts a considerable time and is accompanied by heated discussions among physicists. In order to bring its research to the public, CERN even initiated Dark Matter Day, which is celebrated for the first time today, October 31.
Proponents of the existence of dark matter present quite compelling arguments, confirmed by experimental facts. Its recognition began in the thirties of the 20th century, when the Swiss astronomer Fritz Zwicky measured the speeds at which the galaxies of the Coma cluster move around a common center. As you know, the speed of movement depends on the mass. The scientist's calculations showed that the true mass of galaxies should be much greater than that determined during observations using telescopes. It turned out that a fairly large part of the galaxies was simply not visible to us. Therefore, it consists of matter that does not reflect or absorb light.
The second confirmation of the existence of hidden mass is the change in light as it passes through galaxies. The fact is that any object with mass distorts the rectilinear path of light rays. Thus, dark matter will make its changes to the light picture (image of a distant object), and it will become different from the picture that would be created only by visible matter. There are ten pieces of evidence for the existence of dark matter, but these two are the main ones.
© 2012 The Authors Monthly Notices of the Royal Astronomical Society, 2012 RAS
A photo of a galaxy cluster. The lines show the "outline" of dark matter
Although the evidence for the existence of dark matter is quite convincing, no one has yet found or studied the particles that make it up. Physicists suggest that this secrecy is due to two reasons. The first is that these particles have too high a mass (related to energy through the formula E=mc²), so the capabilities of modern accelerators are simply not enough to “birth” such a particle. The second reason is the very low probability of dark matter appearing. Perhaps we cannot find it precisely because it interacts extremely weakly with the human body and the particles known to us. Even though dark matter is everywhere (according to calculations) and its particles are literally rushing through us every second, we just don't feel it.
To detect dark matter particles, scientists use detectors that are located underground to minimize unnecessary interference. It is assumed that occasionally dark matter particles still collide with atomic nuclei, transfer part of their momentum to them, knock out electrons and cause flashes of light. The frequency of such collisions depends on the probability of interaction of dark matter particles with the core, their concentration and relative speed (taking into account the movement of the Earth around the Sun). But experimental groups, even if they detect some effect, deny that dark matter caused this detector response. And only the Italian experimental group DAMA, working in the underground laboratory of Gran Sasso, reports observed annual variations in the count rate of signals, presumably associated with the movement of the Earth through the galactic hidden mass.
Detector for detecting dark matter
In this experiment, the number and energy of light flashes inside the detector are measured over several years. Researchers have proven the presence of weak (about 2%) annual fluctuations in the count rate of such events.
Although the Italian group confidently defends the reliability of the experiments, the opinions of scientists on this matter are rather ambiguous. The main weakness of the results obtained by the Italian group is their uniqueness. For example, when gravitational waves were discovered, they were detected by laboratories around the world, thereby confirming the data obtained by other groups. In the case of DAMA, the situation is different - no one else in the world can boast of having the same results! Of course, it is possible that this group has more powerful detectors or its own methods, but this uniqueness of the experiment raises doubts among some researchers about its reliability.
“It is still impossible to say exactly what the data collected in the Gran Sasso laboratory relate to. In any case, a group from Italy provided a positive result, and not a denial of something, which is already a sensation. Now the signals found need to be sought for an explanation. And this is a great incentive to the development of a variety of theories, including those devoted to creating a model of hidden mass. But even if a scientist tries to explain why the data obtained in no way relate to dark matter, this can still become a new step in understanding Nature. In any case, the result is and we need to continue the work. But at the moment, I personally cannot completely agree that dark matter has been found,” comments Konstantin Belotsky, leading researcher at the Department of Elementary Particle Physics at NRNU MEPhI.
Dark matter does not emit or absorb light, practically does not interact with “ordinary” matter, scientists have not yet managed to catch a single “dark” particle. But without it, the Universe we know, and even ourselves, could not exist. On Dark Matter Day, which is celebrated on October 31 (physicists decided that this is just the right time to organize a holiday in honor of the dark and elusive substance), N+1 asked the head of the department of theoretical astrophysics at the Astrospace Center of the Lebedev Physical Institute, Andrei Doroshkevich, about what dark matter is and why it is so important.
N+1: How confident are scientists today that dark matter really exists?
Andrey Doroshkevich: The main evidence is observations of fluctuations of the cosmic microwave background radiation, that is, the results that have been obtained by the WMAP and "" spacecraft over the past 15 years.
They measured with high accuracy the temperature disturbance of the cosmic microwave background, that is, the cosmic microwave background radiation. These disturbances have been preserved since the era of recombination, when ionized hydrogen turned into neutral atoms.
These measurements showed the presence of fluctuations, very small, about one ten-thousandth of a kelvin. But when they began to compare these data with theoretical models, they discovered important differences that cannot be explained in any other way except by the presence of dark matter. Thanks to this, they were able to calculate the shares of dark and ordinary matter in the Universe with an accuracy of one percent.
Distribution of matter in the Universe (from left to right) before and after the appearance of data from the Planck telescope
Scientists have made many attempts to get rid of invisible and imperceptible dark matter, creating theories of modified gravity, such as MOND, that try to explain the observed effects. Why are dark matter models preferable?
The situation is very simple: the modern Einsteinian theory of gravity works well on earthly scales, satellites fly in strict accordance with this theory. And it works very well on cosmological scales. And all the modern models that change gravity cannot explain everything. They introduce new constants into Newton's law that help explain the effects of dark matter at the galaxy level, but miss the mark at the cosmological scale.
Could the discovery of gravitational waves help here? Maybe it will help discard some of the theories?
What gravitational waves have now measured is a huge technical, not scientific, success. That they exist was known 40 years ago when gravitational radiation from a double pulsar was discovered (indirectly). Observations of gravitational waves once again confirmed the existence of black holes, although we did not doubt it before, but now we have more or less direct evidence.
The form of the effect, changes in gravitational waves depending on power, can give us very useful information, but we need to wait another five to ten years until we have enough data to refine theories of gravity.
How scientists learned about dark matter
The history of dark matter began in 1933, when astronomer Fritz Zwicky studied the velocity distribution of galaxies in a cluster located in the constellation Coma Berenices. He discovered that the galaxies in the cluster were moving too fast, and if only visible matter were taken into account, the cluster could not be stable - the galaxies would simply be scattered in different directions.
In a paper published on February 16, 1933, Zwicky suggested that they were held together by an invisible gravitational substance - Dunkle Materie.
A little later, other astronomers confirmed the discrepancy between the “visible” mass of galaxies and the parameters of their motion.
In 1958, Soviet astrophysicist Viktor Ambartsumyan proposed his solution to the Zwicky paradox. In his opinion, galaxy clusters do not contain any invisible matter that would hold them gravitationally. We are simply observing clusters in the process of disintegration. However, most astronomers did not accept this explanation, since in this case the lifespan of clusters would be no more than one billion years, and given that the lifespan of the Universe is ten times longer, by today there would simply be no clusters left.
The generally accepted understanding of dark matter is that it consists of WIMPs, massive particles that have little interaction with ordinary matter particles. What can you say about their properties?
They have a fairly large mass - and that’s almost all, we can’t even name the exact mass. They travel long distances without collisions, but density disturbances in them do not die out even on relatively small scales - and this is the only thing we need for models today.
CMB gives us the characteristics of dark matter on large scales, on the scale of galaxy clusters. But in order to “go down” to the scale of small galaxies, we are forced to use theoretical models.
The very existence of small galaxies suggests that even on relatively small scales there were irregularities that arose shortly after the Big Bang. Such inhomogeneities may fade and smooth out, but we know for sure that they do not fade on the scale of small galaxies. This suggests that these dark matter particles must have properties such that these disturbances persist.
Is it correct to say that stars could only arise due to dark matter?
Not really. Without dark matter, galaxies could not form, and stars cannot form outside galaxies. Unlike dark matter, baryons are always hot and interact with the cosmic microwave background radiation. Therefore, they cannot independently assemble into stars; the gravity of stellar mass baryons cannot overcome their pressure.
Dark matter particles act as invisible cement that pulls baryons into galaxies, and then the process of star formation begins in them. There is six times more dark matter than baryons; it “leads”, and the baryons only follow it.
Xenon dark matter particle detector XENON1T
Xenon100 collaboration
Is there a lot of dark matter around us?
It is everywhere, the only question is how much there is. It is believed that in our Galaxy the mass of dark matter is slightly less than 10 percent.
But already in the vicinity of the Galaxy there is more dark matter, we can see signs of the presence around both ours and other stellar systems. Of course, we see it thanks to baryons, we observe them, and we understand that they “stick” there only due to the presence of dark matter.
How scientists search for dark matter
Since the late 1980s, physicists have been conducting experiments in facilities deep underground in an attempt to capture the collisions of individual dark matter particles. Over the past 15 years, the collective sensitivity of these experiments has grown exponentially, doubling on average every year. Two major collaborations, XENON and PandaX-II, have recently launched new, even more sensitive detectors.
The first of them built the world's largest dark matter detector, XENON1T. It uses a 2,000-kilogram target made of liquid xenon, placed in a tank of water 10 meters high. All this is located underground at a depth of 1.4 kilometers in the Gran Sasso National Laboratory (Italy). The PandaX-II installation is buried at a depth of 2.4 kilometers in the Chinese province of Sichuan and contains 584 kilograms of liquid xenon.
Both experiments use xenon because it is extremely inert, which helps keep noise levels low. In addition, the nuclei of xenon atoms are relatively heavy (containing an average of 131 nucleons per nucleus), which provides a “larger” target for dark matter particles. If one of these particles collides with the nucleus of a xenon atom, it will produce a weak but perceptible flash of light (scintillation) and the formation of an electrical charge. Observing even a small number of such events can give us important clues about the nature of dark matter.
So far, neither these nor any other experiments have been able to detect dark matter particles, but this silence can be used to set an upper limit on the probability of collisions of dark matter particles with ordinary matter particles.
Can dark matter particles form clumps like normal matter particles?
They can, but the whole question is what density. From the point of view of astrophysics, galaxies are dense objects, their density is on the order of one proton per cubic centimeter, and stars are dense objects, with a density on the order of a gram per cubic centimeter. But there are 24 orders of magnitude difference between them. Typically, dark matter clouds have a "galactic" density.
Do numerous people have a chance to search for dark matter particles?
They are trying to capture the interactions of individual dark matter particles with atoms of ordinary matter, as they do with neutrinos. But it is very difficult to catch them, and it is not a fact that it is even possible.
The CAST (CERN Axion Solar Telescope) telescope at CERN is looking for hypothetical particles - axions - that could make up dark matter.
Perhaps dark matter generally consists of so-called “mirror” particles, which, in principle, can only be observed by their gravity. The hypothesis of a second “mirror” Universe was proposed half a century ago; this is a kind of doubling of reality.
We only have real observations from cosmology.
Interviewed by Sergey Kuznetsov
MOSCOW, October 31 - RIA Novosti, Olga Kolentsova. Calculations by scientists have shown that 95% of the Universe consists of matter that has not yet been explored by people: 70% is dark energy, and 25% is dark matter. It is assumed that the first represents a certain field with non-zero energy, but the second consists of particles that can be detected and studied. But it’s not for nothing that this substance is called hidden mass - its search lasts a considerable time and is accompanied by heated discussions among physicists. In order to bring its research to the public, CERN even initiated Dark Matter Day, which is celebrated for the first time today, October 31.
Proponents of the existence of dark matter present quite compelling arguments, confirmed by experimental facts. Its recognition began in the thirties of the 20th century, when the Swiss astronomer Fritz Zwicky measured the speeds at which the galaxies of the Coma cluster move around a common center. As you know, the speed of movement depends on the mass. The scientist's calculations showed that the true mass of galaxies should be much greater than that determined during observations using telescopes. It turned out that a fairly large part of the galaxies was simply not visible to us. Therefore, it consists of matter that does not reflect or absorb light.
The second confirmation of the existence of hidden mass is the change in light as it passes through galaxies. The fact is that any object with mass distorts the rectilinear path of light rays. Thus, dark matter will make its changes to the light picture (image of a distant object), and it will become different from the picture that would be created only by visible matter. There are ten pieces of evidence for the existence of dark matter, but these two are the main ones.
© 2012 The Authors Monthly Notices of the Royal Astronomical Society, 2012 RAS
© 2012 The Authors Monthly Notices of the Royal Astronomical Society, 2012 RAS
Although the evidence for the existence of dark matter is quite convincing, no one has yet found or studied the particles that make it up. Physicists suggest that this secrecy is due to two reasons. The first is that these particles have too high a mass (related to energy through the formula E=mc²), so the capabilities of modern accelerators are simply not enough for the “birth” of such a particle. The second reason is the very low probability of dark matter appearing. Perhaps we cannot find it precisely because it interacts extremely weakly with the human body and the particles known to us. Even though dark matter is everywhere (according to calculations) and its particles are literally rushing through us every second, we just don't feel it.
To detect dark matter particles, scientists use detectors that are located underground to minimize unnecessary interference. It is assumed that occasionally dark matter particles still collide with atomic nuclei, transfer part of their momentum to them, knock out electrons and cause flashes of light. The frequency of such collisions depends on the probability of interaction of dark matter particles with the core, their concentration and relative speed (taking into account the movement of the Earth around the Sun). But experimental groups, even if they detect some effect, deny that dark matter caused this detector response. And only the Italian experimental group DAMA, working in the underground laboratory of Gran Sasso, reports observed annual variations in the count rate of signals, presumably associated with the movement of the Earth through the galactic hidden mass.
© Photo: SuperCMDS Collaboration
In this experiment, the number and energy of light flashes inside the detector are measured over several years. Researchers have proven the presence of weak (about 2%) annual fluctuations in the count rate of such events.
Although the Italian group confidently defends the reliability of the experiments, the opinions of scientists on this matter are rather ambiguous. The main weakness of the results obtained by the Italian group is their non-repeatability. For example, when gravitational waves were discovered, they were detected by laboratories around the world, thereby confirming the data obtained by other groups. In the case of DAMA, the situation is different - no one else in the world can boast of having the same results! Of course, it is possible that this group has more powerful detectors or its own methods, but this uniqueness of the experiment raises doubts among some researchers about its reliability.
“It is still impossible to say exactly what the data collected in the Gran Sasso laboratory relate to. In any case, a group from Italy provided a positive result, and not a denial of something, which is already a sensation. Now the signals found need to be sought for an explanation. And this is a great incentive to the development of a variety of theories, including those devoted to creating a model of hidden mass. But even if a scientist tries to explain why the data obtained in no way relate to dark matter, this can still become a new step in understanding Nature. In any case, the result is and we need to continue the work. But at the moment, I personally cannot completely agree that dark matter has been found,” comments Konstantin Belotsky, leading researcher at the Department of Elementary Particle Physics at National Research Nuclear University MEPhI.
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.
