Calculations by scientists have shown that the Universe is 95% composed of matter not yet explored by people: 70% is dark energy, and 25% is dark matter. It is assumed that the first is a kind of field with non-zero energy, but the second consists of particles that can be detected and studied.
But it is not for nothing that this substance is called hidden mass - its search lasts for a considerable time and is accompanied by heated discussions among physicists. In order to bring their research to the public, CERN even initiated Dark Matter Day, which is celebrated for the first time today, October 31st.
Proponents of the existence of dark matter give quite weighty arguments, confirmed by experimental facts. Its recognition began in the 1930s, when the Swiss astronomer Fritz Zwicky measured the speed at which the galaxies in 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 must be much larger than that determined in the process of observations with telescopes. It turned out that a fairly large part of the galaxies is simply not visible to us. Therefore, it consists of matter that does not reflect or absorb light.
The second confirmation of the existence of a hidden mass is the change in light as it passes through galaxies. The fact is that any object with mass distorts the rectilinear course of light rays. Thus, dark matter will make its own changes in the light picture (the image of a distant object), and it will become different from the picture that would be created only by visible matter. There are ten evidences for the existence of dark matter, but the two described are the main ones.
© 2012 The Authors Monthly Notices of the Royal Astronomical Society, 2012 RAS
An image of a cluster of galaxies. The lines show the "outlines" of dark matter
Although the evidence for the existence of dark matter is quite convincing, so far no one has found and studied the particles that make it up. Physicists suggest that such 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 "create" such a particle. The second reason is the very low probability of the appearance of dark matter. Perhaps we cannot find it precisely because it interacts extremely weakly with human body and particles known to us. Although 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 impacts. 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 nucleus, their concentration and relative velocity (taking into account the motion of the Earth around the Sun). But experimental groups, even when detecting some impact, deny that this response of the detector was caused by dark matter. And only the Italian experimental group DAMA, working in the underground laboratory of Gran Sasso, reports the observed annual variations in the count rate of signals, presumably associated with the movement of the Earth through the galactic hidden mass.
Dark matter detector
IN this experiment for several years, the number and energy of flashes of light inside the detector are measured. The researchers proved 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 weak point of the results obtained by the Italian group is their non-reproducibility. 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, there is the possibility that this group has more powerful detectors or their own methods, but this uniqueness of the experiment causes some researchers to doubt its reliability.
“It is not yet possible to say exactly what the data collected in the Gran Sasso laboratory refers to. In any case, the group from Italy provided a positive result, and not a denial of something that is already a sensation. Now the signals found need to be explained. And this is a great incentive to the development of the most different theories, including those devoted to the creation of a hidden mass model. But even if a scientist tries to explain why the data obtained in no way relate to dark matter, this can still be a new step in understanding Nature. In any case, there is a result and we must continue to work. But I personally cannot fully agree that dark matter has been found," comments Konstantin Belotsky, Leading Researcher at the Department of Physics elementary particles NRNU "MEPhI".
MOSCOW, October 31 - RIA Novosti, Olga Kolentsova. Calculations by scientists have shown that the Universe is 95% composed of matter not yet explored by people: 70% is dark energy, and 25% is dark matter. It is assumed that the first is a kind of field with non-zero energy, but the second consists of particles that can be detected and studied. But it is not for nothing that this substance is called hidden mass - its search lasts for a considerable time and is accompanied by heated discussions among physicists. In order to bring their research to the public, CERN even initiated Dark Matter Day, which is celebrated for the first time today, October 31st.
Proponents of the existence of dark matter give quite weighty arguments, confirmed by experimental facts. Its recognition began in the 1930s, when the Swiss astronomer Fritz Zwicky measured the speed at which the galaxies in 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 must be much larger than that determined in the process of observations with telescopes. It turned out that a fairly large part of the galaxies is simply not visible to us. Therefore, it consists of matter that does not reflect or absorb light.
The second confirmation of the existence of a hidden mass is the change in light as it passes through galaxies. The fact is that any object with mass distorts the rectilinear course of light rays. Thus, dark matter will make its own changes in the light picture (the image of a distant object), and it will become different from the picture that would be created only by visible matter. There are ten evidences for the existence of dark matter, but the two described 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, so far no one has found and studied the particles that make it up. Physicists suggest that such 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 "create" such a particle. The second reason is the very low probability of the appearance of dark matter. Perhaps we cannot find it precisely because it interacts extremely weakly with the human body and particles known to us. Although 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 impacts. 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 nucleus, their concentration and relative velocity (taking into account the motion of the Earth around the Sun). But experimental groups, even when detecting some impact, deny that this response of the detector was caused by dark matter. And only the Italian experimental group DAMA, working in the underground laboratory of Gran Sasso, reports the 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. The researchers proved 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 weak point of the results obtained by the Italian group is their non-reproducibility. 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, there is the possibility that this group has more powerful detectors or their own methods, but this uniqueness of the experiment causes some researchers to doubt its reliability.
“It is not yet possible to say exactly what the data collected in the Gran Sasso laboratory refers to. In any case, the group from Italy provided a positive result, and not a denial of something that is already a sensation. Now the signals found need to be explained. And this is a great incentive to the development of a variety of theories, including those devoted to the creation of a model of dark matter.But even if a scientist tries to explain why the data obtained in no way relate to dark matter, this can still be a new step in understanding Nature.In any case, the result is and we need to continue the work, but I personally cannot fully agree that dark matter has been found at the moment,” comments Konstantin Belotsky, a leading researcher at the Department of Elementary Particle Physics, National Research Nuclear University MEPhI.
Dark matter does not emit or absorb light, practically does not interact with "ordinary" matter, scientists have not yet been able to catch a single "dark" particle. But without it, the Universe familiar to us, and we ourselves, could not exist. On Dark Matter Day, which is celebrated on October 31 (physicists have decided that is just the right time to throw a holiday in honor of the dark and elusive substance), N+1 asked Andrey Doroshkevich, Head of the Department of Theoretical Astrophysics of the Astrospace Center of the Lebedev Physical Institute, about what dark matter is and why it is so important.
N+1: How confident are scientists today that dark matter really exists?
Andrei Doroshkevich: The main evidence is observations of fluctuations of the cosmic microwave background radiation, that is, the results that the WMAP and "" spacecraft have received over the past 15 years.
They measured the perturbation of the temperature of the cosmic microwave background, that is, the cosmic microwave background, with high accuracy. These perturbations 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 found important differences that cannot be explained in any other way than the presence of dark matter. Thanks to this, they were able to calculate the proportions of dark and ordinary matter in the Universe with an accuracy of up to a percentage.
The distribution of matter in the universe (from left to right) before and after the data from the Planck telescope
Scientists have made many attempts to get rid of the invisible and imperceptible dark matter, theories of modified gravity, such as MOND, have been created that try to explain the observed effects. Why are dark matter models preferable?
The situation is very simple: modern Einstein's theory of gravity works well on Earth scales, satellites fly in strict accordance with this theory. And it performs very well on cosmological scales. And all the modern models that change gravity cannot explain everything. They introduce new constants into Newton's law, which makes it possible to explain the effects of the presence of dark matter at the level of galaxies, but misses on the cosmological scale.
Could the discovery of gravitational waves help here? Maybe it will help to 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 binary 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 here.
The form of the effect, changes in gravitational waves depending on the power, can give us very useful information, but we need to wait another five to ten years until we have enough data to refine the 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 found that the galaxies in the cluster move too fast, and if only the visible matter is taken into account, the cluster could not be stable - the galaxies would simply be scattered in different directions.
In an article published on February 16, 1933, Zwicky suggested that they were held together by an invisible gravitational substance, the Dunkle Materie.
A little later, the discrepancy between the "visible" mass of galaxies and the parameters of their movement was confirmed by other astronomers.
In 1958, Soviet astrophysicist Viktor Ambartsumyan proposed his own solution to Zwicky's paradox. In his opinion, clusters of galaxies do not contain any invisible matter that would hold them gravitationally. We simply observe clusters in the process of decay. However, most astronomers did not accept this explanation, since in this case the lifetime of clusters would be no more than one billion years, and given that the lifetime of the Universe is ten times longer, there would simply be no clusters left by today.
Generally accepted ideas about dark matter say that it consists of WIMPs (WIMPs), massive particles that hardly interact with particles of ordinary matter. What can be said about their properties?
They have a fairly large mass - and that's almost everything, we can't even name the exact mass. They travel long distances without collisions, but the density perturbations in them do not decay even on relatively small scales - and this is the only thing we need for models today.
The CMB gives us the characteristics of dark matter on large scales, on the scales of galaxy clusters. But in order to "descend" 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 inhomogeneities that arose shortly after big bang. Such inhomogeneities can fade, smooth out, but we know for sure that they have not faded on the scale of small galaxies. This suggests that these dark matter particles must have properties such that these perturbations persist.
Is it correct to say that stars could only be formed 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, they interact with the background radiation. Therefore, they cannot assemble into stars on their own, the gravitation of stellar-mass baryons cannot overcome their pressure.
Dark matter particles act like an 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 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 of it. It is believed that in our Galaxy the mass of dark matter is somewhat 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 others star systems. Of course, we see it thanks to the baryons, we observe them, and we understand that they "hold" there only due to the presence of dark matter.
How scientists are looking for dark matter
Since the late 1980s, physicists have been conducting experiments in facilities deep underground in an attempt to capture the collision of individual particles of dark matter. 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 kg liquid xenon target placed in a 10 meter high water tank. All this is 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, this will give rise to a weak but perceptible flash of light (scintillation) and the formation of an electric charge. Observation of even a small number of such events can give us important data on the nature of dark matter.
So far, neither these nor any other experiments have been able to detect dark matter particles, but even this silence can be used to set an upper limit on the probability of collisions of dark matter particles with ordinary particles.
Can dark matter particles form clusters 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 of the order of a gram per cubic centimeter. But there are 24 orders of magnitude difference between them. As a rule, dark matter clouds have a "galactic" density.
Are there any chances for many to search for dark matter particles?
They are trying to capture the interactions of individual particles of dark matter 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 this is even possible.
The CAST (CERN Axion Solar Telescope) telescope at CERN is looking for hypothetical particles - axions, of which dark matter may consist.
Maybe dark matter generally consists of the so-called "mirror" particles, which in principle can be observed only by their gravity. The hypothesis of the second "mirror" universe was proposed half a century ago, it is a kind of doubling of reality.
We have real observations only from cosmology.
Interviewed by Sergey Kuznetsov
The theoretical construction in physics, called the Standard Model, describes the interactions of all elementary particles known to science. But this is only 5% of the substance existing in the Universe, while the remaining 95% are of a completely unknown nature. What is this hypothetical dark matter and how are scientists trying to detect it? Hayk Hakobyan, a student at the Moscow Institute of Physics and Technology and an employee of the Department of Physics and Astrophysics, talks about this within the framework 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 ordinary particles known to us: leptons, quarks and interaction carriers (bosons and gluons). However, it turns out that all this huge complex theory describes only about 5-6% of all matter, while the rest does not fit into this model. Observations from the earliest moments of the life 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 so far it has not been possible to catch it directly. This phenomenon of hidden mass has been codenamed "dark matter".
Modern science, especially cosmology, works on deductive method Sherlock Holmes
Now the main candidate from the WISP group is the axion, which arises in the theory of strong interaction and has a very small mass. Such a particle is capable of transforming into a photon-photon pair in high magnetic fields, which gives hints of how one can try to detect it. In the ADMX experiment, large chambers are used, where a magnetic field of 80,000 gauss is created (this is 100,000 times more magnetic field Earth). In theory, such a field should stimulate the decay of the axion into a photon-photon pair, which the detectors should catch. Despite numerous attempts, WIMPs, axions or sterile neutrinos have not yet been detected.
Thus, we have traveled through a huge number of different hypotheses that seek to explain the strange presence of a dark mass, and, having rejected everything impossible with the help of observations, we have come to several possible hypotheses that can already be worked with.
A negative result in science is also a result, since it limits the 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 give new, more stringent limits on the mass and other parameters of dark matter particles. Thus, throwing out all the impossible options and narrowing the circle of searches, day by day we are getting closer to understanding what 95% of the matter in our Universe consists of.
