They are looking for dark matter on Earth, underground and in space. Its mysterious particles are invisible to scientific instruments and do not manifest themselves anywhere. However, a solid “evidence base” has been collected in favor of their existence. Do scientists have a chance of ever discovering dark matter.
A key component of the universe
Particles of dark matter were born shortly after the Big Bang, when the universe was a red-hot plasma. As they cooled, they formed clumps that eventually provided the emergence of stars and galaxies. If the plasma contained only ordinary particles that make up atoms, then the radiation would repel them from each other, not allowing them to form any structures. Gravitationally bound objects appeared quickly enough, which means that something was helping them. Some massive substance held them back. Now it does not interact with ordinary matter in any way, does not radiate, therefore we do not observe it by any methods.
This is how scientists reconstruct the evolution of the Universe, which would be incomplete without the participation of dark matter. This conclusion was reached back in the 1930s by the Swiss astronomer Fritz Zwicky. Studying clusters of galaxies, he wondered why they were not scattering. After all, the mass of the visible galaxies is not enough to hold the cluster. Hence, there must be a hidden mass. Later, this hypothesis found numerous confirmations on the anomalies in the rotation rates of galaxies: the parts of the disks far from the center hardly slow down, as it would be if they consisted only of stars.
Gravitational lensing allows to indirectly capture the presence of hidden mass. This effect is created by two massive galaxies located one behind the other. Light from a distant galaxy is bent by the gravitational field of a nearby one, and, as in a lens, its image appears. This provides some insight into the dark matter in galaxies forming a huge, invisible halo around them. Using various models, scientists calculate the density distribution of dark matter in the halo and, on this basis, make guesses about the structure.
On the left - halo of dark matter in the galaxy NGC 4555.
Promotional video:
Dark matter composition
Physicists are inclined to believe that dark matter consists of particles unknown to us.
“Astrophysical methods of observation do not say anything about their properties. It is possible that they do not interact in any way, except for the gravitational method. Maybe neither direct experiments on Earth, nor observations in space will lead to anything. This must always be borne in mind,”says Dmitry Gorbunov, Corresponding Member of the Russian Academy of Sciences, Chief Researcher of the Institute for Nuclear Research, Russian Academy of Sciences.
Candidates for the role of dark particles include ultralight axions, weakly interacting particles (WIMPs), and sterile neutrinos that help explain the mass and oscillation of solar neutrinos.
“The lightest sterile neutrino may well be a particle of dark matter. It is not stable, but it lives for a very long time. In the Galaxy, such particles should decay into neutrinos and a photon. They spin slowly (10-3 times the speed of light), so a peak in the X-ray range is expected in the photon spectrum,”says the scientist.
According to him, a good spectrometer should be sent into orbit to try to register such events.
Two years ago, Gorbunov and his colleagues modeled one of the hypotheses about an unstable component of dark matter to explain the discrepancy in the results of the Planck space telescope experiment, which measured the relic radiation. Perhaps this was a mistake, or perhaps an indication of some property of dark matter. Scientists have suggested that the dark substance is heterogeneous in composition and part of it has not survived to this day.
In search of dark particles
How to capture dark matter particles is one of the key questions in physics. Many theorists and experimenters are trying to answer it. The way of observation depends on the model, in which all the properties of the hypothetical particle are laid. If we assume that dark matter was in equilibrium in the plasma of the early Universe - and there were ordinary particles as well - it means that it somehow interacts with them. Of all the known types of interactions, except for the gravitational one, the most suitable is the weak one, which occurs during the beta decay of an atomic nucleus.
“Under this assumption, after the primary plasma cools down, the required amount of dark matter remains,” explains Dmitry Gorbunov.
Based on this, dark particles can be destroyed with the formation of an electron and a positron. They are looking for traces of these annihilations, but this is in any case circumstantial evidence. In addition, the results are rather fuzzy, particles are deflected, flying around the Galaxy, annihilating, losing energy, and what reaches the Earth is difficult to distinguish against the background of cosmic rays.
They try to observe dark particles directly in underground detectors that register neutrinos. Under the ground, the background from atmospheric particles decreases, the detector substance cools, and you need to wait for a dark matter particle to hit it. These events are rare in themselves, because if a particle interacts, it is weak. The impact causes excitation of the atom and a burst of energy, which is recorded by the detector.
At the same time, it is impossible to infinitely increase the volume of the detector substance in order to increase the probability of the passage of dark particles without loss of sensitivity. In addition, neutrinos interfere with the signal. To cut it off, you may have to build a completely new detector to go below this signal.
“It is necessary to use the detection of the direction of the particle impact. This will significantly suppress the background, because neutrinos fly in the direction from the Sun, and dark matter will strike in other directions,”the scientist specifies.
The third direction is the creation of a dark matter particle as a result of the collision of ordinary particles at the LHC and other accelerators. To the observer, it will look like, for example, a photon flying away to the side. According to the law of conservation of momentum, a particle should also fly out in the other direction, but there is none. So she's invisible.
So far, none of the ways to catch dark matter particles have been successful. It is not even clear which of them is the most promising.
Composition of the Universe / Illustration by RIA Novosti.
Tatiana Pichugina
