Do we ever know what dark matter is?

The quest for elusive matter approaches its limit.

For most astronomers, dark matter is as substantial as stars and planets. We routinely build its distribution maps. We imagine galaxies as pieces of dark matter with many inclusions of luminous matter. We understand the formation of the cosmic structure and the evolution of the entire Universe as a whole from the point of view of dark matter. However, over the decade of the most complicated searches, no one was able to detect dark matter directly. We see the shadow cast by her, but we have no idea what might be hiding in the dark half of the Universe.



These are definitely not ordinary objects or particles - this option has long been excluded. Theoretical arguments speak in favor of a new type of particles interacting weakly with ordinary matter. A huge number of such particles must pass through our planet at each moment of time, and one should expect that one of them should leave a mark. Physicists grew crystals, filled cryogenic tanks, buried them deep underground to eliminate ordinary particles, and looked for tiny impulses of heat and flashes of light that were supposed to give out something that we had not seen before. And so far the results are not encouraging. In the city of Lid in South Dakota, the LUX experiment runs a mile and a half underground in an abandoned gold mine. And found nothing. In China, the PandaX experiment in the underground laboratory Jin-Ping works in a tunnel under a 2.4 km thick layer of stone. He found nothing. In the road tunnel near Frejus in the French Alps, the EDELWEISS experiment, operating at a depth of 1.7 km, did not find anything. This list can be continued.







Zero results quickly narrow areas of the parameter space in which dark matter can hide. Due to the acute lack of data, theoretical physicists began to put forward theories about even more exotic particles, but most of these candidates would be even harder to detect. One could hope instead to get dark matter particles at the particle accelerator, and thus make a conclusion about their presence: looking, whether the energy was lost in the collisions of the particles. But the Large Hadron Collider was doing just that, and so far has not found anything like that. Some theorists suspect that there is no dark matter, and our theory of gravity - Einstein's general theory of relativity - led us astray. The GTR tells us that the galaxies would have scattered if they were not kept together by invisible matter, but perhaps this theory is mistaken. However, the GRT passed all observable tests, and all competing theories have fatal flaws.



Eighty-five percent of all matter is unknown to us. Most of all, we fear that it will always be so.



Although most of the experiments gave nothing, two of them claim to have discovered dark matter. Both statements are extremely contradictory, but for different reasons. They may be wrong, but deserve careful consideration. These cases, at least, demonstrate the difficulty of searching for dark matter among the placers of the matter of space.



The DAMA / LIBRA particle detector at the Gran Sasso National Laboratory, located in a 1.4 km tunnel under the surface of a mountain in northern Italy, searches for flashes of light generated by dark matter particles scattered from atomic nuclei in a sodium iodide crystal. He has been collecting data for thirteen years and has registered something unusual. The number of particle detections seasonally increases and decreases; the maximum is in June, and the minimum is in December.



This is exactly the behavior that can be expected from dark matter. It is believed that it forms a vast cloud enveloping the Milky Way galaxy. Our solar system as a whole is moving through this cloud. But individual planets move through a cloud at different speeds due to their orbital motion around the sun. The speed of the Earth relative to the estimated cloud experiences a maximum in June and a minimum in December. This would determine the speed at which dark matter particles fly through a detector located on Earth.



No one denies that DAMA detects seasonal modulation with very large statistical significance. But many other sources of particles also fluctuate due to the seasons — for example, groundwater flows (affecting background radioactivity) or the production of particles such as muons in the atmosphere. At last count, five other experiments around the world declare restrictions that are inconsistent with DAMA statements. The only way to verify the results is to repeat the experiment with the same detector in other places, and several such experiments are already being prepared. One of them will be located at the South Pole, where seasonal local effects are out of phase and different from those in Italy.



The second intriguing hint of dark matter came from indirect experiments, looking for not elusive particles directly, but secondary particles, which they should have generated when they collide with each other and then annihilate. In 2008, the PAMELA detector (Payload for Antimatter / Matter Exploration and Light-nuclei Astrophysics), installed on the Russian satellite Resurs-DK , and created by experts from Russia, Italy, Germany and Sweden, observed a surprisingly large the number of positrons - analogs of electrons in antimatter - coming from the depths of space. The observation recently confirmed the magnetic alpha spectrometer , located on board the ISS. Meanwhile, the Fermi space gamma telescope reported a diffuse gamma-ray emission from the center of the galaxy. Its shape corresponds to dark matter — spherically symmetrical about the center of the galaxy, with intensity increasing towards the middle.



It is almost too good to be true. Unfortunately, observations of positrons and gamma rays can also be explained by rapidly rotating neutron stars, millisecond pulsars. Positron parameters do not match suitable candidates for dark matter. In order to deal with this case, it is necessary to check whether positrons from directions from known neutron stars do not come. Gamma-ray fluctuations have already been attributed to the multitude of weak pulsars located in the center of the Galaxy. Also, if gamma rays came from dark matter, astronomers would have to detect a similar signal coming from nearby dwarf galaxies, which have a proportionately larger amount of dark matter than ours. No such signals were detected.



Most search attempts focus on the simplest particle candidates, known as wimps , weakly interacting massive particles. The word "weak" here has a double meaning: the interaction is not strong, and occurs through so-called. weak nuclear interaction . Such particles are a natural extension of the Standard Model in particle physics. Even without knowing all the details, from the adverb “weakly” one can understand how many such particles should be in the Universe. In the hot prehistoric soup of the Big Bang, particles were naturally created and destroyed. With the expansion of the universe, the temperature drops, and various types of particles, one after the other, depending on the mass, cease to appear. Particles can continue to be destroyed at a rate that depends on the strength of the interaction, until they are distributed too rarely to collide with each other.



Given the strength of the WIMP interaction, it is possible to make calculations and find that an observable amount of dark matter should have appeared in the cauldron of the early Universe. The resulting particles must weigh hundreds of times larger than a proton. From the calculations associated with the Standard Model and supersymmetry, the existence of a suitable zone of parameters for dark matter particles follows — this fact has been called the “wimp miracle”.



But, probably, this is the case when an ugly fact kills an excellent hypothesis. Despair is growing among physicists, and they are already exploring options that were previously considered second-rate and unlikely possibilities.



Perhaps dark matter particles are extremely massive. There is a natural compromise - the more massive the particle, the less they need to match the total mass observed by astronomers, so there may be so few that our detectors do not notice them. Physicists will need a completely different search strategy, possibly related to the influence of these particles on old neutron stars or other celestial objects.



Conversely, dark matter particles may be too light to leave traces in our detectors. For their search, physicists can use the detector already available to us: the Sun. The sun can capture particles as it moves through the galactic cloud of dark matter. Particles can be scattered on protons in the sun and change its temperature portrait. This will affect the turbulent motion of gas vortices, rising, falling and twisting in the upper layers of the sun. And we need to detect this with the help of helioseismology , the science that studies the disturbances that propagate inside the Sun and their influence on its surface - just like seismology studies earthquakes. It turns out that there are unexplained anomalies in helioseismology, which are difficult to reconcile with the standard model of the Sun.



If dark matter particles accumulate in the sun, they can annihilate in its core. This will lead to the emergence of high-energy neutrinos that can be seen by detectors such as Super-Kamiokande in central Japan and the IceCube at the South Pole. So far, there have been no reports of events suitable for this role.



The most extreme example of a lightweight particle is an axion , a hypothetical weakly interacting particle, with a mass of a trillion times smaller than that of a proton. It will not be completely dark, but will interact with the electromagnetic field and will be able to create microwave photons inside the hollows of strong magnetic fields. Experiments that are trying to detect an axion have been working since the 1980s, and they have no more success than wimp detectors.



Perhaps a dark particle is not a particle at all, but a “non-particle”, as one theorist said. Non-particles are distant relatives of the electromagnetic field, whose energy is not divided into separate packets. They can leave indirect traces in the collider data. Perhaps for the essence of dark matter there is no one solution. After all, ordinary matter also consists of many types of particles. Dark matter can also consist of several participants, making it difficult to search, because the expected signs of any particular candidate particles will be eroded. Perhaps dark matter does not interact at all, except gravity. This will further bring the life of the experimenters closer to the nightmare.



In a sense, we are in a situation that scientists are dreaming of. Old ideas don't work, and new ones are required. They can appear due to the study of new types of particles, or we can discover a new consistent theory of gravity, which allows us to abandon dark matter.



But the constant concern is that nature has hidden the new physics where we cannot find it. And although we have not yet fully exhausted the attempts to find wimps, experiments are not capable of much more. The more sensitive they become to dark matter, the more sensitive they are to trash particles, and they cannot always distinguish one from the other. With the current speed of development in ten years they will be blinded by neutrinos emitted by the Sun, or by cosmic rays colliding with the atmosphere of the Earth.





The sun can be a natural detector of dark matter. Changes in the structure of the layers of the sun under the influence of dark matter can be detected by astronomers. The picture in red shows the areas that are moving away from us, and the blue ones are approaching.



In this case, we can still continue to attempt indirect detection. One of the most promising is the array of Cherenkov telescopes , a collection of more than a hundred telescopes located in Chile and on the island of Palma . Among other tasks, he will search for gamma rays appearing in the annihilation of dark matter particles in our and other galaxies. But at some point this search strategy will face another problem: cost. So far, dark matter detectors belong to the most economical of the main physical experiments, but if we need to increase their size, sensitivity and complexity, their cost may exceed such monsters as the Large Hadron Collider (almost $ 7 billion) and the telescope. James Webb (about $ 8 billion), without any guarantee of success - and it is very difficult to sell to politicians.



The best tool for discovering dark matter particles would be a new collider. Somewhere in three decades, physicists plan to build a collider that exceeds the LHC in power several times. Research is underway in both China and Europe. Roughly, it will cost $ 25 billion dollars today. This can be real if you distribute the load over time and among several countries. But this is likely to be the limit. Even if physicists had unlimited resources, there would be no gains from building something larger. Further, any unknown particle will be so massive that the Big Bang simply could not generate enough of them.



Despite all these incredible attempts, we may not detect the signals. This is a rather dismal prospect. Perhaps there is no dark matter. We continue to look for deviations from GR. So far, none have been found. Conversely, the detection of black holes through gravitational waves in 2016 supported Einstein's theory - and, as a result, the existence of dark matter.



But there are also positive sides. There may be striking secrets and discoveries related to the dark side of nature, which we would never have stumbled upon if it were not for these searches. While we are looking for particles. And we have no choice but to go further.



Joseph Silk is a cosmologist from the University of Oxford, also working at the Institute of Astrophysics in Paris and the University. John hopkins Pioneer research in the field of relic radiation and the formation of space structure.