The emergence of dark matter
Sometimes it seems that it is dark matter itself that is taking revenge on scientists for the inattention with which its discovery was met more than 80 years ago. Then, in 1933, the American astronomer of Swiss origin Fritz Zwicky, observing six hundred galaxies in the Coma cluster located 300 million light years from the Milky Way, discovered that the mass of this cluster, determined based on the speed of movement of galaxies, is 50 times greater than the mass calculated by estimating the luminosity of stars.
Not having the slightest idea of what this mass difference is, he gave it the now official definition - dark matter.
For a very long time, very few people were interested in dark matter. Astronomers believed that the problem of hidden mass would be resolved by itself when it was possible to collect more complete information about cosmic gas and very faint stars. The situation began to change only after the American astronomers Vera Rubin and Kent Ford published the results of measurements of the speeds of stars and gas clouds in the large spiral galaxy M31 - the Andromeda nebula in 1970. Against all expectations, it turned out that far from its center, these velocities are approximately constant, which contradicted Newtonian mechanics and received an explanation only on the assumption that the galaxy is surrounded by a large amount of invisible mass.
When you come across a phenomenon about which nothing is known, then a large number of explanations can be attributed to it, and all that remains is to sort through them one by one, sweeping aside the useless ones and inventing new ones along the way. Moreover, it is not a fact that among all these explanations it will be correct. The improper behavior of peripheral stars could be explained by moving in two directions - by slightly correcting Newton's laws or recognizing that there is matter in the world that is different from ours, which we do not see, because the particles of which it is composed do not participate in electromagnetic interaction, then they do not emit light and do not absorb it, interacting with our world only through gravity.
Was Newton wrong?
The first direction, that is, counter-Newtonian correction, developed rather sluggishly. True, in 1983 the Israeli theorist Mordechai Milgrom created the so-called modified Newtonian mechanics, in which small accelerations react to an acting force somewhat differently than the way we were taught in school. This theory found many followers and was soon developed to such an extent that the need for dark matter disappeared. It is noteworthy that Vera Rubin herself, an internationally recognized pioneer in the study of dark matter, has always been inclined towards the modification of Newton's laws - it seems that she simply did not like the idea of a substance that is plentiful, but which no one ever saw.
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The elusive wimp
There are many candidates for dark matter particles, and for most of them there is a generalizing and almost meaningless name "WIMPs" - this is the English abbreviation WIMPs, derived from the term "Weakly Interacting Massive Particles", or "weakly interacting massive particles". In other words, these are particles that participate only in gravitational and weak interactions - its effect extends to dimensions much smaller than the dimensions of the atomic nucleus. It is on the search for these WIMPs as the most suggestive explanation that the main efforts of scientists are directed today.
WIMP detectors, especially those that capture them for xenon, are similar in principle to neutrino traps. At one time it was even believed that the neutrino is the very elusive WIMP. But the mass of this particle turned out to be too small - it is known that 84.5% of all matter in the Universe is dark matter and, according to calculations, there will not be as many neutrinos on this mass.
The principle is simple. Take, say, xenon as the heaviest of the noble gases, cooled to nitrogen temperatures, and preferably lower, protected from any unnecessary "guests" such as cosmic rays, a lot of photocells are installed around the xenon vessel, and this whole system, located deep underground, proceeds to wait. Because you have to wait a long time - according to calculations, the length of a trap with xenon, which will be able to capture a WIMP passing through it with a 50 percent probability, should be 200 light years!
Here, capture is meant either the flight of the wimp near the xenon atom, and the flight at such a distance at which the weak interaction already works, or a direct hit into the nucleus. In the first case, the outer electron of the xenon atom will be knocked out of its orbit, which will be recorded by the change in charge, in the second, it will jump to another level and immediately return “home” with the subsequent emission of a photon, which is then registered by photomultipliers.
Sensation or mistake?
However, “simple” is not quite the right word when applied to WIMP detectors. It is not very easy and very expensive. One of these detectors under the uncomplicated name Xenon was installed in the underground Italian laboratory of Gran Sasso. To date, it has been twice modified and now bears the name Xenon1T. It is thoroughly cleansed of impurities that can lead to signals similar to signals from dark matter. For example, from one of the typical pollutants - the radioactive isotope krypton-85. Its content in commercial xenon is only a few parts per million, but when looking for WIMPs it is utter filth. Therefore, starting with the second modification of the installation - Xenon100 - physicists additionally purify xenon, reducing the concentration of the pollutant to hundreds of parts per trillion.
XENON100 detector
Photo: Wikimedia Commons
And turning on the detector, they, of course, said the cherished "just about." During the first 100-day observation session, scientists recorded as many as three impulses, very similar to the signals from flying WIMPs. They didn’t believe themselves, although they probably really wanted to believe, but it was 2011, already marked by a strong puncture: physicists discovered that neutrinos arriving at them from CERN in the course of another experiment fly at a speed exceeding the speed of light. Scientists, having checked, it seemed, everything that can only be verified, turned to the scientific community with a request to see what was going wrong. Colleagues looked and could not find errors, saying, however, that this could not be, because it could never be. And so it happened: the puncture, as it turned out, was only one connector with a poor contact, which was difficult to notice.
And now, under the weight of such a fiasco, scientists again faced a choice. If these are WIMPS, then this is a guaranteed Nobel Prize, and an immediate one. And if not? The second time they did not want to be dishonored, and they began to check and recheck. As a result, it turned out that two of the three signals may well be parasitic signals from background pollutant atoms, which were not completely eliminated. And the remaining signal did not get into any statistics at all, so the best thing would be to forget about it and not remember any more.
The detector saw "nothing"
Another "just about" sounded when representatives of the collaboration working on the most sensitive dark matter detector LUX (Large Underground Xenon), which is located in an abandoned gold mine in South Dakota, announced that they had changed the detector's calibration. After that, they had a hope, bordering on certainty, that the long-awaited "just about" would finally come true. The LUX detector, which from the very first day of its existence was much more sensitive than the Italian one, is twice as sensitive to severe WIMPs and 20 times as sensitive to lungs.
LUX detector
Photo: Large Underground Xenon detector
During the first 300-day observation session, which began in the summer of 2012 and ended in April 2013, LUX did not see anything, even where it could see something at least out of politeness. As Yale University's Daniel McKinsey, member of the LUX collaboration, said, "We saw nothing, but we saw this 'nothing' better than anyone before us."
As a result of this "nothing", several promising versions were completely discarded at once, especially in relation to "light" WIMPs. Which did not add to the collaboration of sympathizers from among those whose versions were rejected by LUX. Colleagues attacked them with a whole bunch of reproaches for their inability to set up the experiment correctly - the reaction is quite standard and expected.
Physicists know absolutely nothing about the mass of WIMPs - if they exist at all. Now the search is carried out in the mass range from 1 to 100 GeV (the proton mass is about 1 GeV). Many scientists dream of WIMPs with a mass of one hundred protons, because particles with such a mass are predicted by the supersymmetric theory, which in fact has not yet become a theory, but is only a very beautiful, but speculative model and which many predict the fate of the successor to the Standard Model. This would be a real gift for supporters of supersymmetry, especially now, when the experiment at the Large Hadron Collider has not yet registered any of the particles it predicted.
The second observation session on the LUX detector, which will end next year, should, thanks to the calibrations already mentioned at the beginning, seriously increase the detector sensitivity and help to catch wimps of various masses (previously LUX was tuned to the highest sensitivity of about 34 GeV), detecting their signals where they were previously ignored. In other words, next year another and very decisive “just about” awaits us.
If this “just about” does not happen, then it’s also okay: the next LZ detector, which is much more sensitive, is already being prepared to replace LUX. It is expected to be launched several years later. At the same time, the DARWIN collaboration is preparing a "monster" with a capacity of 25 tons of xenon, in front of which LUX, with its 370 kg of gas, seems "blind" and useless for anything. So it looks like wimpam - if they exist - will simply have nowhere to hide, and sooner or later they will make themselves felt. Physicists give them no more than ten years for this.
Wimp or wisp?
If the wimps continue to persist in their elusiveness, then there is still an axion, which should also be chased. Axions are hypothetical particles introduced in 1977 by American physicists Roberto Peccei and Helen Quinn in order to rid quantum chromodynamics of some symmetry breaking. These are, in fact, also Wimps, belonging to the sub-category of lighter wisps (Weakly Interacting Slim Particles), but they have one peculiarity: in a strong magnetic field, they must induce photons, by which they can be easily detected.
Today, few people are interested in axions, and not even because people do not believe in them too much, and not because their registration is associated with some special difficulties, it is just that their search is associated with too great expenses. In order for the axion to start converting virtual photons into real ones, very strong magnetic fields are needed - interestingly, magnets with the required fields already exist. The market offers 18 Tesla magnets, there are experimental 32 Tesla magnets, but these are very expensive machines and not easy to get. In addition, those on whom the funding of such research depends do not really believe in the reality of the existence of axions. Perhaps someday the need to search for axions will make these financial difficulties surmountable, and by that time the magnets may become cheaper.
Despite the seemingly endless and fruitless pursuit of WIMPs, things are actually going well. To begin with, you need to work out the simplest and most obvious version - wimps. When they are found, and their mass is known, physicists will have to think about what these WIMPs are - are they really heavy neutralinos, a quantum set of superpartners of the photon, Z-boson and Higgs boson, as most physicists now assume, or something- something else. If WIMPs are not found in the entire range of possible masses, it will be necessary to consider alternative options - for example, look for WIMPs in other ways. For example, if this is the famous Majorana fermion, which is itself an antiparticle, then, meeting, such fermions should annihilate, turning into radiation and leaving a memory about themselves in the form of an excess of photons.
If there is no way to detect WIMPs, which actually seems unlikely, then it will be possible to take a closer look at the options with modified Newtonian mechanics. It will also be possible to check (it is not yet clear how) a completely fantastic version associated with the seven extra dimensions predicted by string theory, which are hidden from us, since they are curled up into Planck-sized balls. According to some of the models of such multidimensionality, gravitational force penetrates into each of these dimensions and therefore is so weak in our three-dimensional world. However, this raises the possibility that dark matter is hidden in these curled up dimensions and manifests itself only thanks to the omnipresent gravity. There are also exotic explanations for dark matter associated with topological defects of quantum fields,arising during the Big Bang, there is also a hypothesis explaining dark matter by the fractality of space-time, and there is no doubt that, if necessary, theoretical physicists will come up with something else no less original. The most important thing is to add the only correct explanation to this list.
