The appearance of dark matter
Sometimes it seems that it is the very dark matter researchers revenge for inattention, which was met with its opening over 80 years ago. Then, in 1933, the American astronomer Swiss-born Fritz Zwicky, watching six hundred galaxies located 300 million light-years from the Milky Way cluster Coma found that the mass of this cluster, determined based on the speed of the galaxies 50 times the mass of calculated by estimating the luminosity of stars.
Not having the slightest idea of what is the difference of the masses, he gave her which has now become an official definition – dark matter.
A very long dark matter very few people interested. Astronomers believe that the problem of hidden mass resolve itself when able to collect more complete information about the cosmic gas and very faint stars. The situation began to change only after the 1970 American astronomers Vera Rubin and Kent Ford published the results of measurements of the velocities of stars and gas clouds large spiral galaxy M31 – the Andromeda nebula. Against all expectations it turned out that the distance from its center, these velocities are approximately constant, contrary to Newtonian mechanics and is explained only on the assumption that the galaxy is surrounded by a large number of invisible mass.
When faced with the phenomenon, which does not It is known, then it can be attributed to a large number of explanations, and there is only through them one by one, dismissing unsuitable and coming up with new ways of. And not the fact that, among all of these explanations may be correct. Misconduct peripheral stars could be explained, moving in two directions – slightly correcting Newton’s laws or recognizing that the world is different from our mother, we do not see, because the particles of which it is not involved in the electromagnetic interaction, the there do not emit light and do not absorb it, interacting with our world only through gravity.
Was Newton wrong?
The first direction, that is kontrnyutonovskaya adjustment, developed rather sluggishly. However, in 1983 the Israeli theorist Mordechai Milgrom created a so-called modified Newtonian mechanics, in which the small acceleration of responding to the impact strength somewhat differently than the way we were taught in school. This theory has found many followers and soon was developed to such an extent that the need for dark matter has disappeared. It is noteworthy that the very Vera Rubin, an internationally recognized pioneer in the study of dark matter, has always tended precisely to modify Newtonian laws – it seems she just did not like the idea of substance, which is a lot, but that is because no one saw.
The elusive WIMP ±
Candidates on the particles of dark matter a lot, and most of them have generalized and almost nothing telling name “WIMPs” – is the English abbreviation WIMPs, derived from the term «Weakly Interacting Massive Particles», or “weakly interacting massive particles.” In other words, it is the particles involved only in the gravitational and weak interaction – it applies to the much smaller size than the size of the atomic nucleus. It was in search of WIMPs thing coming to mind as an explanation and sent the main efforts of scientists today.
WIMP detectors, especially those where they are caught in the xenon, in its principle similar to trap neutrinos. At one time even it thought that the neutrino is the most elusive WIMP ±. But the mass of the particles was too small – we know that 84.5% of all matter in the universe account for dark matter, and it is estimated that as many neutrinos to this mass is not reached.
But the principle is simple. Take, for example, xenon as the heaviest noble gas is cooled to liquid-nitrogen temperature, and preferably below, guarded against any unwanted “guests” such as cosmic rays around the vessel with xenon established set of solar cells, and the whole system is located deep underground , starts to wait. Because the wait should be a long time – according to calculations,
length of the trap with xenon, which can with a 50 percent chance to capture flown through it WIMP shall be 200 light years!
Under the capture here means any flight WIMP near the xenon atom, the flight at such a distance, in which the weak interaction is already running, or a direct hit to the core. In the first case the outer electron of the xenon atom is knocked out of its orbit, which will be registered on the change in the charge in the second – it will jump to another level and just return “home”, followed by the release of a photon, which is then registered by photomultipliers.
Sensation or error?
However, “just” – this is not quite the word when applied to WIMP detectors. It is not very easy and very expensive. One of these detectors under the straightforward title Xenon been installed in the underground of the Italian Gran Sasso laboratory. To date, he has twice been modified and is now named Xenon1T. He carefully cleaned of impurities that can lead to the signals, like the signals of dark matter. For example, one of the most common pollutants – radioactive isotope krypton-85. Its content in the commercial xenon is only a few parts per million, but when searching for WIMPs is utter filth. Therefore, since the second modification of the installation – Xenon100 – Physics is further purified xenon, reducing pollutant concentration to hundreds of parts per trillion.
and turn on the detector, they certainly cherished said “is about.” During the first session of the hundred-day observations, scientists recorded three pulse very similar to signals from WIMPs flying. They themselves did not believe, though, probably wanted to believe, but 2011 was already marked by a strong thrust physicists discovered that neutrinos arriving from CERN to them in the course of another experiment to fly faster than the speed of light. Scientists checked, it seemed, all that is possible to check, addressed to the scientific community to see what is going wrong. Colleagues looked and could not find the error, saying, however,
that it can not be, because it can never be. And so it happened: the puncture, as it turned out, consisted of only a single connector with poor contact that it was hard to notice.
And now, under the weight of this fiasco, scientists once again stood in front of goal. If that WIMPs, it guaranteed a Nobel Prize, and immediate. And if not? The second time did not want to disgrace, and they began to check and recheck. The result was that two of the three signals may well be spurious signals from background atoms polluters, from which up to the end and could not get rid of. And the remaining signal is already quite in any statistics did not get, so that for him the best would be to forget and forget.
Detector saw “nothing»
Another “is about “sounded when the representatives of the collaboration working at the now dark matter detector sensitivity LUX (Large Underground Xenon), which is located in an abandoned gold mine in South Dakota, said that the change in the calibration of the detector. After that they had a confidence bordering on the hope that the long-awaited “is about” finally come true. Detector LUX, which from the very first day of its existence was much more sensitive than the Italian, twice as sensitive to heavy WIMPs and 20 times more sensitive to light.
For the first 300-day session of observations, which began in the summer of 2012 and ended in April 2013th, LUX saw nothing even where there is at least out of courtesy could see anything. As stated by a member of the collaboration LUX Daniel McKinsey from Yale University, “we did not see anything, but we saw this” nothing “is better than anyone before us».
As a result of this “nothing” was flatly rejected Several promising versions, especially with regard to “light” WIMPs. What has not added collaboration sympathizers from among those whose release has rejected LUX. Colleagues rushed at them with a whole bunch of accusations in the inability to properly put the experiment – reaction is the standard and expected.
As for the mass of WIMPs – if they exist – physicists do not know absolutely nothing. Now the search is conducted in the mass range from 1 to 100 GeV (the mass of a proton is about 1 GeV). Many scientists dream of WIMPs with a mass of a hundred protons as particles with such a mass is predicted by supersymmetry theory, which is in fact the theory was not yet, but is only very beautiful, but the speculative model and is widely tipped the fate of the successor of the standard model. It would be a real gift for advocates of supersymmetry, especially now, when the experiment at the Large Hadron Collider has not yet registered any particles predicted by it.
The second session of the observations on the detector LUX, which will end next year, due to the already mentioned should be at the beginning of calibrations significantly increase the sensitivity of the detector and help
catch WIMPs very different masses (LUX previously been set to the highest sensitivity around 34 GeV) by detecting their signals where they first were ignored. In other words, next year we will have another very strong “is about».
If this “is about” and did not happen, too, that’s okay: replace LUX already preparing next detector LZ, is much more sensitive. It is expected that within a few years it will be launched. At the same time collaboration DARWIN preparing “monster” with a capacity of 25 tons of xenon to whom LUX with its 370 kg of gas seems to be “blind” and on what does not fit the device. So it seems, WIMPs – if they exist – will simply have nowhere to hide, and sooner or later they will make themselves known. Physicists give them this is not more than ten years.
WIMPs or Visp?
If WIMPs will continue to persist in its elusiveness, then there is the axion, which was also ought to chase. Axions – hypothetical particles introduced in 1977 by American physicists Roberto Peccei and Helen Quinn, to deliver quantum chromodynamics some symmetry breaking. It is, in fact, too, WIMPs, relating to subclass a light wisps (Weakly Interacting Slim Particles), but they have one feature: a strong magnetic field, they should induce the photons, which can be easy to spot.
Today, few people are interested in axions, and not even because people do not believe in them too, and not because their registration is associated with some special challenges, just to find them associated with too high costs. To Axion began to turn virtual photons into real, we need a very strong magnetic fields – that is interesting, magnets with the desired fields already exist. The market offers a 18-teslovye magnets, there are experimental Tesla magnets 32, but it is very expensive cars, and they are not easy to acquire. Besides those on whom the funding of such research, really do not have much faith in the reality of the existence of axions. Perhaps someday need to find axions will make these financial difficulties can be overcome, and the time and magnets can fall in price.
Despite the fact that the pursuit of WIMPs seems endless and fruitless, in fact everything goes as it should. First you need to work out the simplest and most begs version – WIMPs. When they are found, and their mass is known to physicists have to think about the fact that these WIMPs represent a – is this really heavy neutralino, a set of quantum super- partners of the photon, Z-boson and the Higgs boson, as of now assumes the majority of physicists, or something something else. If WIMPs do not find in the entire range of possible masses, it will be necessary to consider alternatives – for example,
search for WIMPs in other ways. For example, if it is known fermion Majorana, which for itself is an antiparticle, it met such fermions must annihilate, turning into light, leaving a memory in the form of excess photons.
If you really no way to detect WIMPs would not that in fact it seems unlikely that it will be possible to consider options closer to a modified Newtonian mechanics. You can also check to be (it is not clear how) absolutely fantastic options related to string theory predicted seven extra dimensions, which are hidden from us, since turned into a glomerulus Planck size. According to some of the models of such a multi-dimensional, the gravitational force penetrates into each of these measurements, and therefore so weak in our three-dimensional world. However, there is a possibility that dark matter is hidden in these dimensions curled and manifests itself only by the omnipresent gravity. There are exotic explanation of dark matter associated with topological defects quantum field that arose during the Big Bang, there is also a hypothesis to explain the dark matter fractal space-time, and there is no doubt that, if necessary, theoretical physicists have come up with something that is not less than original. The most important thing – to add to this list the only true explanation.
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