Dark matter can be distributed over black holes

Scientists are not the first time suggest that black holes may consist of dark matter, but it was believed that this possibility is already excluded . The restoration of an idea is one of the examples of creativity following a new discovery.



When on February 11, 2016, a representative of the aLIGO project [Advanced Laser Interferometric Gravitational Wave Observer - the Advanced Laser Interferometric Gravitational Wave Observatory] announced the discovery of gravitational waves, I was amazed. Of course, we expected that at some point aLIGO would give something interesting, but we thought it would be some cautious assumption. We thought that after many months and even years of data processing, the project will be able to show us a weak signal, barely rising above the noise level.



But no, the charts shown on that fateful day of February were so clear and unambiguous that I didn’t have to convince me of anything. To the naked eye, I could see an incomparable wave pattern of the collision of two large black holes merging into one and emitting gravitational waves as a result into the surrounding space.



And that is not all. ALIGO saw black holes, which should not have been there. We knew about the existence of black holes with masses of a million or trillion times the size of a solar one, and we saw smaller black holes with a mass comparable to that of a solar one. But aLIGO saw BHs that were 30–60 times heavier than the Sun. Some of my colleagues now state that medium-sized BHs, discovered by aLIGO, may turn out to be the same dark matter that has been hiding from us for almost 50 years.

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Scientists are not the first time suggest that black holes may consist of dark matter, but it was believed that this possibility is already excluded . The restoration of an idea is one of the examples of creativity following a new discovery. Ideas that have gone out of fashion can return to it, they can be looked at in a new light and begin to work with enthusiasm - and this, in some cases, replaces the generally accepted opinion. Such revision discoveries sometimes bring together very different areas of research — in our case, dark matter and gravitational waves — and lead to the discovery of fruitful bonds.



In the 1970s, Stephen Hawking and his graduate student, Bernard Carr, suggested that out of the chaos following the Big Bang, a whole sea of ​​tiny prehistoric black holes could appear, and then populate space. Over time, these BHs would grow, serving as seeds of future galaxies. They probably could even contribute to the total energy budget of the Universe. BHs are heavy and difficult to detect, and it is these properties that the missing matter of the Universe can possess.



For several decades, a group of active supporters of this idea developed it. In the 1990s, she seemed to take a mortal blow. The MACHO experiment sent a telescope to the Large Magellanic Cloud and monitored a weak flicker that could occur if an object such as a black hole passes in front of a star. They found that it would be very difficult to type BH enough for them to answer for all the dark matter of the Universe.



Later, Timothy Brandt from the Institute of Advanced Studies in Princeton found out what exactly BHs can do with dense agglomerations of stars, known as globular clusters, existing in dwarf galaxies, that hide in the emptiness around the Milky Way. He showed that if there were too many BHs, these globular clusters would have warmed up, swelled and died very quickly. Substituting the required amount for a certain cluster in the dwarf galaxy Eridan II, he was able to show that only a small part of dark matter can exist in the form of BH. Therefore, BH and TM have become another exotic idea that theorists love to play with, but which is not reflected in the real world.



Instead, the TM quest focused on Wimps [Weakly Interacting Massive Particles, WIMPS]. These are fundamental particles left over from very early times, when the fundamental interactions of nature were combined and behaved quite differently from today. For many of my colleagues, the discovery of wimps is an inevitable event; they must exist. And most cosmologists believe that as soon as we build a powerful enough and large tool for detecting them, we will finally see these strange particles.



That's just this has not happened yet. Over time, the detectors became more powerful and larger, but found nothing. A recent LUX experiment, looking for rare particles that donate their energy to half a ton of liquid xenon, located a mile and a half underground in the city of Lead, South Dakota, could not provide any evidence of the existence of particles not found until now. Richard Geytskel of Brown University, one of the creators of LUX, said: “It would be wonderful if the improvement in sensitivity gave us a clear signal of the presence of dark matter. However, what we observe corresponds to only one background. ”



Given the desperate situation in which the wimps found themselves, it makes sense to return to the old, speculative, rejected ideas that were lying about without moving. Two recent works, one of which was headed by Simeon Bird from Johns Hopkins University, and the other by Misao Sasaki from Yukawa University in Tokyo, are doing just that.



Having received a stimulus in the form of the opening of aLIGO, they worked out the question of whether a BH of several dozen solar masses can be dark matter. There should be about 10 billion of such BHs in the Milky Way, and the closest one is most likely just a few light years from the solar system. Some of these BHs could come together and form binary systems, and some of these systems could be detected with aLIGO. Two teams agree that aLIGO should see from a few pieces to a few dozen such events a year; They should be much more than the usual ways of the BH, for example, by the collapse of stars. In other words, if these black holes are the dark matter of the galaxy, one would expect us to see them in the aLIGO experiment. And we saw them.



The devil is in the details. How prehistoric BH could appear - the question is still open. One of the ideas is that they appeared in a short period of accelerated expansion of the early Universe, known as inflation . The jerks and tremors of such an expansion would concentrate energy in dense pieces, which would lead to the emergence of BH. In order for us to detect them, these BHs must meet and merge, propagating gravitational waves. How and when this will occur depends on the shape of the Milky Way, the density of its mass accumulation and the speed of the BH movement. Reasonable assumptions give a promising answer, but for now it is still assumptions.



So far this is the first time in this field of research, which followed the euphoria of discovery on aLIGO, and anything can happen. The limitations imposed by the MACHO experiment and globular clustering work against this idea, but some tricks can solve the problems generated by observation.



The discovery of aLIGO reminds me of another transformation that I watched during my career. In 1991, the COBE satellite for the first time measured ripples in relic radiation left over from the Big Bang. Full of frustration and almost quixotic search for these waves has been going on for more than 25 years, it was practically transferred to the provincial parts of cosmology. Cosmology itself seemed esoteric and difficult to concretize, a vague area, although very interesting and creative. But when this ripple was finally found, it charged cosmology with a shaft of ideas relating not only to astronomy, but also to particle physics.



For several decades now, we have been trying to link the fundamental laws of nature that worked in the early Universe with the ways in which the galaxies emerge, evolve and die, which have resulted in today's large-scale structures. And if COBE has guided me to the path that I follow to this day, then I can see how aLIGO can do the same with the new generation of physicists in their search for dark matter.



Pedro Ferreira is a cosmological theorist, an astrophysicist from the University of Oxford, a member of Oriel College. Works on the origin of large-scale structures of the Universe, the general theory of relativity and the nature of dark matter and dark energy. His recent book The Perfect Theory is a biography of the general theory of relativity. She was nominated for book prizes from the royal scientific society and the award "book of the year from the field of physics."