Dark matter may be trapped by black holes

It may turn out that the elusive substance does not consist of any new particles.

When on February 11, 2016, the speaker of the aLIGO (Advanced Laser Interferometric Gravitational Wave Observer) project announced the detection of gravitational waves, I was amazed. Of course, we expected that aLIGO at some point would give us something interesting, but we thought about the preliminary results. We believed that the project after complex and difficult calculations for several months will give us a kind of weak signal, a little rising above the noise.

But no, the graphs shown on that fateful day of February were so clear and unambiguous that there was nothing to prove. With my own eyes, I could see a waveform that could not be confused with anything — this is the merging of two black holes, as a result of which gravitational waves set off into the surrounding space-time.
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And that was not all. The black holes seen by aLIGO should not have existed at all. We know about the existence of black holes with masses a million or a trillion times greater than the mass of the Sun, and we saw small black holes with a mass comparable to that of the sun. But the mass of black holes seen by aLIGO was 30-60 times more than solar. Some of my colleagues claim that the medium-sized black holes found by aLIGO may turn out to be the same dark matter that has been hidden from us for almost 50 years.

Not for the first time, scientists have suggested that black holes may be dark matter, but we thought that this possibility was unequivocally rejected. The resurrection of this idea is another example of the rich creative activity that arises after a new discovery. Ideas that have gone out of fashion can return to it if you look at them in a new light and with enthusiasm - and even replace accepted points of view. The revision of discoveries also brings at first glance incomparable areas of research — in our case, dark matter and gravitational waves — and leads to fruitful connections.

In the 1970s, Stephen Hawking and his graduate student Bernard Carr suggested that out of the chaos that had unfolded after the Big Bang, a sea of ​​tiny primary black holes could have appeared. Over time, they could grow and become the basis for the formation of galaxies. They could even contribute to the overall energy budget of the universe. Black holes are heavy and difficult to see - just these properties we need to explain the missing matter of the Universe.

For several decades, loyal supporters of this idea developed it. In the 1990s, she went through a seemingly fatal blow. In the MACHO experiment, scientists sent a telescope to the Large Magellanic Cloud in search of a flicker, which would mean that a black hole passed in front of the star. They found that it would be very difficult to collect enough black holes in order to write off all the dark matter of the universe on them.

Later, Timothy Brandt from the Institute of Advanced Studies in Princeton studied the effect of black holes on dense agglomerations of stars known as globular clusters living in dwarf galaxies hiding in the void around the Milky Way. He showed that in the event of an excess of black holes, these clusters would warm up, swell and quickly die. Substituting specific values ​​for a specific 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 black holes. In this connection, the idea of ​​black holes, playing the role of dark matter, has become another exotic idea that theorists like to play with, but without real support in nature.

The search for dark matter focused on weakly interacting massive particles, WIMP . These are fundamental particles, relics from the earliest times, when fundamental interactions in nature were combined and behaved in a completely different way than they are now. For many of my colleagues, WIMP discovery is inevitable; they must exist. As soon as we build a sufficiently large and powerful tool, then, according to most cosmologists, we will inevitably see these strange particles.

That's just not happening. Over time, our detectors became more powerful and more, but they did not find anything. In a recent experiment, LUX , looking for rare particles that leave their energy in half a ton of liquid xenon buried a kilometer under the ground in the town of Lid in South Dakota, failed to demonstrate evidence of previously unseen particles. Richard Geitskell of Brown University, one of the creators of LUX, said: “It would be wonderful if the improved sensitivity would allow us to see a clear signal of dark matter. However, what we observe corresponds only to the background. ”

Given the desperate situation of WIMP, it makes sense to raise some old, speculative, rejected ideas. In two recent works, one of which was headed by Simeon Bird from the University. John Hopkins, and the other, Misao Sasaki of Yukawa University in Tokyo, did just that.

Spurred on by the discovery of aLIGO, they worked out the question of whether black holes with a mass of several dozens of sun can be dark matter. There should have been 10 billion of such holes in the Milky Way, and the closest of them may be a few light years from our solar system. Some of them had to form binary systems, and some of such systems could be detected by aLIGO. Two teams agree that aLIGO should spot from a few to a few dozen such events a year, and they should prevail over other black holes that appeared in a way such as the collapse of a star. In other words, if these black holes are galactic dark matter, we can expect aLIGO to see them. And he saw them.

The devil is in the details. How prehistoric black holes should have appeared - the question is still open. One of the ideas is that they arose in a short period of accelerated expansion of the early Universe, during inflation. The shocks and vibrations of that period should have concentrated the energy in dense balls, which would give rise to the formation of black holes. In order for us to detect them, these holes must come close enough to merge and emit gravitational waves. How and when this happens depends on the shape of the Milky Way, the density of the mass and the speed of movement of black holes. Reasonable assumptions give a promising answer, but this is still an assumption.

These are only the first steps in the field, following the euphoria of opening aLIGO, and anything can happen. The limitations of the MACHO experiment and the physics of globular clusters work against this idea, but some ingenious thought can solve all the problems associated with observations.

The discovery of aLIGO reminds me of another transformation that I have watched in my career. In 1991, the COBE satellite for the first time measured the relic radiation waves left over from the Big Bang. The disappointing and almost quixotic search for this radiation went more than 25 years, and almost moved to the provincial areas of cosmology. In itself, cosmology seemed an esoteric and hard-to-describe science, a vague, though very interesting and creative topic. But when the radiation was still found, it gave rise to an avalanche of ideas, not only for astronomy, but also for particle physics.

For decades, we are trying to connect the fundamental laws of nature that governed the early Universe with how galaxies appeared to evolve and form large-scale structures visible today. The discovery of COBE has guided me to the path that I have followed to this day, and I can imagine how aLIGO will do the same with the new generation of physicists in their search for dark matter.