Scientists at the University of Amsterdam and the University of California have revealed that the mysterious diffuse gamma-ray radiation emitted by the center of the Milky Way can be generated by exotic objects — light primary black holes that make up most or all of dark matter. The scientists ‘ article, which discusses the possibility of direct observation of Hawking radiation from these holes, is published in the journal Physical Review Letters, a preprint of the study is available on the website arXiv.org.
One of the candidates for the role of dark matter may be light primary black holes (PBH) formed in the early Universe. The mass of PBH, which can make up a different fraction of all the dark matter in the universe, can vary from 10 to the 16th degree of grams to 10 to the 35th degree of grams. However, in this range, there is a “window” of masses-from 10 to the 17th degree to 10 to the 22nd degree of grams-that can have PBH that make up all of the dark matter. Such black holes are comparable in mass to large asteroids.
It is believed that any black hole evaporates by spontaneous Hawking radiation-a stream of particles that originally appeared as part of pairs of virtual particles almost at the event horizon. Such pairs are the result of a quantum fluctuation in a vacuum and they usually annihilate quickly. However, the proximity of the event horizon causes one of the particles in the pair to disappear beyond the event horizon, while the other is carried away, generating Hawking radiation. The smaller the black hole, the more it evaporates, so the PBH should be particularly noticeable.
The existence of PBH is not proven, but the researchers in their new work were able to establish additional restrictions on the parameters of these exotic objects. The fact is that light black holes with the mass of an asteroid should emit a mixture of particles consisting of soft gamma-ray photons, electrons, and pions. The evaporation of the PBH can lead to distortions of the relic radiation, heating of neutral hydrogen, and the interstellar medium in dwarf galaxies. In addition, the number of PBH is limited by the number of electrons and positrons in cosmic radiation. Finally, the gamma radiation produced by black holes should not exceed the background of diffuse gamma radiation, whose origin is not explained by visible sources. It is believed that the cause of such radiation may be dark matter, whose nature is still unknown.
The Hawking radiation from the PBH can be detected by gamma-ray telescopes that measure rays with an energy of the order of megaelectronvolts. Scientists analyzed data collected by the COMPTEL gamma-ray telescope onboard the CGRO space observatory (Compton Gamma Ray Observatory) when observing diffuse radiation from the central region of the Milky Way. It turned out that these data provide the strictest available restrictions on the total mass of the primary black holes that make up dark matter, which can be observed by gamma-ray telescopes. The scientists concluded that many of these instruments have the potential to detect Hawking radiation from the PDS that make up most of the dark matter, with masses ranging from 10 to the 17th degree to 10 to the 18th degree of grams.