Primordial black holes (PBH) are fascinating cosmic bodies that have been widely studied by astrophysicists around the world. As their name suggests, these are black holes that would have appeared in the early days of the universe, less than a second after the Big Bang.
Physics theory suggests that in the split second before the universe was formed, space was not completely homogeneous, so denser and hotter regions could have collapsed into black holes. Depending on when exactly they formed in that split second, these PBHs could have very different masses and associated characteristics.
Some theoretical physicists have explored the possibility that PBHs contribute significantly to the predicted abundance of dark matter in the universe, or in other words, that they are major dark matter candidates. Observations of gravitational waves collected by the LIGO-Virgo-KAGRA collaboration and the constraints imposed by these observations suggest that this is highly unlikely.
Yet, some recent studies have suggested that the clustering of PBHs at the time of their formation could alter their fusion rate, potentially allowing values within the limits set by LIGO-Virgo-KAGRA. This clustering would also potentially affect existing microlensing boundaries, as PBH clusters would act as a massive single lens that cannot be probed by microlensing studies.
Researchers from the University of Geneva, Sapienza University of Rome, and NICPB recently conducted a theoretical study further evaluating the hypothesis that initially clustered PBHs might be candidates for dark matter. Their article, published in Physical examination lettersintroduces a relatively simple argument that seems to exclude this possibility.
“Our work was motivated by the claim, as yet unproven in the literature, that primordial black holes with masses around solar masses could avoid the current strong stresses from microlensing, if strongly clustered,” said Antonio Riotto, one of the researchers who conducted the study, told Phys.org.
“Our study proved that this claim is not correct. The idea is simple: clustered PBHs can avoid the microlensing limit if the clustering is strong enough, but this would contradict another set of data from the Lyman-alpha forest, suggesting that this would require weak clustering.”
In their analyses, Riotto and his colleagues combined microlensing constraints defined by previous astronomical observations with data from the Lyman-alpha forest. The Lyman-alpha forest is an absorption phenomenon that can be observed using astronomical spectroscopy tools, showing up as absorption lines in the spectra of distant galaxies and quasars.
These absorption lines have become a leading probe in astrophysics, particularly in studies of density fluctuations in the Universe. In their paper, the researchers showed that Lyman-alpha forest data suggests that to avoid existing microlensing limits, PBHs should be weakly, rather than strongly, clustered, contradicting the popular theoretical view that they were evaluating.
“Our analysis rules out the possibility that PBHs could be the dark matter of the universe if they have masses similar to stellar masses,” Riotto added. “In our next work, we plan to further investigate the role of PBHs, to see if they can explain other interesting observations, such as the presence of galaxies with high redshifts.”
Valerio De Luca et al, excluding primordial black holes initially clustered as dark matter, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.191302
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