The study rules out primordial black holes accumulating initially as the candidate dark matter
Primordial black holes (PBHs) are fascinating cosmic objects that have been extensively researched by astrophysicists around the world. As their name suggests, these are black holes that are believed to have appeared in the early days of the universe, less than a second after the Big Bang.
Physics theory suggests that within a fraction of a second before the universe formed, space was not completely homogeneous, and so the denser and hotter regions could collapse into black holes. Depending on exactly when they formed within that fraction of a second, these black holes can have very different masses and associated properties.
Some theoretical physicists have been exploring the possibility that surface black holes contribute significantly to the expected abundance of dark matter in the universe, or in other words, that they are prime candidates for dark matter. Observations of gravitational waves collected by the LIGO-Virgo-KAGRA collaboration and the limitations placed by these observations indicate that this is extremely unlikely.
However, some recent studies have suggested that clustering of PBHs at the time of their formation can alter the rate of fusion, which may enable values within the constraints set by LIGO-Virgo-KAGRA. This grouping may also affect the limitations of current microlensing, as PBH assemblies act as a single bulky lens that cannot be examined by microlens studies.
Researchers at the University of Geneva, Sapienza University of Rome, and NICPB recently conducted a theoretical study to evaluate the hypothesis that initially assembled PBH clusters could be candidates for dark matter. Their paper has been published in Physical review letterspresents a relatively simple argument that seems to rule out this possibility.
“Our work was motivated by the claim, not yet substantiated by the literature, that circumsolar-mass primordial black holes could avoid the strong current constraints coming from microlensing, if they were strongly clustered,” says Antonio Rioto, one of the researchers who conducted the study. , he told Phys.org.
“Our study proved this claim to be incorrect. The idea is simple: clustered PBHs may avoid banding of the microlens if aggregation is strong enough, but this would contradict another set of data coming from the Lyman-alpha forest, suggesting that this would require weak aggregation” .
In their analyses, Riotto and colleagues combined fine lensing limitations established by previous astronomical observations with Lyman-alpha forest data. Lyman alpha forests are an absorption phenomenon that can be observed with astronomical spectroscopy instruments, presenting themselves as absorption lines in the spectra of distant galaxies and quasars.
These absorption lines have become a prominent probe in astrophysics, particularly in studies investigating density fluctuations in the universe. In their paper, the researchers show that Lyman-alpha forest data indicate that in order to avoid current microlensing limitations, PBH clusters must be weak, rather than strong, contradicting the widespread theoretical notion they were evaluating.
“Our analysis excludes the possibility that fundamental black holes are the dark matter in the universe if they have masses similar to stellar masses,” Ryoto added. “In our next work, we plan to investigate the role of bright black holes further, to see if they can explain other interesting observations, such as the presence of galaxies at high redshifts.”
Valerio De Luca et al., Ruling out initially clustered primordial black holes as dark matter, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.191302
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