Study rules out primordial black holes as candidates for dark matter

(ORDO NEWS) — Primordial black holes (PBHs) are black holes that are believed to have appeared in the early days of the universe, less than a second after the Big Bang.

Scientists suggest that for a fraction of a second before the formation of the universe, space was not completely homogeneous, so denser and hotter regions could collapse into black holes.

Depending on exactly when they were formed during that fraction of a second, these PBHs could have different masses and associated characteristics.

Some physicists believe that PBHs make a significant contribution to the predicted abundance of dark matter in the universe.

The gravitational wave observations collected as part of the LIGO-Virgo-KAGRA collaboration and the limitations imposed by these observations suggest that this is highly unlikely.

However, some studies have shown that clustering of PBHs during their formation can change their fusion rate, potentially allowing values ​​to be obtained within the limits set by LIGO-Virgo-KAGRA.

This clustering would also potentially affect the current limitations of microlensing, as PBH clusters would act like a massive single lens that cannot be studied with microlensing studies.

Scientists from the University of Geneva, Sapienza University of Rome and NICPB recently conducted a theoretical study that considered the hypothesis that initially clustered PBHs could be candidates for dark matter. The article was published in the journal Physical Review Letters.

“Our work was motivated by the assertion that primordial black holes with masses close to that of the Sun could avoid the current strong limitations associated with microlensing if they were highly clustered,” said Antonio Riotto, one of the authors of the paper.

“Our study has proven that this claim is false. The idea is simple: clustered PBHs can avoid the limitations of microlensing if the clustering is strong enough, but this would be in conflict with another dataset coming from the Lyman-alpha forest, which suggests that this would require weak clustering.”

In their analyses, Riotto and colleagues combined the microlensing limits set 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, appearing as absorption lines in the spectra of distant galaxies and quasars.

In their paper, the researchers showed that the Lyman-alpha forest data suggests that in order to avoid existing microlensing limitations, PBHs should be weakly rather than strongly clustered, contradicting the theory they considered.

“Our analysis rules out the possibility that PBHs could be the dark matter of the universe if they have masses similar to those of stars,” Riotto said.

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