Study rules out initially clustered primordial black holes as dark matter candidates.

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Primordial black holes (PBHs) are interesting cosmic objects that have attracted the attention of astronomers all around the globe. These black holes, as their name implies, are thought to have formed shortly after the Big Bang, in the early stages of the cosmos.

Because space was not fully homogeneous in the fraction of a second before the universe originated, denser and hotter parts may have collapsed into black holes. These PBHs may have quite varied masses and related properties depending on precisely when they generated within this fraction of a second.

Some theoretical physicists have been investigating the potential that PBHs are important dark matter candidates, or that they considerably contribute to the projected abundance of dark matter in the universe. This is improbable, according to constraints derived from gravitational wave measurements made by the LIGO-Virgo-KAGRA team.

However, several recent studies hypothesized that the clustering of PBHs during their production might alter their merger rate, possibly enabling values inside the LIGO-Virgo-KAGRA bounds. Due to PBH clusters acting as a single, enormous lens that cannot be examined by microlensing research, this clustering might also have an impact on current microlensing constraints.

In a recent theoretical investigation, scientists from the Université de Genève, Sapienza University of Rome, and NICPB evaluated the possibility that initially grouped PBHs may be dark matter candidates. Their article, which was published in Physical Review Letters, presents a pretty straightforward argument that seems to disprove this theory.

According to Antonio Riotto, one of the researchers who conducted the study, "Our work was motivated by the claim, not yet proven by literature, that primordial black holes with masses around the solar masses could avoid the current strong constraints coming from microlensing, if they were strongly clustered."

"Our research demonstrated the falsity of this assertion. The notion is straightforward: if the clustering is strong enough, clustered PBHs may escape the microlensing limit. However, this would be inconsistent with another set of data from the Lyman-alpha forest, which indicates that weak clustering would be necessary."

Riotto and his colleagues coupled Lyman-alpha forest data with the microlensing restrictions derived from earlier astronomical observations in their calculations. Using astronomical spectroscopic equipment, one may see the Lyman-alpha forest, an absorption phenomena that appears as absorption lines in the spectra of far-off galaxies and quasars.

In astronomy, these absorption lines have grown to be a well-known probe, especially in research into cosmological density variations. In their paper, the researchers demonstrated that Lyman-alpha forest data suggests that PBHs would need to be weakly, rather than strongly, clustered in order to avoid the current microlensing bounds, which contradicts the pervasive theoretical idea that they were evaluating.

If PBHs have masses similar to stellar masses, "our analysis rules out the possibility that they could be the dark matter of the universe," Riotto added. We intend to further explore the role of PBHs in our upcoming works to determine whether they can account for other intriguing observations, such as the existence of galaxies at high redshifts.

Also Read: Mysteriously bright flash is a black hole jet pointing straight toward Earth, astronomers say.

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