Primordial black holes (PBHs) are fascinating cosmic bodies that have been widely investigated by astrophysicists around the world.
As their name suggests, these are black holes believed to have appeared in the early days of the Universe, less than a second after the Big Bang.
Physical theory suggests that in the split second before the Universe formed, space was not completely homogeneous, so denser and hotter regions could have collapsed into black holes. Depending on exactly when they formed in this fraction of a second, these PBHs could have very different masses and associated characteristics.
Some 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 important candidates for dark matter.
Observations of gravitational waves collected by the LIGO-Virgo-KAGRA collaboration and the constraints set by these observations suggest that this is highly unlikely.
The primordial black holes would have appeared less than a second after the Big Bang
However, some recent studies have suggested that the clustering of PBHs at the time of their formation could alter their merger rate, which could allow values that fall within the constraints set by LIGO-Virgo-KAGRA.
This clumping could also affect the existing limits of microlensing, as PBH clumps would act as a single massive 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 that further evaluates the hypothesis that initially clustered PBH clumps could be candidates for dark matter.
Their work, published in Physical Review Lettersintroduces a relatively simple argument that seems to rule out this possibility.
The study contradicts the widely held theoretical idea
In their analyses, Riotto and his colleagues combined constraints set by previous astronomical observations with data from the Lyman-alpha forest. The Lyman-alpha forest is an absorption phenomenon that can be observed with astronomical spectroscopy instruments, appearing as absorption lines in the spectra of distant galaxies.
These absorption lines have become a prominent probe in astrophysics, particularly in studies investigating density fluctuations in the Universe.
In their paper, the researchers showed that data from the Lyman-alpha forest suggests that to avoid existing limits, PBHs should be weakly, rather than strongly, clustered, which contradicts the widely held theoretical idea, they write Phys.org.