Ruling Out Initially Clustered Primordial Black Holes as Dark Matter

Explaining the nature of dark matter is one of the fundamental goals of modern physics. In recent years, primordial black holes attracted ample attention as they represent a perfect dark matter candidate, whose existence does not necessarily require new physics beyond the standard model of particle physics. 

Primordial black holes can form in the early universe out of the collapse of large overdensities. Many studies have set constraints on their abundance, as a function of their putative mass. Of particular interest is the stellar mass range, as it would be accompanied by the generation of gravitational wave signals in the LIGO/Virgo observable window. This elegant explanation, however, is in contrast with various experiments. Among others, searches of lensing signatures on the electromagnetic radiation reaching us from nearby galaxies and observations of X-rays are incompatible with stellar mass primordial black holes being more than 1% of the dark matter in the stellar mass range. Additionally, requiring the merger rate to be below the upper bound set by the LIGO/Virgo collaboration, forces this fraction to fall below 0.1%.

Nonetheless, it is important to stress that the considerations above are strictly valid assuming the simplest formation scenario, in which primordial black holes are formed randomly in our universe and follow a spatial Poisson distribution at a very high redshift. Recently, it was suggested that initially clustered formation scenarios may give rise to much more complex cosmological evolutions, potentially allowing for primordial black holes to evade the aforementioned constraints.  

In a work published in the prestigious international journal Physical Review Letters [1], an international collaboration between Sapienza University of Rome, the University of Geneva, and the NICPB Research Institute in Tallinn, it was shown that initially clustered primordial black hole dark matter is ruled out as it would be accompanied by too large isocurvature perturbations which are bounded by Lyman-alpha forest observations. On the one hand, this significant result definitively concludes a debate that accompanied researchers in this field for many years. On the other hand, it shifts the attention to the much lighter mass range, around the asteroid mass, where current approaches still fails to set significant bounds. 


``Ruling Out Initially Clustered Primordial Black Holes as Dark Matter''
V. De Luca, G. Franciolini, A. Riotto and H. Veermäe,
Phys. Rev. Lett. 129, no.19, 191302 (2022)
[arXiv:2208.01683 [astro-ph.CO]].

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