Gravitational capture of magnetic monopoles by primordial black holes in the early universe

Abstract

It is intriguing to ask whether the existence of primordial black holes (PBHs) in the early universe could significantly reduce the abundance of certain stable massive particles (SMP) via gravitational capture, after which the PBHs evaporate before BBN to avoid conflict with stringent bounds. For example, this mechanism is relevant to an alternative solution of the monopole problem proposed by Stojkovic and Freese, in which magnetic monopoles produced in the early universe are captured by PBHs, thus freeing inflation from having to occur during or after the corresponding phase transitions that produced the monopoles. In this work, we reanalyze the solution by modelling the capture process in the same way as the coexisting monopole annihilation, which exhibits typical features of a diffusive capture. A monochromatic PBH mass function and a radiationdominated era before PBH evaporation are assumed. We found that for Pati-Salam monopoles corresponding to a symmetry breaking scale between 10 10 GeV and 10 15 GeV, the capture rate is many orders of magnitude below what is needed to cause a significant reduction of the monopole density. The difference with respect to previous literature can be attributed to both the modelling of the capture process and also the assumption on the PBH mass function. Within our assumptions, we also found that the magnetic charge that is large enough to make an extremal magnetic black hole cosmologically stable cannot be obtained from magnetic charge fluctuation via monopole capture. The large magnetic charged required by cosmological stability can nevertheless be obtained from magnetic charge fluctuation at PBH formation, and if later the monopole abundance can be reduced significantly by some non-inflationary mechanism, long-lived near-extremal magnetic black holes of observational relevance might result.

The primordial black holes solution to the cosmological monopole problem

“…Note added As this work was nearing completion, we found new papers discussing similar topics on Arxiv [61,62]. Their calculations differ from ours by still assuming that the PBH's mass remains constant during the whole process, as well as taking a diffusive capture model which makes it less efficient for PBHs to capture magnetic monopoles, and also mandating that the density of the matter cannot exceed the radiation in the early universe which may not be necessary.…”

Linear stability of vector Horndeski black holes

“…In addition, the possible existence of primordial BHs in the early Universe could absorb magnetic monopoles [57][58][59][60][61]. Since the neutralization of magnetic BHs with ordinary matter does not occur in conductive media, it can be a long-lived stable configuration in comparison to the electric BH [62,63].…”