Constraining supermassive primordial black holes with magnetically induced gravitational waves
Theodoros Papanikolaou, Konstantinos N. Gourgouliatos
Abstract
Primordial black holes (PBHs) can answer a plethora of cosmic conundrums, including the origin of cosmic magnetic fields. In particular, supermassive PBHs with masses ${M}_{\mathrm{PBH}}>{10}^{10}{M}_{\ensuremath{\bigodot}}$ and furnished with a plasma disk moving around them can generate through the Biermann battery mechanism a seed primordial magnetic field that can later be amplified to provide the magnetic field threading the intergalactic medium. In this article, we derive the gravitational-wave (GW) signal induced by the magnetic anisotropy of such a population of magnetized PBHs. Interestingly enough, by using GW constraints from big bang nucleosynthesis and an effective model for the galactic/turbulent dynamo amplification of the magnetic field, we set conservative upper bound constraints on the abundances of supermassive PBHs at formation time, ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{PBH},\mathrm{f}}$ as a function of their masses, namely, that ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{PBH},\mathrm{f}}\ensuremath{\le}2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{(\frac{M}{{10}^{10}{M}_{\ensuremath{\bigodot}}})}^{45/22}$. Remarkably, these constraints are comparable and, in some mass ranges, even tighter compared to the constraints on ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{PBH},\mathrm{f}}$ from large-scale structure probes, hence promoting the portal of magnetically induced GWs as a new probe to explore the enigmatic nature of supermassive PBHs.