Late-forming primordial black holes: Beyond the CMB era

Abstract

The intermediate mass black hole range, ${10}^{2}\ensuremath{\lesssim}{M}_{\mathrm{BH}}/{M}_{\ensuremath{\bigodot}}\ensuremath{\lesssim}{10}^{5}$, has long offered enticing possibilities for primordial black holes (PBHs), with populations in this range postulated to be responsible for some of the black hole binary merger detected events as well as the existence of supermassive black holes embedded at galactic centers. However, a prominent bound derived from PBH accretion during recombination severely restricts the mass fraction of intermediate mass PBHs. We address this problem by proposing a formation scenario in which ``primordial'' black holes form late in our cosmological history, beyond the cosmic microwave background era, and bypassing this bound. During this crucial epoch, our population of compact objects exist as thermal balls supported by thermal pressure, which eventually cool to Fermi balls supported by degeneracy pressure and finally collapse to PBHs. Our mechanism is a viable production method for both the mass gap LIGO-VIRGO-KAGRA detections and the James Webb Space Telescope observation of an early time $z>10$ supermassive black hole. Furthermore, we present the remarkable possibility of PBH formation after the present era, which we term future PBH. Such a population would evade most, if not all, bounds on the PBH mass spectrum in the literature and open up previously unthought-of possibilities. Light future PBHs could form below the Hawking evaporation threshold and convert the bulk of the matter in the Universe into radiation.