Formation and Abundance of Late-forming Primordial Black Holes as Dark Matter

Abstract

We propose a novel mechanism where primordial black hole (PBH) dark matter is formed much later in the history of the universe, between the epochs of Big Bang nucleosynthesis and cosmic microwave background photon decoupling. In our setup, one does not need to modify the scale-invariant inflationary power spectra; instead, a late-phase transition in a strongly interacting fermion–scalar fluid (which occurs naturally around redshift 106 ≤ z T ≤ 108) creates an instability in the density perturbation as the sound speed turns imaginary. As a result, the dark matter perturbation grows exponentially in sub-Compton scales. This follows the immediate formation of an early dense dark matter halo, which finally evolves into PBHs due to cooling through scalar radiation. We calculate the variance of the density perturbations and the PBH fractional abundances f(M) by using a nonmonochromatic mass function. We find that the peak of our PBH mass function lies between 10−16 and 10−14 solar mass for z T ≃ 106, and thus that it can constitute the entire dark matter of the universe. In PBH formation, one would expect a temporary phase where an attractive scalar balances the Fermi pressure. We numerically confirm that such a state indeed exists, and we find the radius and density profile of the temporary static structure of the dark matter halo, which finally evolves into PBHs due to cooling through scalar radiation.

Anatomy of single-field inflationary models for primordial black holes

“…[13,32] for reviews). The most commonly considered scenario involves the collapse of large density perturbations generated during inflation , but they may be produced also in cosmological phase transitions [115][116][117][118], by the collapse of cosmic strings [119][120][121][122][123][124][125], false vacuum bubbles [126][127][128][129][130][131] or domain walls [132][133][134][135][136], or post-inflationary scalar field fragmentation [137][138][139]. These…”

Simulations of PBH formation at the QCD epoch and comparison with the GWTC-3 catalog

“…Overall, our work represents the first numerical exploration of PBH formation that fully takes into account the QCD phase crossover. Our approach, in particular the numerical techniques that have been used, could be applied to study the effects of other phase transitions of the very early Universe [22], of lepton flavor violation [117], solitosynthesis [118], a strongly interacting fermion-scalar fluid [133], on the formation of PBHs. More accurate calculations of the gravitational-wave background from PBHs [58,134,135] could also be envisaged.…”

Elastic scattering of cosmological gravitational wave backgrounds: primordial black holes and stellar objects

“…PBHs are black holes that formed early in the Universe via gravitational collapse of primordial density fluctuations. Myriad of mechanisms have been identified that could have produced an appropriate spectrum of primordial density fluctuations, ranging from simple single-field models of inflation to multifield models with non-minimal couplings [1][2][3][4][5][6][7][8][9][10][11][12]. In addition to the vast literature on the production, formation, and abundance of PBHs, constraints on the fraction of DM they can constitute is just as varied [13][14][15][16].…”

“…The optical depth tells us how many gravitons scatter off a PBH in the time interval[zO, zscat]. Thus an optical depth of 0.1 means 1 out of every 10 gravitons will Compton scatter with a PBH at redshift zscat 5. Unlike the CMB, the presence of an infrared divergence in this cross section causes lower frequencies tp get upscattered more than higher frequencies.…”