Primordial black holes and gravitational waves in nonstandard cosmologies

TL;DR

This paper explores how nonstandard early universe histories, like an early kination epoch, can lower the primordial scalar perturbation amplitude needed for primordial black hole formation and enhance gravitational wave signals.

Contribution

It demonstrates that an early kination epoch allows for significant PBH formation with lower perturbation amplitudes and predicts an enhanced GW spectrum compared to standard cosmology.

Findings

Early kination epoch enables percent-level PBH abundance at lower perturbation amplitudes.

Enhanced gravitational wave spectrum due to early kination epoch.

Modified mass ranges for PBH formation in nonstandard cosmologies.

Abstract

For primordial black holes (PBHs) to form a considerable fraction of cold dark matter, the required amplitude of primordial scalar perturbations is quite large ($P_{\zeta}(k) \sim 10^{-2}$) if PBH is formed in radiation epoch. In alternate cosmological histories, where additional epoch of arbitrary equation of state precede radiation epoch, the dynamics of PBH formation and relevant mass ranges can be different leading to lower requirement of primordial power at smaller scales of inflation. Moreover, this alternate history can modify the predictions for the gravitational wave (GW) spectrum, which can be probed by upcoming GW observations. In this paper, we show that an early kination epoch can lead to per cent level abundance ofPBH for a lower amplitude of $P_{\zeta}(k)$ compared to PBH formation in a standard radiation epoch. Moreover, we calculate the effect of early kination epoch on the GW spectrum for first and second orders in perturbation theory which show enhancement in the amplitude of the GW spectrum in a kination epoch with respect to that in a standard radiation epoch.