Primordial Gravitational Waves From Black Hole Evaporation in Standard and Non-Standard Cosmologies

TL;DR

This paper investigates the gravitational wave background from evaporating black holes across various early universe cosmologies, highlighting potential high-frequency signals and constraints from cosmological bounds.

Contribution

It analyzes how different early universe expansion histories affect the gravitational wave spectrum from black hole evaporation, including potential detectability and constraints.

Findings

High-frequency gravitational wave peaks are common in many cosmologies.

Lower-frequency peaks are possible but likely undetectable.

Cosmologies with high pressure-to-density ratios produce stronger signals.

Abstract

Gravitons radiated from light, evaporating black holes contribute to the stochastic background of gravitational waves. The spectrum of such emission depends on both the mass and the spin of the black holes, as well as on the redshifting that occurs between the black hole formation and today. This, in turn, depends on the expansion history of the universe, which is largely unknown and unconstrained at times prior to the synthesis of light elements. Here, we explore the features of the stochastic background of gravitational waves from black hole evaporation under a broad range of possible early cosmological histories. The resulting gravitational wave signals typically peak at very high frequencies, and offer opportunities for proposed ultra-high frequency gravitational wave detectors. Lower-frequency peaks are also possible, albeit with a suppressed intensity that is likely well below the threshold of detectability. We find that the largest intensity peaks correspond to cosmologies dominated by fluids with equations of state that have large pressure-to-density ratios. Such scenarios can be constrained on the basis of violation of $\Delta N_{\rm eff}$ bounds.