Gravitational Waves from Primordial Black Hole Evaporation with Large Extra Dimensions

TL;DR

This paper investigates gravitational waves from primordial black hole evaporation in theories with large extra dimensions, showing the potential for detectable high-frequency signals within planned detector ranges.

Contribution

It models graviton emission during black hole evaporation in large extra dimensions, computing greybody factors and spectral densities to predict observable gravitational wave signals.

Findings

Peak frequency can be in the sub-MHz range for certain parameters.

Gravitational wave signals could be detectable with future high-frequency detectors.

The scenario can be constrained by $ riangle N_{eff}$ bounds.

Abstract

The spectra of gravitational waves from black hole evaporation generically peak at frequencies of order the Hawking temperature, making this signal ultra-high frequency for primordial black holes evaporating in the early universe. This motivates us to consider small black holes in theories with large extra dimensions, for which the peak frequency can be lowered substantially, since the true bulk Planck scale $M_*$ can be much smaller than the effective $M_{\rm Pl}$. We study the emission of brane-localized gravitons during the Hawking evaporation of ultra-light primordial black holes in the context of theories with large extra dimensions, with the ultimate goal of computing the contribution to the stochastic gravitational wave background. To accurately model black hole evolution, we compute greybody factors for particles of spin-0, 1/2, 1, and 2 emitted on the brane and in the bulk, presuming the majority of emission proceeds during the Schwarzschild phase. We then compute the power spectrum and present day spectral density parameter for brane-localized gravitons contributing to a gravitational wave signal. We find that for an optimal choice of parameters, the peak frequency plateaus in the sub-MHz regime, within range of planned high-frequency gravitational wave detectors, making this scenario a target for detection once their sensitivity exceeds $\Delta N_{\rm eff}$ bounds.