Hot Gravitons and Gravitational Waves From Kerr Black Holes in the Early Universe

TL;DR

This paper explores how early universe low-mass black holes, through mergers and evaporation, could produce observable high-energy gravitons and gravitational waves, offering potential signals for future detection.

Contribution

It introduces a novel scenario where early black hole mergers lead to detectable high-energy gravitons and gravitational waves, linking black hole physics to observable cosmological signals.

Findings

Black hole mergers produce high-energy graviton backgrounds.

Mergers generate a stochastic gravitational wave background.

Energy density of signals could be within future detection capabilities.

Abstract

Any abundance of black holes that was present in the early universe will evolve as matter, making up an increasingly large fraction of the total energy density as space expands. This motivates us to consider scenarios in which the early universe included an era that was dominated by low-mass ($M < 5\times 10^8$ g) black holes which evaporate prior to primordial nucleosynthesis. In significant regions of parameter space, these black holes will become gravitationally bound within binary systems, and undergo mergers before evaporating. Such mergers result in three potentially observable signatures. First, any black holes that have undergone one or more mergers will possess substantial angular momentum, causing their Hawking evaporation to produce significant quantities of high-energy gravitons. These products of Hawking evaporation are predicted to constitute a background of hot ($\sim$eV-keV) gravitons today, with an energy density corresponding to $\Delta N_{\rm eff} \sim 0.01-0.03$. Second, these mergers will produce a stochastic background of high-frequency gravitational waves. And third, the energy density of these gravitational waves can be as large as $\Delta N_{\rm eff} \sim 0.3$, depending on the length of time between the mergers and evaporation. These signals are each potentially within the reach of future measurements.