Charged black holes in de Sitter space: superradiant amplification of charged scalar waves and resonant hyperradiation

TL;DR

This paper investigates how charged scalar waves are amplified around charged black holes in de Sitter space, revealing enhanced superradiant effects and resonant phenomena linked to quasinormal modes.

Contribution

It demonstrates that a positive cosmological constant increases superradiant amplification factors and identifies resonant peaks associated with long-lived quasinormal modes.

Findings

Superradiant amplification factors are higher in de Sitter space.

Frequency range of superradiance is reduced with positive cosmological constant.

Resonant peaks occur when wave frequency matches quasinormal mode frequencies.

Abstract

Black holes typically inhabit highly dynamical galactic environments and are frequently permeated by accretion media. The inevitable scattering of scalar, electromagnetic and gravitational waves off rotating and charged black holes provides a remarkable demonstration of the energy extraction and subsequent amplification of scattered waves under the expense of the compact object's rotational and electromagnetic energy; a phenomenon known as superradiance. Certain circumstances tremendously favor the energy extraction process in such extent that linear superradiant instabilities are triggered. We examine the superradiant amplification of monochromatic charged scalar fields impinging Reissner-Nordstr\"om-de Sitter black holes which are known to exhibit superradiantly unstable quasinormal modes. We find that even though the frequency range of superradiance is reduced when a positive cosmological constant is incorporated, the amplification factors are significantly elevated with respect to those occurring in Reissner-Nordstr\"om spacetime. Intriguingly, we confirm that the long-lived quasinormal resonances that reside in the superradiant regime induce resonant peaks when the wave's frequency matches the real part of the quasinormal mode, regardless of the spacetime's modal stability.