Superradiant amplification by rotating viscous compact objects

TL;DR

This paper investigates how rotating viscous stars can amplify incoming gravitational waves through superradiance, using relativistic hydrodynamics, and finds that this amplification is universal but does not lead to instability.

Contribution

It introduces a relativistic hydrodynamics framework for viscous stars and demonstrates superradiant amplification of gravitational waves in this context.

Findings

Rotating viscous stars amplify low-frequency gravitational waves.

Superradiant amplification is likely universal.

No instability is triggered in uniformly rotating stars.

Abstract

We study fluctuations of rotating viscous stars, using the causal relativistic hydrodynamics of Bemfica, Disconzi, Kovtun, and Noronha. We derive, in a slow-rotation approximation, a coupled system of equations describing the propagation of axial gravitational waves through the star, which couple to internal viscous modes. We show that rotating viscous stars amplify incoming low-frequency gravitational waves, a phenomenon which we argue to be universal. Superradiant amplification does not seem to trigger an instability for uniformly rotating stars, even if the object is compact enough to have light rings.