Damping of gravitational waves by matter

TL;DR

This paper presents a unified Boltzmann equation approach to analyze how matter causes damping of gravitational waves, identifying collisional and Landau damping mechanisms, and exploring effects in flat and expanding spacetimes.

Contribution

It introduces a comprehensive framework for gravitational wave damping by matter, including collisions and Landau damping, extending previous models to cosmological contexts.

Findings

Collisional damping is significant only near the horizon scale.

Collisions suppress anisotropic stresses, reducing damping effects.

Landau damping can occur in an expanding universe, but not in flat spacetime.

Abstract

We develop a unified description, via the Boltzmann equation, of damping of gravitational waves by matter, incorporating collisions. We identify two physically distinct damping mechanisms -- collisional and Landau damping. We first consider damping in flat spacetime, and then generalize the results to allow for cosmological expansion. In the first regime, maximal collisional damping of a gravitational wave, independent of the details of the collisions in the matter is, as we show, significant only when its wavelength is comparable to the size of the horizon. Thus damping by intergalactic or interstellar matter for all but primordial gravitational radiation can be neglected. Although collisions in matter lead to a shear viscosity, they also act to erase anisotropic stresses, thus suppressing the damping of gravitational waves. Damping of primordial gravitational waves remains possible. We generalize Weinberg's calculation of gravitational wave damping, now including collisions and particles of finite mass, and interpret the collisionless limit in terms of Landau damping. While Landau damping of gravitational waves cannot occur in flat spacetime, the expansion of the universe allows such damping by spreading the frequency of a gravitational wave of given wavevector.