Damping of Gravitational Waves and Density Perturbations in the Early Universe

TL;DR

This paper investigates how viscous cosmic media affect gravitational waves and density perturbations in the early universe, concluding that damping effects are negligible for long wavelengths, preserving their usefulness as probes of early cosmological conditions.

Contribution

It provides a detailed analysis showing that viscous damping of long-wavelength gravitational waves and density perturbations is negligible, highlighting their robustness as early universe probes.

Findings

Long wavelength gravitational waves experience negligible damping.

Density perturbations are similarly unaffected by viscosity at long wavelengths.

Long wavelength perturbations remain reliable probes of early universe conditions.

Abstract

Since the discovery of the large angular scale anisotropies in the microwave background radiation, the behaviour of cosmological perturbations (especially, density perturbations and gravitational waves) has been of great interest. In this study, after a detailed and rigorous treatment of the behaviour of gravitational waves in viscous cosmic media, we conclude that the damping of cosmological gravitational waves of long wavelengths is negligible for most cases of physical interest. A preliminary analysis suggests that similar results hold for density perturbations in the long wavelength limit. Therefore, long wavelength cosmological perturbations have not been practically affected by viscous processes,and are good probes of the very early Universe.