Tensor Modes Damping in Matter and Vacuum Dominated Era

TL;DR

This paper develops an integro-differential equation to analyze how free streaming neutrinos dampen cosmological gravitational waves during matter and vacuum dominated eras, highlighting small amplitude reductions and background effects.

Contribution

It introduces a new integro-differential framework for gravitational wave propagation considering neutrino anisotropic stress in matter-dominated era, including effects of closed spacetime.

Findings

Neutrino anisotropic stress reduces GW amplitude by 0.03% during matter era.

Amplitude reduction is less during Lambda-dominated era.

Closed spacetime background influences GW amplitude.

Abstract

The present paper has developed an integro-differential equation to propagate cosmological gravitation waves in matter-dominated era in accounting for the presence of free streaming neutrinos as a traceless transverse tensor part of the anisotropic stress tensor. Its focus is on short and long wavelengths of GWs that enter the horizon in matter-dominated era. Results show that the anisotropic stress reduces the squared amplitude by $ 0.03\%$ for wavelengths, entering the horizon during matter-dominated phase. This reduction is less for those wavelengths that enter the horizon at $ \Lambda $ dominated era in flat spacetime. All of the calculations have been done in closed spacetime and the results have been compared with the radiation-dominated case for both flat and closed spacetimes. Finally the paper investigates the effect of closed background on the amplitude of the gravitational waves.