Non-Markovian open quantum system approach to the early universe: I. Damping of gravitational waves by matter

TL;DR

This paper develops a non-Markovian quantum Boltzmann equation to analyze early universe processes, specifically showing how gravitational waves are damped or amplified when interacting with a neutrino medium.

Contribution

It introduces a new non-Markovian quantum Boltzmann framework extending previous models to better describe irreversible processes in the early universe.

Findings

Gravitational wave intensity is damped in a neutrino medium.

Linear polarization of gravitational waves is damped.

Circular polarization (V-mode) can be amplified during propagation.

Abstract

By revising the application of the open quantum system approach to the early universe and extending it to the conditions beyond the Markovian approximation, we obtain a new non-Markovian quantum Boltzmann equation. Throughout the paper, we also develop an extension of the quantum Boltzmann equation to describe the processes that are irreversible at the macroscopic level. This new kinetic equation is, in principle, applicable to a wide variety of processes in the early universe. For instance, using this equation one can accurately study the microscopic influence of a cosmic environment on a system of cosmic background photons or stochastic gravitational waves. In this paper, we apply the non-Markovian quantum Boltzmann equation to study the damping of gravitational waves propagating in a medium consisting of decoupled ultra-relativistic neutrinos. For such a system, we study the time evolution of the intensity and the polarization of the gravitational waves. It is shown that, in contrast to intensity and linear polarization which are damped, the circular polarization (V-mode) of the gravitational wave (if present) is amplified by propagating through such a medium.