On the coupling between spinning particles and cosmological gravitational waves

TL;DR

This paper investigates whether spin in classical particles can couple with primordial gravitational waves in a cosmological setting, concluding that no such coupling occurs without other perturbations.

Contribution

It demonstrates that, in a flat FRW universe with tensor perturbations, spin does not influence gravitational wave evolution when other perturbations are absent.

Findings

No spin-gravitational wave coupling in the studied model.

Spin does not affect the distribution function without precession.

Gravitational waves are unaffected by particle spin in this context.

Abstract

The influence of spin in a system of classical particles on the propagation of gravitational waves is analyzed in the cosmological context of primordial thermal equilibrium. On a flat Friedmann-Robertson-Walker metric, when the precession is neglected, there is no contribution due to the spin to the distribution function of the particles. Adding a small tensor perturbation to the background metric, we study if a coupling between gravitational waves and spin exists that can modify the evolution of the distribution function, leading to new terms in the anisotropic stress, and then to a new source for gravitational waves. In the chosen gauge, the final result is that, in the absence of other kind of perturbations, there is no coupling between spin and gravitational waves.