Gravitational birefringence of light in Robertson-Walker cosmologies

TL;DR

This paper investigates how light with spin, specifically photons, deviates from null geodesics in an expanding universe, potentially allowing experimental tests of cosmic acceleration through gravitational birefringence effects.

Contribution

It derives equations of motion for spinning massless particles in Robertson-Walker cosmologies and performs numerical simulations to analyze photon trajectory deviations.

Findings

Photon worldlines deviate from null geodesics by an amount proportional to wavelength.

Numerical integration confirms deviations in standard cosmological models.

Perturbative solutions extend analysis to more general cases.

Abstract

The spacetime evolution of massless spinning particles in a Robertson-Walker background is derived using the deterministic system of equations of motion due to Papapetrou, Souriau and Saturnini. A numerical integration of this system of differential equations in the case of the standard model of cosmology is performed. The deviation of the photon worldlines from the null geodesics is of the order of the wavelength. Perturbative solutions are also worked out in a more general case. An experimental measurement of this deviation would test the acceleration of our expanding universe.