The Music of the Aetherwave - B-mode Polarization in Einstein-Aether Theory

TL;DR

This paper investigates how the vector field in Einstein-Aether theory influences CMB B-mode polarization, showing it can dominate over inflationary gravitational waves for certain parameters.

Contribution

It derives linear perturbation equations in covariant form and modifies the CAMB code to include effects of the aether vector field on CMB polarization.

Findings

Aether vector field can produce a B-mode signal surpassing inflationary gravitational waves.

The shape of the B-mode spectrum is explained using a tight coupling approximation.

The amplitude of the B-mode signal depends on model parameters.

Abstract

We study how the dynamical vector degree of freedom in modified gravity affects the CMB B-mode polarization in terms of the Einstein-aether theory. In this theory, vector perturbations can be generated from inflation, which can grow on superhorizon scales in the subsequent epochs and thereby leaves imprints on the CMB B-mode polarization. We derive the linear perturbation equations in a covariant formalism, and compute the CMB B-mode polarization using the CAMB code modified so as to incorporate the effect of the aether vector field. We find that the amplitude of the B-mode signal from the aether field can surpass the contribution from the inflationary gravitational waves for a viable range of model parameters. We also give an analytic argument explaining the shape of the spectrum based on the tight coupling approximation.