Generation of magnetic fields in Einstein-Aether gravity

TL;DR

This paper explores how Einstein-Aether gravity can generate primordial magnetic fields through vector perturbations, providing a potential explanation for observed intergalactic magnetic fields and predicting a characteristic spectrum with a peak at specific scales.

Contribution

It introduces a novel mechanism for magnetogenesis via Einstein-Aether gravity, where the aether field sources vector metric perturbations leading to magnetic field generation.

Findings

Magnetic field amplitude can reach ~10^{-22} Gauss at recombination.

The magnetic field spectrum peaks at k=0.1 h Mpc^{-1}.

Constraints on aether parameters are derived from CMB observations.

Abstract

Recently the lower bounds of the intergalactic magnetic fields $10^{-16} \sim 10^{-20}$ Gauss are set by gamma-ray observations while it is unlikely to generate such large scale magnetic fields through astrophysical processes. It is known that large scale magnetic fields could be generated if there exist cosmological vector mode perturbations in the primordial plasma. The vector mode, however, has only a decaying solution in General Relativity if the plasma consists of perfect fluids. In order to investigate a possible mechanism of magnetogenesis in the primordial plasma, here we consider cosmological perturbations in the Einstein-Aether gravity model, in which the aether field can act as a new source of vector metric perturbations and thus of magnetic fields. We estimate the angular power spectra of temperature and B-mode polarization of the Cosmic Microwave Background (CMB) Anisotropies in this model and put a rough constraint on the aether field parameters from latest observations. We then estimate the power spectrum of associated magnetic fields around the recombination epoch within this limit. It is found that the spectrum has a characteristic peak at $k=0.1 h{\rm Mpc^{-1}}$, and at that scale the amplitude can be as large as $B\sim 10^{-22}$ Gauss where the upper bound comes from CMB temperature anisotropies. The magnetic fields with this amplitude can be seeds of large scale magnetic fields observed today if the sufficient dynamo mechanism takes place. Analytic interpretation for the power spectra is also given.