Primordial vorticity and gradient expansion

TL;DR

This paper derives the evolution equations for vorticities in a pre-decoupling plasma using a fully inhomogeneous, relativistic framework, estimating the magnetic fields generated and discussing implications for early universe magnetogenesis.

Contribution

It introduces a novel fully inhomogeneous relativistic approach to analyze vorticity transfer and magnetic field generation in the early universe plasma.

Findings

Total vorticity source from spatial gradients of geometry and electromagnetic sources.

Estimated magnetic field strength of up to 10^{-37} G on Mpc scales.

Results suggest negligible seeding for galactic dynamos but demonstrate a systematic inhomogeneous treatment.

Abstract

The evolution equations of the vorticities of the electrons, ions and photons in a pre-decoupling plasma are derived, in a fully inhomogeneous geometry, by combining the general relativistic gradient expansion and the drift approximation within the Adler-Misner-Deser decomposition. The vorticity transfer between the different species is discussed in this novel framework and a set of general conservation laws, connecting the vorticities of the three-component plasma with the magnetic field intensity, is derived. After demonstrating that a source of large-scale vorticity resides in the spatial gradients of the geometry and of the electromagnetic sources, the total vorticity is estimated to lowest order in the spatial gradients and by enforcing the validity of the momentum constraint. By acknowledging the current bounds on the tensor to scalar ratio in the (minimal) tensor extension of the $\Lambda$CDM paradigm the maximal comoving magnetic field induced by the total vorticity turns out to be, at most, of the order of $10^{-37}$ G over the typical comoving scales ranging between 1 and 10 Mpc. While the obtained results seem to be irrelevant for seeding a reasonable galactic dynamo action, they demonstrate how the proposed fully inhomogeneous treatment can be used for the systematic scrutiny of pre-decoupling plasmas beyond the conventional perturbative expansions.