Generation of Primordial Magnetic Fields on Linear Over-density Scales

TL;DR

This paper proposes a mechanism for generating primordial magnetic fields on linear scales in the early universe through vorticity induced by temperature fluctuations, providing a potential seed for observed cosmic magnetic fields.

Contribution

It introduces a novel linear-scale generation process for primordial magnetic fields via vorticity from temperature fluctuations, differing from previous nonlinear or post-recombination models.

Findings

Estimated magnetic field strength of 1e-25 to 1e-24 G at redshift a few tens.

Field generation occurs even without dark matter-gas relative velocity.

Predicted fields are amplified during structure formation and reionization.

Abstract

Magnetic fields appear to be present in all galaxies and galaxy clusters. Recent measurements indicate that a weak magnetic field may be present even in the smooth low density intergalactic medium. One explanation for these observations is that a seed magnetic field was generated by some unknown mechanism early in the life of the Universe, and was later amplified by various dynamos in nonlinear objects like galaxies and clusters. We show that a primordial magnetic field is expected to be generated in the early Universe on purely linear scales through vorticity induced by scale-dependent temperature fluctuations or equivalently, a spatially varying speed of sound of the gas. Residual free electrons left over after recombination tap into this vorticity to generate magnetic field via the Biermann battery process. Although the battery operates even in the absence of any relative velocity between dark matter and gas at the time of recombination, the presence of such a relative velocity modifies the predicted spatial power spectrum of the magnetic field. At redshifts of order a few tens, we estimate a root mean square field strength of order 1e-25 -1e-24 G on comoving scales~10 kpc. This field, which is generated purely from linear perturbations, is expected to be amplified significantly after reionization, and to be further boosted by dynamo processes during nonlinear structure formation.