Seed Magnetic Fields

Magnetic fields appear to be present in all galaxies and galaxy
clusters, and perhaps 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 showed that primordial
magnetic fields are expected to be generated in the early Universe on
purely linear scales through vorticity induced by scale-dependent
temperature fluctuations. Residual free electrons left over after
recombination tap into this vorticity to generate magnetic field via
the Biermann battery process. At redshifts of order a few tens, we
estimate a root mean square field strength of order 1e-25 – 1e-24G on
comoving scales ~10kpc. This field, which is generated purely from
linear perturbations, is expected to be amplified significantly after
deionization, bringing it close to the 10e-18 G estimated from observations,
and also to be further boosted by dynamo processes during
nonlinear structure formation.