The primordial magnetic field in our cosmic backyard

TL;DR

This paper reconstructs the 3D structure of primordial magnetic fields seeded during the early Universe's radiation epoch, evolving through cosmic structure formation, and estimates their present-day strength and morphology.

Contribution

It introduces a novel method combining Bayesian density field inference with MHD simulations to map primordial magnetic fields in 3D.

Findings

Lower limit of primordial magnetic field strength at recombination is ~10^{-23} G.

Evolved magnetic fields today are estimated to be above ~10^{-27} G in clusters.

Magnetic field morphology and Faraday rotation maps are provided for local universe objects.

Abstract

We reconstruct the 3D structure of magnetic fields, which were seeded by density perturbations during the radiation dominated epoch of the Universe and later on were evolved by structure formation. To achieve this goal, we rely on three dimensional initial density fields inferred from the 2M++ galaxy compilation via the Bayesian $\texttt{BORG}$ algorithm. Using those, we estimate the magnetogenesis by the so called Harrison mechanism. This effect produced magnetic fields exploiting the different photon drag on electrons and ions in vortical motions, which are exited due to second order perturbation effects in the Early Universe. Subsequently we study the evolution of these seed fields through the non-linear cosmic structure formation by virtue of a MHD simulation to obtain a 3D estimate for the structure of this primordial magnetic field component today. At recombination we obtain a reliable lower limit on the large scale magnetic field strength around $10^{-23} \mathrm{G}$, with a power spectrum peaking at about $ 2\, \mathrm{Mpc}^{-1}h$ in comoving scales. At present we expect this evolved primordial field to have strengthts above $\approx 10^{-27}\, \mathrm{G}$ and $\approx 10^{-29}\, \mathrm{G}$ in clusters of galaxies and voids, respectively. We also calculate the corresponding Faraday rotation measure map and show the magnetic field morphology and strength for specific objects of the Local Universe.