A large scale coherent magnetic field: interactions with free streaming particles and limits from the CMB

TL;DR

This paper investigates how free-streaming particles like neutrinos influence the anisotropy caused by large-scale magnetic fields in the early universe, showing that neutrinos can significantly suppress CMB anisotropies if their masses are below 0.3 eV.

Contribution

It demonstrates that free-streaming relativistic particles can isotropize a universe with a large-scale magnetic field, reducing CMB anisotropies depending on neutrino masses.

Findings

Neutrino free-streaming suppresses magnetic field-induced anisotropy.

CMB temperature anisotropy is significantly reduced for neutrino masses below 0.3 eV.

The universe becomes more isotropic due to anisotropic pressure from relativistic particles.

Abstract

We study a homogeneous and nearly-isotropic Universe permeated by a homogeneous magnetic field. Together with an isotropic fluid, the homogeneous magnetic field, which is the primary source of anisotropy, leads to a plane-symmetric Bianchi I model of the Universe. However, when free-streaming relativistic particles are present, they generate an anisotropic pressure which counteracts the one from the magnetic field such that the Universe becomes isotropized. We show that due to this effect, the CMB temperature anisotropy from a homogeneous magnetic field is significantly suppressed if the the neutrino masses are smaller than 0.3 eV.