Magnetic fields and chiral asymmetry in the early hot universe

TL;DR

This paper analytically investigates the generation and evolution of lepton chiral asymmetry and helical magnetic fields in the early universe, revealing their mutual support, scaling laws, and eventual decay due to electron chirality-changing scattering.

Contribution

It provides a detailed analytical model of the coupled evolution of chiral asymmetry and magnetic fields, including scaling laws and termination conditions.

Findings

Chiral asymmetry and magnetic fields support each other, prolonging their existence.

Magnetic fields undergo inverse cascade, transferring power to larger scales.

Chiral asymmetry decays exponentially at lower temperatures due to electron chirality-changing scattering.

Abstract

In this paper, we study analytically the process of external generation and subsequent free evolution of the lepton chiral asymmetry and helical magnetic fields in the early hot universe. This process is known to be affected by the Abelian anomaly of the electroweak gauge interactions. As a consequence, chiral asymmetry in the fermion distribution generates magnetic fields of non-zero helicity, and vice versa. We take into account the presence of thermal bath, which serves as a seed for the development of instability in magnetic field in the presence of externally generated lepton chiral asymmetry. The developed helical magnetic field and lepton chiral asymmetry support each other, considerably prolonging their mutual existence, in the process of `inverse cascade' transferring magnetic-field power from small to large spatial scales. For cosmologically interesting initial conditions, the chiral asymmetry and the energy density of helical magnetic field are shown to evolve by scaling laws, effectively depending on a single combined variable. In this case, the late-time asymptotics of the conformal chiral chemical potential reproduces the universal scaling law previously found in the literature for the system under consideration. This regime is terminated at lower temperatures because of scattering of electrons with chirality change, which exponentially washes out chiral asymmetry. We derive an expression for the termination temperature as a function of the chiral asymmetry and energy density of helical magnetic field.