Phase Transition Generated Cosmological Magnetic Field at Large Scales

TL;DR

This paper investigates how primordial magnetic fields generated during phase transitions can influence cosmological observations, especially through relic gravitational waves, constrained by nucleosynthesis bounds.

Contribution

It introduces a method to constrain phase transition generated primordial magnetic fields using big bang nucleosynthesis bounds and explores their observable signatures.

Findings

PMFs can leave detectable gravitational wave signatures.

Amplitude at 1 Mpc can be small yet still impactful.

Relic gravitational waves can serve as probes for PMFs.

Abstract

We constrain a primordial magnetic field (PMF) generated during a phase transition (PT) using the big bang nucleosynthesis bound on the relativistic energy density. The amplitude of the PMF at large scales is determined by the shape of the PMF spectrum outside its maximal correlation length scale. Even if the amplitude of the PMF at 1 Mpc is small, PT-generated PMFs can leave observable signatures in the potentially detectable relic gravitational wave background if a large enough fraction ($1-10%$) of the thermal energy is converted into the PMF.