Primordial Magnetic Field from Gravitationally Coupled Electrodynamics in Bouncing Scenario

TL;DR

This paper investigates the generation of primordial magnetic fields during a non-singular bouncing cosmology by coupling electromagnetism to gravity, revealing a blue-tilted spectrum and constraints on bouncing models.

Contribution

It introduces a scale-factor-dependent coupling model for electromagnetic fields in bouncing scenarios, analyzing its effects on primordial magnetic field generation and constraints.

Findings

Primordial magnetic field spectrum is blue-tilted on large scales.

Observational constraints favor the ekpyrotic-bounce scenario.

Back reaction imposes theoretical limits on the bouncing model.

Abstract

We in this paper study the generation of primordial magnetic field (PMF) in the non-singular bouncing scenario, through the coupling of the electromagnetic field to gravity. We adopt an electrodynamic model with a coupling coefficient as a function of the scale factor $a$, i.e. $f=1+(a/a_\star)^{-n}$, with $a_\star$ and $n>0$ being constants. The result implies that in this mechanism, the power spectrum of PMF today is always blue tilted on large scales from $1$ Mpc to the Hubble length, and the observational constraints favor the ekpyrotic-bounce scenario. Furthermore, the back reaction of the energy density of PMF at the bouncing point yields theoretical constraints on the bouncing model.