Magnetic fields from inflation: The CMB temperature anisotropies

TL;DR

This paper investigates how primordial magnetic fields generated during inflation influence CMB temperature anisotropies, revealing an inflationary magnetic mode that dominates the anisotropy spectrum and tightens constraints on magnetic field properties.

Contribution

It introduces and analyzes an inflationary magnetic mode in the curvature perturbation, showing its dominance in CMB anisotropies and its impact on constraining magnetic field parameters.

Findings

The inflationary magnetic mode dominates the CMB anisotropy spectrum.

This mode leads to stronger constraints on magnetic field amplitude and spectral index.

The mode is scale invariant and detectable even for non-scale-invariant spectra.

Abstract

The cosmic microwave background (CMB) temperature anisotropies from primordial magnetic fields are studied. In addition to the known passive and the compensated mode we discuss an inflationary magnetic mode in the curvature perturbation, present when magnetic fields are generated during inflation. This mode is absent if the generation mechanism is causal, e.g. a phase transition. We compute and discuss the effect of this mode on the observed CMB anisotropy spectrum, in comparison with the passive and compensated ones. We find that it dominates the CMB anisotropy, and consequently leads to stronger constraints on the amplitude $B_{1\,{\rm Mpc}}$ and spectral index $n_B$ of the magnetic field than what usually found in CMB analyses from the compensated mode. This happens in particular for spectral indexes $n_B>-3$: the inflationary magnetic mode is always scale invariant, therefore through this mode even a magnetic field with a spectrum which is not scale invariant can leave a detectable signal in the CMB at large scales.