Adding helicity to inflationary magnetogenesis

TL;DR

This paper introduces a generalized inflationary magnetogenesis model incorporating a pseudoscalar term, enabling the generation of helical magnetic fields and gravitational waves consistent with observations at various inflationary energy scales.

Contribution

The model extends previous work by adding a pseudoscalar invariant, allowing for helical magnetic fields and relaxing fine-tuning constraints in inflationary magnetogenesis.

Findings

Generates helical magnetic fields compatible with intergalactic observations.

Produces a chiral spectrum of primordial gravitational waves.

Can achieve a high tensor-to-scalar ratio even at low inflationary energies.

Abstract

The most studied mechanism of inflationary magnetogenesis relies on the time-dependence of the coefficient of the gauge kinetic term $F_{\mu\nu}\,{F}^{\mu\nu}$. Unfortunately, only extremely finely tuned versions of the model can consistently generate the cosmological magnetic fields required by observations. We propose a generalization of this model, where also the pseudoscalar invariant $F_{\mu\nu}\,\tilde{F}^{\mu\nu}$ is multiplied by a time dependent function. The new parity violating term allows more freedom in tuning the amplitude of the field at the end of inflation. Moreover, it leads to a helical magnetic field that is amplified at large scales by magnetohydrodynamical processes during the radiation dominated epoch. As a consequence, our model can satisfy the observational lower bounds on fields in the intergalactic medium, while providing a seed for the galactic dynamo, if inflation occurs at an energy scale ranging from $10^5$ to $10^{10}$ GeV. Such energy scale is well below that suggested by the recent BICEP2 result, if the latter is due to primordial tensor modes. However, the gauge field is a source of tensors during inflation and generates a spectrum of gravitational waves that can give a sizable tensor to scalar ratio $r={\cal O}(0.2)$ even if inflation occurs at low energies. This system therefore evades the Lyth bound. For smaller values of $r$, lower values of the inflationary energy scale are required. The model predicts fully helical cosmological magnetic fields and a chiral spectrum of primordial gravitational waves.