Palatini approach and large-scale magnetogenesis

TL;DR

This paper explores how modifications to gravity via the Palatini approach influence large-scale magnetogenesis, showing potential for generating significant primordial magnetic fields without affecting inflationary scalar modes.

Contribution

It introduces a nonlinear Einstein-Hilbert term in the Palatini framework and analyzes its effects on magnetogenesis and inflationary dynamics.

Findings

Tensor-to-scalar ratio is suppressed by nonlinear terms.

Large-scale magnetic fields can reach nanogauss levels.

A stiff phase can follow inflation with a quintessential potential.

Abstract

Large-scale magnetogenesis is analyzed within the Palatini approach when the gravitational action is supplemented by a contribution that is nonlinear in the Einstein-Hilbert term. While the addition of the nonlinear terms does not affect the scalar modes of the geometry during the inflationary phase, the tensor-to-scalar ratio is nonetheless suppressed. In this context it is plausible to have a stiff phase following the standard inflationary stage provided the potential has a quintessential form. The large-scale magnetic fields can even be a fraction of the nG over typical length scales of the order of the Mpc prior to the gravitational collapse of the protogalaxy.