Inflationary magneto-(non)genesis, increasing kinetic couplings, and the strong coupling problem

TL;DR

This paper investigates inflationary magnetogenesis with increasing kinetic couplings, demonstrating that while it avoids the strong coupling problem, it cannot produce scale-invariant magnetic fields due to amplitude suppression.

Contribution

It shows that increasing kinetic couplings during inflation circumvent the strong coupling problem but prevent the generation of scale-invariant magnetic fields.

Findings

Strong coupling problem can be avoided with moduli destabilization.

Magnetic fields dominate over electric fields during inflation.

Amplitude of magnetic fields is suppressed after moduli destabilization.

Abstract

We study the generation of magnetic fields during inflation making use of a coupling of the inflaton and moduli fields to electromagnetism via the photon kinetic term, and assuming that the coupling is an increasing function of time. We demonstrate that the strong coupling problem of inflationary magnetogenesis can be avoided by incorporating the destabilization of moduli fields after inflation. The magnetic field always dominates over the electric one, and thus the severe constraints on the latter from backreaction, which are the demanding obstacles in the case of a decreasing coupling function, do not apply to the current scenario. However, we show that this loophole to the strong coupling problem comes at a price: the normalization of the amplitude of magnetic fields is determined by this coupling term and is therefore suppressed by a large factor after the moduli destabilization completes. From this we conclude that there is no self-consistent and generic realization of primordial magnetogenesis producing scale-invariant fields in the case of an increasing kinetic coupling.