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

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.

Primordial circular polarization in the cosmic microwave background

“…The spectral tilt of the resulting electromagnetic spectra will depend on the value of n. In the absence of the axionic coupling, these are given by (see e.g. [19,20])…”

“…From which one can compute the spectral indices (20). As in the minimal axion model [13], the amplitude of P V is enhanced by the exponential factor e 2πξ , Ignoring backreaction considerations, the power spectrum of V-mode anisotropies is given by…”

Viable inflationary magnetogenesis with helical coupling

“…The general difficulty with scenarios of this kind is the evolution law of magnetic field B ∝ I/a 2 (with a being the scale factor) on the super-Hubble spatial scales. As I decreases from large values back to unity, the gain in the magnetic field obtained during its preceding growth is lost [22].…”

“…Numerous versions of this model have been under consideration in the literature (see [15,16] for recent reviews). Scenarios based on the kinetic coupling to electromagnetism meet with the issues of back-reaction and strong gauge coupling [20][21][22]. Essentially, if one assumes the function I to be monotonically decreasing with time, then it is electric field that is predominantly amplified, causing the problem of back-reaction on inflation and making it difficult to generate magnetic fields of plausible values.…”

Inflationary Magnetogenesis with Helical Coupling

“…Scenarios with the kinetic coupling to electromagnetism face the problems of back-reaction and strong gauge coupling [11][12][13]. Essentially, if the function is monotonically decreasing with time, then it is electric field that is predominantly enhanced, causing the problem of back-reaction on inflation and preventing generation of magnetic fields of plausible strengths.…”