Cosmic magnetization out from the vacuum

TL;DR

This paper proposes a quantum vacuum effect during inflation as a mechanism for generating large-scale cosmic magnetic fields, showing it can produce observable fields without common theoretical problems.

Contribution

It demonstrates that breaking conformal invariance during inflation can produce scale-invariant magnetic fields and resolves issues like strong coupling and backreaction in simple models.

Findings

Scale-invariant magnetic fields of about 10^{12} G can be generated.

The model avoids strong-coupling and backreaction problems.

Post-inflationary electric currents do not freeze superhorizon magnetic fields.

Abstract

The large-scale magnetic fields we observe today in galaxies and galaxy clusters could be the result of a pure quantum effect taking place during inflation, to wit, the creation of particles (photons) out from the vacuum in a curved spacetime. We show that, whenever the conformal invariance of electromagnetism is broken during inflation, the actual magnetic field spectrum, in the classical limit, is given by $B_k \simeq k^2 \sqrt{n_{\mathbf{k}} }$, where $n_{\mathbf{k}} \gg 1$ is the number of created photons with wavenumber $k$. In particular, a scale-invariant magnetic field of order of $10^{12}$G can emerge in the simplest model of cosmic magnetogenesis, the one where the inflaton is kinetically coupled to the photon. Moreover, and contrarily to the general belief, we show that such a model is free from the so-called strong-coupling and backreaction problems. This conclusion follows, indirectly, from the observation that post-inflationary electric currents, which in the literature are incorrectly supposed to freeze superhorizon magnetic fields after inflation, are indeed vanishing on superhorizon scales due to causality.