Correlation of inflation-produced magnetic fields with scalar fluctuations

TL;DR

This paper investigates how breaking conformal invariance during inflation can produce correlated magnetic fields and scalar fluctuations, providing a potential new probe of non-Gaussianity in the early universe.

Contribution

It models the coupling between electromagnetism and a scalar field during inflation, calculating the resulting power spectra and cross-correlations, and shows how scale-free magnetic fields can be generated.

Findings

Magnetic fields with ~nG amplitude at Mpc scales can be produced.

Cross-correlation amplitude can reach up to ~500 H_I/M.

The model demonstrates a scale-free magnetic spectrum compatible with observations.

Abstract

If the conformal invariance of electromagnetism is broken during inflation, then primordial magnetic fields may be produced. If this symmetry breaking is generated by the coupling between electromagnetism and a scalar field---e.g. the inflaton, curvaton, or the Ricci scalar---then these magnetic fields may be correlated with primordial density perturbations, opening a new window to the study of non-gaussianity in cosmology. In order to illustrate, we couple electromagnetism to an auxiliary scalar field in a de Sitter background. We calculate the power spectra for scalar-field perturbations and magnetic fields, showing how a scale-free magnetic field spectrum with rms amplitude of ~nG at Mpc scales may be achieved. We explore the Fourier-space dependence of the cross-correlation between the scalar field and magnetic fields, showing that the dimensionless amplitude, measured in units of the power spectra, can grow as large as ~500 H_I/M, where H_I is the inflationary Hubble constant and M is the effective mass scale of the coupling.