Correlation of inflation-produced magnetic fields with scalar
  fluctuations
  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.
