Causality, initial conditions, and inflationary magnetogenesis
  The post-inflationary evolution of inflation-produced magnetic fields,
conventional or not, can change dramatically when two fundamental issues are
accounted for. The first is causality, which demands that local physical
processes can never affect superhorizon perturbations. The second is the nature
of the transition from inflation to reheating and then to the radiation era,
which determine the initial conditions at the start of these epochs. Causality
implies that inflationary magnetic fields dot not freeze into the matter until
they have re-entered the causal horizon. The nature of the cosmological
transitions and the associated initial conditions, on the other hand, determine
the large-scale magnetic evolution after inflation. Put together, the two can
slow down the adiabatic magnetic decay on superhorizon scales throughout the
universe's post-inflationary evolution and thus lead to considerably stronger
residual magnetic fields. This is "good news" for both the conventional and the
non-conventional scenarios of cosmic magnetogenesis. Mechanisms operating
outside standard electromagnetism, in particular, do not need to enhance their
fields too much during inflation, in order to produce seeds that can feed the
galactic dynamo today. In fact, even conventionally produced inflationary
magnetic fields might be able to sustain the dynamo.
