Inflationary Magnetogenesis without the Strong Coupling Problem
  The simplest gauge invariant models of inflationary magnetogenesis are known
to suffer from the problems of either large backreaction or strong coupling,
which make it difficult to self-consistently achieve cosmic magnetic fields
from inflation with a field strength larger than $10^{-32} G$ today on the
$\Mpc$ scale. Such a strength is insufficient to act as seed for the galactic
dynamo effect, which requires a magnetic field larger than $10^{-20} G$. In
this paper we analyze simple extensions of the minimal model, which avoid both
the strong coupling and back reaction problems, in order to generate
sufficiently large magnetic fields on the Mpc scale today. First we study the
possibility that the coupling function which breaks the conformal invariance of
electromagnetism is non-monotonic with sharp features. Subsequently, we
consider the effect of lowering the energy scale of inflation jointly with a
scenario of prolonged reheating where the universe is dominated by a stiff
fluid for a short period after inflation. In the latter case, a systematic
study shows upper bounds for the magnetic field strength today on the Mpc scale
of $10^{-13} G$ for low scale inflation and $10^{-25} G$ for high scale
inflation, thus improving on the previous result by 7-19 orders of magnitude.
These results are consistent with the strong coupling and back reaction
constraints.
