Adding helicity to inflationary magnetogenesis
  The most studied mechanism of inflationary magnetogenesis relies on the
time-dependence of the coefficient of the gauge kinetic term
$F_{\mu\nu}\,{F}^{\mu\nu}$. Unfortunately, only extremely finely tuned versions
of the model can consistently generate the cosmological magnetic fields
required by observations. We propose a generalization of this model, where also
the pseudoscalar invariant $F_{\mu\nu}\,\tilde{F}^{\mu\nu}$ is multiplied by a
time dependent function. The new parity violating term allows more freedom in
tuning the amplitude of the field at the end of inflation. Moreover, it leads
to a helical magnetic field that is amplified at large scales by
magnetohydrodynamical processes during the radiation dominated epoch. As a
consequence, our model can satisfy the observational lower bounds on fields in
the intergalactic medium, while providing a seed for the galactic dynamo, if
inflation occurs at an energy scale ranging from $10^5$ to $10^{10}$ GeV. Such
energy scale is well below that suggested by the recent BICEP2 result, if the
latter is due to primordial tensor modes. However, the gauge field is a source
of tensors during inflation and generates a spectrum of gravitational waves
that can give a sizable tensor to scalar ratio $r={\cal O}(0.2)$ even if
inflation occurs at low energies. This system therefore evades the Lyth bound.
For smaller values of $r$, lower values of the inflationary energy scale are
required. The model predicts fully helical cosmological magnetic fields and a
chiral spectrum of primordial gravitational waves.
