Gravitational waves from gauge preheating
  We study gravitational wave production during Abelian gauge-field preheating
following inflation. We consider both scalar and pseudoscalar inflaton models
coupled directly to Abelian gauge fields via either a dilatonic coupling to the
gauge-field kinetic term or an axial coupling to a Chern-Simons term. In both
cases gravitational waves are produced efficiently during the preheating phase,
with a signature louder than most cosmological signals. These gravitational
waves can contribute to the radiation energy budget of Universe at a level
which will be probed by upcoming cosmic microwave background experiments
through $N_{\rm eff}$. For axially coupled fields the resulting gravitational
wave spectrum is helically polarized---a unique feature that can be used to
differentiate it from other stochastic gravitational wave backgrounds. We
compute the gravitational topological charge and demonstrate that gauge
preheating following axion inflation may be responsible for the
matter-antimatter asymmetry of the Universe via gravitational leptogenesis.
