Gravitational-Wave Mediated Preheating
  We propose a new preheating mechanism through the coupling of the gravitational field to both the inflaton and matter fields, without direct inflaton-matter couplings. The inflaton transfers power to the matter fields through interactions with gravitational waves, which are exponentially enhanced due to an inflation-graviton coupling. One such coupling is the product of the inflaton to the Pontryagin density, as in dynamical Chern-Simons gravity. The energy scales involved are constrained by requiring that preheating happens fast during matter domination. 
Stephon Alexander, Sam Cormack, Antonino Marcian\`o, Nicol\'as Yunes
gr-qc hep-ph hep-th