Parity violation in stochastic gravitational wave background from inflation

Understand

We study the inflationary implications of a novel parity-violating gravity model, which modifies the teleparallel equivalent of general relativity by introducing the Nieh-Yan term coupled to an axion-like field.

• The parity-violating Nieh-Yan term results in the velocity birefringence of gravitational waves (GWs) and triggers the tachyonic instability only for one of the two circular polarization states.
• We consider that the inflaton is identified as the coupled axion-like field with a wiggly potential characterized by steep cliffs connected by smooth plateaus.
• During inflation, the temporary fast roll of axion on the cliff-like region leads to the significant enhancement of the tensor perturbations in one polarization state with the wave numbers that exit the horizon around this period.