Bridging inflation and reheating: chiral gravitational waves from aHz to GHz

Abstract

In this paper, we investigate chiral gravitational wave (GW) signals generated from inflation to reheating, driven by a parity-violating (PV) term coupled to the inflaton, $\dotϕε^{ijk} h_{il}\partial_j h_{k}^l$, which naturally arises in PV extensions of teleparallel gravity. During inflation, the PV term reduces the sound horizon for right-handed circularly polarized GWs, and amplifies their power spectra relative to left-handed GWs. At CMB scales, these chiral GWs induce BB as well as non-vanishing EB and TB correlations in CMB, which are potentially detectable by LiteBIRD. During reheating, subhorizon modes undergo tachyonic instability, leading to fully circularly polarized GWs with enhanced amplitudes, which may be probed by future high-frequency GW experiments, such as resonant cavity. The absence of backreaction effect of enhanced chiral GWs imposes constraints on the energy scale of the PV term, the inflationary potential, and the reheating history. Our findings highlight the potential of multi-frequency GW experiments to offer a unique probe of the parity violation and early Universe.