Resonant Primordial Gravitational Waves Amplification

Observational signatures of the parametric amplification of gravitational waves during reheating after inflation

“…However, this is not necessarily the case if the tensor perturbation is subject to some growth after inflation. Indeed, there are models in which the existence of a time-dependent tensor mass leads to the amplification of GWs during the reheating stage after inflation [16] 2 .…”

“…The massive gravity scenario studied in Ref. [16], which was originally advocated in Refs. [18,19], is constructed under the internal SO(3) symmetry φ i → Λ i j φ j , and φ i → φ i + Ξ i (φ 0 ), where Ξ i is a general function of its argument.…”

“…The internal symmetry φ i → φ i + Ξ i (φ 0 ) forbids the propagation of vector modes [18,19], so we are left with only three dynamical degrees of freedom: one scalar mode and two tensor modes. 3 We minimally extend our inflationary scenario by adding this tensor mass m g into our theory, and identify the scalar mode with the inflaton scalar field φ [16]. Since the scalar mode does not give rise to a ghost state, our theory is free from the Higuchi bound [22], which is always an issue in cosmology for theories that respect de Sitter symmetry.…”

“…Since the scalar mode does not give rise to a ghost state, our theory is free from the Higuchi bound [22], which is always an issue in cosmology for theories that respect de Sitter symmetry. Further we impose the global scaling symmetry φ i → λφ i , which guarantees the non-dynamical nature of φ i in the cosmological background [16,23].…”

“…To recover the local Lorentzinvariance after reheating, we require that m g eventually vanishes. The φ-dependent tensor mass leads to the parametric resonant amplification of GWs at the early stage of reheating [24,25] due to coherent oscillations of the inflaton around the potential minimum [16].…”