Enhancement of gravitational waves induced by scalar perturbations due to a sudden transition from an early matter era to the radiation era

Stochastic gravitational wave background is produced from the primordial curvature perturbations at the second order of perturbations. We revisit such induced gravitational waves produced around a sudden reheating transition from an early matterdominated era to the radiation-dominated era. We find significant enhancement of the induced gravitational waves because of fast oscillations of scalar modes already inside the horizon just after the transition with unsuppressed amplitude. The enhancement is so significant that the future gravitational wave observations can detect the gravitational waves if the reheating temperature is in the range T_R ≲ 7 × 10⁻²GeV or 20 GeV ≲ T_R ≲ 2 × 10⁷ GeV. Thus, the induced gravitational waves can be a useful probe of the reheating dynamics of the universe. This talk is based on Ref. [1].