Primordial gravitational waves offer unique insights into the inflationary period and subsequent thermal history of the Universe. The spectrum of primordial high-frequency gravitational waves is...Show morePrimordial gravitational waves offer unique insights into the inflationary period and subsequent thermal history of the Universe. The spectrum of primordial high-frequency gravitational waves is highly sensitive to the processes in the early Universe and can be significantly suppressed during an epoch of early matter domination (EMD) induced by new long-lived massive particles. This damping effect is studied with numerical and analytic methods. The relative energy density of gravitational waves today is found to scale with the wavenumber k as k^(-2) for waves crossing the horizon during the EMD epoch. The overall damping between the start and the end of the EMD epoch is given by m^(4/3) Γ^(-2/3)M^(-2/3), where m and Γ are the mass and decay width of the long-lived particles correspondingly, and M is the Planck mass. For concrete examples of EMD, models with inflaton decay and heavy neutral leptons are considered. Experimental observation of stochastic gravitational wave background could probe early cosmological events and constrain new physics scenarios.Show less
