Blue-tilted tensor spectrum and thermal history of the Universe

Abstract

We investigate constraints on the spectral index of primordial gravitational\nwaves (GWs), paying particular attention to a blue-tilted spectrum. Such\nconstraints can be used to test a certain class of models of the early\nUniverse. We investigate observational bounds from LIGO+Virgo, pulsar timing\nand big bang nucleosynthesis, taking into account the suppression of the\namplitude at high frequencies due to reheating after inflation and also\nlate-time entropy production. Constraints on the spectral index are presented\nby changing values of parameters such as reheating temperatures and the amount\nof entropy produced at late time. We also consider constraints under the\ngeneral modeling approach which can approximately describe various scenarios of\nthe early Universe. We show that the constraints on the blue spectral tilt\nstrongly depend on the underlying assumption and, in some cases, a highly\nblue-tilted spectrum can still be allowed.\n