Blue-tilted Tensor Spectrum and Thermal History of the Universe

TL;DR

This paper examines how observational data constrains the spectral index of primordial gravitational waves, especially for blue-tilted spectra, considering effects like reheating and entropy production, revealing that constraints are highly model-dependent.

Contribution

It provides a comprehensive analysis of observational bounds on blue-tilted gravitational wave spectra, incorporating various early Universe scenarios and effects.

Findings

Constraints vary significantly with reheating temperature.

Highly blue-tilted spectra can still be consistent with observations.

Model assumptions critically influence the allowed spectral tilt.

Abstract

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