Decoding the phases of early and late time reheating through imprints on primordial gravitational waves

TL;DR

This paper investigates how different reheating scenarios after inflation imprint on primordial gravitational waves, potentially allowing us to decode early universe physics through GW observations.

Contribution

It introduces models of reheating with abrupt and gradual transitions, analyzing their distinct effects on the primordial GW spectrum and implications for recent observations.

Findings

Perturbative inflaton decay causes oscillations in the GW spectrum.

Secondary reheating phases can produce GW signals consistent with NANOGrav data.

Different reheating EoS parameters significantly alter the GW spectrum shape.

Abstract

Primordial gravitational waves (GWs) carry the imprints of the dynamics of the universe during its earliest stages. With a variety of GW detectors being proposed to operate over a wide range of frequencies, there is great expectation that observations of primordial GWs can provide us with an unprecedented window to the physics operating during inflation and reheating. In this work, we closely examine the effects of the regime of reheating on the spectrum of primordial GWs observed today. We consider a scenario wherein the phase of reheating is described by an averaged equation of state (EoS) parameter with an abrupt transition to radiation domination as well as a scenario wherein there is a gradual change in the effective EoS parameter to that of radiation due to the perturbative decay of the inflaton. We show that the perturbative decay of the inflaton leads to oscillations in the spectrum of GWs, which, if observed, can possibly help us decipher finer aspects of the reheating mechanism. We also examine the effects of a secondary phase of reheating arising due to a brief epoch driven possibly by an exotic, non-canonical, scalar field. Interestingly, we find that, for suitable values of the EoS parameter governing the secondary phase of reheating, the GWs can be of the strength as suggested by the recent NANOGrav observations. We conclude with a discussion of the wider implications of our analysis.