Blue-tilted inflationary tensor spectrum and reheating in the light of NANOGrav results

TL;DR

This paper explores how a blue-tilted inflationary gravitational wave spectrum, influenced by the universe's thermal history, can explain NANOGrav results while satisfying other observational constraints.

Contribution

It demonstrates that considering reheating and entropy production allows a blue tensor spectrum to account for NANOGrav signals without conflicting with existing bounds.

Findings

Low reheating temperature enables blue tensor spectrum to fit NANOGrav data.

Late-time entropy production relaxes constraints, allowing instant reheating scenarios.

The spectral shape of IGWs is sensitive to the universe's thermal history.

Abstract

We discuss the possibility of explaining the recent NANOGrav results by inflationary gravitational waves (IGWs) with a blue-tilted primordial spectrum. Although such IGWs can account for the NANOGrav signal without contradicting the upper bound on the tensor-to-scalar ratio at the cosmic microwave background scale, the predicted spectrum is in strong tension with the upper bound on the amplitude of the stochastic gravitational wave background by big-bang nucleosynthesis (BBN) and the second LIGO-Virgo observation run. However, the thermal history of the Universe, such as reheating and late-time entropy production, affects the spectral shape of IGWs at high frequencies and permits evading the upper bounds. We show that, for the standard reheating scenario, when the reheating temperature is relatively low, a blue tensor spectrum can explain the recent NANOGrav signal without contradicting the BBN and the LIGO-Virgo constraints. We further find that, when one considers a late-time entropy production, the NANOGrav signal can be explained even for an instant reheating scenario.