Implications of the NANOGrav results for inflation

TL;DR

The paper explores whether the NANOGrav gravitational wave signal can be explained by an inflationary origin, considering constraints on tensor spectra, reheating scale, and existing observational limits.

Contribution

It demonstrates that an inflationary explanation for NANOGrav is plausible under specific conditions on the tensor spectral index and reheating temperature.

Findings

Inflationary models with blue tensor spectra can explain NANOGrav data.

Reheating temperature constraints are compatible with the inflationary interpretation.

An inflationary origin of the NANOGrav signal is not excluded under power-law spectrum assumptions.

Abstract

The NANOGrav pulsar timing array experiment reported evidence for a stochastic common-spectrum process affecting pulsar timing residuals in its 12.5-year dataset, which might be interpreted as the first detection of a stochastic gravitational wave background (SGWB). I examine whether the NANOGrav signal might be explained by an inflationary SGWB, focusing on the implications for the tensor spectral index $n_T$ and the tensor-to-scalar ratio $r$. Explaining NANOGrav while complying with upper limits on $r$ from BICEP2/Keck Array and Planck requires $r \gtrsim {\cal O}(10^{-6})$ in conjunction with an extremely blue tensor spectrum, $0.7 \lesssim n_T \lesssim 1.3$. After discussing models which can realize such a blue spectrum, I show that this region of parameter space can be brought in agreement with Big Bang Nucleosynthesis constraints for a sufficiently low reheating scale, $T_{\rm rh} \lesssim 100\,{\rm GeV}-1\,{\rm TeV}$. With the important caveat of having assumed a power-law parametrization for the primordial tensor spectrum, an inflationary interpretation of the NANOGrav signal is therefore not excluded.