Implications of the NANOGrav results for inflation
  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.
