Speaker
Description
We investigate whether an Early-Universe stochastic gravitational–wave back-
ground (SGWB) can account for the common-spectrum process reported by
NANOGrav, while also being consistent with current and projected CMB measure-
ments of extra radiation. We compute the contribution of effective number of rela-
tivistic species, ∆Neff, for a number of Early-Universe models proposed to explain
the pulsar timing array (PTA) spectrum. We demonstrate that models predicting
∆Neff above the CMB limit would be firmly excluded, implying that the NANOGrav
signal in tension with these bounds must instead arise from astrophysical sources.
We find that current NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a
negligible contribution to ∆Neff and remains well below the present and future
CMB detection threshold. However, when we project future PTA capabilities reach-
ing upto 1 µHz, even with our conservative estimate we find that Inflation, Scalar
Induced Gravitational Waves (SIGW), and metastable cosmic strings can induce a
∆Neff large enough for >3.5σ detection by the Simons Observatory.
