Cosmological Background Interpretation of Pulsar Timing Array Data

We discuss the interpretation of the detected signal by pulsar timing array (PTA) observations as a gravitational wave background of cosmological origin. We combine NANOGrav 15-years and EPTA-DR2new datasets and confront them against backgrounds from supermassive black hole binaries (SMBHBs), and cosmological signals from inflation, cosmic (super)strings, first-order phase transitions, Gaussian and non-Gaussian large scalar fluctuations, and audible axions. We find that scalar-induced, and to a lesser extent audible axion and cosmic superstring signals, provide a better fit than SMBHBs. These results depend, however, on modeling assumptions, so further data and analysis are needed to reach robust conclusions. Independently of the signal origin, the data strongly constrain the parameter space of cosmological signals, for example, setting an upper bound on primordial non-Gaussianity at PTA scales as $|f_{\mathtt{nl}}|\lesssim 2.34$ at 95% C.L.

Figure 1

(a) 1D posteriors of field theory CSs and SMBHBs with $n_t=2/3$, (b) 2D posterior for a PL signal (black and gray lines indicate the error band for $n_t=0$ and $2/3$, respectively) (c) 2D posterior for AA, (d) 2D posterior for superstrings, (e) and (f) 2D posteriors for Gaussian and non-Gaussian SIGWB bump, respectively.

Figure 2

$h^2{\mathrm{\Omega }}_{\mathrm{GW}}^{(0)}(f)$ for various cases. Solid-colored lines represent central values, while lighter (lightest) regions the 68% (95%) C.L. intervals. Dark and light gray violins are the EPTA and NANOGrav free spectrum data, respectively.