Primary origin of the PTA nanohertz stochastic gravitational-wave background

Determine whether the stochastic gravitational-wave background reported by pulsar timing array collaborations (NANOGrav, EPTA/InPTA, IPTA) at frequencies around O(10) nanohertz is astrophysical in origin (e.g., supermassive black hole binaries) or primordial in origin (e.g., early-universe processes), by establishing observational and modeling criteria that discriminate between these possibilities in a manner consistent with current datasets.

Background

The paper reviews current gravitational-wave observations, noting recent PTA data releases that provide tentative evidence for a low-frequency stochastic background at nanohertz scales. The authors emphasize that the nature of this background remains uncertain, with possibilities including astrophysical sources like supermassive black hole binaries and primordial cosmological origins.

Resolving the origin of the PTA signal is critical for interpreting early-universe physics and for assessing whether the detected background can constrain post-inflationary dynamics. This uncertainty is external to the paper’s main modeling contributions but is explicitly flagged by the authors as an outstanding question in the field.

References

The recent data release by the PTA observations, such as the NANOGrav collaboration, the EPTA/InPTA and IPTA provide tentative evidence for the detection of a background of stochastic GWs at low frequencies (~ O(101) nHz range), whose primary origin, whether astrophysical or primordial, is yet to be established.

— Inflationary Gravitational Waves as a probe of the unknown post-inflationary primordial Universe  (2407.07956 - Soman et al., 2024) in Section 1 (Introduction)

While the interpretation of the PTA signal remains open, this has sharpened the broader question of whether future gravitational-wave observations can distinguish standard cosmic strings from superstring-motivated extensions.

— Bayesian Forecasts on Cosmic Superstring Searches with LISA  (2608.19406 - Datta et al., 19 Aug 2026) in Section 2, paragraph beginning “The possibility that cosmic strings or superstrings could contribute to a nanohertz stochastic background”