> If that's what this is No, this is about the nanohertz gravitational wave "background", which comes from massive binaries with orbital periods of on the orde

> If that's what this is No, this is about the nanohertz gravitational wave "background", which comes from massive binaries with orbital periods of on the order of months to years. Sources are expected to be be randomly arranged in spherical coordinates (up to an unfortunately low radial distance cutoff), like the (low-redshift) galaxies they likely live in the middle of. Primordially, most still-viable types of inflation produce gravitational waves on all length scales, so some very short wavelength primordial GWs are in principle accessible to us despite the large cosmological redshift (z >> 10^10; 10^10 is about the redshift at neutrino decoupling, a good guess is that z ~ 10^25 is the ballpark for the latest high-amplitude-peak (~ 10^-17 Hz) inflation-produced gravitational radiation[1]). If multimessenger and other techniques can help subtract the "foreground" of binary black holes and the like, then perhaps we will see them as redshifted-to-light-year-wavelengths not traceable to a hard SMBH binary or foreground mimicers. The CMB, particularly in its B-mode polarization, is already a useful probe of primordial gravitational radiation as was anticipated at least as early as BOOMERanG's 2003 flight. A better measurement of (dust and gas, and also some gravitational lensing) foregrounds is important there too. [1] Grischchuk 2005 §3, most easily obtained at https://arxiv.org/abs/gr-qc/0504018v4 https://arxiv.org/abs/gr-qc/0504018v4 (see Comments section below abstract for its provenance). PS: Bécsy, Cornish & Kelley 2022 (open access at The Astrophysical Journal, but easiest to grab the arxiv version which matches the published paper) https://arxiv.org/abs/2207.01607 https://arxiv.org/abs/2207.01607 is a very good overview of the nanohertz GW background(s) and how they're accessible to pulsar timing arrays. A couple of the recent coordinated-release papers by the various intl PTA members are also good overviews https:///www.ipta4gw.org/ https:///www.ipta4gw.org/