The millisecond pulsars in the international pulsar timing arrays https://ipta4gw.org/ https://ipta4gw.org/ are all within the Milky way. There is no metric ex

The millisecond pulsars in the international pulsar timing arrays https://ipta4gw.org/ https://ipta4gw.org/ are all within the Milky way. There is no metric expansion of space within the Milky way. The pulsars (and our solar system) are all on ~Keplerian orbits through the galaxy, and relative motion between us and them is small. > If the distance is great enough It's not, in this context. > Stars don't stay put Close enough, in this context. The uncertainties ("red noise") in PTA timing residuals from scattering in the interstellar medium and local properties (mostly internal structure) of the pulsars dwarf the uncertainties in relative motion. PTAs also care about the pulse timing not the carrier of the pulse. That is, the beam points at us with a predictable period and that period evolves (in parts per million) in the presence of nanohertz gravitational waves. The beam itself is broadband (radio to gamma rays) and any spectral line structure could be anything (most likely the interstellar medium). Detailed redshift studies look at the spread of spectral lines, particulary the Lyman and Balmer series, and pulsars are what you get when there's no hydrogen left to fuse. ("Finally, pulsars have broadband continuum spectra, so if there are gas clouds along our line of sight, pulsars can be used to probe the ISM via absorption by spectral lines of Hi or molecules. Such absorption spectra can be used to estimate pulsar distances" -- end of §6.2 of Condon & Ransom's Essential Radio Astronomy advanced undergraduate one-semester-course textbook (2016 ed.), web version at https://www.cv.nrao.edu/~sransom/web/Ch6.html https://www.cv.nrao.edu/~sransom/web/Ch6.html ) I have a related comment in this thread if you're interested.