From the paper: > "Spectroscopic observations of the afterglow and the host galaxy provided a redshift of z = 0.151 (de Ugarte Postigo et al. 2022; Castro-Tira

From the paper: > "Spectroscopic observations of the afterglow and the host galaxy provided a redshift of z = 0.151 (de Ugarte Postigo et al. 2022; Castro-Tirado et al. 2022; Izzo et al. 2022), corresponding to a distance of 749.3 Mpc." 3.262e6 (so about 3.3 million light years per megaparsec) * 749.3 gives 2.445 billion light years, so in the ballpark. The key thing in getting these estimates is the complex business of standard candles, which are astronomical objects believed to have a well-defined luminosity so one can calibrate distances by looking at how faint they are. There are two major kinds used, Cepheid variables (which only cover the range from 1 kpc to 50 Mpc, so not useful here) and type 1a supernovae (which are rare): > "Beyond the question of whether conditions in the early universe affect the luminosity of a type 1a supernova, a second problem besets this standard candle that severely limits its use: supernovae are rare in any given galaxy. They therefore cannot be used to determine the distance to any galaxy that we may be interested in. Their only uses are in calibrating other distance measures, such as the cosmological redshift, in testing cosmological theories, and in studying the surrounding of these supernovae. Current studies with these standard candles are examining the expansion of the universe at redshifts between z = 0.01 and 1." https://astrophysicsspectator.org/topics/overview/DistanceExtragalactic.html https://astrophysicsspectator.org/topics/overview/DistanceEx... It might be interesting to look at those cited papers to see exactly how they got that measurement but it's bedtime. P.S. Crash Course Astronomy #25 (Distances) is a fun 10 min watch, great series overall: https://youtu.be/CWMh61yutjU https://youtu.be/CWMh61yutjU