Hopefully someone more knowledgeable about this chimes in but I believe calculating the redshift by knowing the expected spectrum and comparing to the observed

Hopefully someone more knowledgeable about this chimes in but I believe calculating the redshift by knowing the expected spectrum and comparing to the observed spectrum gives us the info needed for distance (since along the way the light is constantly stretched by the expansion of the universe). From Wikipedia on supernovae: > Because of the expansion of the universe, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more-distant objects recede with greater velocity than those nearby, and so have a higher redshift. And a bit on Hubble's law / the early discoveries around cosmological redshift: https://en.m.wikipedia.org/wiki/Hubble%27s_law https://en.m.wikipedia.org/wiki/Hubble%27s_law