[dleviwing]

You are getting into detail that requires one to understand the effects of temperature and pressure on the behavior of emission and absorption lines. These are usually considered synonymous to Doppler effect just as gravitational redshift is synonymous to pressure redshift. Absorption however is more sensitive to the state of the hydrogen molecules (space) as opposed to individual hydrogen atoms (Earthbound experiments). The redshift referred to in cosmology is not only based on hydrogen but on the heavier more stable elements not found in the interstellar regions of space; this also allows us to separate Doppler and expansion redshifts. I think most people just assume it is based only on hydrogen lines since this is what we see most on the Discovery Channel.