Sidian

thanks alot for the answers.

Friar Bellows

Cosmological redshift is definitely not the same as the Doppler effect. The former is due to expansion of space, the latter is due to movement through space. The wavelength of light from a light-emitting object in space is modified by the following three (*) effects:

1) Cosmological redshift (i.e. expansion of the universe, which is not a motion through space)

2) Gravitational redshift (i.e. a sort of loss of energy as the photon struggles out of the gravitational field, but don't quote me on that!)

3) Doppler effect, can be redshift or blueshift (i.e. due to the relative movement of source and detector)

Once you go past the Local Group of galaxies (and probably somewhat further than that, I can't remember now) the effect that dominates is number 1, cosmological redshift. I guess there might be a few exceptions from relativistic phenomena like jets.

(*) I think it's 3, there may be others I forget.

DigitalDruid

Yes, cosmological redshift is due to expnasion of space.The reason I discussed only the doppler effect is because it does not need quantum mechnaics( gravitaional redshift) or general relativity (cosmological redshift), yet it adequately explains the redshift of the local galaxies.

But for a full understanding of the redshift of remote galaxies the other two kinds of redshifts need to be considerd as well.

I find gravitational redshift easier to understand than cosmological redshift. For gravitational redshift, since photons lose energy working against gravitational field, from Plank's law (Energy =h x frequency), its frequency drops leading to redshift. I think this was confirmed in laboratory experiments at Harvard University some 30 years ago. Though this needs to be taken into account for remote galaxies, the gravitational redshift by itself is not an evidence for expansion of the universe. It would happen in a non-expanding universe too. That's what my understanding is. Is that right?

Or even cosmological redshift? There would be cosmological redshift even if the universe expanded earlier, but has since stopped expanding. Light from remote galaxies reaching us now would still show redshift being "stretched" en-route.

[ November 15, 2002: Message edited by: DigitalDruid ]</p>