demagogue on 21/6/2009 at 17:46
I think because the doppler effect occurs when things are physically moving away from each other. In the case of distant galaxies, they are stationary (or not moving so much). It's just that the space between them is expanding. Uh, should probably be more graphic ...
In the case of the doppler effect, it's the object moving away that makes each successive wave of light (sound, water...) farther apart in the aft direction, because the object moved a little away in the interem (and in the forward direction the waves get bunched up and blue-ify). In the redshift effect, the galaxy isn't moving, so the light waves are coming off of them normally, in perfect circles of ripples. It's just that as the space in between the galaxies is expanding, the waves inside it themselves get expanded with that space. So it has the effect of stretching the waves in a way that looks like the doppler effect, and the light similarly reddens. But technically they're different.
E.g., there's no bunching up of waves on one side like with the doppler effect; you get the red shift from every direction. Also, in the doppler effect, the streching of the wave happens immediately as the waves leave the object. In the redshift, the light gradually grows redder and redder the older it gets (i.e., the farther away it moves). That's why the original light from a bit after the big bang has now been stretched so much it's now microwave sized (hence background microwave radiation).
Edit: Ah, about "speed is the same in every direction." That was just working with Tocky's phrasing, since he asked the question like that, and in a way that is telling to Tocky's basic confusion, since nothing is really moving in the space (like from a blast) so there is no true "speed" to even measure. Everything is still.
What heywood meant was the redshift is the same from every direction. The galaxies themselves aren't moving; it just looks like they are because space is expanding (and in effect moving them only relative to one another). And the speed it looks like they're going is the same everywhere (because the expansion of space is the same everywhere; at least there's reasons to say it's the same everywhere).
heywood on 21/6/2009 at 18:22
Yup, the expansion of space stretches the light waves causing the red shift. The older the photon, the more space has expanded since it was emitted, so the longer the wavelength. That's why more distant sources of light exhibit more red shift. Before Hubble, people assumed the red shift was due to the Doppler effect. If it were, the red shift should be the same no matter how far away the source because the age of the photon would not matter. But Hubble discovered that the red shift is proportional to the distance of the source (and thus the age of the photon) and the only way we can explain that is through expansion of space.
If the Big Bang were an explosion of matter out into space, then the red shift would not be time dependent but it would vary depending on where we look.
And Tocky, curved space time does not necessarily come back on itself. That is a closed spherical universe. The universe could also be open (unbounded) and hyperbolic. It depends on the curvature, which in turn depends on the mass density of the universe, and on the effect of mysterious "dark energy".
Vivian on 22/6/2009 at 08:22
Holy shit, that's how red shift works? My mind just popped.
