Talk:Redshift

Previous talk at talk:Red Shift.

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I wonder if the redshift of light from distance galaxies is also a consequence of the fact that the space between the source and us has expanded as the light travelled through, "stretching" the waves out. Is that effect at work at all, and if so, how big is its effect compared to the Doppler effect? --AxelBoldt

I think (but don't quote me on this) that they are just two different ways of looking at the same thing. - AdamW

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I believe that Big Bang says that the whole universe started in a singular point, not just the observable universe. Even an infinite thing can start in a point: initially, the distances between all the infinitely many things were zero, then they took off. Isn't that true for the FRW models? --AxelBoldt

No, it's not. In the case of an infinite universe, the Big Bang starts from an infinite collection of points (infinite manifold is prefered by theorists, that I'm not).Remember that the universe has had a finite time to grow, if it was a point, now every point could only be at most at the distance it has had time to expand from that point. That is only true in the case of a closed, finite universe.

But it is true that even in an infinite universe, the distance between any two points is finite. Consider a one-dimensioinal universe: the real numbers R. We let this universe "expand" by defining the distance between the points x and y at time t to be d_t(x, y) = t |x - y|. As time t increases, the distances between any two points increases. The whole universe is of course infinite, but any two points, at any time t, have a finite distance. Now as you go back in time (t goes to zero), any two points come arbitrary close. At time t=0 you hit a singularity where all points have distance zero from each other, which one would typically call "a point". I'll have to look at Wald's book again. --AxelBoldt

Um, no. Try this: keep one point finite and take the lim as the other goes to infinity (perfectly valid in an infinite universe). Now take the limit as t goes to zero. In principle, the value could be anything (even zero), but because they are both linear terms order one, I'm inclined to say that the limit should be finite and non-zero. --BlackGriffen