Re: Fully formed 8-11 billion year old galaxies observed.
From: starman (starman_at_tech.net)
Date: 07/09/04
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Date: Fri, 09 Jul 2004 00:33:28 -0400
Brian Tung wrote:
>
> starman wrote:
> > True but at what phase of the universe's evolution was it effectively
> > expanding faster than the speed of light?
>
> This question is a bit ill-formed. It's like asking at what speed is a
> rubber band being stretched. The answer depends on what section of the
> rubber band you're talking about. In the case of the rubber band, a
> natural section is to take two points at opposite "ends" of the rubber
> band, and to measure the rate at which the distance between those points
> is increasing.
>
> In the case of the universe, however, there isn't an obvious choice of
> two points, especially since the topology of the universe isn't well
> established. Rather, because the expansion of the universe appears to
> be fairly uniform (that is, the rate of "stretching" is the same here
> as it is anywhere), we give the expansion in terms of some abstract
> "scale."
>
> For instance, we say that in a flat universe, the scale goes as time
> raised to the two-thirds power. That is to say, if two points in the
> universe are separated by distance d at some time t after the Big Bang,
> then after time 8t, they are separated by distance 4d, because 8 raised
> to the two-thirds power is 4:
>
> 8^(2/3) = (8^2)^(1/3) = cbrt(8^2) = cbrt(64) = 4
>
> where cbrt is the cube root function.
>
> This means that whether or not two points are separating at a speed
> faster than that of light, at some time t, depends on how far apart
> they are at that time. The galaxies in the Virgo cluster have never
> receded from us at the speed of light, because they are too close to
> us. Even now, a typical distance to a galaxy in the Virgo cluster is
> "only" 60 million light-years or so. But the galaxy whose distance
> now is estimated to be 32 billion light-years--obviously, that galaxy
> has (at least for part of the history of the universe) been receding
> from us at more than the speed of light, since the universe is thought
> to be only about 13.6 billion years old.
>
> It seems paradoxical that we should be able to detect that galaxy, if
> it is receding from us at more than the speed of light. One can liken
> the photons from the galaxy as bugs that travel along a rubber band
> that is constantly stretching out. Amazingly, one can show that even
> if the ends of the rubber band are separating faster than the bugs can
> crawl, the bugs can *still* make it from one end to the other--the
> photons still make it from that distant galaxy all the way to us.
>
> Brian Tung
Thanks for that insight. It certainly stretches the imagination.
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