[Arp220]

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Okay, new question about whether there is a center point to the universe. I understand the idea that space itself expands. The universe itself may be endless, but what about the matter originating in the BB? None if it is more than 14 billion or 47 billion apart. Or I guess that would be actually 94 BLY apart, if it's 47 in each direction. So within the infinite, shapeless universe, isn't there an expanding globe of stuff -- our observable universe -- that has a center point? If the universe is 94 billion LY wide, can't 47 BLY in from the frontier be considered a center point? And where in the sky would it be?


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I agree it is slightly subtle - on the other hand, it IS basic information for someone setting themselves up as any kind of 'authority' on matters cosmological

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He understands that. The critics are just pouncing on something ambiguous, inadequately explained prose. It would be much more adult to say "Don't you mean 47 BLY?" rather than "gotcha, you wrong, wrong, wrong, bow before my superior intellect for all the world to see."

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Ok:

The 'comoving radial distance' is about 47 billion light years. Now, from this we can infer that there exist (say) two galaxies that are 94Glyr apart, but those two galaxies cannot see each other. We can see both of them though.

This alludes to this 'center of the universe' stuff. Its often stated that the universe has no center, but this is not exactly true. For all practical purposes, *we* are at the center of the universe, because we see everything expanding away from us in all directions. Now, its also true that, if we were to go live in another galaxy far far away, we would also see everything expanding away from us. In other words, wherever we are, we are at the center of the universe!

This sounds a touch egotistical, but in fact what its saying is that we are at the center of the *observable* universe. Which makes much more sense.

As to whether the universe has an absolute center - well, its rather a moot question, because, unless we work out a way to look beyond the particle horizon, its almost impossible to answer. We have no observed evidence for an absolute 'center', therefore we can say there is no center. Of course, if evidence comes along to contradict this then things will change.

There is however an observable absolute 'rest-frame' for the universe. If we look at the cosmic microwave background, we see that it is marginally hotter in one direction. This is called the 'dipole anisotropy', and is caused by our movement relative to the CMB.

As for Gugel - I applaud his efforts, but the fact remains that he said something demonstrably incorrect. It wasnt in the least bit ambiguous.