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Old September 28 2012, 12:14 AM   #59
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Re: About planetary gravity

The Borgified Corpse wrote: View Post
Christopher wrote: View Post
No, because there's no center of the universe. The universe is effectively infinite and its expansion is uniform.
How does that reconcile with the Big Bang Theory? Isn't all matter in the universe thought to have expanded outward from a single pinpoint? Wouldn't the original site of that pinpoint be the ostensible center of the universe (even if we're currently incapable of observing where that would be)?
That's not what it means at all. "Big Bang" is a misnomer, coined by Fred Hoyle in order to ridicule the theory, since he was a fan of the continuous-creation cosmological model which has since been definitively debunked. It shouldn't be interpreted as a literal explosion into space. After all, it was space itself that was being created. It didn't expand into anything, it just expanded -- it started out infinitely dense and then things within it got further apart, uniformly.

A useful analogy is to imagine a microbe living on the surface of a balloon. When you blow up a balloon, the whole surface expands, but the center of that expansion is nowhere on the surface itself; it's in the center of the balloon. The microbe will perceive the balloon expanding outward from itself in all directions, and imagine itself (it's an unusually thoughtful microbe) as being at the center of the ballooniverse. But another microbe on the other side of the balloon will see the same thing, and so will every other microbe no matter where it lives on the surface. None of them has any more claim to calling its location the center of their 2-dimensional universe than any other, because the surface is expanding outward from every point within it, not just one.

It used to be thought that our universe might be a 3-dimensional "surface" of a 4-dimensional closed shape, like the skin of a balloon with a dimension added, so that there'd be no center of expansion within the universe itself. We now think that the universe is "flat" but infinite -- and something that's infinite can't have a definable center, because it has no edges relative to which a center could be defined.
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