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How Galaxies Are Disappearing From Our Universe
StartsWithABang writes: You've heard of dark energy before, and you know that it causes the expansion of our Universe to be accelerating. Instead of slowing down, distant galaxies are speeding up in their recession from us, rendering them unreachable from our point of view. But even though we can't see the light emitted from them today, we can still see the galaxies themselves! This article explains how this works, how no information gets lost, and what it means for the Big Bang.
What always bothered me about the balloon analogy was the implication that this expansion of space is mostly taking place where there's little actual matter, ie the space between galaxies. If it really was expanding like dots on a balloon, we'd see equivalent expansion within galaxies and as far as I'm aware we don't, at least not to any significant degree.
At that scale, gravity massively dwarfs expansion. For any system which is gravitationally bound, you can assume the "force" of expansion is trivial.
Actually the whole thing is bothersome, if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?
I think you understand. Yes, the hypothesis is that space itself is being created, and that this is a fundamental law of physics. There's no fundamental law for it to violate, there's conservation mass and energy, no conservation of space.
I am personally doing my part to conserve galaxies and I hope that all of you are too. Please, please, please help do your part to conserve this valuable resource before it is too late. Not just for today because it's Universe Day, but for life.
No. At least I don't think so. I'll admit my understanding is fuzzy, but a few points to think on:
As I understand it (poorly), virtual particles appear to violate mass/energy conservation by their very nature - but they don't actually possess mass or energy themselves. Instead they sort of run on an "energy debt" for a few moments before either annihilating or capturing some "normal" energy so that they can persist. I think the energy is believed to be borrowed from the vacuum energy field - so that essentially you've temporarily lowered the energy of space itself in order to create some virtual particles, and there is no net energy change within the volume.
Vacuum energy is *not* conserved - like dark energy it is created along with new space-time. Again this simply appears to be one of the fundamental laws of the universe - mass/energy conservation apparently doesn't apply to space-time itself, only to things *within* space-time.
And finally there's a real possibility that matter can be created from whole cloth without violating conservation: In creating mass you also create a gravitational field, which in turn reduces the gravitational potential energy of everything else in the universe (gravitational energy is always negative). That reduction in the potential energy of the entire universe *may* perfectly balance the increase in mass-energy of the new matter. This is actually one of the theories about the big bang - it may be that the entire universe, taken as a whole, contains exactly zero net mass-energy: in essence everything really was created from nothing, and if you squeezed it all back together again you'd find there was still nothing there. It's just that the nothingness has been separated from itself in such a manner that things can be built out of the pieces.
--- Most topics have many sides worth arguing, allow me to take one opposite you.