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Is this a postulate? I get the expansion of the universe, the addition of discrete bits of space time between me and a distant galaxy, until very distant parts of the universe are moving relative to me, faster than the speed of light. But the other sure seems like (giant steaming load of convenience) I think I get reasons why, because they don't have any other way to explain the size of the universe and so on. Just seems it would be much easier to say the universe is eternal and cyclical in nature, and call it a day!

Qmechanic
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Todd Burkett
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    from this and your other question about black holes, it seems like you are basing a lot of your conceptions on pop-science descriptions of phenomena. If you haven't learned the mathematics yet that's fine, but trying to poke holes in theories based on layman's analogies is a fairly futile endeavour –  Jan 26 '12 at 23:17
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    I'd have sworn that we'd done this already but I can't find a duplicate. – dmckee --- ex-moderator kitten Jan 26 '12 at 23:31
  • Possible duplicates: http://physics.stackexchange.com/q/26549/2451 and links therein. – Qmechanic May 22 '13 at 20:29

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There are rules about things moving through space faster than light - there is no rule about space expanding faster than light.

As long as it can't be used to transfer information then there is no problem with relativity.

Martin Beckett
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  • So basically, this doesn't violate special relativity since information isn't moved FTL, it is created FTL? I might have understood it wrongly. – OmnipresentAbsence Feb 20 '13 at 18:34
  • Special Relativity is only valid locally. SR is, you can say, a special case of General Relativity in kinda the same way Newtonian Mechanics is a special case of Special Relativity. You can look at it like raisins in a rising dough. If the dough is large enough and the rising is uniform, some raisins are bound to be moving away from each other faster than light. This is perfectly fine, since what Special Relativity tells you is that you cannot move between any two raisins faster than light. – Thriveth Jun 06 '13 at 15:33
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The recession rates of cosmologically distant objects are not relative velocities, so there's no reason to compare them to the speed of light.

They can be defined as follows. Conceptually, imagine a chain of galaxies that leads to some target galaxy. Each galaxy along the chain has some small velocity relative to the galaxy before it. If you add all of those relative velocities together, that should give you the velocity of the target galaxy, right? However, velocities in relativity add in a special way; see the relativistic velocity addition formula. The cosmological recession rate is computed by instead just adding the relative velocities naively, without properly using relativistic velocity addition. That's why we should not be concerned that it can exceed the speed of light.

It's also worth noting that the relative velocity between cosmologically distant objects doesn't even have a unique meaning. You can just as well say that it is very high or that it is zero.

(Some text was adapted from one of my other answers.)

Sten
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