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Here's an excellent explanation on cosmic inflation from Sean Carroll: http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-mi

Here's an excellent explanation on cosmic inflation from Sean Carroll: http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-microwave-background/ http://www.preposterousuniverse.com/blog/2014/03/16/gravitat...

mcguire 13y ago
...which seems to be unreachable for me at the moment. I'll have to check it out later. In the mean time.... Cosmic inflation has always bothered me. I just don't get it. I'm not a physicist at all, though. I get why it's postulated. (The uniformity of the universe requires it to all be in close proximity and the time scales don't have enough time for that part, IIUC.) But the expansion of inflation is significantly faster than the speed of light. What's up with that? I know, spacetime is expanding rather than things moving within spacetime, but still, during inflation, this particle here is watching that particle there recede at >> C. I doan geddit. (Grump.)
- GeneralMayhem 13y ago
  The way I understand it is that the problem is with the definition of velocity. If you want to measure speed, you have to measure distance, which eventually comes down to the magnitude of the vector between two points - subtract the X's, subtract the Y's, subtract the Z's. Problem is, at cosmic scales, expansion means that those two points aren't in the same reference frame, because more space has appeared between them. You can try to measure it, but by the time you do the axes themselves have stretched again. The two X's can't be subtracted directly anymore, because they're on different reference frames. You get a result of >>C only by assuming that the relevant bits of space are relatively well-behaved Euclidean R^3, but that's not the case.
- marcosdumay 13y ago
  Current models state that the Universe is still expanding "faster than the speed of light" (that is, fitting your explanation) if you look far enough. (What, of course you can't do, because light from there is redshifted into zero.)
- gibybo 13y ago
  > this particle here is watching that particle there recede at >> C Technically speaking, the particle can't really observe the other particle moving away from it faster than C since the light (or any other signal from the particle) will never be able to reach the observing particle. It would just see it disappear.

mcguire 13y ago

...which seems to be unreachable for me at the moment. I'll have to check it out later. In the mean time.... Cosmic inflation has always bothered me. I just don't get it. I'm not a physicist at all, though. I get why it's postulated. (The uniformity of the universe requires it to all be in close proximity and the time scales don't have enough time for that part, IIUC.) But the expansion of inflation is significantly faster than the speed of light. What's up with that? I know, spacetime is expanding rather than things moving within spacetime, but still, during inflation, this particle here is watching that particle there recede at >> C. I doan geddit. (Grump.)

GeneralMayhem 13y ago
The way I understand it is that the problem is with the definition of velocity. If you want to measure speed, you have to measure distance, which eventually comes down to the magnitude of the vector between two points - subtract the X's, subtract the Y's, subtract the Z's. Problem is, at cosmic scales, expansion means that those two points aren't in the same reference frame, because more space has appeared between them. You can try to measure it, but by the time you do the axes themselves have stretched again. The two X's can't be subtracted directly anymore, because they're on different reference frames. You get a result of >>C only by assuming that the relevant bits of space are relatively well-behaved Euclidean R^3, but that's not the case.
marcosdumay 13y ago
Current models state that the Universe is still expanding "faster than the speed of light" (that is, fitting your explanation) if you look far enough. (What, of course you can't do, because light from there is redshifted into zero.)
gibybo 13y ago
> this particle here is watching that particle there recede at >> C Technically speaking, the particle can't really observe the other particle moving away from it faster than C since the light (or any other signal from the particle) will never be able to reach the observing particle. It would just see it disappear.