What does "many times speed of light" really mean in this context? For a layman it's easy to draw wrong conclusions here. Is this just bad journalism or is there a good scientific explanation for this?

  • $\begingroup$ You ask a good question, one whose answer lies in the subtle difference between expansion that is faster than the speed of light and the propagation of information that is faster than the speed of light. The latter is forbidden by fundamental physical laws, but the former is allowed; that is, as long as you are not transmitting any information (like a light pulse), you can make something happen at a speed that is faster than that of light. $\endgroup$
    – MoonKnight
    Mar 24, 2014 at 17:10
  • $\begingroup$ You mean recessional velocity? $\endgroup$
    – user14742
    Mar 24, 2014 at 17:12
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    $\begingroup$ possible duplicate of Can space expand with unlimited speed? $\endgroup$
    – Kyle Kanos
    Mar 24, 2014 at 17:40
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/26549/2451 and links therein. $\endgroup$
    – Qmechanic
    Mar 24, 2014 at 17:41
  • $\begingroup$ Thanks for possible duplicate links, seems to me that this has been answered already. $\endgroup$
    – user14742
    Mar 24, 2014 at 18:00

1 Answer 1


In curved spacetime, you can no longer compare velocities at different points in the straight-forward manner we use in flat spacetime. Thus the claim that recession velocities should not be considerer 'real' (as in relative) velocities, but rather rates of expansion of space. If you want to get at the former, you need to parallel transport the source's four-velocity along the light path to the observer. This should get you meaningful values below $c$ that correspond to the observed redshift (but I have to admit that I never went through these calculations myself).

Also note that superluminal expansion isn't really the defining characteristic of the inflationary epoch: Hubble's law relates recession velocities to distance, and if you move out far enough, the expansion in still superluminal today - there's not really any special significance to the speed of light in this particular context. Case in point: In principle, we should be able to observe galaxies with recession velocities of about $4c$ or so when their light got emitted.

  • $\begingroup$ On your final point, I think those galaxies were $<c$ relative to Earth when the light was emitted. "Now" they're moving $>c$, and they're unreachable by light we emit now. $\endgroup$ Mar 24, 2014 at 17:29
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    $\begingroup$ @AlanSE: no, I do mean that they had recession velocities $> c$ at time of emission; basically, the Hubble sphere is arbitrary - as long as we don't move beyond the event horizon, we're good to go $\endgroup$
    – Christoph
    Mar 24, 2014 at 17:34

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