It appears that the universe expanded faster than light speed during the inflationary period.

How fast was the universe expanding during the inflationary period?

Inflation factor and doubling time

We also know that what we observe is the past as we look out into the cosmos. Coupling these two led me to a (layman's) supposition some time ago:

If the universe was collapsing, right now, faster than the speed of light we will never know.

Is this a correct assertion (i.e no violation of physical law)?

  • $\begingroup$ I think, you are referring to the event horizon. Have a look here: arxiv.org/abs/astro-ph/0310808v2 $\endgroup$ – Photon Mar 30 '17 at 12:47
  • $\begingroup$ Not, really. Your reference relates to galaxies disappearing as the universe expands. But I want to know about a collapse. i.e. the galaxies coming together faster than light speed. $\endgroup$ – Athena Widget Mar 30 '17 at 12:58
  • $\begingroup$ All the field in the universe is contained in the spacetime geodesics. If this geodesics were to change their spacetime shape but the limiting speed of changing coordinate is still speed of light. But if more points on geodesics that had nothing to do with each other before are joined by a different geodesic we could call this something like parallel joining -once they were far apart now they are close together.But the rate at which this new join is happening does not overcome the speed of light.Space is something that can be acted upon and can act himself on nearby objects, at max rate $c$ $\endgroup$ – Mihai B. Mar 30 '17 at 13:39

If the universal dynamics obeys the cosmological principle, then the answer is we could tell if it was contracting right now. The cosmological principle is the key simplification of modern cosmological theories, and it says that the universe is doing the same thing everywhere at any given time (on large enough scales), so if it is contracting on the scales of the most distant galaxy clusters, then it is also contracting on the scales of the nearest clusters (though not within galaxy clusters, they have their own gravitational dynamics that is not ruled by the cosmological principle). What this means is, there is a concept of a "scale parameter," often called a, which depends only on the age of the universe, called t, where the function a(t) is what multiplies all current distances (so a=1 at t=now) to give all future and past distances (on the largest scales). The existence of the a(t) function generates what is called a "Hubble law", which simply means that the rate of change with t of all distances from the observer at any time t is proportional to the distance at that time t. The way to tell if the universe is contracting at any time t is if the Hubble law slope is negative, which means you would see blueshifts rather than the redshifts we see now (again at large enough distances, not within our own galaxy cluster).

It makes no difference if the rate of change of distance exceeds the speed of light, that always happens at large enough distance for any Hubble law. The only way to avoid that is if the universe has only a finite extent, but whatever is the extent of the universe it seems amply large to make any expansion or contraction exceed c at all suitably large distances.

Now, there is a wrinkle, which your question may be alluding to. To plot the rate of change of distance now against the distance now, which is always a straight line if we have a cosmological principle, we cannot just take the redshift or blueshift we observe and interpret that as a rate of change of distance. That's what Hubble did originally, but this only gives the rate of change of distance for relatively nearby sources, and that method only gives a straight line for those nearby sources. For more distant sources, the redshift is different from the rate of change of distance now, because the redshift gives the total factor by which the scale parameter a has changed since the light was emitted, so integrates over all the expansion and contraction since the light was emitted to give the redshift/blueshift. To connect that to the rate of change of distance now, you need to use the redshift/blueshift of very source at all distances between the one you are observing and yourself, and then you can back out the connection between the expansion and the redshift, as a function of t. You just can't get the rate of change of a(t) at any t by looking at a single redshift and a single distance, the redshift does not tell you that.

What this means is, if the expansion transitions from an expansion to a contraction as t elapses, then the redshift you might see at very large distances, due to the history of early expansion, could give over into blueshifts at nearer distances, due to the more recent history of contraction. That would still obey a Hubble law, because to be a straight line, the Hubble law requires we plot the rate of change of distance now against the distance now, not the redshift/blueshift now versus the distance now. We'd have to piece together all those nearby blueshifts and distant redshifts to tell the story of the early expansion followed by later contraction. And again, it wouldn't matter if any of that was faster than c, in this empirical approach c has no significance beyond connecting distances to t due to the finite speed of light.

However, so far when we do this empirical exercise, we find that a given theory, general relativity with dark energy included, does a good job of describing the data. It also gives the result that the universe is nearly spatially flat, and it fits with a universe that has always been expanding. To get that to change over into contraction, you'd have to put in some new physics that is not observed, something hypothetical. If you don't break the cosmological principle, then we could still determine that contraction was happening by seeing blueshifts from nearby galaxy clusters, though presumably we'd still see redshifts from distant ones due to the need to have a Big Bang.

  • $\begingroup$ Very good explanation, especially the bit about contracting everywhere. To see the blue shift at all, that information has to get to us (even at a near distance). If at some point the collapse exceed that information propagation, how would we know? (Assuming there is a reason for the collapse in the first place.) (I have this image of letting air out of a balloon really fast!). Let me ponder your answer some more. $\endgroup$ – Athena Widget Mar 30 '17 at 14:04
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    $\begingroup$ Bear in mind that the light does not have to overtake the contraction, it is like ants walking on a contracting rubber sheet, or a person walking down a down escalator-- the light gets here all the quicker. $\endgroup$ – Ken G Mar 30 '17 at 21:06

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