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I'm aware of the Big Crunch theory, that once at capacity, the universe may collapse in on itself. Hawking once theorized that time may go backwards during this crunch. So, that got me thinking: how would we perceive backward time?

That leads me a question: considering what we know about thermodynamics, relativity and entropy, is it scientifically plausible that the universe is not currently expanding, but is actually in the process of a Big Crunch, and that we are simply perceiving time as moving forward, kind of like the way your eyes see the world upside down but your brain corrects the image?

What could the implications of this be? Would any phenomena that we've observed in the universe make more sense from the perspective of this theory?

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    $\begingroup$ There is no "backward time" in the big crunch. Time would keep running forward as it alway had. The most obvious effect would be the reversal of the doppler shifts of galaxies. Galaxies moving away would slow down and then start to move towards us, again, i.e. the Hubble constant would reverse its sign. Whether there will be a big crunch or not is, at the moment, completely open. We would have to know the exact dynamic behind the cosmological constant, which keeps the universe accelerating. If that dynamic were to reverse itself, for whatever reason, the universe may collapse again. $\endgroup$ – CuriousOne Dec 22 '14 at 13:19
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    $\begingroup$ Having said that, the movie "Mr. Nobody" had a rather nerdy and, for my taste, satisfying conclusion. $\endgroup$ – CuriousOne Dec 22 '14 at 13:25
  • $\begingroup$ Related: physics.stackexchange.com/q/128603/23473 $\endgroup$ – Jim Dec 22 '14 at 15:32
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The Big Crunch is a prediction of the FLRW metric when the matter/energy density is above the critical value i.e. a closed universe. The FLRW metric gives us the scale factor of the universe as a function of comoving time, and for a closed universe the scale factor increases smoothly with time from zero at the Big Bang, through a maximum and back to zero at the Big Crunch. There is no sense in which comoving time reverses and runs backwards as we approach the Big Crunch.

The argument about time is usually related to the entropy of the universe. If the entropy started off as zero (or small) at the Big Bang, maybe it would reach a maximum at the maximum size then decrease again as the universe contracted towards the Big Crunch. Given that entropy generally increases with time, does this mean that as the entropy decreased towards the Big Crunch time, or maybe just our perception of time, would run backwards?

The problem with this idea is that there is no reason to suppose that entropy would decrease as the universe approached the Big Crunch. Quite the reverse really. In fact this is one of the arguments commonly made against cyclic universes, as without some mechanism to reset the entropy each cycle would have a higher entropy than the one before.

So your question can't be answered, because there is no reason to suppose that time runs backwards at the approach to a Big Crunch.

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  • $\begingroup$ John, I know this is an old post but I was curious. (1) Other than the entropy problem you mentioned above is there any other reasons why a cyclical universe wouldn't work? (2) As for the entropy I always assumed Big Bang and Big Crunch where like two ends of a pendulum. At one end things are so defused they become perfectly uniform. At the other end things are so compacted they're in perfect order. Both amplitudes could have The lowest entropy. Thanks $\endgroup$ – Bill Alsept Jul 25 '16 at 20:43
  • $\begingroup$ "The problem with this idea is that there is no reason to suppose that entropy would decrease as the universe approached the Big Crunch." Isn't there? Entropy is a state function. If as we get closer and closer to the Big Crunch, the state of the universe gets closer and closer to the state during the big bang, then the entropy would also get closer to the value it was during the big bang. The question then is: for this not to work, the state during a big crunch would have to be different in some way as that during the big bang. What would be the difference? And where would the entropy go? $\endgroup$ – Marses Sep 26 '19 at 11:38
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As far as I'm aware, the fact that entropy increases over time is the reason why we can say that time has a direction. Generally, physical laws and forces are reversible in time. In Newtonian physics with a simple system like a bunch of billiard balls, if you reverse all velocities, you would end up looking like you're just going backwards through time as you retrace all previous positions.

If time started going backwards, the only way you'd be able to notice that things were different would be that entropy would be decreasing rather than increasing. Or in other words, if you did reverse time, the only physical law that would appear to change would be the second law of thermodynamics.

So if total entropy starts decreasing rather than increasing, it doesn't necessarily mean everything is going backwards, but by the above argument you could conclude you must be going backwards in time.

I think the question about whether entropy would decreasing going into the Big Crunch is interesting. I'm sure it's more complicated, but since entropy is a state function, if the Big Crunch sends the universe into the same state it was in during the Big Bang (i.e a very compressed hot soup of energy), surely the entropy would be the same as in the Big Bang. If the entropy in the Big Crunch doesn't decrease and go back to what it was in the Big Bang, then what would actually cause this difference?

What would be different during the big crunch (other than the rate of change of the scale factor of the universe)? If we imagine two hot soups of particles, one in a rapidly expanding spacetime like the Big Bang, and one in a rapidly collapsing spacetime like the Big Crunch, if the entropy of the former is less than that of the latter, does that mean the extra entropy of the latter is stored in the fact that spacetime is collapsing?

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