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Q & A: The Big Crunch

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Most recent answer: 04/21/2013
Could it be that the Big Crunch already happened, and we are living forwards in a world that is traveling backwards in time?
- Patrick (age 19)
Hi Patrick,

I guess it is possible that the universe could have undergone a Big Crunch prior to the Big Bang. Itís really quite hard to tell these things because information about the state of the universe before the Big Bang is erased in the process. One of the possible hypotheses advertised a few years back is that the universe continually explodes and then recollapses under its own weight, only to explode again. And again.

The most recent observations of how fast the universe is expanding and how clumpy the stuff in it is seem to indicate that we are not in such a cycle. The expansion of the universe, if anything, appears to be accelerating! Of course people could find out later that the measurements were wrong, but more and more independent ways of looking at things seem to be indicating that the universe will keep expanding, and even do it faster.

This still doesnít tell us about previous Big Crunches, it only indicates that we donít believe one is coming up.

As for your second question, Iím not too sure what it means to "live forwards" and what it means for a world to "travel backwards" in time. Time is a number you need, in addition to three numbers for space, to identify where and when something happens.

One of the principles of physics that doesnít seem to have any experimental evidence against is that of causality. An event can only affect events in the future, and can have no effect on prior events. This principle seems to apply to all interactions of all kinds of stuff with no exceptions, so "we" and "the world" should agree on which way in time the future is.


(published on 10/22/2007)

Follow-Up #1: entropy and big crunch

You said, ".. because information about the state of the universe before the Big Bang is erased in the process." Can "the process" also erase the increased total "entropy" of the entire universe or not?
- Anonymous
Very good question. The inability of any known process to erase entropy makes an enormous problem for Big Crunch style cyclic models. You can't have simple cycles if entropy keeps increasing.

There are models that run eternally and repetitively but avoid this problem. Sean Carroll discusses one in From Eternity to Here. New universes would be spawned by quantum fluctuations from old, cold, flat universes. The process of spawning these bubble universes would increase some net cosmological entropy, but each universe would be born with low entropy and no contact with anything else, and thus would have a history of pure entropy increase.

Mike W.

(published on 04/21/2013)

Follow-up on this answer.