I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows.
1) How one reconciles it with the fact that it is now experimentally established that the universe is expanding in an accelerating manner?
2) The entropy of the universe should have been very very low (maybe zero) at the time of the big bang/ big bounce. What was the entropy "before" that event? Was it even lower? If it was lower then doesn't that mean entropy decreases indefinitely? On the other hand if we accept the entropy was higher then doesn't that violate the second law of thermodynamics?
3) What observational evidence can confirm pre-bounce physics? According to GR there are some points near the bang/bounce where one simply can't easily define time in a meaningful way. Even for a quantum gravity theory, is it possible for any theory to extrapolate itself "before" the "bounce"? If there are no observational consequences after the bounce shouldn't one apply Occam's razor?
Answer
This connects in some ways to what I wrote in:
Did spacetime start with the Big bang?
The other side of the de Sitter “throat” is a cosmology which implodes. Now it is important that everyone not take the solutions to Einstein’s field equations literally. Johannes is right in fact, the entropy increases from the minimal region in the throat with a time arrow heading in opposite directions. Then according to our time arrow entropy decreases on the “other side.” This is not different from the white hole part of the Schwarzschild solution. The white hole horizon rapidly decreases and the entropy declines as well. However, we do not have white holes in the current state of the universe, so they are not taken as physically the same as a black hole. They may have existed in the inflationary universe, where the rewind of anisotropy in the reverse time direction might lead to black holes --- or white holes in the forwards time direction.
The other side may correspond to some quantum “anti-universe.” An electron may be thought to tunnel through a potential barrier by being annihilated by a positron, which in turn is in a pair $e,~e^+$ and the electron on the other side of the barrier escapes free. This other half of the universe, or “biverse” might be compared to such tunneling. So this other universe should not be taken in some concrete sense, but more as a QFT or mathematical gadget.
The data does suggest our universe will expand endlessly and will no recollapse. So the cyclic model appears to be ruled out. Our observable universe is not likely to recollapse unless there is some unexpected potential function in the mini-superspace which reverses the current accelerated expansion.
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