A fundamental issue with a cyclical universe is entropy, which shows even if the universe was cyclical, it could not have been eternal.
A cyclic universe needs no beginning. It always was, always is, and always will be. But the model isn’t without its problems. One of the main ones is the problem of entropy. Entropy is a measure of disorder in a system, and according to the laws of thermodynamics can never decrease. In a simple cyclic universe model, the entropy of any given universe must be at least a little greater than its parent universe. So if universes cycle to an infinite past, the current universe would have infinite entropy, which it doesn’t. So there must have been some initial universe with low entropy, and we’re back to the beginning.
New research highlights a troubling problem with concepts of a cyclical universe that experiences infinitely alternating periods of rapid expansion and contraction, known as ‘bouncing universe’ models.
These bouncing universe models suggest the cosmos has no beginning, eliminating the need for a troubling singularity prior to the initial period of rapid inflation — commonly known as the Big Bang — needed by ‘beginning of time’ models.
University at Buffalo researchers say a newly suggested bouncing universe recipe that attempts to deal with the problem of entropy — the measure of unusable energy in the universe, which can only increase — suffers from a problem that has plagued previous models of endless inflation and contraction. It still needs a beginning.
However, work by Richard C. Tolman in 1934 showed that these early attempts failed because of the cyclic problem: according to the Second Law of Thermodynamics, entropy can only increase. This implies that successive cycles grow longer and larger. Extrapolating back in time, cycles before the present one become shorter and smaller culminating again in a Big Bang and thus not replacing it.