Is there any evidence that supports or disproves the Oscillating Universe theory? What kind of observations could provide this evidence?
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15$\begingroup$ Basic point - a theory cannot be proved, but can be disproved. $\endgroup$– ProfRobFeb 9, 2016 at 9:35
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$\begingroup$ If one doesn't agree that the universe is indeed cyclical, one is most likely stuck in the wrong frame of reference. $\endgroup$– user13762Aug 11, 2016 at 21:21
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$\begingroup$ In order to have a cyclical universe, apart from M theory, which while well thought out remains completely theoretical (and will probably remain so for a very long time, as unfortunately experimental analysis on this subject is quite beyond our reach), you would need a universe that is not visibly flat. If you had a curved spacetime, even slightly so, you could end up with a sperical, toroidal, or other geometry that would allow continuous cycling. However as far as we can see, the Universe for now at least seems flat. $\endgroup$– Drunken Code MonkeyAug 14, 2016 at 3:40
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$\begingroup$ @DrunkenCodeMonkey That doesn't fix the fact that the second law of thermodynamics would be broken. $\endgroup$– Sir CumferenceOct 4, 2016 at 4:14
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2 Answers
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The main obstacle with the Oscillating Universe model is the pesky second law of thermodynamics — that entropy always increases within an isolated system, and never the reverse. Thus, as time progresses, entropy increases, until it (as far as we know) reaches its maximum — the heat death or Big Rip.
Consider the Big Bang, when entropy was at its minimum, or at least incredibly low. As the Universe evolved, entropy increased, as dictated by the second law. If the Big Crunch were to happen, we'd see a decrease in entropy as conditions in the Universe recede back to how they were at $t = 0$. As we've stated, the Universe's entropy can never decrease.
But say we ignore the whole "Big Crunch problem" and instead focus on the cycles. As stated before, the entropy always increases with time. I'll let Baum & Frampton (2006) explain the problem:
...one principal obstacle is the second law of thermodynamics which dictates that the entropy increases from cycle to cycle. If the cycles thereby become longer, extrapolation into the past will lead back to an initial singularity again, thus removing the motivation to consider an oscillatory universe in the first place.
As they described, the entropy in the Universe would increase with each cycle. At this point we realize that each cycle in the Universe's history would be different — and if we extrapolated back into the past, we would find that at some point, entropy must have been at its minimum (the Big Bang!) The fact that we would be back to the Big Bang would remove any need for an Oscillating Universe hypothesis — which tries to explain what preceded the Big Bang.
That problem led many astrophysicists to abandon the Oscillating Universe hypothesis. But nonetheless, the idea that the Universe is reborn is quite attractive, and we still know very little about cosmology — including dark energy, which we think has a vital role in the Universe. Some cosmologists continue to research the Oscillating Universe model in hopes that, as we learn more about dark energy and the Universe, we may be able to avoid the problems with the second law of thermodynamics.
answered Oct 4, 2016 at 3:24
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2$\begingroup$ The entropy argument is difficult, because you need to set the second law of thermodynamics as an axiom. There is a school of thinking that wants to explain the second law of thermodynamics as a consequence of initial conditions. Following this school of thought, the entropy decreases while the universe crunches again. $\endgroup$ Oct 25, 2016 at 13:09
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$\begingroup$ @jknappen It is a scientific given that, when presented only with assumptions, we usually trust the one with the fewest hypotheses. The second law seems to hold, so it is only natural to talk about it here. However, I also mentioned that our research on dark energy may give us a possible way to avoid these problems in the future. $\endgroup$ Oct 25, 2016 at 15:25
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The evidence suggests, strongly, that the expansion of the universe is accelerating. If an oscillating universe theory ruled that out then this would, in turn, dismiss this theory. As it is, it does strongly indicate that the idea that gravitational attraction would eventually slow the expansion of the universe and then send the expansion into reverse, is a false one.
But, you really need to be more specific in your question: which theory of an oscillating universe is it that you are asking about?
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$\begingroup$ The problem is less observational and more theoretical. In some models expansion stops just short of the Big Rip, causing the Big Crunch. It's not necessarily dependent on whether the expansion of the Universe is accelerating or not. $\endgroup$ Oct 4, 2016 at 3:36