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sci-guy
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I have only a basic understanding of science, but don't the Second Law of Thermodynamics and the concept of entropy preclude a cyclical universe (i.e. one that will eventually contract to a singularity)?
bcrowell said:There is no well established answer to this question, mainly because there is no well established way of defining the entropy of a gravitating system in general relativity. Roger Penrose has a model (CCC) whose main selling point is that he thinks it makes sense out of the entropy issues -- more than non-cyclical models. The idea is that in a standard cosmology, exceptional fine-tuning is required to explain why we didn't have a maximum-entropy big bang (in which case we would have a universe that experienced heat death right when it was born).
skydivephil said:Can you elaborate on what you mean by "no well established way of defining the entropy of a gravitating system in general relativity."
This is really not true. Observations don't tell us this at all. They tell us that the current epoch is undergoing an accelerated rate of expansion. Sure, if you extrapolate this behavior into the future, then there will never be a contracting phase again. But that's a big if, given that we don't currently have a good understanding of what's causing the accelerated expansion.Dmitry67 said:This is why scientists don't believe in that model.
Also, based on the observations, our Universe will never ever contract again. So if it was cyclical, then current cycle is the very last one.
This is not completely established. Penrose's CCC is cyclical and is consistent with the evidence that our universe will never contract again.Dmitry67 said:Also, based on the observations, our Universe will never ever contract again. So if it was cyclical, then current cycle is the very last one.
Well, it certainly disproves any overly-naive cyclical universe model. But there are a number of ways around it. For instance, if you make it so that the post-bounce universe is different in such a way that entropy increases the whole time, then there is no problem (e.g. if the volume of each subsequent bounced universe is vastly larger).sci-guy said:I have only a basic understanding of science, but don't the Second Law of Thermodynamics and the concept of entropy preclude a cyclical universe (i.e. one that will eventually contract to a singularity)?
Yeah, but it's also not a full model (there is no explanation whatsoever for the remapping of the large universe onto a small early one...it just happens).bcrowell said:This is not completely established. Penrose's CCC is cyclical and is consistent with the evidence that our universe will never contract again.
But it's important to point out that this kind of universe is not truly cyclic, since it is not past-eternal.Chalnoth said:Well, it certainly disproves any overly-naive cyclical universe model. But there are a number of ways around it. For instance, if you make it so that the post-bounce universe is different in such a way that entropy increases the whole time, then there is no problem (e.g. if the volume of each subsequent bounced universe is vastly larger).
Perhaps, but there are no guarantees this is the case. Unfortunately, the properties of inflation are such that is is all too easy to completely and utterly wipe away any trace of what happened before.Chronos said:I think a cyclical universe should leave an imprint in the CMB. Cosmologists have been looking for this for years ... and still looking. Anisotropies in the CMB are apparent, but, no reasonable explanation has yet been offered, IMO.
Entropy is a measure of the disorder or randomness in a system. In the context of the cyclical universe, entropy plays a crucial role in determining the fate of the universe. As the universe expands and cools, the entropy increases, leading to the eventual heat death of the universe. However, in a cyclical universe, entropy is reset after each cycle, allowing for the possibility of a never-ending series of cycles.
One of the main pieces of evidence for a cyclical universe is the cosmic microwave background radiation, which is the leftover radiation from the Big Bang. This radiation is uniform in all directions, suggesting that the universe has undergone multiple cycles of expansion and contraction. Additionally, studies of galactic clusters and the distribution of matter in the universe also support the idea of a cyclical universe.
The beginning and end of each cycle in a cyclical universe are explained by the concept of a "bounce." This is when the universe reaches a point of maximum expansion and then starts to contract, eventually leading to a "big crunch." This contraction then causes the universe to bounce back and start a new cycle of expansion.
Yes, there are alternative theories to the cyclical universe model, such as the Big Bang theory and the steady-state theory. The Big Bang theory proposes that the universe began with a single event and has been expanding ever since. The steady-state theory suggests that the universe has always existed and is continually expanding, with new matter being created to maintain a constant density.
At this point, it is impossible to know for certain if the universe is truly cyclical. While there is evidence that supports this idea, there is also evidence that contradicts it. Further research and advancements in technology may one day provide a definitive answer, but for now, the cyclical universe remains a fascinating and debated concept in the scientific community.