Cyclic Universe Models: Agreed terms help sensible discussion


The devil is in the detail
I am giving this a thread of its own because it overlaps other areas. For example, it replaces the idea of a singularity (at least in some cases), and to put it into Universe would mix it with cases where the singularity is needed. My comments are in blue.

Whilst this model (better than theory) avoids the singularity problem, by replacing the t = 0 point with a nexus through ("backwards") to another previous phase (for want of a better word), there is a potential problem with entropy. If the Universe passes cyclically through "phases" does this require an increase in entropy with each cycle?

The State of the Universe Pedro G Ferreira Phoenix 2007
Discussing a closed Universe in terms of scale factor:
The Universe would then expand, stop, and then contract until it collapsed into a point. This might then be followed by a subsequent expansion, until once again it reached a maximum size, and started to collapse again: the Universe would evolve in a series of cycles repeated over and over again ad infinitum.

All About Space What happened before the Big Bang Kulvinder Singh Chadha 2016
Backed by Sir Roger Penrose
"Big Bang and expansion.
In the cyclic model, a universe may have a true origin as normal and go through an inflation and expansion phase.
Everything now stalls.
In a cyclic universe the cosmos may expand to a point and then stall. This may happen from the gravitational effect of the matter within it.
Universe in reverse.
Gravity takes over
and the universe now contracts, with galaxies moving towards one another instead of moving further away.
Big Bounce and next phase.
Inevitably, the universe can only contract so far. A 'big bounce' initiates the next expansion phase. The process continues, with each phase getting larger and slower.
I have some comments about this. Clearly, we are outside the realms of science, and many speculations are possible. One of these may turn out to be accepted - maybe, maybe not.
One difficulty is the reversal from expansion to contraction and repeated "passage" through a nexus (which takes the place of the singularity). To return from the point of maximum expansion to the nexus ('singularity') must involve reduction in entropy. Thus it seems that a contraction might be accompanied by decreasing entropy. Whilst appreciating the dangers of analogies, one might remember the condensation of steam (maximum volume), through water (closer contact) to ice (crystalline), with decreasing entropy throughout. Whatever, it is difficult to go from maximum entropy to effectively zero entropy without some intervening stage.

A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 theorized a universe following an eternal series of oscillations, each beginning with a Big Bang and ending with a Big Crunch; in the interim, the universe would expand for a period of time before the gravitational attraction of matter causes it to collapse back in and undergo a bounce. [Wikipedia]

Cyclic universe theory - AccessScience from McGraw-Hill Education Current 2 June 2021
The big bang is not the beginning of everything, but a point, or nexus, of transition between phases. Thus big bangs occur periodically. Each cycle may take 10^12 years.
Although the cyclic model differs considerably from the conventional big bang model
“both theories match all current observations with the same degree of precision.” Any crucial differences occur close to t = 0. “However, the two pictures differ in their predictions of primordial gravitational waves and the fine-scale statistical distribution of matter; experiments over the next decade will test these predictions and determine which picture survives.”

All About Space What happened before the Big Bang? by Kulvinder Singh Chadha 2016
"Dr Cai and Professor Brandenberger McGill's High-Energy Theory Group
What he means is that in the multidimensional scenario, although ekpyrosis can 'smooth' out some problems like cosmic inflation, the singularity is still present and the physics surrounding that are as vague and problematic as ever. But Dr Cai's work, . . . . . . . . . does away with singularities entirely."
"In their model, a previous universe collapsed until it could go no further, and then bounced out as a new universe. 'In our scenario, the whole of cosmic evolution then becomes smooth. The physics around the bounce, including the background [CMB] and perturbation, are well controlled and calculable," he says. By removing the singularity, a lot of associated problems (such as infinite densities and zero dimensions - which have little support in physics) are also removed."

WORK IN PROGRESS. Latest: 6th June 2021 13.00 BST approx.
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  • Like
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Dec 9, 2020
For me it's amazing just how quickly physics and cosmology concepts are being refined, updated and revised. I only wish I had more time to even keep current. Cat's and other's posts are very helpful; my thanks to all.
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Apr 23, 2020
For me it's amazing just how quickly physics and cosmology concepts are being refined, updated and revised. I only wish I had more time to even keep current. Cat's and other's posts are very helpful; my thanks to all.
The "mirror-image universe" mentioned in the recent blog on parallel universes is identical to an idea I wrote about roughly a decade ago in replies to various physics blogs such as Sean Carroll's Cosmic Variance. In addition to what was written here, I suggested this could possibly explain what happened to all the (missing) antimatter in our universe, i.e. from our perspective the antimatter travels backwards in time as the mirror-image universe. However, I took it even further, suggesting that the Time Dimension of Spacetime may be finite but circular, such that if you travel far enough forward or backward in time, you wind up back where, or rather "when," you started. This is analogous to the idea that space is curved such that traveling far enough you would eventually return to your starting point.

At the time, I never got any professional physicist (such as Sean Carroll) to respond to my suggestion. It's interesting to see that my idea is now being seriously considered by a leading science research institute.