The discussion revolves around the entropy of the universe at the time of the Big Bang, exploring whether it began in a state of low or high entropy. Participants reference theories and concepts from cosmology, including Penrose's Conformal Cyclic Cosmology and the implications of the second law of thermodynamics.
Participants express differing views on the initial entropy state of the universe, with no consensus reached. Some support the idea of low initial entropy, while others question aspects of this interpretation and its implications for thermal equilibrium and gravitational systems.
Participants note that the discussion involves complex concepts that may depend on specific definitions and assumptions, particularly regarding entropy, thermal equilibrium, and the nature of gravitational systems.
Penrose doesn't believe the inflation theory, which is that space expanded incredibly rapidly right after the Big Bang. He says his conformal cyclic cosmology theory explains the things that inflation was invented to explain: it explains correlations in temperature in the cosmic microwave background between areas that are separated by large angles, and the scale invariance in the temperature fluctuations. The CCC theory also requires Weyl curvature to be zero at the Big Bang. This apparently explains why we don't see magnetic monopoles, another thing that inflation is invoked to explain, although Penrose doesn't discuss this in his book.
The CCC theory seems much more appealing than the inflation theory. It's more parsimonious, not requiring extra fields or an incredibly rapid expansion of spacetime. The universe would have expanded at the normal rate, only over a very very long time before the Big Bang.
The big hole in Penrose's theory is that our universe can only lose track of the scale of space and time if rest mass disappears. Rest mass gives a scale to spacetime. So it's necessary that all particles should eventually decay into massless particles like photons, or lose their rest-mass some other way. He hasn't come up with any good explanation for how this would happen. His best attempt at a theoretical framework for the decay of rest mass is:
"A standard procedure for addressing the idea of an 'elementary particle' is to look for what are termed the 'irreducible representations of the Poincare' group'. Any elementary particle is supposed to be described according to such an irreducible representation. The Poincare' group is the mathematical structure describing the symmetries of the Minkowski space M, and this procedure is a natural one in the context of special relativity and quantum mechanics. The Poincare' group possesses two quantities referred to as Casimir operators, these being rest-mass and intrinsic spin, and accordingly the rest-mass and spin are deemed to be 'good quantum numbers', which remain constant so long as the particle is a stable one and does not interact with anything. However, this role of M appears to be less fundamental when there is a positive cosmological constant L (Greek letter Lambda in the book) present in physical laws (as L=0 for M), and it would seem that, when we are concerned with matters related to cosmology, it should be the symmetry group of de Sitter space-time D, rather than of M, that should ultimately be our concern. However, it turns out that rest-mass is not exactly a Casimir operator of the de Sitter group (there being a small additional term involving L), so that its ultimate status is more questionable in this case, and a very slow decay of rest mass seems to me to be not out of the question."
I don't know how convincing this is. Does rest mass need to be a Casimir operator of the spacetime, to be a good quantum number, so that it's conserved for a particle as long as it exists? Apparently nobody's worked out what becomes of quantum mechanics and the Standard Model of particle physics in de Sitter spacetime. Until they do, and rest mass really does turn out to fade away in the expanding universe, Penrose's theory will limp badly.
the matter after the big bang is not a black hole.
the entirety of matter after the big bang is a gravitating system.
...why when looking at the CMB we notice there is thermal equilibrium?
...The big hole in Penrose's theory is that our universe can only lose track of the scale of space and time if rest mass disappears. Rest mass gives a scale to spacetime. So it's necessary that all particles should eventually decay into massless particles like photons, or lose their rest-mass some other way. He hasn't come up with any good explanation for how this would happen.
At end of our expansion we come to masslessness and as black holes evaporate and as information is lost, we return to a low entropy state…
In the past, Penrose has investigated cyclic cosmology models because he has noticed another shortcoming of the much more widely accepted inflationary theory: it cannot explain why there was such low entropy at the beginning of the universe. The low entropy state (or high degree of order) was essential for making complex matter possible. The cyclic cosmology idea is that, when a universe expands to its full extent, black holes will evaporate and all the information they contain will somehow vanish, removing entropy from the universe. At this point, a new aeon with a low entropy state will begin.
Marcus posted this in another discussion:As time goes on cosmological acceleration is increasing…that expansion means a colder and colder and emptier universe. Back holes are colder, but expansion will eventually cause universe to be colder and black holes will disappear via Hawking radiation…black holes disappear in a ‘pop’ [magnitude of an artillery explosion] and release entropy and DO swallow information….so our notion of entropy changes….must redefine definition entropy….. [explanation not clear].. second law still works, but definition changes due to swallowing information by black holes…very end is low entropy state…..similar to big bang….
Mass finally fades away is Penrose’s theory…..sort of an anti Higgs mechanism….
In addition there is good reason to believe entropy cannot be well defined without defining an observer...and the number of degrees of freedom also have major impact on defining entropy...further Penrose has changed his mind about black hole information loss.. Penrose has returned believing it IS lost...if the information were NOT lost, as black holes spew forth their contents [ 99% of the information of the universe] it would be recovered at the end of this aeon...hardly a low entropy stateI heard Penrose give this argument in March 2006 to an audience of math and physics people at the MSRI. He was charming and had great slides but the argument was handwaving and not convincing. You cannot use entropy in a rigorous math argument unless you can define it and he was not able to define the global entropy through the course of the LQC bounce. So he used vague suggestive language and did not claim certainty.