Watsisname wrote:We're more than 99.7% sure that w is between -1.2 and -0.8. And something like 99.9 and about 90 more 9's percent sure that it is less than zero. Basically, a negative w means the energy density is diluted more slowly than what you would expect for the increase in volume with the expanding universe. w = -1 means the energy density isconstantas the universe expands, which is the simplest model for dark energy and most consistent with observations.

Heat death is just what happens if the expansion goes on forever, which applies to any open or flat universe model. So I think the cyclic universe model with dark energy must have some more complicated requirement than just w being negative -- i.e. some more exotic form of dynamical dark energy.

Something called phantom energy I think. I will read up again and repost. edit- I just did, note the part where it says that W can be close to but must be less than -1. Imagine if it was something like -1.1 or -1.2 !

https://en.wikipedia.org/wiki/Cyclic_model

The Baum–Frampton model[edit]

This more recent cyclic model of 2007 makes a different technical assumption concerning the equation of state of the dark energy which relates pressure and density through a parameter w.[7][10] It assumes w < −1 (a condition called phantom energy) throughout a cycle, including at present. (By contrast, Steinhardt–Turok assume w is never less than −1.) In the Baum–Frampton model, a septillionth (or less) of a second (i.e. 10−24 seconds or less) before the would-be Big Rip, a turnaround occurs and only one causal patch is retained as our universe. The generic patch contains no quark, lepton or force carrier; only dark energy – and its entropy thereby vanishes. The adiabatic process of contraction of this much smaller universe takes place with constant vanishing entropy and with no matter including no black holes which disintegrated before turnaround.

The idea that the universe "comes back empty" is a central new idea of this cyclic model, and avoids many difficulties confronting matter in a contracting phase such as excessive structure formation, proliferation and expansion of black holes, as well as going through phase transitions such as those of QCD and electroweak symmetry restoration. Any of these would tend strongly to produce an unwanted premature bounce, simply to avoid violation of the second law of thermodynamics. The surprising w < −1 condition may be logically inevitable in a truly infinitely cyclic cosmology because of the entropy problem. Nevertheless, many technical back up calculations are necessary to confirm consistency of the approach. Although the model borrows ideas from string theory, it is not necessarily committed to strings, or to higher dimensions, yet such speculative devices may provide the most expeditious methods to investigate the internal consistency. The value of w in the Baum–Frampton model can be made arbitrarily close to, but must be less than, −1.

Cyclic steady-state model[edit]

Astrophysicist Geoffrey Burbidge proposes a realistic cyclic steady-state model (2008) in which the universe continuously goes through stages of expansion and contraction, each cycle on a length far greater than 1012 years, with very hot and massive active galaxies and black holes playing roles in the formation of elements in each cycle. The model attempts to address issues with standard Lambda-CDM cosmology, including the lack of viable or confirmed non-baryonic dark matter sources, uncertainty regarding dark energy, early galaxy formation and constraints on inflation, while also reconciling and taking account with recent observations and element abundances without the need of additional new physics beyond the Standard Model.[11]

Other cyclic models[edit]

Conformal cyclic cosmology—a general relativity based theory due to Roger Penrose in which the universe expands until all the matter decays and is turned to light—so there is nothing in the universe that has any time or distance scale associated with it. This permits it to become identical with the Big Bang, so starting the next cycle.

Loop quantum cosmology which predicts a "quantum bridge" between contracting and expanding cosmological branches.