Marshall wrote:You and I seem to have come to essentially similar ideas from different directions. If you look at the summaries of the papers in those links they are almost all about bounce cosmology.
(not necessarily cyclic, may just be one bounce)
I thought the whole premise of the bounce, or bang/crunch was to give a degree of explaination to the expanding Universe? So the bounce you are referencing is a low energy, (low relative to the energy of the singularity, and explainable in theory)...
The "Loop" approach (LQG) is one of several competing attempts to formulate a quantum theory of spacetime geometry that reproduces classical GR in the limit.
That name was appropriate in the 1990s, when it was formulated using the idea of tiny loops which sense and express spatial geometry. By the mid-1990s they no longer used loops---there were more general ways to express and calculate quantum states of geometry (mixtures of uncertain shapes, so to speak). But they kept on calling it LQG.
The bounce was not a premise
but rather an unexpected thing that came out of the math during the struggle to quantize GR, around 2001.
When you apply classic (i.e. non-quantum, deterministic) GR to cosmology the Einstein field equation boils down to a simpler equation called the FRIEDMANN EQUATION that governs the evolution of the broad outlines of the universe's expansion and spatial curvature as it relates to overall matter and energy density.
In 2001 the LQG method was applied to cosmology to obtain a QUANTUM FRIEDMANN EQUATION, at low density it reproduced the classic expanding geometry we know and love. But at very high density there were "quantum corrections" that made it behave differently.
In 2005 some problems were pointed out with that original version of LQG cosmology (LQC for short) and in 2006 it was re-derived and reformulated, primarily by an Indian guy named Abhay Ashtekar. Then you got a very clear quantum Friedmann equation, with clear quantum correction terms. The quantum effects were small at low density and could be ignored. At very high density the correction terms became large and became dominant in the equation. In effect gravity became repellent!
After 2006 the universe's geometric behavior was studied in many different cases, both using computer simulation methods and analytical "solvable equation" methods. In every case they always got a bounce.
Ashtekar and friends are still checking this, relaxing restrictive assumptions made at first, generalizing etc.
The bounce seems to be "robust" in the sense that it keeps on recurring in whatever variation they try.
It tends in many cases to happen when the energy density reaches about 40% of the socalled "Planck" density. That energy is very hard to imagine. Think of the gamma-ray light in the core of a star. Very short wavelength photons. Think of light that is a trillion trillion times hotter and brighter than that starcore light.
Very short wavelength photons, many many packed together. Each photon with enough energy to drive your car 100 miles. Unbelievable energy in each photon and unbelievably many crowded together. That is the energy density at Planck-like scales where the LQC quantum corrections become dominant and bounce happens.
It is high but it is not infinite. So there is no singularity. A singularity is where a theory breaks down and gives infinite answers---answers that therefore do not mean anything. No definite number. No physics.
The bounce naturally triggers a period of inflation, or actually socalled superinflation. Ordinary Alan Guth inflation was only exponential expansion that gradually slid down from initial the exponential rate. In superinflation the exponential rate actually increases---you get faster than exponential inflation for a while after bounce but then it quickly slows down to ordinary (if you have usual Guth-type assumptions).
This does not predict a cyclic pattern. You can get that under certain assumptions but the only robust conclusion from the theory is that OUR start of OUR expansion was a bounce. There was an immediately preceding contraction phase. Where that prior contracting universe came from the theory does not say.
Also the theory has to be tested and it predicts some features of the ancient light, the microwave background, which may or may not be observed. So it is not something to believe, it is something to be tested by observation and possibly discarded or improved-on.
I have to go. haven't finished answering, but no more time for now.