Big Bounce Theory

[Dyothelite]

Does any one out there understand the Big Bounce Theory and Quantum Loop Gravity enough to explain it to me well? And do you feel it could be a valid cosmology and if so why?

[CraigD]

The big bounce is a pretty simple cosmological concept, regardless of the more fundamental physics of a particular comprehensive cosmology. It can be a feature of a big bang cosmology, a “continuous recycling” one, or something more off-beat. All the concept requires is that most of the visible universe not achieve escape velocity relative to the visible universe – that is, that it all eventually fall back together. Though some “big crunch” sub-variants of a big bounce predict the universe collapsing into a hot, dense, nearly homogenous state like the early universe according to the big bang model, this isn’t a requirement for all big crunches. Versions in which the universe falls together and bounces by “missing”, in the manner of a globular cluster, are also valid incorporators of the big bounce concept.

 

In short, in its most generic form, the big bounce is just a sort of synonym for a gravitationally closed (AKA bound, [imath]\Omega > 1[/imath], etc.) universe. A big bounce can be a predicted one-time occurrence (in which case it’s more commonly called a big crunch) or many-time (in which case it’s more common called a cyclic model, and also still a big crunch).

 

The big problem with big bounce models is that, even after allowing for the large amounts of missing/dark/invisible mass that appear to be required to account for the observed motion of the visible universe, the universe doesn’t appear to be gravitationally closed. A few fringe theorists, such as Tippler with his Omega point theory, hypothesize vaguely that some sort of far-future magic-like science will artificially force the universe to be gravitationally closed, or discover that it is so naturally due to some subtle, at-present-not-understood principle of time, space, and gravity, but such ideas are very far out in the fringes of science, arguably even beyond into the wasteland of pseudoscience.

 

Loop quantum gravity is one of those family of theories, like string theory, that it’s reasonable to say nobody really understands even well enough to have a good guess as to whether any of its theories are right or wrong. As I see it – a view largely taken on faith from reading Smolin – the big problem with LQG is that it’s a bottom-up approach promising, at least in principle, to yield testable predictions on a very tiny scales. Scaling it up to make cosmological predictions – predictions about postdictions about the origin and/or ultimate fate of the universe – requires a vast amount of calculation. So expecting LQG to inform cosmology is somewhat like expecting quantum mechanics to predict the exact path of a forest fire – they can, in principle, do so, but no currently conceivable strategy of practical calculation could actually do this with them.

 

Since reading post #1, I surveyed a few sources on LQG, and couldn’t find any mentions on the subject about open/closed universe / no big bounce/big bounce cosmologies. Given how difficult and esoteric LQG is, this could simply be my failure to know enough about it to even read it well.