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Was There Only One Big Bang?
goldaryn writes "Physorg.com is running an interesting story about the work of Oxford-based theoretical physicist Roger Penrose. Penrose has been studying CWB radiation and believes it's possible that space and time did not come into being at the Big Bang but that our universe in fact continually cycles through a series of 'aeons.' He believes that he has found evidence supporting his theory that the universe infinitely cycles."
That's not really so much "consensus" as "the result of the model we're currently using". No-one *likes* the standard model of cosmology; it's obviously just phenomenology, but it happens to fit all the data at least as well as any alternative. The standard model of cosmology is "lambda CDM", the lambda being a cosmological constant which drives an accelerating expansion in the current universe, and the CDM being cold dark matter which was responsible for the clustering of matter and the formation of galaxies and so forth.
The problem is that if it *is* just a cosmological constant then it will grow to dominate the universe and things will, indeed, expand forever.
But it's probably not a cosmological constant. The "best" prediction from quantum field theory -- and it's not really a prediction so much as the only way of estimating the size of the constant -- comes from evaluating the vacuum energy. Doing this suggests that it should be about 10^120 times bigger than we see in reality. That's a pretty big difference. Weinberg described it as the most embarrassing mismatch between theory and observation in the history of science, and he's got a point. The conclusion is that there's probably some mechanism (coming from trans-Planckian physics, maybe, or something else) that cancels the cosmological constant. If that's true, then it's almost certain to cancel it perfectly because the fine-tuning necessary to produce the *observed* constant is horrific, whereas a symmetry principle could wipe the whole thing much more easily.
That leaves you open to more general dark energy models to explain the accelerating expansion and that's where you have more fun. There are plenty of ways to get an observed acceleration. Some of them lead to big rips, which is where eventually the universal expansion will tear galaxies, then solar systems, then stars, planets and eventually even atoms and nuclei apart. Others lead to the decay of whatever field is responsible for acceleration -- like if you couple a scalar field into dark matter you can tune it such that the scalar field grows to dominate and then starts transferring its energy into dark matter, which would cause the universe to reclump again (and then probably the dark matter would dump energy back into the scalar field causing more acceleration). Those models are horribly contrived and unrealistic, but at least they're alternatives.
And the cosmological constant is pretty contrived and unrealistic in the first place...
Anyway. Before I got side-tracked my point was that there isn't really a consensus so much as a model that fits observations and predicts eternal expansion, but that it's not the only model and it's not even the best motivated model, merely the simplest. Other models can still lead to crunches while fitting pretty much all the data as well. And others can lead to cyclic universes, which is ultimately what Penrose is talking about in one form or another.
Disclaimer: I am a cosmologist but I've not actually read the article. This is Slashdot, after all.