Is this a legitimate attempt to posit a dramatic change in thinking about the early universe (or lack thereof), or is it more of a thought experiment where they are demonstrating interesting results when they solve equations in a different way?
Is there a way to explain how this model accounts for dark matter and energy without getting into the details of the equations?
And at the risk of getting in IMHO territory, what do the resident SDMB physicists think of all this?
Not to be nit-picky, but wouldn’t it be “suggests no Big Bang,” rather than “predicts no Big Bang,” since you cannot predict something that already happened (or didn’t happen)?
Heh, I used that phrase originally but changed it on preview to match the headline of the article I linked to. I figured the writer knew the right terminology better than I. Maybe not a good assumption.
It was pointed out to me on Twitter the other day that the phys.org article linked above looks a lot like a press release rather than actual science journalism: it doesn’t quote anyone other than the authors of the paper, and doesn’t present any critiques of the idea (even to rebut them). It’s a little frustrating when such thin coverage of a scientific idea gets viral traction, but that’s the world we live in, I suppose.
As far as the content goes, it would take me a while to decipher what the authors are actually doing. I’ll note from their abstract that their approach is based on something called Bohmian mechanics, which is a decidedly non-mainstream approach to physics. (See here for a critique of that article from a mainstream perspective.) This approach isn’t wrong, per se, but so far nobody has been able to come up with an experiment that can unambiguously distinguish between this approach and standard quantum mechanics; so it’s usually thought of as a reformulation of standard quantum mechanics with some weird extra entities thrown in.
However, as physicists such as these authors try to push the Bohmian perspective into new realms, they might come up with some new and interesting ideas that could lead to distinguishing experiments. Maybe this idea is a step towards a new perspective on quantum mechanics, but as of yet it makes no new predictions (just postdictions), and so I remain mildly interested but skeptical for the time being.
To be fair, they don’t dispute these. In their model, the whole Universe was in a hot, dense state at some point nearly fourteen billion years ago, and then it expanded. The difference between this new model and the conventional “Big Bang” model is that in their model, there wasn’t an “initial singularity”, i.e., the Universe was never the size of a point. Why? Because quantum. (At least, that’s my level of understanding for the “why” of it.)
The only way that this model differs from the conventional model is that it avoids the Universe being infinitely dense and hot at some initial instant. And, to be fair, we’re pretty sure that’s true already; real infinities simply don’t occur in nature, and if you ask 100 physicists whether the density of the Universe was really infinite at some initial instant, if they’re being honest they’ll say “probably not; we don’t yet understand how physics works in such circumstances”. Whether this is the correct explanation for how the Universe “avoided” being infinitely dense is another question entirely.
Is this some sort of cyclical universe without the “infinities” of the big bang and big crunch? If not cyclical, why in a universe of infinite age would something special happen about 14 Bya?
> In their model, the whole Universe was in a hot, dense state at some point nearly
> fourteen billion years ago, and then it expanded.
Yeah, but we know now that it will never pause and start to go the other way, collapsing ever inward, so our best and brightest no longer figure that there will be an even bigger and brighter bang.
Their basic idea is to approximate the corrections to the classical equation of state due to quantum gravity. Now what they come up with is a little all-singing and all-dancing in that they claim to both derive the cosmological constant (dark energy) and identify a prime dark matter candidate. Of course a bit of fine-tuning to agree with observation is hardly a sin, but always take models that claim to solve several mysteries at once with a pinch of salt as it often means the parameters and elements of the model have been chosen out of a wide range of possible parameters/elements that specifically get the desired answer.
It’s well-known that altering the equation of state in the early Universe can avoid the big bang singularity - a classic example would be a cosmological model which contracts at a decelerating rate before reaching a finite minimum size and expanding. Classically these models are ruled out by Hawking’s singularity theorem but this is true only if certain energy conditions are true and quantum fields are known to violate energy conditions which are reasonable for classical fields.
Bohmian mechanics itself is hardly outre, its a well established interpretation of QM and like all of the main interpretations has several desirable features as well as a few not so desirable features Bohmian mechanics is often useful alternative way of viewing quantum mechanical problems for example John Bell was particularly keen on Bohmian mechanics and investigating limitations of hidden variable theorems (like Bohmian mechanics) led him to his famous theorem. The question here is whether Bohmian mechanics has been applied correctly in a relativistic/general relativistic context. Obviously the authors would say yes, but that doesn’t mean everyone would agree.
So basically the model is legitimate attempt to apply derive a quantum-corrected view of the early Universe (though it doesn’t mean there are flaws, but it’d take someone with more expertise than me to judge this), but also fairly speculative.
We do not know this. If the dark energy (whatever it is) keeps going at its current strength, then no, it will never re-collapse. But if it turns off, or reverses sign, then it might. And we know so little about what the dark energy is and how it works that we can’t even make a meaningful guess about whether this might happen.
