Why this universe? A new calculation suggests our cosmos is typical.

#1
C C Offline
https://www.quantamagazine.org/why-this-...-20221117/

INTRO: Cosmologists have spent decades striving to understand why our universe is so stunningly vanilla. Not only is it smooth and flat as far as we can see, but it’s also expanding at an ever-so-slowly increasing pace, when naïve calculations suggest that — coming out of the Big Bang — space should have become crumpled up by gravity and blasted apart by repulsive dark energy.

To explain the cosmos’s flatness, physicists have added a dramatic opening chapter to cosmic history: They propose that space rapidly inflated like a balloon at the start of the Big Bang, ironing out any curvature. And to explain the gentle growth of space following that initial spell of inflation, some have argued that our universe is just one among many less hospitable universes in a giant multiverse.

But now, two physicists have turned the conventional thinking about our vanilla universe on its head. Following a line of research started by Stephen Hawking and Gary Gibbons in 1977, the duo has published a new calculation suggesting that the plainness of the cosmos is expected, rather than rare. Our universe is the way it is, according to Neil Turok of the University of Edinburgh and Latham Boyle of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, for the same reason that air spreads evenly throughout a room: Weirder options are conceivable, but exceedingly improbable.

The universe “may seem extremely fine-tuned, extremely unlikely, but [they’re] saying, ‘Wait a minute, it’s the favored one,’” said Thomas Hertog, a cosmologist at the Catholic University of Leuven in Belgium.

“It’s a novel contribution that uses different methods compared to what most people have been doing,” said Steffen Gielen, a cosmologist at the University of Sheffield in the United Kingdom.

The provocative conclusion rests on a mathematical trick involving switching to a clock that ticks with imaginary numbers. Using the imaginary clock, as Hawking did in the ’70s, Turok and Boyle could calculate a quantity, known as entropy, that appears to correspond to our universe. But the imaginary time trick is a roundabout way of calculating entropy, and without a more rigorous method, the meaning of the quantity remains hotly debated. While physicists puzzle over the correct interpretation of the entropy calculation, many view it as a new guidepost on the road to the fundamental, quantum nature of space and time.

“Somehow,” Gielen said, “it’s giving us a window into perhaps seeing the microstructure of space-time.” (MORE - details)
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#2
Zinjanthropos Offline
Quote: And to explain the gentle growth of space following that initial spell of inflation, some have argued that our universe is just one among many less hospitable universes in a giant multiverse.

Humbling enough with just one universe but it could be we’re more insignificant than what many believe.
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#3
Kornee Offline
Can hardly wait for the fierce counter attacks from rival theorists. And no-one can win because there is absolutely no chance of experimental confirmation either way. It's ivory towers all the way down.

One incontestably true show stopper is something left out of the article. 'Eternal' inflation cannot be past eternal.
So at the end of the day, Turok & Co still have to wrestle with explaining how it all got boot-strapped going at some finite time in the past.

An interesting problem since it's known the conditions allowing inflation to begin at all are extremely fine tuned:
https://en.wikipedia.org/wiki/Eternal_in...velopments
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#4
Zinjanthropos Offline
Hardest thing for me to visualize is what all the universes exist in and the boundaries that might separate them. If ours is typically plain then what is an atypical universe?

I mean if they’re all typical then why have a boundary? Why have other universes? To suggest other universes co-exist says they’re different from another and are separated for probably that reason.
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#5
confused2 Offline
If the total mass of the universe is finite there ought to be place where you can stand with most of it behind you .. with just darkness ahead. I've said this before (SF?) and got "Oh no no no - you don't know what you're talking about." . Any thoughts anyone?
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#6
Kornee Offline
(Nov 23, 2022 06:44 PM)confused2 Wrote: If the total mass of the universe is finite there ought to be place where you can stand with most of it behind you .. with just darkness ahead. I've said this before (SF?) and got "Oh no no no - you don't know what you're talking about." . Any thoughts anyone?
While I don't subscribe to the idea of a spatially infinite universe, all available data indicates it has to be many orders of magnitude larger than the observable one, currently indistinguishable from being perfectly spatially flat.
There is no model taken seriously by cosmologists that allows any kind of 'edge' or actual 'center'. The Copernican Principle requires everywhere to look much the same in every direction. But only on a large enough scale.
We all know and have seen illustrations of the 'cosmic web' structure that is hundreds of millions of light years in characteristic dimensions.
https://astronomy.swin.edu.au/cosmos/L/L...+Structure
https://www.darkenergysurvey.org/support...structure/
https://en.wikipedia.org/wiki/Shape_of_the_universe

PS - Surely the 2D analogy of e.g. an ant on the surface of a sphere has been pointed out. A finite sized 'universe' but without any edge. That's a closed universe analog but something similar applies to a flat or open one.
So evidently the leap to a 3D case is confounding. Being hard or impossible to visualize doesn't equate to 'can't be true'.
The modern conception of Big Bang is not of an explosion into an empty void, but of space and time co-forming and evolving together with matter/radiation/dark energy.
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#7
Kornee Offline
Recent findings of a competent cosmologist team that plumps for a closed universe:
https://arxiv.org/abs/2210.09865
Unless one likes wading through troves of data and equations, just go straight to 'B. Concluding remarks and the road ahead' bottom of p36 on.
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