Far out cosmology: an only possible universe (Introduction)

by David Turell , Friday, November 18, 2022, 17:13 (524 days ago) @ David Turell

New calculations:

https://www.quantamagazine.org/why-this-universe-new-calculation-suggests-our-cosmos-is...

"...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.

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"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.”

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"...they turned to a technique developed in the 1940s by the physicist Richard Feynman.

"Aiming to capture the probabilistic behavior of particles, Feynman imagined that a particle explores all possible routes linking start to finish: a straight line, a curve, a loop, ad infinitum. He devised a way to give each path a number related to its likelihood and add all the numbers up. This “path integral” technique became a powerful framework for predicting how any quantum system would most likely behave.

"As soon as Feynman started publicizing the path integral, physicists spotted a curious connection with thermodynamics, the venerable science of temperature and energy. It was this bridge between quantum theory and thermodynamics that enabled Turok and Boyle’s calculation.

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"Entropy gives physicists a sharp way of comparing the odds of different outcomes: The higher the entropy of a macrostate, the more likely it is. There are vastly more ways for air molecules to arrange themselves throughout the whole room than if they’re bunched up in a corner, for instance. As a result, one expects air molecules to spread out (and stay spread out). The self-evident truth that probable outcomes are probable, couched in the language of physics, becomes the famous second law of thermodynamics: that the total entropy of a system tends to grow.

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"By Wick-rotating the roller-coaster expansion history of a more realistic class of universes, they got a more versatile equation for cosmic entropy. For a wide range of cosmic macrostates defined by radiation, matter, curvature and a dark energy density (much as a range of temperatures and pressures define different possible environments of a room), the formula spits out the number of corresponding microstates. Turok and Boyle posted their results online in early October.

"Experts have praised the explicit, quantitative result. But from their entropy equation, Boyle and Turok have drawn an unconventional conclusion about the nature of our universe. “That’s where it becomes a little more interesting, and a little more controversial,” Hertog said.

Boyle and Turok believe the equation conducts a census of all conceivable cosmic histories. Just as a room’s entropy counts all the ways of arranging the air molecules for a given temperature, they suspect their entropy counts all the ways one might jumble up the atoms of space-time and still end up with a universe with a given overall history, curvature and dark energy density.

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"Their census reveals that the overwhelming majority of the marbles [universes] have just one color — blue, say — corresponding to one type of universe: one broadly like our own, with no appreciable curvature and just a touch of dark energy. Weirder types of cosmos are vanishingly rare. In other words, the strangely vanilla features of our universe that have motivated decades of theorizing about cosmic inflation and the multiverse may not be strange at all.

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“'What our calculation does is provide huge extra motivation for people who are trying to build microscopic theories of quantum gravity,” Turok said. “Because the prospect is that that theory will ultimately explain the large-scale geometry of the universe.'”

Comment: Only one possible universe? Which is fine-tuned for life. From one Big Bang. More evidence of a designer universe.