Why this universe? Maybe it’s not special – just probable

10 months ago

Cosmologists spent decades trying to figure out why our universe is so amazingly vanilla. As far as we can see, it is not only smooth and flat, but expanding at an ever-increasing rate, where naive calculations suggest that after the Big Bang, space must have been compressed by gravity and torn apart by repulsive dark energy.

To explain the flatness of the cosmos, physicists have added a dramatic introductory chapter to cosmic history: they suggest that space rapidly inflated like a balloon at the start of the Big Bang, flattening out any curvature. And to explain the smooth growth of space after an initial period of inflation, some argue that our universe is just one of many less hospitable universes in the giant multiverse.

But now two physicists have turned the conventional wisdom about our vanilla universe on its head. Following a line of research begun by Stephen Hawking and Gary Gibbons in 1977, the duo published new calculations suggesting that the simplicity of the cosmos is expected, not rare. Our universe is what it is, according to Neil Turk Edinburgh University and Latham Boyle The perimeter of the Institute for Theoretical Physics in Waterloo, Canada, for the same reason that air spreads evenly around the room: weirder options are conceivable, but extremely unlikely.

The universe “may appear extremely finely tuned, highly improbable, but [they’re] saying: “Wait, this is the most beloved,” said Thomas Hertogcosmologist from the Catholic University of Leuven in Belgium.

“This is a new contribution that uses methods that are different from what most people have done,” he said. Steffen Gielencosmologist from the University of Sheffield in the United Kingdom.

The provocative conclusion is based on a mathematical trick with switching to a clock that ticks with imaginary numbers. Using an imaginary clock, as Hawking did in the 1970s, Turok and Boyle were able to calculate a quantity known as entropy that appears to fit our universe. But the imaginary time trick is a roundabout way of calculating entropy, and without a more rigorous method, the value of the quantity remains the subject of heated debate. While physicists are puzzling over the correct interpretation of the calculation of entropy, many see it as a new benchmark on the way to the fundamental, quantum nature of space and time.

“Somehow,” Gielen said, “it gives us the ability to see the microstructure of space-time.”

imaginary paths

Turok and Boyle, frequent collaborators, are known for their creative and unorthodox ideas about cosmology. Last year, to study how likely our universe might be, they turned to a technique developed in the 1940s by physicist Richard Feynman.

In an effort to capture the probabilistic behavior of particles, Feynman imagined that a particle explores all possible paths connecting the beginning and the end: a straight line, a curve, a loop, and so on ad infinitum. He came up with a way to give each path a number associated with its probability and add up all the numbers. This “path integral” method has become a powerful framework for predicting the likely behavior of any quantum system.

As soon as Feynman began publishing the path integral, physicists noticed a curious connection with thermodynamics, the venerable science of temperature and energy. It was this bridge between quantum theory and thermodynamics that allowed Turok and Boyle to do the calculations.

South African physicist and cosmologist Neil Turok is a professor at the University of Edinburgh.Photograph: Gabriela Secara/Perimeter Institute

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