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

Cosmologists have spent decades of striving to understand why our universe is such a beautiful vanilla color. Not only is it as smooth and flat as we can see, but it is also expanding at an increasingly slow rate, as naive calculations suggest that—out of the Big Bang—space should have been shrinking. by gravity and exploded by repulsive dark energy.

To explain the flatness of the universe, physicists have added an impressive opening chapter to the history of the universe: They propose that space inflates rapidly like a balloon at the start of the Big Bang, smooth any curvature. And to explain the gentle growth of space after that initial inflation, some have argued that our universe is just one of many less hospitable universes in a giant multiverse. giant.

But now, two physicists have changed the conventional wisdom about our vanilla universe. After a series of studies started by Stephen Hawking and Gary Gibbons in 1977, the duo published a new calculation showing that the simplicity of the universe is expected, rather than rare. Our universe is like that, 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 across the room: More exotic but extremely unlikely choices are conceivable.

The universe “seems extremely sophisticated, extremely unlikely, but [they’re] say, ‘Wait a minute, that’s the priority,’” said Thomas Hertoga cosmologist at the Catholic University of Leuven in Belgium.

“It’s a novel contribution that uses different methods than what most people are doing,” says Steffen Gielena cosmologist at the University of Sheffield in the United Kingdom.

The provocative conclusion is based on a mathematical trick that involves switching to a ticking clock with imaginary numbers. Using an imaginary clock, as Hawking did in the 70s, Turok and Boyle were able to calculate a quantity, called entropy, that seems to correspond to our universe. But the imaginary time trick is a roundabout way of calculating entropy, and without a more rigorous method, the quantity’s meaning is still hotly debated. While physicists are confused about exactly how to interpret the calculation of entropy, many see it as a new guide on the way to the fundamental, quantum nature of space and time.

“Somehow,” said Gielen, “it gives us a window into the microstructure of space-time.”

Imaginary path

Turok and Boyle, frequent collaborators, are known for devising innovative and unorthodox ideas about cosmology. Last year, to study how our universe might exist, they turned to a technique developed in the 40s by physicist Richard Feynman.

Aiming to capture the probabilistic behavior of particles, Feynman imagined that a particle explores all possible routes from start to finish: a line, a curve, a loop, to infinity. . He devised a way to give each path a number relative to its probability and add all the numbers together. This “path integration” technique has become a powerful framework for predicting how any quantum system should behave.

Just as Feynman began to publish path integrals, physicists discovered an uncanny 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 calculations.