Cosmologists have spent decades of struggling to understand why our universe is so incredibly vanilla. Not only is it smooth and flat as far as we can see, it’s also expanding at an ever-slower pace, as naive calculations suggest that – coming out of the Big Bang – space should have been puckered by gravity and shredded by repulsive dark energy.
To explain the flatness of the cosmos, physicists have added a dramatic opening chapter to the cosmic story: they propose that space inflated rapidly like a balloon at the start of the Big Bang, eliminating any curvature. And to explain the smooth growth of space after that initial period of inflation, some have argued that our universe is just one of many less hospitable universes in a giant multiverse.
But now two physicists have turned 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 published a new calculation suggesting that the simplicity of the cosmos is expected, not 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 across a room: stranger options are conceivable, but extremely unlikely.
The universe “may seem extremely fine-tuned, extremely improbable, but [they’re] saying, ‘Wait a minute, it’s the favourite,'” said Thomas Hertog, a cosmologist at the Catholic University of Leuven in Belgium.
“It’s a groundbreaking contribution that uses different methods compared to what most people have done,” said Steffen Gielen, a cosmologist at the University of Sheffield in the UK.
The provocative conclusion is based on a mathematical trick involving switching to a clock that runs on imaginary numbers. Using the imaginary clock, as Hawking did in the 1970s, Turok and Boyle were able to calculate a quantity, known as entropy, that appears to correspond to our universe. But the imaginary time trick is an indirect way of calculating entropy, and without a more rigorous method, the meaning of the quantity remains hotly debated. While physicists are confused about the correct interpretation of the entropy calculation, many see it as a new milestone on the way to the fundamental quantum nature of space and time.
“In some way,” Gielen said, “it’s giving us a window to maybe see the microstructure of spacetime.”
Turok and Boyle, frequent collaborators, are known for coming up with creative and unorthodox ideas about cosmology. Last year, to study the probability of our universe, they resorted to a technique developed in the 1940s by physicist Richard Feynman.
In order to capture the probabilistic behavior of particles, Feynman imagined that a particle explores all possible routes connecting the beginning to the end: a straight line, a curve, a loop, ad infinitum. He devised a way to give each path a number related to its probability and add all the numbers together. This “path integral” technique has become a powerful framework for predicting how any quantum system is likely to behave.
Once Feynman began to publicize the path integral, physicists noticed a curious connection to thermodynamics, the venerable science of temperature and energy. It was this bridge between quantum theory and thermodynamics that allowed Turok and Boyle’s calculus.