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Our Universe is finite and is one of many similar universes, according to Stephen Hawking’s final paper on the nature of the cosmos.
The paper, titledA Smooth Exit From Eternal Inflation?? is the end result of Hawking’ss longstanding collaboration with Thomas Hertog, a physicist based at the Catholic University of Leuven in Belgium, and was submitted for publication just days before the physicist’s death earlier this year.
There are several competing theories regarding exactly how the Universe came to be, but most agree that in the fractions of a second following the Big Bang the Universe expanded incredibly rapidly in all directions, much like a balloon does when it’s being blown up. This is known as cosmic inflation and accounts for the fact that the Universe has a large-scale structure that appears to be the same in every direction. There is far less of a consensus, however, on what happened next.
According to the theory of eternal inflation, one of the leading theories, after the Big Bang some pockets of space stopped expanding while others continued. This gave rise to many so- called bubble universes, which are separated from one another by vast areas of expanding space.
In our Universe, expansion ended. This enabled the galaxies and stars to form, but it’s just one small pocket of space embedded in a much larger expanding area within which there are countless other bubble universes. According to the theory, the laws of physics in these bubble universes could be different from ours, making them very strange worlds indeed.
But this idea has never sat well with Hawking. “The usual theory of eternal inflation predicts that, globally, our Universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean,” he said. “The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can’t be tested.”
“THE NEW THEORY IS BASED ON THE SOMEWHAT ABSTRACT CONCEPT THAT THE UNIVERSE ACTS LIKE A VAST HOLOGRAM”
The new theory is based on the somewhat abstract concept that the Universe acts like a vast hologram. Physical reality in certain 3D spaces can, thanks to some very clever maths, be reduced to 2D projections on a surface, much like a hologram can display a 3D image on a 2D surface.
After hashing through some complicated equations, Hertog and Hawking came to the conclusion that our Universe is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.
“We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it’s not a fractal structure,” said Hawking. “We’re not down to a single, unique Universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes.” This means it’s possible to test the theory through experiment. Hertog believes that primordial gravitational waves, ripples made in space-time by some of the most violent events in the cosmos, may provide a means of doing so.
ABOVE: Prof Stephen Hawking worked with Thomas Hertog (to his left) on his final paper
RIGHT: Eternal inflation enables the possibility that many bubble universes may exist