May 2 (UPI) -- The late Stephen Hawking's last paper, published Wednesday in the Journal of High Energy Physics, attempts to reign in the multiverse theory, a theory the physicist considered unwieldy and unprovable.
"We are 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," Hawking said, in a Cambridge University press release.
The multiverse theory posits that our immediate universe is simply a small pocket of a seemingly infinite mosaic of parallel universes. The theory is based on the idea that much of the universe kept expanding, infinitely, in the wake of the Big Bang, and only in small bubbles did expansion slow down and allow a physical order to be established.
Hawking explained the theory in an interview last year: "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."
"The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse," Hawking said. "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."
Several physicists have expressed their frustrations with the theory and its immunity to scrutiny. In 2007, Nobel laureate Steven Weinberg offered a critique similar to Hawking's.
"The hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our Big Bang is doomed, for their values would be an accident of the particular part of the multiverse in which we live," Weinberg said.
In the newest paper, which features a complicated combination of quantum physics and string theory math, Hawking and his research partner Thomas Hertog, a professor at Belgium's Catholic University of Leuven, suggested there are likely multiple universes but only universes with physical laws like our own.
"We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure," said Hawking.
Unlike the multiverse theory, the new theory offers physicists a chance of testing its conclusions. The theory, according to Hertog, allows scientists to test its predictions and also prove the existence of other universes.