Before the eminent physicist died at age 76, Hawking completed a paper that describes a reality made up of multiple universes, but which are not too different to our own. This is Stephen Hawking’s final theory of the cosmos, a work that proposes to simplify the evolution of the last 13.8 billion years since time exists. 

Stephen Hawking at Gonville & Caius College, Cambridge. Credit: Lwp Kommunikáció / Flickr.

The new work is the result of a collaboration with Thomas Hertog, who is a Belgian physicist at the Catholic University of Leuven. The pair’s theory is sophisticated (and quite speculative) but not all that difficult to explain. You might have heard about the Big Bang, but modern physics has actually multiple, alternative theories that explain how our universe came to be. One multiverse theory suggests that right after the Big Bang, repeated bursts of ‘cosmic inflation’ occurred which seeded an endless number of pocket universes.

“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,” Hawking said in late 2017. 

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.”

In their new paper, Hawking and Hertog came up with a different take. They propose that the pocket universes that fill space do not employ radically different laws of physics but rather these alternate universe may not actually vary that much from one another. By reducing the multiverse down to a more manageable set of universes which all resemble each other, scientists now have a better chance of arriving at a fully predictive framework of cosmology.

According to the two physicists’ new paper, the eternal inflation model is wrong since Einstein’s theory of general relativity breaks down on quantum scales.

“The problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein’s theory of general relativity and treats the quantum effects as small fluctuations around this,” Hertog explained.

“However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein’s theory breaks down in eternal inflation.”

The new paper is based on work done by Hawking and US physicist James Hartle in the 1980s. The theory was revisited and updated with more powerful mathematical techniques used in string theory, where reality is described through the interaction of one-dimensional objects called cosmic strings. Ultimately, this allowed the physicists to reduce eternal inflation to a timeless state on a spatial surface.

“When we trace the evolution of our universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning,” said Hertog.

Hertog says that the work he’s done with Hawking brings humanity one step closer to understanding the origin of the cosmos. Their assertions could be experimentally tested one day since the theory predicts that if the universe evolved as described, then telltale signs should be recorded in gravitational waves or in the cosmic microwave background, the radiation released by the Big Bang.

“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,” said Hawking.

Stephen Hawking died on 14 March, at age 76, at his home in Cambridge. His ashes will be laid near the grave of Sir Isaac Newton at Westminster Abey during a thanksgiving service at the end of 2018.

S.W. Hawking and Thomas Hertog. ‘A Smooth Exit from Eternal Inflation?’’ Journal of High-Energy Physics (2018). DOI: 10.1007/JHEP04(2018)147