So, what does Hawking and Hertog's research have to do with this unrelenting multiverse?
In the multiverse, our universe is merely a pocket universe where inflation has ended, and, despite the odds, it found calm to create a bounty of stars and galaxies and a bunch of humans living on some random rock pondering the cosmos. What's going on beyond our pocket of calm is, however, somewhat different.
"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," said Hawking in an interview last year. "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 problem, according to Hawking and Hertog, lies with the incompatibility of Einstein's general relativity (that governs the evolution of the universe) and quantum mechanics (that seeds the creation of new universes through quantum fluctuations). The eternal inflation model of the multiverse "wipes out the separation between classical and quantum physics," Hertog said in the accompanying press release. "As a consequence, Einstein's theory breaks down in eternal inflation."
Their study doesn't go so far as reconciling general relativity with quantum physics (a quest that has, so far, been unsuccessful), but they use the math of string theory to help simplify the multiverse model. Quick recap: String theory predicts that all the subatomic particles in our universe are in fact composed of one-dimensional strings that propagate through space. The vibrational state of these strings is what gives these particles their quantum state (such as charge, spin and mass). But string theory also predicts the existence of the hypothetical graviton, a quantum particle that carries the force of gravity. String theory would therefore provide an explanation of how Einstein's general relativity (gravity) jibes with quantum physics.
Using the mathematical framework of string theory, this study simplifies the multiverse. Hawking and Hertog used the string theory concept of holography to reduce our three-dimensional universe down to a two-dimensional "surface," from which the universe we know and love is projected. By doing this, they were able to describe eternal inflation without general relativity, creating a "timeless state."
"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 in a statement.
The math is complex, but the result is interesting. The calculations have the effect of turning the infinite and fractal multiverse into a far simpler (and finite) situation than eternal inflation predicts.
"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.