The theory of eternal inflation that Hawking and Hertog put forward is based on string theory: a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, in part by describing the fundamental constituents of the universe as tiny vibrating strings. Their approach uses the string theory concept of holography, which postulates that the universe is a large and complex hologram: physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.
Hertog and Hawking used their new theory to derive more reliable predictions about the global structure of the universe. They predicted the universe that emerges from eternal inflation on the past boundary is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.
Their results, if confirmed by further work, would have far-reaching implications for the multiverse paradigm. “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.
This makes the theory more predictive and testable. Paper. open access – S. W. Hawking, Thomas Hertog. A smooth exit from eternal inflation? Journal of High Energy Physics, 2018; 2018 (4) DOI: 10.1007/JHEP04(2018)147 More.
Well yes, of course. It is always possible that there are other universes out there, as the scaled down theory suggests. But that is not what the multiverse is about. It is about all states being true somewhere, which means that no definitive statement can be made about whether any fact about our universe, like fine-tuning, is significant. An infinity of other universes might not appear fine-tuned, so…
In the unlikely event that we could study a finite number of other universes, if they appear fine-tuned, one wonders what explanation will be offered… Creativity is a must for the job of explaining that away.
See also: Sabine Hossenfelder: Hawking’s final theory is just one of “some thousand” speculations
Did Stephen Hawking discover a means of detecting parallel universes just before he died? This sounds a lot like grief talking but we’ll see.
Post-modern physics: String theory gets over the need for evidence
Cosmic inflation theory loses hangups about the scientific method
The multiverse is science’s assisted suicide