I’ve been following this topic for years. Just some notes before I post something more complete — prompted by Stephen Hawking’s final research paper, much in the news. Multiverse: infinite or countable verses?
• Prof Stephen Hawking’s multiverse finale (May 2, 2018), BBC News, includes a video.
The study was submitted to the Journal of High-Energy Physics 10 days before Prof Hawking died.
A crisis arises because if there are infinite types of universes with infinite variations in their laws of physics then the theory cannot predict what kind of universe we should find ourselves in.
Prof Hawking joined forces with Prof Thomas Hertog at KU Leuven in Belgium, who is funded by the European Research Council to try to resolve this paradox.
• Stephen Hawking’s Final Paper Cuts the Multiverse Down to Size (May 3, 2018), Space.com.
Stephen Hawking’s final paper, which aims to test a theory that proposes parallel universes, appeared today (May 2) in the Journal of High Energy Physics.
… it shares Hawking’s final look at one of his earliest theories, the so-called “no-boundary proposal.” This idea describes the conditions in the very early universe. … Hertog called their conclusions a “departure” from the “no-boundary proposal” as Hawking originally presented it.
… most significantly that it [the original proposal] rendered most basic scientific ideas about the multiverse impossible to test.
“What they’ve done in this paper is to use what they call a toy model — it’s not fully rigorous and complete,” Mack said. “They admit that there’s a lot more work to be done.” [Katie Mack is a North Carolina State University cosmologist.]
To get to that point, Mack said, physics will need to overcome some significant hurdles. Most importantly, they have to develop a thorough unification of the theories of gravity and quantum mechanics.
• Stephen Hawking’s final work turns theories about the universe on their head (May 2, 2018), Cambridge News.
His latest theoretical work, a collaboration with Professor Thomas Hertog from KU Leuven, … is based on string theory and turns science on its head, predicting the universe is finite, rather than infinite as previously suggested, and far simpler than many current theories about the big bang say.
“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 autumn.
“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,” said Hertog.
“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.”
“We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure,” said Hawking.
Hawking’s earlier ‘no boundary theory’ predicted that if you go back in time to the beginning of the universe, the universe shrinks and closes off like a sphere, but this new theory represents a step away from the earlier work.