What is the multiverse theory about

Hawking's posthumously published theory "simplifies" the multiverse

Vienna - When the astrophysicist Stephen Hawking died on March 14th at the age of 76 in Cambridge, he left behind one last theory for posterity that had not yet been published. The paper that he wrote together with his Belgian colleague Thomas Hertog (Catholic University of Leuven) has recently appeared in the "Journal of High Energy Physics" - and promises a "simpler theory" of the universe.

In the specialist article entitled "A smooth exit from eternal inflation?" theorists deal with the central phenomenon immediately after the Big Bang: After a few fractions of a second, our universe expanded extremely quickly, which is known in astrophysics as "inflation". And according to the popular theory of many cosmologists, this inflation lasts forever thanks to quantum effects and only came to a standstill in a few "pockets" of the universe.

Is the multiverse an infinite fractal ...

"The usual theory of perpetual inflation predicts that our universe is global like an infinite fractal, with a mosaic of different pocket universes, separated by a billowing ocean," said Hawking in an interview last fall. The observable part of our universe would therefore only be a hospitable pocket universe, a region in which inflation has ended and stars and galaxies could form.

... or is it much easier?

On the basis of string theory, Hawking and Hertog now state in their new publication that this notion of perpetual inflation as a theory of the Big Bang is likely to be wrong. "The problem with the usual description of perpetual inflation is that it assumes an existing background universe that evolves according to Einstein's general theory of relativity and treats the quantum effects as small fluctuations around them," says Hertog.

"We predict that our universe is relatively smooth on the largest scales and globally finite. So it is not a fractal structure," said Hawking when he came up with the new theory last fall.

This also claims to be able to derive more reliable predictions about the global structure of the multiverse. According to their assumptions, this is finite and much simpler than the infinite fractal structure predicted by the old theory of perpetual inflation. Logically, that would have far-reaching implications for the multiverse paradigm.

Possible empirical review

Hertog now plans to study the consequences of the new theory on smaller scales that are within range of our space telescopes. He believes that the original gravitational waves - waves in space-time - at the exit of eternal inflation are the cheapest "smoking gun" to test the new model and, ideally, to confirm it.

Such gravitational waves would have very long wavelengths outside the range of the current Ligo detectors. But they could be heard from the planned European space gravitational wave observatory Lisa or seen in future experiments measuring the cosmic microwave background. (Klaus Taschwer, May 2nd, 2018)