Stephen Hawking has a final goodbye: a study decades in the making that aims to restore order in the increasingly chaotic multiverse.

The paper was submitted just 10 days before Hawking's death. It is coauthored by the physicist's longtime collaborator Thomas Hertog.

The Multiverse

An official release from the University of Cambridge reports that the new theory is based on the string theory and paints a simpler and finite universe than previous concepts of what followed the Big Bang.

Modern theories predict that the local universe came about when, a fraction of a second after the Big Bang, the universe expanded at an exponential rate in a burst of inflation. It's believed that inflation is endless once it begins, keeping on going in certain regions of the universe and consequently creating multiple pocket universes throughout space.

"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 explains in an interview last autumn. "The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse."

However, the late physicist adds, he's never been a fan of the multiverse theory.

In an interview with BBC News, Hertog seconds his friend's sentiment, saying the prospect of infinite universes with infinite variations of the laws of nature and science did not appeal to the pair.

Hertog admits to BBC News that the two of them were not happy with the scenario.

"It suggests that the multiverse emerged randomly and that we can't say very much more about that," he continues, then adds that while they considered simply living with it, he and Hawking eventually decided not to give up.

The pair used the mathematical techniques of the string theory to develop a new definition of eternal inflation, reducing it "to a timeless state defined on a spatial surface at the beginning of time." With their recent work, Hawking and Hertog move away from their earlier "no boundary theory" to a theory saying there's a boundary to the past, to the beginning of the universe.

This theory predicts that the universe emerging from the eternal inflation on this boundary is finite and simpler with a much smaller range of potential universes.

In this new Hawking-Hertog assessment, the pocket universes in the multiverse will all share the same laws of physics as our universe, BBC notes.

Critics On Hawking's Final Paper

While the findings of Hawking and Hertog are widely appreciated, fellow scientists have tempered the hype due to its theoretical nature at the moment.

Avi Loeb, chair of the Harvard Astronomy Department, tells Popsci that the findings of the pair is stimulating, yet not revolutionary.

"The main conclusion of the paper is a conjecture and not proven mathematically," Loeb points out. 

In Gizmodo, he goes on to explain that while it is conjecture at this point, it's one that he prefers to traditional concepts of the multiverse.

"The traditional formulation of the multiverse theory could not be falsified if 'anything that can happen will happen an infinite number of times,'" Loeb says. "I like the Hawking and Hertog concept much more. But more work is needed to flesh it out in more detail."

Andrei Linde, theoretical physicist, also explains to Gizmodo that the two authors offer a new approach to eternal inflation that's based on conjectures they formulated, which is why the title of the paper is in question marks.

Further studies are necessary to formulate a full theory.

"Thus in his last paper, Hawking approached one of the most important and challenging problems of modern physics, going beyond the realm of our universe, and proposed a possible way towards its solution," Linde says. "Can we rise to the challenge?"

The study "A smooth exit from eternal inflation?" is published in the Journal of High Energy Physics.