Stephen Hawking's final theory could prove the existence of the multiverse

Weeks before he died, renowned physicist Stephen Hawking finished laying out the groundwork on a theory he hoped would prove the existence of other universes outside our own.

The paper, which has not yet been published or peer reviewed, lays out the mathematics needed to build a space probe capable of detecting evidence of parallel universes — also known as the multiverse. 

"Think of it as many universes. So not just many solar systems, but really an ensemble of separate worlds," Thomas Hertog, a Belgian physicist who co-wrote the paper with Hawking, told As It Happens host Carol Off.

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"Some of these universes are completely empty, and others are full of black holes, and yet others have stars and galaxies and life."

The concept of the multiverse stems from the big bang theory — Albert Einstein's once controversial, but now widely accepted, idea that the universe instantaneously expanded from a tiny point called a singularity.

Infinite big bangs 

Hawking predicted that our big bang was just one in an infinite number of big bangs that occurred simultaneously — each of them creating its own separate universe.

But if other universes exist, we have no way to find them, and no way to test the theory.

That loose thread had long been a sore point for Hawking, Hertog said. 

"This was very much on the top of his mind. Hawking did not like the multiverse. But on the other hand, he realized it's very hard to avoid. Pretty much any reasonable model of the big bang which we could come up with led us to a multiverse," he said.

"If anything is possible — if a multiverse is too gigantic, too wild — then our theory won't say anything about our own universe, and so it's useless as a scientific theory."

'Control the multiverse'

So, he said, Hawking came to him a few years ago and said, "Alright, let's try to control the multiverse."

That's what they set out to do in what would become Hawking's last scientific paper. 

He and Hertog laid out the mathematics needed to build a space probe that would be capable of detecting powerful gravitational waves created by multiple big bangs.

This, Hertog says, gives scientists a way to actually test the multiverse theory. 

The most recent draft of the paper — called A Smooth Exit from Eternal Inflation?— was submitted for review on March 4, just over two weeks before Hawkings died.

Some in the scientific community have hailed the paper as groundbreaking, suggesting it would have earned the renowned physicist a Nobel Prize if it had been published while he was still alive. Nobel Prizes are not awarded posthumously.

"These ideas offer the breathtaking prospect of finding evidence for the existence of other universe," Carlos Frenk, professor of cosmology at Durham University, told The Sunday Times.

Others more sceptical

Avi Loeb, chair of the Harvard Astronomy Department and director of Harvard's Institute for Theory and Computation, told Mashable the paper puts forward "a very interesting idea," but uses "an approach that's questionable."

"More work is needed to flesh it out in more detail," Loeb said.

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But Hertog says Hawking's ideas are not as radical as they appear.

Rather, he was trying to take this huge, controversial idea that encapsulates infinite possibilities, and turn it into a "testable, verifiable scientific framework."

"Stephen always had interesting new ideas," Hertog said.

"Science was an adventure for him, and you were never sure where it would take him."

Written by Sheena Goodyear. Interview produced by Katie Geleff.

Correction: An earlier version of this story said incorrectly that Stephen Hawking developed the big bang theory. In fact, it was Albert Einstein.