Ten days before he died, Stephen Hawking sent one more written insight out into the cosmos — a paper, co-written with physicist Thomas Hertog of the Catholic University of Leuven in Belgium, tackling the problem of a multiverse.
The paper had previously been posted on a pre-print site, where scientists share drafts of papers before they are published in peer-reviewed journals. It inspired a lot of breathless reports that he had predicted the end of the world and offered a way to detect alternate universes.
The actual study, published in the Journal of High Energy Physics, isn't quite so sensational. It deals with a paradox that the physicist himself helped raise: If the big bang created infinite universes with an inexhaustible number of variations on the laws of physics, then how can scientists hope to answer fundamental questions about why our universe looks the way it does?
Hawking, who died March 14 at age 76, overcame a devastating neurological disease to publish groundbreaking insights into black holes and other mysteries of the cosmos.
“It's very Stephen,” Michael Turner, a cosmologist at the University of Chicago, said of the new research. “To the very end, he was not afraid to take on the toughest problems, and the problem he's taking on is the one that gives me a headache.”
Scientists believe that when the universe began, some 13.8 billion years ago, it underwent a process called inflation — exponential expansion in a very short amount of time. Over the course of that rapid expansion, tiny quantum fluctuations in space were magnified to cosmic size, creating the seeds of the structures that would become galaxies and light up the universe.
Some of the architects of inflation theory, including Stanford University physicist Andrei Linde, have proposed a parallel idea: Quantum fluctuations during inflation gave rise not only to galaxies but to entire universes. Weirder yet, Linde suggests that inflation is still happening. In an interview Wednesday, he compared the cosmos to an ever-expanding block of Swiss cheese. The holes are “pocket universes,” places where inflation has stopped locally, allowing matter to condense and stars and galaxies to form. We may well live in one of those pockets, encased in dense dairy product, disconnected from and blissfully unaware of the infinite alternate universes that exist around us.
If that idea has you raising your eyebrows, you're not alone. Some cosmologists are wary of “eternal inflation” — and the multiverse that might stem from it. For one thing, if the various pocket universes are disconnected, how will we ever verify that they are there? For another, an infinite multiverse would defy mathematical analysis, making it hard to use the model to understand how the cosmos should work.
In their new paper, Hawking and Hertog “give the multiverse a haircut,” Turner said. To do so, they draw on Hawking's “no boundary" theory — an idea developed in the 1980s to explain how the universe arose — and a concept known as the “holographic universe,” which suggests that the universe can be seen as a hologram and that a three-dimensional reality can be mathematically collapsed into only two dimensions to make calculations easier.
As a result, they were able to bring some measure of order to the sprawling, incomprehensible multiverse. Pocket universes in this model would be less numerous, and they would share certain fundamental qualities, making them easier to analyze.
“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,” Hawking said in an interview with Cambridge University last fall.
The proposed model isn't fully developed, said Katie Mack, a cosmologist at North Carolina State University. “It's more like a simplified version of something just to see what happens,” dependent on concepts that are not yet widely accepted and mathematical tools that are still new.
She also pointed out that Hawking and Hertog do not offer any quantifiable predictions in their paper, making it hard to test the idea.
“It's not 42,” Turner agreed — referencing a joke from the novel “A Hitchhiker's Guide to the Galaxy,” in which the number is “the Answer to the Ultimate Question of Life, the Universe, and Everything.”
But for scientists, it's something just as good: Not a solution to all the problems in the universe, but an intriguing — if somewhat wacky-sounding — idea. A potential path to follow, though they don't know where it might lead.
In that, it's a fitting legacy for the physicist.
“Stephen Hawking was a human being,” Linde said. “He was not a genius who said all correct things every day of the week. He was struggling through the same scientific problems through which we all are struggling” — albeit amid greater adversity and with sharper insight than most people can hope to achieve.
With this paper — which Cambridge University says is his “final theory on the origin of the universe” — Hawking has given his colleagues one more thing to think about as they tackle the problems of “life, the universe and everything.”
“But it is very sad that Stephen is not here to talk about it with us,” Linde said.