Days before his death on March 14, famed theoretical physicist and cosmologist Stephen Hawking completed what would be his final research paper. Initially made available via the arXiv preprint service, it passed peer review and was published online in the Journal of High Energy Physics on April 27.

Written with co-author Thomas Hertog, a theoretical physicist at the University of Leuven, Belgium, the paper adds another facet to the understanding of the universe we live in. Needless to say, it's complicated. Titled "A smooth exit from eternal inflation?" the publication discusses an enigmatic problem facing cosmologists.

But before we delve into the crux of the study, let's go back to when our universe was a baby — some 13.8 billion years ago.

### Big Bang and Inflation

A lot of evidence suggests that our universe originated from a singularity, an infinitely dense point where all the universe as we know it was born. We call that event the Big Bang. But how the singularity came to be and why the Big Bang happened isn't of concern right now. We're interested in what happened immediately after our universe was spawned, a period known as "inflation."

Cosmologists predict that inflation occurred over a vanishingly small period right after the Big Bang — or during our universe's first 10^{-32} seconds! During inflation, the universe expanded exponentially and much faster than the speed of light. After only a second, the energy from this inconceivably gargantuan explosion condensed to form the subatomic particles that, over millions of years, created the stars, galaxies, planets and (after 3.8 billion years) life. Once this inflationary period ended, the universe's rate of expansion slowed, but it continues to expand to this day.

Because inflation powered a faster-than-light-speed expansion, the "observable universe" that we see today isn't the *entire* universe. Rather we just exist inside a region of the cosmos that light has had time to reach. It's like dropping a pebble into a calm swimming pool. The first circular ripple to propagate from the splash travels at a fixed speed across the surface of the pool. If we imagine the limit of our observable universe is that ripple — traveling across the pool at the speed of light — it's not that nothing exists beyond that ripple (there's lots more pool, or universe, beyond it), we just can't see it yet.

So, the consequence of inflation is that there should be a LOT more universe beyond what we can see with our most powerful telescopes.

### Enter the Multiverse

Cosmologists have long been grappling with the possibility that our universe isn't the *only* universe. In fact, we could be nothing more than a single bubble in an infinite, frothy ocean, a concept known as the "multiverse." You see, inflation didn't happen once; it's *always* happening via some infinitely vast chain reaction known as "eternal inflation." One universe will appear, inflation will take over, expanding that universe, and that universe will have its own quantum instabilities that will spawn more singularities that go on to create more universes. It's like blowing up a party balloon that itself spawns many other party balloons that erupt from its rubber surface seemingly at random. This situation sounds chaotic, and it is. Proponents of this hypothesis think that eternal inflation is unstoppable, vastly complex and continually generating new universes. The math of this situation suggests the multiverse acts like a fractal.

It's worth noting that each successive universe in the multiverse doesn't likely share the same physics as *our* universe. One universe may not have gravity. Another may not support the forces that hold matter together. There are a lot of stillborn universes that just don't amount to much. We humans are simply lucky to have a universe that has the right environment to create what we see, a philosophical argument known as the anthropic principle.

The problem with eternal inflation is that it's messy and infinite, and the hypothesis is, ultimately, untestable. What's the point of having a wonderful mathematical model for the universe (or universes) if we can't at least find some observational evidence that supports the multiverse hypothesis?

### Hawking Wasn't a Fan of the Multiverse

So, what does Hawking and Hertog's research have to do with this unrelenting multiverse?

In the multiverse, our universe is merely a pocket universe where inflation has ended, and, despite the odds, it found calm to create a bounty of stars and galaxies and a bunch of humans living on some random rock pondering the cosmos. What's going on beyond our pocket of calm is, however, somewhat different.

"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 year. "The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can't be tested."

The problem, according to Hawking and Hertog, lies with the incompatibility of Einstein's general relativity (that governs the evolution of the universe) and quantum mechanics (that seeds the creation of new universes through quantum fluctuations). The eternal inflation model of the multiverse "wipes out the separation between classical and quantum physics," Hertog said in the accompanying press release. "As a consequence, Einstein's theory breaks down in eternal inflation."

Their study doesn't go so far as reconciling general relativity with quantum physics (a quest that has, so far, been unsuccessful), but they use the math of string theory to help simplify the multiverse model. Quick recap: String theory predicts that all the subatomic particles in our universe are in fact composed of one-dimensional strings that propagate through space. The vibrational state of these strings is what gives these particles their quantum state (such as charge, spin and mass). But string theory also predicts the existence of the hypothetical graviton, a quantum particle that carries the force of gravity. String theory would therefore provide an explanation of how Einstein's general relativity (gravity) jibes with quantum physics.

Using the mathematical framework of string theory, this study simplifies the multiverse. Hawking and Hertog used the string theory concept of holography to reduce our three-dimensional universe down to a two-dimensional "surface," from which the universe we know and love is projected. By doing this, they were able to describe eternal inflation without general relativity, creating a "timeless state."

"When we trace the evolution of our universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning," said Hertog in a statement.

The math is complex, but the result is interesting. The calculations have the effect of turning the infinite and fractal multiverse into a far simpler (and finite) situation than eternal inflation predicts.

"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," said Hawking.

### How Do We Test It?

To put it in perspective, Hawking's final paper doesn't revolutionize our understanding of how the universe (and, indeed, the multiverse) works, but it is a valuable addition to a huge field of theoretical work. Specifically, Hertog hopes that this study may help us search for ancient gravitational waves that were generated by eternal inflation. These ripples in spacetime are far too weak for current gravitational wave detectors to detect, however. We'd need to wait until advanced space-based observatories – such as the planned European Space Agency's LISA mission – are launched.

Regardless of whether this study leads to groundbreaking discoveries about the cosmos we live in, it's a testament to a great scientist who worked tirelessly his entire life to answer some of the biggest questions humanity has pondered. And on Hawking's shoulders, other great minds will build on this work to hopefully decipher if our universe is unique — or if it's one bubble chaotically floating in the ocean of the multiverse.