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Just a few days before he died, Stephen Hawking submitted one last research paper using string theory math to talk about the multiverse.

Host: Hank Green

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[ ♪ Intro ].

Just a few days before he died, Stephen Hawking submitted one last research paper. And now, it’s officially public.

After making it through the peer-review gauntlet, the paper was published last week in the Journal of High Energy Physics. And in it, Hawking and his co-author Thomas Hertog attempt to upend the one of the most popular theories of not just the universe, but the entire multiverse. Yeah.

We’re getting a bit brain-bendy here. Thanks to astronomical observations, we know that our universe’s structure is surprisingly uniform given its size. On a large scale, there’s roughly the same amount of matter everywhere.

To explain this, cosmologists introduced the idea of inflation around 1980. It says that, almost immediately after the Big Bang, the universe started expanding faster than the speed of light -- And that’s not allowed for stuff in space, but is allowed for space itself. It doubled in size over and over again in a teeny tiny fraction of a second.

Then, for some reason physics can’t really explain, that super-fast inflation stopped. At least, it did in our neck of the universe. According to an idea called the theory of eternal inflation, it’s still going on somewhere.

The idea says that separate parts of space too far away for us to observe have continued to inflate. But other pockets have stopped inflating, too. So you end up with isolated bubbles of space -- or, whole other universes -- in a giant, ever-inflating multiverse.

If that hypothesis is correct, it means there are an infinite number of universes out there, and they’d be so far apart that each would have its own laws of physics. Now, if this sounds like the parallel universes that sometimes show up in science fiction, that’s because that’s kind of what they are. Infinite, coexisting universes would mean there’d be no real significance behind the laws that govern our reality.

That includes the exact speed of light, the strength of gravity, or any of the other components of physics that allow us to exist. They’d be just one random version of the way things could have gone. The biggest problem with this multiverse idea, though, is that you can’t really test it.

After all, if there’s an infinite number of universes, then any experiment that made predictions about what the universe should look like would be guaranteed to find a match somewhere. If everything is possible, there’s no way to falsify your hypothesis. Unsurprisingly, many scientists, Hawking and Hertog included, just weren’t comfortable with this.

The two were actually working on the problem for decades, but for this paper, they borrowed some newer, special math from string theorists. These researchers are trying to pin down a single set of laws that govern everything in the universe -- which we don’t have yet, but if we did, would help us understand the Big Bang. To make their calculations easier, Hawking and Hertog’s math ignores the dimension of time, so that’s not a perfect reflection of the real world.

But other researchers have used it in their own work, so it’s a decent estimate. Their model suggests that, mathematically speaking, the multiverse doesn’t have to be infinite. And any other universes that do exist have similar laws to our own.

Their work also suggests that time itself didn’t exist at the very beginning of our universe. Which might be even weirder. It's all theoretical for now, but if this hypothesis proves correct, studying the really early universe would allow cosmologists to understand a lot.

It would help them figure out where the laws of physics come from, how they came to be what they are today, and whether they’re unique among the multiverse. And also, whether the multiverse actually even exists. As of right now, though, we don’t have the technology to test this.

So several of Hawking’s cosmology peers are skeptical, and he and Hertog have also stated their ideas need further development. Hertog believes evidence either supporting or refuting their work could be found in gravitational waves generated during our universe’s inflation. These are similar to the ripples in space-time generated by things like black holes, but they’d be much older and longer.

That means we couldn’t detect them with LIGO -- the instrument we currently use to detect gravitational waves. But the European Space Agency’s Laser Interferometer Space Antenna, or LISA, should be up for the task after it launches in 2034. Still, even that might not be enough.

We really won’t be able to get an accurate story of our origins until we can find that batch of equations people like string theorists are looking for: one that unites general relativity and quantum mechanics, or the physics of really large and the physics of the really small. Right now, those two theories don’t play well together, but we need them to in order to describe the Big Bang, which is simultaneously super massive and super tiny. So we might be waiting a long time to learn exactly what’s going on with this multiverse.

Or if there even is one. But whether he turns out to be right or wrong, it’s clear Hawking’s work will continue to resonate with current and future cosmologists. And as a bonus, when he passed, Hawking also had several other papers in the publishing pipeline.

So even if this was the last one he submitted, there should be even more science to come. Thanks for watching this episode of SciShow Space! This paper might not end up in the cosmology hall of fame, but some of Stephen Hawking’s other work has definitely changed how we think about the universe.

If you’d like to learn more about some of his biggest accomplishments, you can watch our episode all about them. [ ♪ Outro ].