Few days before Professor Stephen Hawking died, he submitted one last research paper. He works with Professor Thomas Hertog. Now the paper is officially public. The paper was published last week in the Journal of High Energy Physics.
Hawking and his co-author Thomas Hertog attempt to spend one of the most popular theories of not just the universe, but the entire multiverse.
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.
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.
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 —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, co-existing 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 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.
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