The paper, which was published in the Journal of High Energy Physics, solves a nagging problem with a widely accepted theory called eternal inflation. Introduced in the early 1980s, eternal inflation itself solved some problems with the previous model of the universe, which said that after the Big Bang, the universe inflated exponentially for a while, but eventually slowed down while all that energy turned into matter and radiation. But if that were true, why is every point in the universe roughly the same temperature, and why is the universe so perfectly flat?

Eternal inflation solved that by declaring that the universe stops inflating in certain patches — and those, like ours, can form all the matter you see around you — and just goes on inflating forever in others. An eternally inflating universe would be the same temperature everywhere and perfectly flat. Plus, tiny quantum fluctuations in the energy of the universe become a way for matter to clump together and become planets, stars, and galaxies. But while it solves those problems, it brings up others. The eternally inflating patches in this theory would be additional universes in one big multiverse, and they'd all play by different rules. Some would be flat, others would be curved; some would be the same temperature all over, others wouldn't be. Even the laws of physics and chemistry would change in these alternate universes.

That leads to a difficult question: Why this universe? How did we end up here, where everything is so perfectly balanced for life? And worst of all — if you're a scientist, anyway — how do you test it? "... I have never been a fan of the multiverse," Hawking said in an interview. "If the scale of different universes in the multiverse is large or infinite the theory can't be tested."

If something isn't testable, is it even science?

The big issue with eternal inflation, Hawking and his co-author Professor Thomas Hertog say, is that while it assumes the multiverse evolves according to classical physics, it also relies on quantum effects — and these two areas of physics don't play nice together. "... the dynamics of eternal inflation wipes out the separation between classical and quantum physics," Hertog explains. "As a consequence, Einstein's theory breaks down in eternal inflation."

Instead, Hawking and Hertog explained these issues using string theory. You can read more about this far-out concept right here, but in essence, string theory says that every particle in existence is just a very tiny one-dimensional string vibrating in its own way. Even wilder, it says that you can mathematically describe a volume of space in a lower dimension, kind of like a two-dimensional hologram can produce the image of a three-dimensional shape.

The universe we live in is assumed to have four dimensions, time being the fourth. Hawking and Hertog took that time dimension in eternal inflation and reduced the whole thing to three-dimensional space without time, right at the beginning of the universe. Importantly, that let them describe eternal inflation without using classical physics.

It also told them that the universe did indeed have a beginning, contrary to an earlier theory of Hawking's. That allowed the researchers to make more reliable — and even better, testable — predictions about the universe. Those predictions say that the universe is a lot simpler than we thought. "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.

With his co-author sadly gone, Hertog's next step is to find evidence for this theory. Eternal inflation should have produced ripples in spacetime known as primordial gravitational waves, and Hertog believes these would be the "smoking gun" for his model. These waves are too long to be detected by our old gravitational-wave-detecting pal LIGO, but spacecraft like LISA could one day find evidence of these echoes from our cosmic origin.

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