Renowned physicist Stephen Hawking passed away earlier this year, but his legacy to science will live on. His final theory on the origin of the universe has now been published, and it offers an interesting departure from earlier ideas about the nature of the "multiverse."

Ideas about how the universe came to exist the way we see it today have been adapted and built on for decades. The new paper, authored by Hawking and Professor Thomas Hertog, adds to the literature with a new understanding of a theory known as eternal inflation.

After the Big Bang kickstarted the universe, it expanded exponentially for a brief fraction of a fraction of a second. When that inflationary period ended, the universe continued to expand at a much slower rate. But according to the eternal inflation model, quantum fluctuations mean that in some regions of the universe, that rapid inflation never stopped. That results in a gigantic "background" universe full of an infinite number of smaller pocket universes – including the one we live in.

"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," Hawking has previously said. "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."

For an idea to be considered scientifically sound, it needs to make predictions that are testable, so experiments can either prove or disprove it. Otherwise, science starts to sound a whole lot like religion – after all, the notion that God created everything in seven days is also elegant and answers all the questions about our existence, but is inherently untestable.

With that in mind, some scientists, including Hawking and Hertog, take that to mean the eternal inflation theory can't be considered without some modification. And that modification is the focus of the new paper.

"The problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein's theory of general relativity and treats the quantum effects as small fluctuations around this," says Hertog. "However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein's theory breaks down in eternal inflation."

Hawking and Hertog's new theory branches off from the holographic principle, which itself is part of string theory. String theory states that each particle in the universe is actually the end point of a one-dimensional string, and the different vibration patterns of each string is what gives a particle its specific properties. Holography, meanwhile, is the idea that the entire universe exists as a hologram, described as 3D spaces that can be mathematically reduced to a 2D projection on a surface.

In their new version of the holographic principle, the authors have wrapped the fourth dimension, time, into the picture. At the beginning of time, eternal inflation can be reduced to a two-dimensional, timeless state on a spatial surface. Doing so allowed them to bypass Einstein's theory of general relativity, which breaks down in previous versions of eternal inflation.

The new theory goes against Hawking's "no boundary" theory, first proposed in 1983. That stated that the universe didn't have what we could understand as a beginning, because the singularity that existed pre-Big Bang had no initial time or space boundaries.

"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," says Hertog.

As a result, the universe that sprung from the Big Bang in that scenario must be infinite as well, and the pocket universes it contains represent every single possibility.

But according to Hawking and Hertog's new theory of eternal inflation, there was a boundary in the past, so time and space had a beginning. The universe that emerged from that state is also finite, making it far simpler to deal with – and test scientifically. If that turns out to be the case, it deals a blow to the concept of a multiverse containing endless pocket universes.

"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," says Hawking.

So how might this new theory be put to the test? Hertog says the most promising "smoking gun" for the model would be the detection of primordial gravitational waves, ripples through spacetime generated at the beginning of time. These waves would have extremely long wavelengths, meaning existing detectors like LIGO wouldn't be able to pick them up, but Hertog says that LISA, the ESA's planned orbital gravitational wave observatory, could be sensitive enough to detect them when it launches in 2034.

The research was published in the Journal of High Energy Physics.

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