Stephen Hawking’s death in March incited us all take a moment and think about the famed physicist’s impact on the scientific world, and the myriad ways his research affected the way we think about the universe. As it turns out, he wasn’t exactly done. Hawking’s final paper was finally published Wednesday, in the Journal of High Energy Physics, and while it’s not exactly the science-shattering work many outlets are reporting it to be, it still puts a pretty interesting, Hawking-esque spin on one of theoretical physics’ most discussed concepts: the multiverse.
The idea that multiple parallel universes exist originates out of inflation, the incredibly rapid expansion of the universe right after the Big Bang, over repeated bursts at speeds faster than light. Many scientists think during these bursts, the smallest blips in energy at the quantum level swelled into larger pockets of space-time—effectively entire individual universes which are possibly, conceivably found everywhere, within an ever-expanding larger multiverse that houses them.
Subscribing to that view essentially means assuming that, if the multiverse continues to inflate, individual universes are being created ad infinitum. For some, that’s a tough pill to swallow. And you can count Hawking and his co-author, Thomas Hertog from the University of Leuven in Belgium, among those skeptics.
So Hawking and Hertog created a framework for a simpler model of the multiverse that limits how many new universes could form, and ensures they adhere to the same laws of physics as our known universe. As opposed to older theories of multiverse that called for universes empty and full, volatile and boring, dead in an instant or with long lives ahead of them, these would be truer to the layman’s conception of parallel universes.
The new paper is sort of an update of the “no-boundary” proposal, something Hawking and American physicist James Hartle worked on in the 1980s. Using new mathematics derived from string theory that weren’t available in the 80s, Hawking and Hertog reach the conclusion our own universe is compatible with this idea, and that our multiverse is smaller than what we might expect from eternal inflation.
“Our model fits in nicely with the theory of inflation that says our universe underwent a very rapid period of expansion in its earliest stages,” says Hertog. “But our model goes radically against the prevailing extrapolation of inflation that led to a multiverse.”
It’s a pretty neat idea! It’s just not as exceptional as one outside the field might think at first glance. For one, it remains a theoretical paper; there’s no real way to test it out or make any sort of observations of this cosmology. In practical terms, it’s not practical at all. The original “no-boundary” proposal is speculative, and by extension so are these latest conclusions.