**His last article, detailing his theory on the origins of the Universe, and co-authored by physicist Thomas Hertog, was the final stroke of brilliance achieved by British physicist Stephen Hawking, not long before his death. **

Published in early May, the article proposes that the Universe could in fact be far less complex than current theories on the multiverse suggest. The paper draws on a concept called “eternal inflation”, presented for the first time in 1979. After the Big Bang, the Universe went through a period of exponential inflation before slowing down, when the energy converted into matter and radiation. This expansion phase however may not have stopped all over. According to the theory of “eternal inflation”, our Universe could be “a bubble” among others, all separated by pieces of space-time in which inflation continues.

According to this theory, everything we see in our observable Universe is contained in one of these bubbles, in which inflation has stopped, allowing for the formation of stars and galaxies. *“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,” *explained Stephen Hawking last Autumn. *“The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse.”*

Could we thus imagine a potential infinity of Universe pockets? Not exactly, according to the theory. Going back to the very first moments of the Universe, new calculations show a different exit from this phase of inflation: *“we conjecture that the exit from eternal inflation does not produce an infinite fractal-like multiverse, but is finite and reasonably smooth”,* wrote the two physicists. Furthermore, the array of potential “pocket universes” may not be as numerous as we had thought. The theory does not go against the idea of a multiverse, but suggests a significant reduction in the multiverse and a much smaller range of potential universes.

So could this new theory one day be proved by observations? The content of the article outlines the mathematical data necessary to send a probe into distant space, to collect proof for the existence of other universes. This highly theoretical work stipulates that proving the existence of the multiverse should be possible by measuring the background radiation dating back to the beginning of the Universe.

Does this mean we will soon have a revolutionary mathematical framework, allowing humans to test for the existence of other universes? Professor Hertog now needs to study the implications of this theory on smaller scales, which could be within the reach of space telescopes. He believes in particular that primordial gravitational waves, waves in space-time generated when the universe exits this famous inflation, could allow for the model to be tested. These waves are too large to be detected by LIGO, but the future Laser Interferometer Space Antenna (LISA) could well be capable of doing so.

The duo’s research was published in the *Journal of High Energy Physics* and can be read in full on arXiv.