With a science paper published after his death, Stephen Hawking has revived debate on a deeply divisive question for cosmologists: Is our Universe just one of many in an infinite, ever-expanding “multiverse”? According to one school of thought, the cosmos started expanding exponentially after the Big Bang.

In most parts, this expansion or “inflation” continues eternally, except for a few pockets where it stops.

These pockets are where universes like ours are formed — multitudes of them that are often likened to “bubbles” in an ever-expanding ocean dubbed the multiverse.

Many scientists don’t like the idea, including Hawking, who said in an interview last year: “I have never been a fan of the multiverse.”