Hawking’s final publication, which was published in the journal High-Energy Physics, revisited one of his earlier (and equally mind-blowing) speculations. The “no-boundary idea” takes into account Einstein’s theory that the pre-Big Bang world was a singularity, an extraordinarily dense and hot micro-speck of matter where the laws of physics did not apply. Hawking hypothesized that time as we know it did not exist in this singularity, which had no beginning and no end and was endless and spherical rather than finite and linear. During a phase known as cosmic inflation, the embryonic universe is considered to have expanded rapidly and created parallel worlds.

Something this unfathomable to the human brain eventually made Hawking want to quantify it. Infinite universes open portals to infinite possibilities, but that paradoxically means that there can be no accurate way to test theories about the origins of our own universe or its future.

He and physicist Thomas Hertog set out to put theoretical boundaries on his own no-boundary proposal by trying to isolate every unique type of universe that may be floating out there.

“Hawking was not satisfied with this state of affairs,” Hertog, who co-authored that paper with Hawking, said to Live Science. “‘Let’s try to tame the multiverse,’ he told me a year ago. So, we set out to develop a method to transform the idea of a multiverse into a coherent, testable scientific framework.”

Holography, the concept of encrypting three-dimensional information on a two-dimensional surface, was ultimately what Hawking and Hertog used to establish a link between Einstein’s theoretical singularity and our universe, which allegedly emerged from it. Some scientists believe we live in a holographic universe. They reconciled the singularity’s crazy quantum physics with the classical laws of physics that allow us to understand the behaviour of the universe. Knowing that there are a finite number of universes in the multiverse means that the nature of those universes can be predicted.

We still need evidence of a multiverse before we can prove that other worlds exist somewhere in time and space. The cosmic microwave background is a cold microwave radiation that contains traces of primordial gravitational waves from the Big Bang (CMB).

If an advanced enough satellite ever beams back a CMB energy signature that matches Hawking’s hypothesis of cosmic inflation, it could indicate that the multiverse is more than just a science fiction concept.

While additional research is needed to support Hawking and Hertog’s theory, the mere prospect of being a life-form in one of many worlds is enough to keep you awake at night.