Before he passed away in March, renowned physicist Stephen Hawking had published more than 230 articles on the birth of the universe, black holes and quantum mechanics. But, ten days before his death, Hawking finished his final theory on the origin of the universe - now published posthumously - and it offers an interesting departure from earlier ideas about the nature of the "multiverse."

As PBS reports, the new report, co-authored by Belgian physicist Thomas Hertog, counters the longstanding idea that the universe will expand for eternity.

If you asked an astrophysicist today to describe what happened after the Big Bang, he would likely start with the concept of “cosmic inflation.” Cosmic inflation argues that right after the Big Bang — we’re talking after a teeny fraction of a second — the universe expanded at breakneck speed like dough in an oven. But this exponential expansion should create, due to quantum mechanics, regions where the universe continues to grow forever and regions where that growth stalls. The result would be a multiverse, a collection of bubblelike pockets, each defined by its own laws of physics. “The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse,” Hertog said in a statement. “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.” Along with being difficult to support, the multiverse theory, which was co-developed by Hawking in 1983, doesn’t jibe with classical physics, namely the contributions of Einstein’s theory of general relativity as they relate to the structure and dynamics of the universe. “As a consequence, Einstein’s theory breaks down in eternal inflation,” Hertog said.

Einstein spent his life searching for a unified theory, a way to reconcile the biggest and smallest of things, general relativity and quantum mechanics.

He died never having achieved that goal, but leagues of physicists like Hawking followed in Einstein’s footsteps. One path led to holograms.

Diagram of evolution of the (observable part) of the universe from the Big Bang (left) to the present. After the Big Bang and inflation, the expansion of the universe gradually slowed down for the next several billion years, as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image and caption by NASA

Instead of the 'standard' description of how the 'universe' unfolded (and is unfolding), the authors argue the Big Bang had a finite boundary, defined by string theory and holograms.

The new theory - which sounds simplistically like the world of the red-pill-blue-pill Matrix movies - embraces the strange concept that the universe is like a vast and complex hologram. In other words, 3D reality is an illusion, and that the apparently "solid" world around us - and the dimension of time - is projected from information stored on a flat 2D surface.

The Telegraph reports that Prof Hertog, from the Katholieke Universiteit Leuven (KT Leuven), said:

"It's a very precise mathematical notion of holography that has come out of string theory in the last few years which is not fully understood but is mind-boggling and changes the scene completely."

Applied to inflation, the newly published theory suggests that time and "the beginning" of the universe arose holographically from an unknowable state outside the Big Bang.

Prof Hawking said before his death:

"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."

And believe it or not, there’s actually evidence that the world works this way.

Hawking's final paper can be read in full below...

As PBS concludes, some physicists point out that the Hawking-Hertog theory is preliminary and should be considered speculation until other mathematicians can replicate its equations.

Sabine Hossenfelder, a theoretical physicist with the Frankfurt Institute for Advanced Studies, said on her blog that the ideas put forward in this paper join others that are currently pure speculation and don't yet have any evidence to support them. She makes it clear that while the proposals aren't uninteresting, Hawking and Hertog haven't found a new way to detect the existence of universes other than our own.

"Stephen Hawking was beloved by everyone I know, both inside and outside the scientific community," she wrote. "He was a great man without doubt, but this paper is utterly unremarkable."

Here is Ars Technica's John Timmer with more details on the holographic universe.