Sorry: Stephen Hawking's last paper doesn't favour the so-called “multiverse”, but there's some cool stuff in it if you ignore the headlines.

The late professor Hawking and colleague Belgian theoretical physicist Thomas Hertog first published this paper at arXiv in July last year, and posted an updated version on 4 March, 10 days before Hawking's death.

The paper looked at one of the enduring challenges Hawking created for himself – how to resolve the conflict between how Albert Einstein's models of the universe work (gravity, for example, at the very large scale), and how quantum mechanics works at the smallest scale.

For decades, Hawking's work on the mathematics of black holes had led him to speculate on how quantum mechanics interacted with the large-scale universe.

For example, he proposed “Hawking Radiation” in 1974 as one such interaction: if a pair of virtual particles came into being in the right circumstances, near the event horizon of a black hole, one might escape, stealing a tiny amount of mass and energy from the black hole, leading to its evaporation (he later abandoned this model, because it led to other quantum paradoxes).

Such problems, as his last paper demonstrated, occupied his prodigious mind to the very end.

### Bye-bye, fractal multiverse

However, in spite of headlines boosting “the multiverse”, even the paper's abstract leaned away from such a conclusion: “we conjecture that the exit from eternal inflation does not produce an infinite fractal-like multiverse, but is finite and reasonably smooth”.

The inflationary universe is an accepted theory for the history of space-time: shortly after the Big Bang (10^{-36} seconds after), the universe underwent a brief period of exponential inflation, ending somewhere after 10^{-33} seconds.

This sparked a conjecture that by the end of the inflationary period, there wasn't one universe – the one we see – but a potentially infinite number (the multiverse).

Your correspondent isn't a physicist, but *El Reg* has obtained permission to include tweets from someone who's both an astrophysicist and a science communicator, Katie Mack. Here are some key points in chronological order (top to bottom).

REALLY short summary of the paper: Hawking & Hertog found, using some VERY preliminary calculations, that the rapid expansion in the early universe (inflation) might not always create a big mess of a multiverse, but might be more likely to make a cosmos like what we see.

— Katie Mack (@AstroKatie) March 20, 2018

If it holds up, it might make it possible to draw a fairly straight line between the beginning of the cosmos and today, rather than trying to explain why we're in a Universe like ours and not one of the countless others that look totally different.

— Katie Mack (@AstroKatie) March 20, 2018

This doesn't (as some coverage has suggested) lead us to ways of testing the multiverse hypothesis, or space probes that can do cool new measurements. It's a preliminary model for now; there may be ways to back it up in the future.

— Katie Mack (@AstroKatie) March 20, 2018

The paper supported this. We've emphasised key point in the extract below:

“We find that the amplitude of surfaces with conformal structures far from the round one is exponentially small. Finally we put forward a general argument that indicates that the amplitude is zero for all highly deformed conformal boundaries with a negative Yamabe invariant.

This raises doubt about the widespread idea that eternal inflation produces a highly irregular universe with a mosaic structure of bubble like patches separated by inflationary domains.”

The emphasised passage is a description of the fractal multiverse.

Since the paper leaned away from the existence of the multiverse, Mack said, the idea that it provides a test platform is also misguided; and the paper's mention of an “end of the world brane” (membrane) isn't talking about how the universe ends, but rather, a certain set of boundary conditions.

However:

I should add that the calculations in the paper, while, again, very preliminary, connect to some really cool ideas, like ways to bring gravity and quantum mechanics together.

— Katie Mack (@AstroKatie) March 20, 2018

It should be noted that the paper is harshest on one particular multiverse model – that in which there are a large number of alternative universes.

It's still possible for there to be more than one universe: “Our conjecture strengthens the intuition that holographic cosmology implies a significant reduction of the multiverse to a much more limited set of possible universes.”

The Hawking-Hertog conjecture has, they wrote, “important implications for anthropic reasoning”, the idea of a relationship between the universe and the sapient life observing it. Only under some conditions, they write, do anthropic arguments hold. ®