Many thanks to Sabine Hossenfelder for her efforts to debunk the attempt to use Hawking’s death as a platform for multiverse hype. See her posting at Backreaction for a good explanation of what is going on here.
To summarize the problem, there are loads of news stories out there telling the public that Stephen Hawking’s ‘breathtaking’ final multiverse theory completed two weeks before he died, Stephen Hawking’s Final Paper Proposes Way to Detect the ‘Multiverse’, etc., etc. Cosmologist Carlos Frenk and theorist Thomas Hertog seem to be among those encouraging this nonsense.
This is all based on this recent paper by Hawking and Hertog, which contains nothing like a way to “detect the ‘Multiverse'”. It’s a toy model of bubble universe formation, one the authors admit they can’t even solve:
However, the setup we have considered does not allow us to describe the transition from the quantum realm of eternal inflation to a universe in the semiclassical gravity domain. This is because our duals are defined in the UV and live at future infinity. It therefore remains an open question whether the conjectured smoothness of global constant density surfaces impacts the eternity of eternal inflation. To answer this will require a significant extension of holographic cosmology to more realistic cosmologies
Their calculations inspire them to state: “… we conjecture that eternal inflation produces universes that are relatively regular on the largest scales”, but this is just an extremely vague conjecture without much backing it. Using it to get press stories published claiming to have found a way to “detect the ‘Multiverse'” is just absurd, and it’s sad to see Hawking’s passing memorialized with a cloud of ridiculous hype.
Update: Another detailed explanation of what is going on here, from Ethan Siegel. His summary:
There are no observable consequences; there is nothing to measure; there is nothing to test. There’s no prediction about the end of the Universe, and there are no robust conclusions we can draw about its beginning. There are tremendous limitations to the implications of this work, and there are few compelling reasons to believe that their toy model has relevance for our physical Universe. It is a seed of an idea that itself is controversial, based off of an also-controversial foundation, and this is a very small step in its development. Furthermore, all of what they do is based on the Hartle-Hawking no-boundary conjecture, which is still not generally accepted as true. The authors go so far as to admit, in the discussion of this paper, that even within their toy model, they have not shown that there is a non-Multiverse-inducing exit to eternal inflation:
“It therefore remains an open question whether the conjectured smoothness of global constant density surfaces impacts the eternity of eternal inflation.”
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