Hawking's last post

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Hawking's last post

#1  Postby newolder » Mar 20, 2018 4:31 pm

After his passing, a paper with Thomas Hertog has appeared at the arXiv and awaits peer review. The subject concerns inflation that may not be eternal but ends smoothly, with all that implies for a multiverse.
arXiv source
Abstract

The usual theory of inflation breaks down in eternal inflation. We derive a dual description of eternal inflation in terms of a deformed CFT located at the threshold of eternal inflation. The partition function gives the amplitude of different geometries of the threshold surface in the Hartle- Hawking state. Its local and global behavior in dual toy models shows that the amplitude is low for surfaces which are not nearly conformal to the round three-sphere and essentially zero for surfaces with negative curvature. Based on this we conjecture that the exit from eternal inflation does not produce an infinite fractal-like multiverse, but is finite and reasonably smooth.

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Re: Hawking's last post

#2  Postby Calilasseia » Mar 23, 2018 2:04 pm

Bugger, you beat me to it!

A suitably readable non-technical account, but one which still nicely reports relevant details, can be read here.

From that article:

According to The Sunday Times, the contents of the paper sets out the mathematics necessary for a deep-space probe to collect evidence which might prove that other universes exist.

The highly theoretical work posits that evidence of the multiverse should be measurable in background radiation dating to the beginning of time. This in turn could be measured by a deep-space probe with the right sensors on-board.

Thomas Hertog, a physics professor who co-authored the paper with Hawking, said the paper aimed "to transform the idea of a multiverse into a testable scientific framework."


Oh look - this paper takes to the limit, that principle I keep informing people of - physical processes leave behind them physical evidence of their occurrence, with some evidence being suitably persistent across time.
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Re: Hawking's last post

#3  Postby newolder » Mar 23, 2018 4:13 pm

Having read through it a couple of times (still without total comprehension) the title seems to be referring to the spectrum of gravitational radiation at very early time that would, in principle, be detectable by a LISA-like device in space. I haven't yet understood where the spectrum from the 'toy' model is actually predicted but I'm thinking that the absolute values of the power spectral density of any gravitational fluctuations coming out of any earlier inflationary epoch will be small. The smooth end to inflation here is distinct from earlier inflationary scenarios in this and other respects (e.g. fewer universes than the earlier infinite measure).

But, of course, I could be a mile or gigaparsec off.

ETA Another popular write up at space.com shows I could indeed be way off. :dunno:
...
But other scientists are skeptical. There are other models of inflation, and most of these models also include some primordial gravitational waves, said Katie Mack, a cosmologist at North Carolina State University who was not an author on the new paper.

"The ideas brought up in this paper are not giving any new kind of signature that's different from other models of inflation," Mack told Live Science.

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