Hawking’s “Final Theory” is not groundbreaking

Yesterday, the media buzzed with the revelation that Stephen Hawking had completed a paper two weeks before his death. This paper supposedly contains some breathtaking insight.

The headlines refer to a paper titled “A Smooth Exit from Eternal Inflation” in collaboration with Thomas Hertog. The paper was originally uploaded to the arXiv in July last year, but it was updated two weeks ago. It is under review with “a leading journal” which I suspect but do not know is Physical Review D. Thomas Hertog gave a talk about this at the conference which I attended last summer. You can watch the video of Hertog’s talk here.

According to The Independent the paper contains “a theory explaining how we might detect parallel universes and a prediction for the end of the world.” Furthermore, we learn, “Hawking also theorised in his final work that scientists could find alternate universes using probes on space ships, allowing humans to form an even better understanding of our own universe, what else is out there and our place in the cosmos.”

In the Sunday Times you can read that the paper “shows how we might find other universes”  and in The Telegraph you find a quote by Carlos Frenk, professor of cosmology at Durham University who said: “The intriguing idea in Hawking’s paper is that [the multiverse] left its imprint on the background radiation permeating our universe and we could measure it with a detector on a spaceship.”

Since the paper doesn’t say anything about detecting parallel universes, I was originally confused whether the headlines were referring to another paper. But no, Thomas Hertog confirmed to me that the paper in question is indeed the paper that is on the arXiv. There is no other paper.

So what does the paper say?

The paper is based on an old idea by Stephen Hawking and Jim Hartle called the “no-boundary” proposal. In the paper, the authors employ a new method to do calculations that were not previously possible. Specifically, they calculate which type of universes a multiverse would contain if this theory was correct. The main conclusion seems to be that our universe is compatible with the idea, and also that this particular multiverse which they deal with is not as large as the usual multiverse one gets from eternal inflation.

It’s not entirely uninteresting if you are into multiverse ideas, because then you need this information to calculate the probability of our universe. But it is also a very theoretical paper that does not say anything about observational consequences.

The only thing that the paper does say is that inflation took place. And inflation predicts that gravitational waves produced in the early universe should leave an imprint in the cosmic microwave background (CMB). This is the CMB polarization signal that BICEP was looking for but didn’t find. There are, however, some satellite missions in the planning that will look for it with better precision.

So how do we detect parallel universes? By detecting the CMB polarization. I do not kid you.

Here’s what Hertog said about this:

“This model predicts that our universe came into existence with a burst of rapid expansion called cosmic inflation. A big bang of this kind amplifies gravitational waves which in turn show up in satellite images of [the pattern of temperature fluctuations in] the cosmic microwave background. Future satellite missions should see this, if the theory is correct.

Observational evidence for the no-boundary model [in the form of gravitational waves from the big bang] would yield strong evidence for a multiverse. This paper provides a step towards a mathematically sound and testable model of the multiverse. That constitutes a significant extension of our notion of physical reality.

Some cosmologists have argued against the multiverse on the basis it can’t be tested. However our model shows that observations in our own universe can provide strong evidence for the existence of other universes. ”

Allow me put this into perspective.

Theoretical physicist have proposed some thousand ideas for what might have happened in the early universe. There are big bangs and big bounces and brane collisions and string cosmologies and loop cosmologies and all kinds of weird fields that might or might not have done this or that. All of this is pure speculation, none of it is supported by evidence. The Hartle-Hawking proposal is one of these speculations.

The vast majority of these ideas contain a phase of inflation and they all predict CMB polarization. In some scenarios the signal is larger than in others. But there isn’t even a specific prediction for the amount of CMB polarization in the Hawking paper. In fact, the paper doesn’t so much as even contain the word “polarization” or “tensor modes.”

The claim that the detection of CMB polarization would mean the multiverse exists makes as much sense as claiming that if I find a coin on the street then Bill Gates must have walked by. And a swarm of invisible angels floated around him playing harp and singing “Ode To Joy.”

In case that was too metaphorical, let me say it once again but plainly. Hawking has not found a new way to measure the existence of other universes.

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

Note added May 2nd 2018: The paper was now published in JHEP.