A couple months ago Scientific American published an article by Ijjas, Steinhardt and Loeb (also available here), which I discussed a bit here. One aspect of the article was its strong challenge to multiverse mania, calling it the “multimess” and accusing multiverse explanations of being untestable and unscientific.
Yesterday Scientific American published, under the title A Cosmic Controversy, a rebuttal signed by 33 physicists, together with a response from the authors, who have also set up a webpage giving further details of their response. Undark has an article covering this: A Debate Over Cosmic Inflation (and Editing at Scientific American) Gets Heated.
As Ijjas, Steinhardt and Loeb point out on their webpage, the story of this letter is rather unusual. It was written by David Kaiser and three physicists well-known for their outspoken promotion of the multiverse (Guth, Linde and Nomura). Evidently these authors decided they needed reputational support on their side, and sought backing from other prominent names in the field (I’m curious to know who may have refused to sign if asked…). Their letter starts out with a claim to represent the “dominant paradigm in cosmology” and notes the large number of papers and researchers involved in studying inflation.
If you read carefully both sides (IS&L and GKL&N) of this, I think you’ll find that they are to a large degree speaking past each other, with a major problem that of imprecision in what one means by “inflation”. To the extent that there is a specific identifiable scientific disagreement, it’s about whether Planck data confirms predictions of the “simplest inflationary models.” IS&L write:
The Planck satellite results—a combination of an unexpectedly small (few percent) deviation from perfect scale invariance in the pattern of hot and colds spots in the CMB and the failure to detect cosmic gravitational waves—are stunning. For the first time in more than 30 years, the simplest inflationary models, including those described in standard textbooks, are strongly disfavored by observations.
whereas GKL&N respond:
there is a very simple class of inflationary models (technically, “single-field slow-roll” models) that all give very similar predictions for most observable quantities—predictions that were clearly enunciated decades ago. These “standard” inflationary models form a well-defined class that has been studied extensively. (IS&L have expressed strong opinions about what they consider to be the simplest models within this class, but simplicity is subjective, and we see no reason to restrict attention to such a narrow subclass.) Some of the standard inflationary models have now been ruled out by precise empirical data, and this is part of the desirable process of using observation to thin out the set of viable models. But many models in this class continue to be very successful empirically.
I take this as admission that IS&L are right that some predictions of widely advertised inflationary models have been falsified. Of course, if these had worked they would have been heavily promoted as “smoking gun” proof of inflation, as was demonstrated by the BICEP2 B-mode fiasco. After BICEP2 announced (incorrectly) evidence for B-modes, Linde claimed this was a “smoking gun” for inflation (see here) and the New York Times had a front page story about the “smoking gun” confirmation of inflation vindicating the ideas of Guth and Linde. A couple months later, before the BICEP2 result was shown to be mistaken, Guth, Linde and Starobinsky were awarded the $1 million Kavli Prize in Astrophysics.
GKL&N don’t mention the sorry story of the BICEP2 B-modes, what they have to say about this is
the levels of B-modes, which are a measure of gravitational radiation in the early universe, vary significantly within the class of standard models…
The B-modes of polarization have not yet been seen, which is consistent with many, though not all, of the standard models.
About the IS&L “unexpectedly small (few percent) deviation from perfect scale invariance” all GKL&N have to say is
The standard inflationary models… predict the statistical properties of the faint ripples that we detect in the cosmic microwave background (CMB). First, the ripples should be nearly “scale-invariant”
This doesn’t seem to address at all the IS&L claims, which they make in more detail as
The latest Planck data show that the deviation from perfect scale invariance is tiny, only a few percent, and that the average temperature variation across all spots is roughly 0.01 percent. Proponents of inflation often emphasize that it is possible to produce a pattern with these properties. Yet such statements leave out a key point: inflation allows many other patterns of hot and cold spots that are not nearly scale-invariant and that typically have a temperature variation much greater than the observed value. In other words, scale invariance is possible but so is a large deviation from scale invariance and everything in between, depending on the details of the inflationary energy density one assumes. Thus, the arrangement Planck saw cannot be taken as confirmation of inflation.
GKL&N argue for three other confirmed predictions of inflationary models:
Second, the ripples should be “adiabatic,” meaning that the perturbations are the same in all components: the ordinary matter, radiation and dark matter all fluctuate together. Third, they should be “Gaussian,” which is a statement about the statistical patterns of relatively bright and dark regions. Fourth and finally, the models also make predictions for the patterns of polarization in the CMB, which can be divided into two classes, called E-modes and B-modes. The predictions for the E-modes are very similar for all standard inflationary models
On these issues I don’t see anything from IS&L and would love to hear from an expert.
The main issue here comes down to the question of the flexibility vs. rigidity of inflationary models. Is the inflationary paradigm rigid enough to make solid predictions, or so flexible that it can accommodate any experimental result? GKL&N are making the case for the former, IS&L for the latter, and they point out the following quote from Guth himself:
when asked via email if they could name any pro-inflation scientists who believe that the theory is nonetheless untestable, the trio pointed to a video of a 2014 panel during which Loeb asks Guth directly whether it’s possible to do an experiment that would falsify inflation.
“Well, I think inflation is a little too flexible an idea for that to make sense,” Guth replied.
A fair take on all this would be to note that it’s a complicated situation, and I doubt I’m the only one who would like to see an even-handed technical discussion of exactly what the “simplest” models are and a comparison of their predictions with the data. Claims to the public from one group of experts that Planck data says one thing, from others claiming it says the opposite are generating confusion here rather than clarity about the science.
I’m strongly on the side of IS&L on one issue, that of the danger of theories that invoke the multiverse as untestable explanation. I don’t think though that they make a central issue clear. The simple inflationary models whose “predictions” for Planck data are being discussed involve a single inflaton field, with no understanding of how this is supposed to couple to the rest of physics. One is told that eternal inflation implies a multiverse with different physics in different universes, but in a single inflaton model this physics should just depend on a single parameter, and such a theory should be highly predictive (once you know one mass, all others are determined). What’s really going on is that there is no connection at all between the simple single field models that GKL&N and IS&L are arguing about, and the widely promoted completely unpredictive string theory landscape models (involving large numbers of inflaton-type fields with dynamics that is not understood).
I think IS&L made a mistake by not pointing this out, and that Guth, Linde, Nomura and some of the signers of their letter (e.g. Carroll, Hawking, Susskind, Vilenkin) have long been guilty of promoting the defeatist pseudo-scientific idea that “evidence for inflation is evidence for a multiverse with different physics in each universe, explaining why we can’t ever calculate SM parameters”. By defending the predictivity of “inflation” while ignoring the “different physics in different parts of the multiverse” question, I think many signers of the GKL&N letter were missing a good opportunity to make common cause with IS&L on defending their science against an ongoing attack from some of their fellow signatories.
Update: There are sources with technical details of the arguments being made by both parties:
I’ll try to find time to read these carefully and try and understand exactly what claims are being made. Would love to hear from experts who may have looked at these and are better placed to evaluate what the arguments are.
This controversy continues to involve an unusual level of PR rather than science. The Stanford press office has just put out this, where Linde makes it clear that he sees this as a political and PR fight:
Linde added that he worries about the younger generation of scientists getting the wrong impression from this story. “I don’t want them to read this article and think that they are spending their time on inflationary theory in vain. But the enthusiastic support that we are receiving makes us optimistic that this is not going to happen,” he said.
There’s no mention in this press office story of their last press office story about Linde and inflation, which promoted the BICEP2 “smoking gun” vindication of Linde and inflation.
Update: Another article about this is at the Atlantic. A crucial issue here is whether inflation has now entered the realm of unfalsifiability. Given any likely new data that could appear, is there any way it could falsify inflation, or can one just come up with some version of inflation that will match it? Guth and Linde seem at times to be taking the attitude that this is fine, I take Steinhardt et al. as pointing out that this is no longer conventional science. Replacing falsifiability by arguments about how many prominent people have signed your letter is a worrisome development. From the article:
In 2014, for example, Loeb asked Guth during a panel discussion if inflation was falsifiable—whether you could design an experiment to disprove it. Guth called that a “silly question,” suggesting that inflation was an umbrella over many theories, making it very hard to knock them all out at once. The hope right now, he says, is to use observation and further theory to winnow inflation down to just one specific version.
“Our point is that this kind of reasoning is inconsistent with normal science and cannot be resolved by invoking authority,” Ijjas, Steinhardt, and Loeb wrote to The Atlantic.
Update: More about this from John Horgan.
Update: It seems that Andrei Linde is a Lubos Motl fan. This is getting very weird. It’s not normal to respond to a scientific argument by enlisting letter writers on your behalf, even less normal to put your university press office to work on a response, and truly far out there to think that it’s helpful to have Lubos announce that you have eaten from the tree of knowledge and that your opponents are imbeciles.
Update: For a sensible take on this that I think likely reflects well the views of most mainstream cosmologists, see Peter Coles.
Update: IJS have put together a Fact-Checking page. It lists the four “predictions” of inflation claimed to agree with experiment by Linde et al. and gives four references to papers published by Linde touting different “predictions” for the same quantities, predictions not agreeing with experiment.
This month’s Scientific American has a bizarre cover story by Nomura on “The Quantum Multiverse”, see here.
All in all, I don’t know what other people’s reactions to this have been, but before this started I was a lot more sympathetic to the argument that Steinhardt was treating the case for inflation unfairly.
Update: Yet more coverage trying to make sense of this, from Nick Stockton at Wired.