It occurred today that the past year or so there haven’t been as many editions here of This Week’s Hype, with in particular the previously common “Scientists finally find a way to test string theory!” stories now less common than they used to be. What is often replacing bogus claims about testability though is just unvarnished hype about string theory, minus any claims that it can ever be tested. Examples I’ve run into today include Cumrun Vafa’s Fundamental Lessons From String Theory presentation at the April APS Meeting, which tells us that
String theory is leading to a revolutionary revision of many fundamental and long held principles of physics
despite a lack of any connection to experiment, either now or in the future.
In the local bookstore I took a look at Christophe Galfard’s The Universe in Your Hand, which builds up to a final chapter with some sort of rather incomprehensible voyage with a robot to the string theory multiverse. Nothing anywhere to be seen there about whether this might be science or fantasy. Jennifer Ouellette has a review in the New York Times here.
While mulling over these thoughts about the new prediction-free environment for string theory, I noticed that an article has just appeared that seemed to contradict such thoughts, Natalie Wolchover’s Physicists Hunt for the Big Bang’s Triangles, with a headline claiming that “evidence for string theory” could be found in the sky. It’s by far the best popular piece I’ve seen about “string cosmology”, giving an excellent idea of what people in that field are up to these days (which includes large amounts of hype, coming from the scientists, not the journalist).
In summary, here’s what we learn about current string cosmology. One of the main targets is a “prediction” of the level of non-gaussianity in the CMB, something which all observations so far have shown to be unobservably small:
- Matthew Kleban and Eva Silverstein are described as “cosmological clocksmiths”, working out the non-Gaussianity “predictions” of a large range of string cosmology models. It seems that you can get any number you want this way by an appropriately complicated model. Kleban likes unwinding inflation and we’re told:
“I think it’s pretty plausible that some version of this happens,” he said.
Though Kleban acknowledges that it’s too soon to tell whether he or anyone else is on to something, plans are under way to find out.
Silverstein has “many string inflationary models” of all sorts, so can get you one with any amount of non-gaussianity you might want. As far as she is concerned, having plenty of complicated models with zero evidence for any of them, such that, no matter what you see or don’t see, there’s always lots more and more “predictions”, is a perfectly traditional kind of science. Her reaction to people pointing out the problems with this?
I find it surreal, because we are currently doing some traditional science with string theory.
- Her Stanford colleagues Andre Linde and Renate Kallosh are taking a different approach, promoting theories with no observable non-gaussianity, and, it seems, no observable effects at all:
Linde isn’t bothered by this. In supporting the alpha-attractor models, he and Kallosh are staking a position in favor of simplicity and theoretical beauty, at the expense of ever knowing for sure whether their cosmological origin story is correct. An alpha-attractor universe, Linde said, is like one of the happy families in the famous opening line of Anna Karenina. As he paraphrased Tolstoy: “Any happy family, well, they look in a sense alike. But all unhappy families — they’re unhappy for different reasons.”
- On the East Coast, there’s Arkani-Hamed and Maldacena, with a paper last year on Cosmological Collider Physics. It described observable signatures of new physics at the inflationary scale, but the only comment I could find about how you would actually observe such things was
In terms of measurability, if the couplings are Planck suppressed, then it seems impossible to measure this through the CMB or large scale structure. (See e.g. [58, 59, 60, 61] for a discussion of measuring these effects via large scale structure.) But it might be possible using the 21cm tomography .
Arkani-Hamed explains to Wolchover that there’s a “cosmic variance” problem making things inherently unobservable, but he removed most discussion of this from the paper, hoping to get around it by changing the laws of quantum mechanics:
In his paper with Maldacena, Arkani-Hamed initially included a discussion of this issue, but he removed most of it. He finds the possibility of a limit to knowledge “tremendously disturbing” and sees it as evidence that quantum mechanics must be extended. One possible way to do this is suggested by his work on the amplituhedron, which casts quantum mechanical probabilities (and with them, unitarity) as emergent consequences of an underlying geometry. He plans to discuss this possibility in a forthcoming paper that will relate an analogue of the amplituhedron to non-Gaussianities in the sky.
There has been further work on this by Kamionkowski and collaborators, and we’re told:
Observing the signals predicted by Arkani-Hamed, Maldacena and Kamionkowski would be like striking gold, but the gold is buried deep: Their strength is probably near the gravitational floor and will require at least 1,000 times the sensitivity of current equipment to detect.
No word on prospects for more than 1,000 times more sensitive experiments.
For about as long as I can remember, string cosmologists have been promising that their ideas would be tested by the Planck experiment. Now that negative results are in from that, we’re told to instead look forward to a new generation of experiments. The ones mentioned in the article are SPHEREx and the LSST, with results next decade. SPHEREx claims that they may be able to push down bounds on non-gaussianity by a factor of 10, which would be an impressive result. This would rule out lots of string cosmology models, but of course there would still be plenty more.
The next experiment all involved are talking about is looking for signals in the 21cm hydrogen line, see Sabine Hossenfelder here for more about that. No time estimates on that one. As far as I can tell, the plan for this starts off with “First, build a base on the other side of the moon…”