Pop Goes the Universe and Other Cosmic Microwave Background Games

(With apologies to whoever came up with this “book”.)

Back in February, Ijjas, Steinhardt, and Loeb wrote an article for Scientific American titled “Pop Goes the Universe” criticizing cosmic inflation, the proposal that the universe underwent a period of rapid expansion early in its life, smoothing it out to achieve the (mostly) uniform universe we see today. Recently, Scientific American published a response by Guth, Kaiser, Linde, Nomura, and 29 co-signers. This was followed by a counterresponse, which is the usual number of steps for this sort of thing before it dissipates harmlessly into the blogosphere.

In general, string theory, supersymmetry, and inflation tend to be criticized in very similar ways. Each gets accused of being unverifiable, able to be tuned to match any possible experimental result. Each has been claimed to be unfairly dominant, its position as “default answer” more due to the bandwagon effect than the idea’s merits. All three tend to get discussed in association with the multiverse, and blamed for dooming physics as a result. And all are frequently defended with one refrain: “If you have a better idea, what is it?”

It’s probably tempting (on both sides) to view this as just another example of that argument. In reality, though, string theory, supersymmetry, and inflation are all in very different situations. The details matter. And I worry that in this case both sides are too ready to assume the other is just making the “standard argument”, and ended up talking past each other.

When people say that string theory makes no predictions, they’re correct in a sense, but off topic: the majority of string theorists aren’t making the sort of claims that require successful predictions. When people say that inflation makes no predictions, if you assume they mean the same thing that people mean when they accuse string theory of making no predictions, then they’re flat-out wrong. Unlike string theorists, most people who work on inflation care a lot about experiment. They write papers filled with predictions, consequences for this or that model if this or that telescope sees something in the near future.

I don’t think Ijjas, Steinhardt, and Loeb were making that kind of argument.

When people say that supersymmetry makes no predictions, there’s some confusion of scope. (Low-energy) supersymmetry isn’t one specific proposal that needs defending on its own. It’s a class of different models, each with its own predictions. Given a specific proposal, one can see if it’s been ruled out by experiment, and predict what future experiments might say about it. Ruling out one model doesn’t rule out supersymmetry as a whole, but it doesn’t need to, because any given researcher isn’t arguing for supersymmetry as a whole: they’re arguing for their particular setup. The right “scope” is between specific supersymmetric models and specific non-supersymmetric models, not both as general principles.

Guth, Kaiser, Linde, and Nomura’s response follows similar lines in defending inflation. They point out that the wide variety of models are subject to being ruled out in the face of observation, and compare to the construction of the Standard Model in particle physics, with many possible parameters under the overall framework of Quantum Field Theory.

Ijjas, Steinhardt, and Loeb’s article certainly looked like it was making this sort of mistake. But as they clarify in the FAQ of their counter-response, they’ve got a more serious objection. They’re arguing that, unlike in the case of supersymmetry or the Standard Model, specific inflation models do not lead to specific predictions. They’re arguing that, because inflation typically leads to a multiverse, any specific model will in fact lead to a wide variety of possible observations. In effect, they’re arguing that the multitude of people busily making predictions based on inflationary models are missing a step in their calculations, underestimating their errors by a huge margin.

This is where I really regret that these arguments usually end after three steps (article, response, counter-response). Here Ijjas, Steinhardt, and Loeb are making what is essentially a technical claim, one that Guth, Kaiser, Linde, and Nomura could presumably respond to with a technical response, after which the rest of us would actually learn something. As-is, I certainly don’t have the background in inflation to know whether or not this point makes sense, and I’d love to hear from someone who does.

One aspect of this exchange that baffled me was the “accusation” that Ijjas, Steinhardt, and Loeb were just promoting their own work on bouncing cosmologies. (I put “accusation” in quotes because while Ijjas, Steinhardt, and Loeb seem to treat it as if it were an accusation, Guth, Kaiser, Linde, and Nomura don’t obviously mean it as one.)

“Bouncing cosmology” is Ijjas, Steinhardt, and Loeb’s answer to the standard “If you have a better idea, what is it?” response. It wasn’t the focus of their article, but while they seem to think this speaks well of them (hence their treatment of “promoting their own work” as if it were an accusation), I don’t. I read a lot of Scientific American growing up, and the best articles focused on explaining a positive vision: some cool new idea, mainstream or not, that could capture the public’s interest. That kind of article could still have included criticism of inflation, you’d want it in there to justify the use of a bouncing cosmology. But by going beyond that, it would have avoided falling into the standard back and forth that these arguments tend to, and maybe we would have actually learned from the exchange.


6 thoughts on “Pop Goes the Universe and Other Cosmic Microwave Background Games

  1. Lubos Motl

    Hmm, the fact that you see the clear differences between the status of SUSY, string theory, and inflation is a result of your insider’s perspective.

    The critics don’t really want to look at any of these nuances. For them, it is a populist fight against science that has run too abstract and detached from the practical needs of the people.

    So I do see all the differences when I focus on the actual science but when I focus on the conceptual “wars about the meaning of science”, I see that those three fields are much more “equivalent” than you’re admitting.


  2. Giotis

    Certain individuals must have hallucinations of infinity grandeur and they are ready to pull the plug on both String theory and Inflation though (and I guess SUSY)🙂

    In practice they want to terminate research in high energy theoretical Physics research all together. BTW do you know where your local unemployment office is? You may have to visit it soon enough 🙂


    But the problem is not these folks, crackpots and quacks always existed and will always exist; the problem is the scientific outlets harbouring them.


        1. 4gravitonsandagradstudent Post author

          Fair enough. Not sure I would call Horgan a quack/crackpot either, since that term is usually reserved for people at least pretending to do science. He’s a journalist. Whether he’s trying to kill HEP is of course another question.


          1. Lubos Motl

            OK, I think that calling Horgan a journalist is about as problematic as calling him a scientist. Check a definition of journalism, e.g. at Wikipedia. It is the production of the “news of the day” – the information should flow from the real world to those who are interested in it.

            Horgan is an activist because he primarily wants to affect the things he is writing about. So while he might agree that he is not a scientist, he wants to place himself as a meta-scientist who is doing an upper level of science that determines what other scientists shold be doing.

            I don’t understand how any of these specifics could lead you to “remove him from the list of those who deserve criticism”, 4gravitons. He needs to face at least the same counter-pressures and “review” as scientists do not because he’s a scientist according to meritocratic criteria but because he is trying to have a similar influence on science as scientists. I picked Horgan as an example, the point is obviously much more general.



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