The Inflationary Bubble

The Summer School I’m attending on Inflation and the CMB got under way yesterday morning with a couple of lectures (90 minutes each) by Andrei Linde, one of the pioneers of the theory of cosmic inflation. I enjoyed the first part of the session, but then he went off into the technical details of a specific model for which there seemed previous little in the way of physical motivation or testable consequences. There’s an occupational hazard for people working on inflation which is that they become so absorbed by their calculations that they forget that they’re supposed to be doing science. It sometimes appears that this field has reached a critical density of activity which means that it’s in danger of forming a closed universe completely incapable of communicating with the world outside and perhaps of collapsing in on itself.

The other thing I didn’t like was the evangelism about the multiverse, which is widespread amongst theorists these days. I’ve stated my position about this before so I won’t repeat my objections here. I will, however, lodge an objection to the way Prof. Linde answered a question about whether the multiverse theory was a testable of various fine-tuning problems in cosmology by saying

Ihe multiverse is the only known explanation so in a sense it has already been tested.

I don’t mind particularly if theories are not testable with current technology. New ideas often have to wait a very long time before equipment and techniques are developed to test them, but Linde’s response is rather symptomatic of a frame of mind that does not consider testability important at all. The worst offenders in this regard are certain string theorists who seem to thing string theory is so compelling in its own right that it just has to be the one true description of how the Universe works, even if the framework it provides is unable to make any predictions at all.


9 Responses to “The Inflationary Bubble”

  1. Anton Garrett Says:

    If it’s not testable then it’s philosophy not physics.

  2. Andrew Liddle Says:

    It’s certainly testable in principle. Weinberg used it in 1987 to predict a non-zero cosmological constant, subsequently verified, which I suppose is what Andrei Linde refers to. That’s a pretty striking achievement in my view, since in the 80s hardly anyone thought the cosmological constant was worth considering. Had the cosmological constant been zero, the multiverse as presently conceived would have been ruled out. It’s not been too easy to make other predictions so far though, because the `theory’ is not yet terribly well specified.

    On the more philosophical side of the coin, the multiverse on one hand expands the range of questions we can try to address in a scientific manner, eg through Bayesian probability, while on the other hand is a reminder that there may be phenomena that simply do not have testable explanations. I don’t think the Universe is under any obligation to let us see all its workings.


    • IIRC, Weinberg assumes the vacuum-energy sourceo of the cosmological constant beloved of particle physicists and combines this with a traditional, but negative, cosmological constant of slightly less magnitude, so that the resulting value, anthropically selected, is the value we observe (quite small in particle-physics terms). Does anyone other than Weinberg think of the cosmological constant in this fashion?

    • Anton Garrett Says:

      Maybe not, but if we assume that we can’t then we never shall even if we actually could.

      • Andrew Liddle Says:

        I do agree with that. So as well as working on ideas which are manifestly testable, as Peter advocates, we need to think about how questions/explanations get classified into testable versus untestable. From that perspective, I don’t think I would be that bothered if it turned out that the multiverse were untestable (though as I said above I don’t think that is the case), because it might still have the status of a valid, though possibly non-unique, explanation of what is observed.

  3. […] The latest on the philosophy of science from Linde (see here), who is at a workshop in Bad Honnef this […]

  4. Hi Andrew, hope you’re well. I presume you’re referring to inflation in the first paragraph of your first comment? V interesting if so.

    On the multiverse, I’m no fan either, but I’ve always liked the theme of sober scientists being driven by mathematical generalizations to unwelcome ideas! On testability, I like the word ‘yet’ in Peter’s post, it’s not used often enough in such discussions (though ‘yet’ sort of presumes something will later occur, we need a word to express that we don’t know whether some theories will or will not be testable in the future)

  5. Gitanes… good choice.

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