Last week Princeton hosted what seems to have been a fascinating conference, celebrating the 50th anniversary of studies of the CMB. Hopefully videos and slides will be posted, but one can get some idea of the highlights of the talks from live tweeting that was going on, that is gathered together here. The third day of the conference featured a panel where sparks flew on the topics of inflation and the multiverse, including the following:

Neil Turok: “even from the beginning, inflation looked like a kluge to me… I rapidly formed the opinion that these guys were just making it up as they went along… Today inflation is the junk food of theoretical physics… Inflation isn’t radical enough – it’s too much a patchwork. It all rests on rare initial conditions… Akin to solving electron stability with springs… all we have is proof of expansion, not that the driving force is inflation… “because the alternatives are bad you must believe it” isn’t an option that I ascribe to, and one that is prevalent now… inflation is pretty but we should encourage young to think about its problems & be creative (not just do designer inflation)

David Spergel: papers on anthropics don’t teach us anything – which is why it isn’t useful.. sometimes we need to surrender (to anthropics) but that time is not yet now.

Slava Mukhanov: inflation is defined as exponential expansion (physics) + non-necessary metaphysics (Boltzmann brains etc)… we should separate inflation from the landscape… exponential inflation is very useful, the rest [of the metaphysical stuff] is not for scientific discussion… In most papers on initial conditions on inflation, people dig a hole, jump in, and then don’t succeed in getting out… unfortunately now we have three new indistinguishable inflation models a day – who cares?

Paul Steinhardt: inflation is a compelling story, it’s just not clear it is right… I’d appreciate that astronomers presented results as what they are (scale invariant etc) rather than ‘inflationary’… Everyone on this panel thinks multiverse is a disaster.

Roger Penrose: inflation isn’t falsifiable, it’s falsified… BICEP did a wonderful service by bringing all the Inflation-ists out of their shell, and giving them a black eye.

Marc Davis: astronomers don’t care about what you guys are speculating about at all (mulitiverses, pre-big bang, etc).

I was encouraged by Steinhardt’s claim that “Everyone on this panel thinks multiverse is a disaster.” (although I think he wasn’t including moderator Brian Greene). Perhaps as time goes on the fact that “the multiverse did it” is empty as science is becoming more and more obvious to everyone.

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15 Responses to CMB@50

  1. Geoff says:

    Although I’m not particularly a fan of inflation and certainly not the multiverse, I can’t help but wonder if any theory of the early universe runs into the inevitable problem of initial and/or boundary conditions. It seems that the best one can do in physics is write down a physical law, but there really doesn’t seem to be a way to derive the initial / boundary conditions from first principles. Is there any consensus that they are somehow outside the purview of physics?

  2. Mesmar Djehha says:


    From what I understand about the Ekpyrotic scenario, it just claims to do so: eliminate the need for initial/boundary conditions by removing any beginning for the Universe and replacing it with a cyclic history.

  3. Peter Woit says:

    I think Steinhardt’s argument is against claims that inflation solves fine-tuning problems (why is the universe so flat?), that all inflation models with a scalar field do is move the fine-tuning problem from one place to another.

  4. Tim Howells says:

    As a naive outsider those quotes really surprised me. It looks like something radical has happened since Leonard Susskind was claiming victory in a cafeteria food-fight over the multiverse. That was in 2005, so ten years ago. Was there a turning point, or is this the turning point? I was particularly suprised that they questioned inflation so strongly. Any pointers would be appreciated. Genuinely curious – not flame-bait!

  5. Anonymous says:

    This panel is full of (in)famous critics with various agendas. Not very representative of the field, in contrast to the observational talks which looked amazing.

  6. Peter Woit says:

    Tim Howells/Anonymous,
    The panel did over-represent critics of inflation, but I think the reason for multiverse skepticism is that string theorists were not on the panel, just cosmologists (except for the moderator, who seems to have been the only one defending the multiverse). Susskind is not a cosmologist…

  7. I’m trying to parse Penrose’s remarks. Does Penrose think that the BICEP2 results have somehow falsified inflationary cosmology? I don’t see how a lack of a statistically significant signal above the dust background is a strong statement one way or the other. Is there some context here that I’m missing?

  8. Student says:

    Peter or anyone,

    Where can we watch the videos?

    Are they available online?


  9. Peter Woit says:

    As far as I know, the talks were livestreamed, so in principle there is video. However, it does not seem to be available now, don’t know if there are plans to make it available in the future.

    Doug Natelson,
    I have no idea what Penrose had in mind, perhaps someone else does.

  10. Student says:

    Ed Witten defends inflation:

    Ed Witten: the criticisms of inflation are too a priori. Inflation is an incredibly simple theory. The beauty: it’s weakly coupled. #CMBat50

    Ed Witten: the same (simple, weakly coupled) cannot be said about the competing theories (to inflation) #CMBat50

    Why no mention of this Peter? Perhaps you missed it?

  11. Peter Woit says:

    The extracts I gave were only a small fraction of the discussion, the “sparks flew” ones where people were saying something controversial. There were many other interesting comments, including Witten’s, anyone interested should read the whole thing. I don’t think it’s surprising that Witten (who is not a cosmologist, by the way…) and many others would defend inflation, I’m much more curious whether he’d be willing to defend the multiverse…

  12. Cormac says:

    I think Mukhanov has a point. The word ‘inflation’ is used in several different ways, and much of the criticism isn’t really aimed at the basic notion of an exponential- expansion in the early moments of the cosmos (a hypothesis that gives a very ‘natural’ and hard-to-rule-out explanation for several features of the observable universe).
    Most criticisms of inflation are leveled at the apparent consequences of various models of inflation, from the problem of fine-tuned initial conditions to the notion of the multiverse – but if a successful model of inflation ever emerges, these predictions may not part of it. In the meantime, there are few theories that can model current observations the way the hypothesis of an exponential expansion can…

  13. milkshaken says:

    As a naive outsider, I believe Penrose comments (about bringing out all hidden inflation partisans and giving them black eye) was about the premature celebrations of B-mode polarization result that soon turned out to be analysis error…

  14. chris says:

    “The beauty: it’s weakly coupled.” … speaks volumes. translation: the beauty is that we can use perturbation theory.

    this is an unfortunate “search by the lamppost” bias that has influenced physics for far too long now.

  15. Andrew Tho as says:

    Very true, Chris.

    “Ed Witten: the same (simple, weakly coupled) cannot be said about the competing theories (to inflation)”

    That’s no argument in favour of the existence of inflation!

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