Big Bang Blunder Bursts the Multiverse Bubble

This week’s Nature has an article by Paul Steinhardt, with the title Big Bang blunder bursts the multiverse bubble. The subtitle of the piece describes the BICEP2 frenzy of last March as “premature hype”, and the description in the body of the article is:

The results were hailed as proof of the Big Bang inflationary theory and its progeny, the multiverse. Nobel prizes were predicted and scores of theoretical models spawned. The announcement also influenced decisions about academic appointments and the rejections of papers and grants. It even had a role in governmental planning of large-scale projects.

Given recent arguments that BICEP2 may be seeing dust, not primordial gravitational waves, the March media frenzy quite possibly was highly premature, if not completely misguided. Steinhardt goes on to argue that in the future

announcements should be made after submission to journals and vetting by expert referees. If there must be a press conference, hopefully the scientific community and the media will demand that it is accompanied by a complete set of documents, including details of the systematic analysis and sufficient data to enable objective verification.

He also takes the occasion to note the odd fact that while BICEP2 results have been claimed to be proof of inflation and the multiverse, if they turn out to be wrong, that’s fine too:

The BICEP2 incident has also revealed a truth about inflationary theory. The common view is that it is a highly predictive theory. If that was the case and the detection of gravitational waves was the ‘smoking gun’ proof of inflation, one would think that non-detection means that the theory fails. Such is the nature of normal science. Yet some proponents of inflation who celebrated the BICEP2 announcement already insist that the theory is equally valid whether or not gravitational waves are detected. How is this possible?

The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests. First, inflation is driven by a hypothetical scalar field, the inflaton, which has properties that can be adjusted to produce effectively any outcome. Second, inflation does not end with a universe with uniform properties, but almost inevitably leads to a multiverse with an infinite number of bubbles, in which the cosmic and physical properties vary from bubble to bubble. The part of the multiverse that we observe corresponds to a piece of just one such bubble. Scanning over all possible bubbles in the multi­verse, every­thing that can physically happen does happen an infinite number of times. No experiment can rule out a theory that allows for all possible outcomes. Hence, the paradigm of inflation is unfalsifiable…

Taking this into account, it is clear that the inflationary paradigm is fundamentally untestable, and hence scientifically meaningless.

Steinhardt was on a panel last Friday night here in New York at the World Science Festival, which can be watched here. The panel included Guth and Linde (who earlier in the week got $1 million for their work on inflation), as well as John Kovac of BICEP, and Amber Miller, Dean of Science here at Columbia. The last part of the video includes an unsuccessful attempt by Steinhardt to pin down Kovac on the significance of the BICEP2 evidence for primordial gravitational waves claim, as well as an exchange with Guth and Linde. They both defend inflation as the best model of the alternatives.

Multiverse promotion continues apace, with Steinhardt one of a rather small number of physicists publicly objecting. On Monday Alexander Vilenkin will explain to the public at the American Museum of Natural History that “the Big Bang was not a unique event in cosmic history and that other Big Bangs constantly erupt in remote parts of the universe, producing new worlds with a great variety of physical properties” (see here). A recent story on livescience has Brian Greene on the multiverse. Over at Massimo Pigliucci’s Scientia Salon Coel Hellier is starting a multipart series arguing against multiverse skeptics with The multiverse as a scientific concept — part I. Nothing in Part I about the problematic issues (untestable claims that fundamental physics is “environmental”), maybe in Part II…

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27 Responses to Big Bang Blunder Bursts the Multiverse Bubble

  1. AcademicLurker says:

    After watching how carefully managed the release of the LHC results was, I was kind of surprised at how they jumped the gun with BICEP2. I know they’re completely different projects and organizations, but still…

    Maybe the BICEP2 team could take some PR lessons from CERN.

  2. IMHO the paper in Nature is too harsh. See here the reaction of Andrei Linde to the original BICEP2 news:

    Pay particular attention to 1:57-2:14 This is the reaction of an intellectual honest person. The original excitement and the media hype on BICEP2 was premature, but I feel the Nature paper is also a knee jerk reaction but in the opposite direction.

  3. anonymous says:

    The BICEP2 youtube video with Andrei Linde was so awkward. He seemed wise enough to be skeptical of the results, yet was forced to act nice and happy because people brought this celebration to his front door and video taped it.

  4. Peter Woit says:

    Linde has inflationary models with any value of r you want, so he had no particular reason to set great store in the BICEP2 number being right. This is the same Linde quoted by SciAm as saying
    “suddenly we have something which I firmly believe is experimental evidence in favor of the multiverse,” Linde says. “Those people who say the theory of the multiverse does not have any experimental confirmation have not paid enough attention.”
    I noticed that when engaged in discussion with Steinhardt he doesn’t say nonsense like that, sticks to the “my theory sucks less” argument.

  5. Peter Woit says:

    I think of lot of the questionable choices BICEP made can be attributed to their competition with Planck. A defensible claim from BICEP would have been “we have observed the right kind of B-modes for the first time, but we’ll need Planck’s results to know if this is dust or primordial gravitational waves.” Instead they went for what now seems a not very solid argument that “we don’t need Planck”, and a claim for a Nobel for themselves.

    It would have gotten very interesting at CERN if ATLAS and CMS had very different characteristics, and one of them saw something that could be the Higgs, but knowing this would require a study of backgrounds that only the other could do. Things would have gotten interesting…

  6. physicsphile says:


    I think you and Steinhardt are being a bit harsh. The key thing is that we can learn about inflation by doing experiments. Any improved measurement of r will cut into the space of possible potentials for the inflaton field. If r is sufficiently large then we can hone into a very small subset of possible potentials by using improved observations. So I don’t think its right to say inflation is just philosophy. Observations have and can further be used to get a better understanding of how inflation happened. Inflation can be falsified if a better theory is found that predicts new observations which only a very contrived version of inflation could produce. It would be great if one could take a simple Popperian view on falsability but I think the reality of how science actually works is a bit more complex.

  7. Curious Mayhem says:

    The BICEP2 hype may be have been premature. But proof of inflation is not in any way proof of the “multiverse,” which is unprovable by its nature.

    Inflation is by the far the best extension of the hot Big Bang we have. While it’s consistent with Linde’s “eternal inflation,” it doesn’t require it.

    And, again, inflation has nothing whatever to do with the multiverse in the “landscape” sense.

  8. Person says:

    No matter if the BICEP2 results turn out to be true or not, they are not good science. Therefore, the claim that the hype was premature is quite accurate and not too harsh at all. Publications that are based on scraping the presentation slides of other groups should be flat out rejected. That is no clean process and should not be allowed within the science community.

    On an other note, I dont understand how one can conclude from inside our universe on the existence of other universes or bubbles. That makes no sense. We are unable to understand how the natural constants come to their values, even if we accept the (any) inflation model to be correct. Taking our not-understanding as proof of a multiverse concept is grotesque. It is religious at best, comparable to the invention of a creator of the world as a model for the existence of the world in ancient times. The multiverse is in its nature not falsifiable. It should have no in science.

  9. Garbage says:

    There’re so many misleading statements in Steinhardt’s article there’s no point really to argue. One should write a similar argument complaining about how he’s changing his mind every time a new bound on local non-Gaussianity comes about… We all know inflation makes well defined predictions, however, as usual with physical theories, one has to explore parameter space. One could have said the same about the Higgs 40 years ago, until experiments pushed to the ultimate “limit” and found it, or whatever else would’ve replaced it. We know there’s a GW signal at large angles from inflation, gravity is universal, that’s why this is so exciting! Moreover, we know there’s an ultimate level of non-Gaussianity with a very specific shape, provided inflation happened with vacuum fluctuations. Under very mild assumptions we have more than enough observables to test the theory. So far inflation is the only framework where we can *calculate*, the scale invariance of the perturbations comes out beautifully and many well motivated assumptions make predictions than can be tested, like a large value for r. I don’t see how else are we supposed to do science. Inflation is not a unique theory, so isn’t the standard model for that matter, no one has ever found a way so far to predict the values of masses and mixing angles. This is not a very constructive article, on the contrary. We all know we don’t understand Lambda and the hierarchies in physics, but proposing no alternative explanation and bashing the attempts which have produced the most amazing fit to the data so far, is just absurd IMHO…

  10. Patrick M. Dennis says:

    Peter, is it your opinion that inflation resides in the same “not even wrong” boat as string theory and multiverse(s)? …. Also, do you find the Everett version of the latter to be equally “not even wrong”?

  11. Anonyrat says:

    Why worry? In some alternate universe, multiverse is recognizably not science.

  12. Peter Woit says:

    Patrick M. Dennis,
    About inflation, see some comments in the latter part of
    “Many worlds” is a completely different (off)-topic.

    I’m getting on a plane soon and will be traveling for a few days, unclear how much I’ll be able to deal with the blog. Please don’t feed the trolls…
    If you want to discuss this topic, I highly recommend the Scientia Salon site I linked to, where Massimo Pigliucci does a great job of moderating the discussion.

  13. Jeffrey Herrmann says:

    Steinhardt argues “Scanning over all possible bubbles in the multi­verse, every­thing that can physically happen does happen an infinite number of times. No experiment can rule out a theory that allows for all possible outcomes.”
    Then why not quit. If every possible experiment is incapable of distinguishing between Theory X and the theory of eternal inflation because even if Theory X is right, both theories predict the same observed data, what is the point of continuing to observe more and more data?

  14. TomH says:

    is Steinhardt still a strong proponent of the “Ekpyrotic” big bang model?
    If so, it surely accounts for part of his harsh criticism of inflationary models.

    But Ekpyrotic model relies on existence of unobserved and/or unobservable higher -order dimensions , “branes” floating in the “bulk” , and such — yes?

    To this layman, that seems almost as speculative and unfalsiable as any multiverse hype … although if BICEP2 results are validated, doesnt that falsify Ekpyrotic models??

  15. Monty says:

    I watched the video from the World Science Festival. Could I ask Peter, or the commenters, what would be wrong with the following response to Steinhardt’s complaint that whatever the BICEP2 results had been, they could be made to fit with some variant of inflationary theory: yes, the observation of B-mode polarisation (let’s assume it’s not an artifact of foreground dust–this will be shown one way or the other soon in any case) cannot by itself prove inflation. But it lets us choose appropriate candidate theories from within the previous set of inflationary theories, and, more importantly for inflation-backers, it adds another item to the list of things requiring explanation for any competing theory. So now we have not only isotropy, flatness, absence of relics, and large-scale structure to explain, but we also have otherwise unexplained B-mode polarisation of the CMB. An appropriately narrowed inflation model can account for all of those things at once; that makes it correspondingly harder for an alternative theory to be equally successful. Doesn’t that make it a stronger theory than it was this time last year?

  16. JG says:


    Not really, if the observable B-mode signal was a favoured prediction of inflation then the BICEP2 observation would certainly have a strong impact on the credibility of inflation. But there are many models of inflation which predict a non-observable B-mode polarisation, so the BICEP2 result just narrows the parameter space for inflation models – it doesn’t say anything definitive about their truth – despite all the hype about a “smoking gun”.


    Steinhardt has admitted the Ekpyrotic model is dead if BICEP2 are correct.

    Jeffrey Herrmann

    Because Steinhardt doesn’t believe the multiverse is correct, that is why there is still a point in making observations and doing science!


    Maybe if they didn’t (practically) claim the scientific debate is over and award million pound prizes in hollywood style ceremonies to the “victors” the opponents might not make such a public “fuss”. Pointing out obvious deficiencies in the science is a necessary, and useful public service. :-)

  17. Monty says:

    JG – thanks, but I acknowledged all that you said in my comment. Let me try again: the BICEP2 observation doesn’t confirm anything, because the range of inflationary theories would have encompassed any result of those observations. But isn’t it true that (if the polarisation holds up as being in the background) it adds to the list of things that inflation takes care of, and that a competing theory would need to address? I thought inflation was widely held to be an appealing theory because it explained so many things at once. Now it explains “so many plus one”. A competing theory now doesn’t just have to account for flatness, isotropy of the CMB and large-scale structure, and the lack of magnetic monopoles, but it also has to explain where the heck these swirls in the polarisation of the CMB come from.

  18. JG says:


    Ok, sure, inflation was a pretty nice theory already before BICEP2 announcement. It’s just that most of the popular inflation models inspired by string theory disfavoured an observable B-mode signal. So I don’t see how an observation by BICEP2 which wipes out the majority of inflation models is a good argument in favour of inflation. In fact, ironically, it may lead to a non-inflationary model like string gas cosmology becoming a serious contender, and note that sgc solves a really big problem that inflation does not – why 3 large spatial dimensions?

    Even if Planck raise major doubts about the BICEP2 observation the string theorists have loads of other inflation models which “predict” weak B-modes.

    So although I think inflation is a nice (and even compelling) idea, I’m not convinced BICEP2 should have been sold to the public as confirmation of it.

    But I also agree, that any competing model (such as sgc) must solve the problems of flatness, isotropy, lack of relics at least as cleanly as inflation does.

  19. Neophyte says:

    What is a “relic”?

  20. Mark says:

    I was astonished by Steinhardt’s comments. I have great respect for him, as one of the few physicists who has made major contributions in both cosmology and condensed matter physics (quasicrystals). But he seems to have jumped the shark with his statement that the unfalsifiability of inflation removes it from the scientific paradigm.

    Paul (and I hope you read this): Supersymmetry makes many predictions and is the basic of most particle physics research over the past 25 years. Yet if it is not seen in the next run at the LHC, many of its proponents will focus on slivers of remaining parameter-space, or make small modifications. Thus, low scale supersymmetry isn’t really falsifiable. Does that mean you believe that supersymmetry is not part of the scientific paradigm?

  21. Monty says:

    Neophyte: relic particles would be the surviving exotic particles predicted by hot big bang models to have been created in the extremely energetic conditions of the very early universe. Magnetic monopoles are one type of relic. One thing inflation does is explain why we don’t actually observe any such thing.

  22. Yatima says:

    One thing inflation does is explain why we don’t actually observe any such thing.

    Not really. Explaining that unicorns live in faraway lands over the rainbow “explains” why we don’t actually see unicorns in the woods. But in very wrongheaded away.

  23. Monty says:

    Yatima: would you say it’s incorrect that hot big bang theories without inflation predict exotic relics?

    Because if they do, then your analogy isn’t very fair. There’s no reason to believe we should find unicorns in the woods in the first place.

  24. GoletaBeach says:

    “And this time, the announcements should be made after submission to journals and vetting by expert referees.”

    Has the Flauger/Hill/Spergel paper been vetted by expert referees?

    Sometimes theories have unknown parameters like r… r didn’t/doesn’t have to be big to support inflation.

    Just re-read the story of some of the first new scientific results from E.O.Lawrence’s program of cyclotrons circa 1933. His team totally messed up the parameters of the deuteron & neutron… they thought the neutron was lighter than the proton. Rutherford’s team corrected them with results from a linear accelerator. The Berkeley team eventually got it right, and indeed, circular machines dominated particle physics until the present (OK, there is SLAC too).

    So BICEP2 maybe messed up. Big whup. They pushed the envelope. Wouldn’t be the first time something went wrong, but they still deserve commendation and respect for getting the envelope moved forward.

    Without hype, there will be insufficient funding. The real world knows this. The real world also knows that overly strong bloodletting over mistakes is counterproductive.

  25. Peter Woit says:

    About the argument that inflation “explains” the non-observation of magnetic monopoles. One should keep in mind that magnetic monopoles are not expected in the standard model. The lack of magnetic monopoles is a problem that only occurs in various GUT models. Given the lack of success of GUTs on all other fronts (e.g. no proton decay) there’s no need for an “explanation” by inflation of why we don’t see magnetic monopoles, given the much simpler explanation that GUTs don’t work.

  26. Monty says:

    Peter, thanks, I wasn’t aware of that and retract my suggestion that Yatima’s unicorn analogy was unfair! I’m assuming it’s still fair to say that inflation provides a nice explanation for those other things (large-scale structure, homogeneity of the CMB, flatness).

  27. Rob Heusdens says:

    If we are comparing Inflation theory and String theory, then at least I would say that inflation theory makes testable predictions which after observation confirm the predictions. There are different models, each have different properties and lead to different observables, so we can rule out some models. That is far better as we can think of in String theory.
    Inflation theory is not saying in first instance that ‘there is a multiverse’ and/or that ‘any outcome is correct’, but that comes as a by product of it. It was not ‘invented’ for that reason. Even so, a theory could in principle be correct, without being thoroughly falsifiable. All claims inflation theory makes about other universes are untestable. Only the claims inflation makes about our universe we can test and see if they conform to that model of inflation. Standard science practice. We should not by definition rule out a theory because it makes more predictions then can be tested for.

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