Do You Think Inflation Probably Happened?

I was at a meeting in Princeton a short while ago, a small and focused workshop for people who are working on fundamental questions in inflationary cosmology. I hope to talk more about the meeting once the website is up (talks were not recorded), but here’s a simple question: what is the likelihood you would attach to the idea that some form of cosmic inflation occurred in the early universe?

My answer was 75%, which I thought was generous. It’s very hard to give a high probability to a speculative theory about what happened at energy scales to which we currently have no experimental access. But I found myself on the low end of opinions at the meeting, where the median was about 90% confidence. Of course, these are people who work on inflation professionally, and have chosen to do so. When I came home to ask the same question of my lunch crowd at Caltech, the answers were more like 25%.

An interesting glimpse into the non-unanimity of scientific opinion when it comes to untested theories. So, just for fun, let’s ask what your personal likelihoods are for the following theoretical ideas.

  1. Inflation
  2. Supersymmetry
  3. String theory
  4. Some form of Higgs boson
  5. Large extra dimensions
  6. WIMP dark matter
  7. Any non-cosmological-constant explanation for cosmic acceleration

I’m not defining these very carefully, and let’s posit that we’re not interested in weaseling about what the definitions mean. We’re asking what you think the probability is that, if you were to ask an omniscient being who knew everything about the workings of Nature whether these ideas were part of how the world works, would they answer in the affirmative. What do you think? (It’s helpful if you say a bit about what kind of perspective you are coming from.)

178 Comments

178 thoughts on “Do You Think Inflation Probably Happened?”

  1. 1. Inflation 35%
    2. Supersymmetry 85%
    3. String theory 45%
    4. Some form of Higgs boson 95%
    5. Large extra dimensions 65%
    6. WIMP dark matter 0%
    7. Any non-cosmological-constant explanation for cosmic acceleration 10%

    Just a layman with a studied interest in Cosmology. I think the whole Dark Matter/Dark Engergy thing is completely wrong. Any of you read Reinventing Gravity, by physicist John W. Moffat?

  2. Torbjörn Larsson, OM

    1. Inflation – At least 95 % certain, since IIRC WMAP later data releases has tested inflation on its lonesome to something like 2.6 sigma.
    2. Supersymmetry – Symmetries works, when not broken: ~ 10 %?
    3. String theory – Dunno, not my scene.
    4. Some form of Higgs boson – Standard Model stuff, have been correct earlier, so simplest explanation for particle mass: ~ 70 % (“1 sigma”, LOL).
    5. Large extra dimensions – Dunno, not my scene.
    6. WIMP dark matter – Simplest DM IIRC, again ~ 70 %.
    7. Any non-cosmological-constant explanation for cosmic acceleration – What would an alternative be: ~ 0 %.

  3. Galactic dynamics background

    1. Inflation: 70%
    2. Supersymmetry: 40%
    3. String theory: 5%
    4. Some form of Higgs boson: 99% (“some form” is important here)
    5. Large extra dimensions: 5%
    6. WIMP dark matter: 30%
    7. Any non-cosmological-constant explanation for cosmic acceleration: 75%

  4. first year grad student in particle physics:

    1. Inflation: :-/
    2. Supersymmetry: 🙂
    3. String theory: 😐
    4. Some form of Higgs boson: 😀
    5. Large extra dimensions: 🙁
    6. WIMP dark matter: 😛
    7. Non-cosmological constant explanation: :-0

  5. 1. Inflation: 95%
    2. Supersymmetry: 75%
    3. String theory: 90+%
    4. Some form of Higgs boson: 50+%
    5. Large extra dimensions: 50+%
    6. WIMP dark matter: 90%
    7. Any non-cosmological-constant explanation for cosmic acceleration: 50%

    String theory has such organic links to the physics we know about, that it would be very surprising for it to be unrelated to the truth. However, it seems very possible that even M-theory is just the supersymmetric phase of a deeper theory, which is why I rated strings above supersymmetry.

    My relative skepticism about the Higgs and enthusiasm for (genuinely) large extra dimensions predate any serious study of string theory and particle physics. Heavily favoring a Higgs boson, when there are Higgsless ways of breaking electroweak symmetry, seemed like bias in favor of the first and simplest idea that anyone had; while having all the extra dimensions stably small seemed unnatural and due to lack of imagination. I preferred the idea that extra dimensions would be genuinely large (infinite or at least cosmological in scale) even before the ADD and RS models came along. It’s possible that these are legacy opinions that I will revise as I understand the theoretical options better; or maybe not.

    I gave WIMPs such a high rating because I was thinking dark matter was overwhelmingly likely to be composed of superparticles, but clearly that 90% is inconsistent with giving susy itself odds of just 75%. So that figure should be renormalized to about 70%. On reflection, I’d also want to revise #7 down to maybe 25%.

  6. Retired physicist who is reading cosmology for relaxation….

    1. Inflation – 60% (more thoroughly worked through than, for instance, Penrose’s story
    2. Supersymmetry – 50% – as with most ideas yet to be tested
    3. String theory – 0% (or 100% says God, if it can never be tested)
    4. Some form of Higgs boson – 70% (but declining as its phase space shrinks)
    5. Large Extra Dimensions – o% (God says four is magic enough)
    6. WIMP dark matter – 50% (but it outranks the others)
    7. non-CC – 20% (God likes things simple, if very small)

  7. Particle phenomenologist here.

    1. Inflation- 80%. Inflation’s virtue is its ability to resolve many different problems simultaneously, not that any one resolution is overwhelming convincing. That being said, I tend to take economical explanations as very suggestive.
    2. Supersymmetry- 20%. This is a very biased answer- life is a lot harder if supersymmetry is correct.
    3. String theory- 5%. I am 90% confident some aspects of string theory (extra dimensions, ADS/CFT) will turn out to be correct, bot not the theory as a whole. (not that it’s a whole theory!)
    4. Some form of Higgs boson- 85%. My guess is that a vanilla Higgs will show up at the LHC.
    5. Large extra dimensions- 1%
    6. WIMP dark matter- 75% I think the answer IS just going to be a new particle. Just like neutrinos don’t seem so mysterious anymore (even though there’s still a lot we don’t know about them), I think we’ll look back and see dark matter as just another particle
    7. Any non-cosmological-constant explanation for cosmic acceleration- 10%

  8. 1. Inflation: 95%. Inflation seems pretty general and plausible. I think the chances of anybody having come up with anything close to the correct inflationary model are pretty low though.
    2. Supersymmetry: 5%. It just seems way too messy, what with the huge parameter space for symmetry breaking and the doubling of the number of particles and all. That, and I never thought gauge coupling unification was that compelling a reason to believe in it.
    3. String theory: 1%. I really don’t know much about it, so this is almost pure bias.
    4. Some form of the Higgs: 15%. At least for the Higgs as a fundamental scalar. The fact that we’ve never seen a fundamental scalar before really makes me doubt that we’ll see one in the future. Since a big part of the reason for supersymmetry is solving the hierarchy problem, my low answer for this helps keep #2 low too.
    5. Large extra dimensions: ?. I know very little about this, but it sure would be neat.
    6. WIMP dark matter: 33%. But 99% for any form of particle dark matter, 33% for axions, and 33% for some other unthought of particle.
    7. Non-CC explanation for cosmic acceleration: 95%. I’m not sure why everyone else has this so low. Is the alternative that the CC is just a God-given parameter set in the theory? There has to be something a little deeper than that, although again I doubt anyone has got the right idea yet.

    I’m a 3rd year grad student in particle/astroparticle theory. I’m at the point in my grad career where skepticism and doubt are running high, which kind of explains my low scores for #2–4.

  9. angryphysics, you don’t really sound like much of a fan. Go back to watching tedtalks.

    fortunately, your answers are such outliers that i knew to ignore them immediately.

    The only reason I read these posts was to get a sense of what effects people were basing their answers on. Thanks, guys 😀

    –math overgrad

  10. I’m a mathematician.
    I follow physics and astronomy primarily through popular books and blogs, though I keep trying to go through the “real science” when possible.

    My ratings:

    1. Inflation: 90%.
    From what I’ve read, it has a lot of observational support.
    Was rather surprised that Caltech physicists rated it so low. Maybe they were speaking of a particular model – like Eternal Inflation ?

    2. Supersymmetry: 30%.
    My impression is, this is like a working fusion reactor – always on the verge of being discovered, but never quite there….

    3. String theory: Sqrt( -1) %.
    My problem is, I’m not even sure what string theory IS, anymore !!
    Is it a coherent theory like General Relativity ? Or a framework for building theories ?
    Does it involve strings, or membranes, or something else altogether ?
    Does it predict a multiverse ?
    Or is a multiverse conveniently introduced to explain why it has made no testable predictions so far ?

    My position: “First tell me clearly what you mean by String Theory, and I’ll tell you why I don’t believe in it”. 🙂

    4. Some form of Higgs boson: 95%. The Standard Model seems really powerful.
    It would be strange if it suddenly breaks at this point.

    5. Large extra dimensions: 0.1%
    I may be mistaken, but it seems to be one branch of stringy nonsense.

    6. WIMP dark matter: 75%
    I understand that dark matter is pretty much in the bag.
    Don’t know what the alternatives to WIMPs are. Making some allowances for that.

    7. Any non-cosmological-constant explanation for cosmic acceleration: 50%
    I know the evidence points otherwise, but a cosmological constant is just dead boring.
    So here’s hoping. 🙂

    I’d really be interested to see how the answers differ between physicists and non-physicists.
    Would give a good idea of how popular science books are shaping opinions.

  11. Peer Stritzinger

    Theoretical Physicist working in software development:

    1. Inflation 80%
    2. Supersymmetry 70%
    3. String theory 10% — other than by that many parameters you can fit anything 😉
    4. Some form of Higgs boson 25%
    5. Large extra dimensions 5%
    6. WIMP dark matter 70%
    7. Any non-cosmological-constant explanation for cosmic acceleration 50%

  12. Research scientist in observational cosmology (with an embarrassing lack of fundamental understanding of hep-th / field theory stuff).

    1. Inflation – 75%. Goes up if you generalize to “period of very rapid expansion in the early universe plus nearly gaussian initial conditions.” That is very well observationally established (in my opinion), but I don’t think it’s out of the realm of possibility that a non-scalar-field-driven mechanism could be responsible.

    2. Supersymmetry – 50%.

    3. String theory – 10%. As many others have pointed out, this depends on one’s definition of “string theory”. However, I take issue with the claim that if we have no idea, we should say “50%” (I’m looking at you, Dr. “Colbert Fan” Lim). With a flat prior on the modelverse, the probability that any theory with no observational evidence is right is vanishingly small.

    4. Some form of Higgs boson – 95%. My particle friends tell me that this “Standard Model” they’ve come up with has been quite successful so far.

    5. Large extra dimensions – 5%. Weight this answer as you would if you had asked a fortune teller or a chimpanzee.

    6. WIMP dark matter – 80%. I may have been to too many DM detection talks and journal clubs lately, but it seems to me like we’re in the “several interesting 1-2 sigma results” era that often precedes a slam-dunk detection. Blame Dan Hooper.

    7. Any non-cosmological-constant explanation for cosmic acceleration – 5%. And I think most of that probability space is occupied by modified gravity, not quintessence.

  13. I’d really be interested to see how the answers differ between physicists and non-physicists.
    Would give a good idea of how popular science books are shaping opinions.

    based on the comments so far (with pretty graphs too!)

    Average & Standard Deviation of predictions by all vs. by physics backgrounds

    Inflation: 69.60, 26.57 || 74.21, 18.45 (+7, -9) – physicists think it’s more likely

    SuSy: 43.76, 29.02 || 44.73, 23.99 (+1, -6) – just as likely

    Strings: 30.94, 30.22 || 24.82, 24.92 (-6, -5) – less likely

    Higgs: 78.22, 27.41 || 86.26, 19.69 (+8, -8) – more likely

    Large xD: 11.41, 19.76 || 8.42, 14.38 (-3, -5) – a little less likely

    WIMPs: 61.09, 27.95 || 66.21, 21.54 (+5, -6) – more likely

    non-CC exp: 27.16, 28.33 || 24.94, 26.18 (-2, -2) – a little less likely

  14. Inflation — 25%
    Supersymmetry — 10%
    String theory — 10%
    Some form of Higgs boson — 50%
    Large extra dimensions — 0.2%
    WIMP dark matter — 50%
    Any non-cosmological-constant explanation for cosmic acceleration — 0%

    The real question, of course, is how many people believe in inflation because Rocky Kolb believes in inflation and they are scared of Rocky Kolb. 🙂

  15. Here are my answers
    Inflation — 25%
    Supersymmetry — 0% (would have seen it by now)
    String theory — 100% (any observation is consistent with string theory)
    Some form of Higgs boson — 50%
    Large extra dimensions — 0 % (we would have seen it by now)
    WIMP dark matter — 0 % (else we would have seen it by now)
    Any non-cosmological-constant explanation for cosmic acceleration — 0%

  16. I am a senior PhD student working in hep-ph. Here are my answers…

    1. Inflation – 80%
    2. Supersymmetry – 99%
    3. String theory – 70%
    4. Some form of Higgs boson – 99%
    5. Large extra dimensions – 10%
    6. WIMP dark matter – 99%
    7. Any non-cosmological-constant explanation for cosmic acceleration – 20%

    I am pretty surprised how SUSY gets a low average. I think it’s too beautiful not to be true.

  17. I’m a layman so all I can do is to have some fun with the poll. Which I will do. So how I would justify my choices based on what I know I don’t know:

    Inflation — 10% – why do we care? If happened, happened once in the distant past and will not happen again (maybe some “reverse” inflation just before Big Crunch). If happened, there is something fishy about invoking exotic, one time phenomenon just to make data seam less puzzling.
    Supersymmetry — 70% – everything with “super” in the name sounds cool and has to be true.
    String theory — 10% – another step down on the ladder of reductionism. I’m quite sure that string theory will be successful in predicting that strings are composed of 0-dimensional fibers that clap their small hands and each particular clapping sequence corresponds to vibrational mode of the string, that corresponds to …….
    Some form of Higgs boson — 80% – everything with somebody’s’ last name in the name has to be legitimate. Also, we’ve just spent couple billions dollars to find out what is going on.
    Large extra dimensions — 0.2% I firmly believe in extra dimensions of moderate size though.
    WIMP dark matter — 65% but only if “weakly” in the name means weakly interacting through all four forces. If it means “only interacting trough weak force” let’s give it 0.4%.
    Any non-cosmological-constant explanation for cosmic acceleration — 60%. It will be big.

  18. Frenchman answer:

    1. Inflation: 10%
    2. Supersymmetry: 95%
    3. String theory: 10%
    4. Some form of Higgs boson: 80%
    5. Large extra dimensions: 80%
    6. WIMP dark matter: 10%
    7. Any non-cosmological-constant explanation for cosmic acceleration: 20%

  19. 1. Inflation: 40%
    2. Supersymmetry : 60%
    3. String theory: 10%
    4. Some form of Higgs boson: 75%
    5. Large extra dimensions: 15%
    6. WIMP dark matter: 50%
    7. Any non-cosmological-constant explanation for cosmic acceleration: 50%

  20. I’m a PhD mathematician with casual (not professional) interests in astronomy, cosmology, and particle physics. Not sure which camp that puts me in, but “layman” seems more appropriate to me because I’m not an expert in these specific areas.

    1. Inflation – 99%
    2. Supersymmetry – 80%
    3. String theory – 50%
    4. Some form of Higgs boson – 99%
    5. Large extra dimensions – 1%
    6. WIMP dark matter – 90%
    7. Any non-cosmological-constant explanation for cosmic acceleration – 10%

    Inflation and Higgs seem to be the simplest explanation of genuine physical observations, so they’re almost certainly true. I think the chance of having supersymmetry without WIMP-based dark matter is close to zero. In other words, if supersymmetry is a symmetry of nature, then I think the lightest supersymmetric particle is almost surely a correct explanation of dark matter. So WIMP > SUSY, and as others have pointed out, SUSY > Strings. There could be other explanations of WIMP dark matter, of course. No opinion on cosmic acceleration. I give “Large extra dimensions” 1% because the idea seems patently absurd to me, but this might just reflect my lack of knowledge.

  21. Astrophysicist
    1. Inflation 98%
    2. SUSY 85%
    3. ST 85%
    4. “Higgs” mechanism 99%
    5. Large extra d 0.1%
    6. WIMP 30%
    7. Non-Lambda 20%

  22. 5th-year grad student in theoretical cosmology.

    1. Inflation: 60%
    There’s good observational evidence in its favor, but it’s pretty generic stuff that you can reproduce with other theories. It has passed every test we’ve thrown at it, though, so that’s something.
    2. SUSY: 40%
    I’m out of my element here, but the fact that nobody’s ever seen a proton decay seems fishy. On the other hand, it’s a compelling idea.
    3. String Theory: 25%
    Out of my element again, but it has to be strictly lower than the SUSY odds. I’d give it (slightly) better than 50-50 odds if SUSY is true, hence 25%.
    4. Higgs: 95%
    The Standard Model predicts it, and the SM seems to work *very* well, so I’d be surprised — but not totally floored — if we don’t find it.
    5. Large extra dimensions: ???
    I have no idea. This is so far outside my area of expertise that it’s not even funny.
    6. WIMP dark matter: Depends. If you just mean “particle dark matter as the primary explanation for excessively deep potential wells on galactic and super-galactic scales,” then I’m going to go with something like 90%. But if you make the stronger claim of “WIMPs, specifically, (and not, say, sterile neutrinos or some other particle) are the sole cause of the excess potential wells,” then I’d have to lower it to something more like 50%.
    7. Non-CC explanation for cosmic acceleration: 30%.
    Lambda is our best bet, but it’s hardly our only one.

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