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.)

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178 Responses to Do You Think Inflation Probably Happened?

  1. Chris Duston says:

    I guess I’ll start…

    1. 85%
    2. 15%
    3. 15%
    4. 75%
    5. 5%
    6. 25%
    7. 0%

    Looks pessimistic. Can I add 80% for LQG?

  2. matthiasr says:

    1. 70 %
    2. 50 %
    3. 15 %
    4. 80 %
    5. 10 %
    6. 95 %
    7. 5 %

  3. Sarah says:

    1. Inflation – 25%
    2. Supersymmetry – 45%
    3. String theory – 40%
    4. Some form of Higgs boson – 95%
    5. Large extra dimensions – 10%
    6. WIMP dark matter – 20%
    7. Any non-cosmological-constant explanation for cosmic acceleration – 10%

  4. ossicle says:

    That’s interesting to me, as an ignorant layman. I thought cosmic inflation was much more widely accepted than that, to the point where it was practically considered settled. THAT’S WHY ONE READS BLOGS, though, I guess!

  5. Greg says:

    1. Inflation – 90%
    2. Supersymmetry – 50%
    3. String theory – 1%
    4. Some form of Higgs boson – 95%
    5. Large extra dimensions – 1%
    6. WIMP dark matter – 85%
    7. Any non-cosmological-constant explanation for cosmic acceleration – 1%

    I can’t bring myself to say 0% chance.

    I guess for fair comparison, it helps to provide background. I’m 5 years removed from a bachelors degree in physics and don’t work in the field professionally.

  6. Alex says:

    1. Inflation: 70%
    2. Supersymmetry: 40%
    3. String theory: 3%
    4. Higgs: 80%
    5. Large extra dimensions: 1%
    6. WIMP: 50%
    7. Complex Dark Energy: 30%

  7. Lonely Flower says:

    Inflation 50%
    Supersymmetry 70%
    String theory 2%
    Some Form of Higgs bosons 80%

  8. PSP says:

    HAH! I was digit-for-digit with Greg, except:

    7. Any non-cosmological-constant explanation for cosmic acceleration – 50%
    Jury is still out, IMHO.

  9. Ian says:

    I’d be interested to see whether there’s a theorist/observer/experimentalist bias in the answers. My answers (trying hard to give snap answers so I don’t over-analyze)

    1. 70%
    2. 50%
    3. 30%
    4. 40%
    5. 10%
    6 90%
    7 20%

    Having thought about it for a second, I wonder why I’m so hesitant to put less than 10% odds on anything…

  10. Tom says:

    1. 30%
    2. 50%
    3. 5%
    4. 70%
    5. 10%
    6. 50%
    7. 50%

  11. MPS17 says:

    Who do you eat lunch with? The correct answers are:

    1 (inflation): 95%
    2 (susy): 20% (I assume you mean low-energy)
    3 (string theory): 80%
    4 (Higgs): 80%
    5 (large extra dims): 5%
    6 (WIMP DM): 60%
    7 (non-CC DE): 15%

    (String theory percentage is not that it’s the ultimate theory of reality, but that it’s a “correct” theory in a sense extending “correct” theories like GR and SM.)

    And yes they are all consistent with total probabilities adding up to one. 😉

  12. I wish this was done with a web based poll, to see the results graphically.

    1. Inflation – 50%
    2. Supersymmetry – 50%
    3. String theory –25%
    4. Some form of Higgs boson – 90%
    5. Large extra dimensions – 10%
    6. WIMP dark matter – 25%
    7. Any non-cosmological-constant explanation for cosmic acceleration – 1%

  13. From a layman’s perspective who can really only apply percentages based on the reasoning and tone of the arguments that I’ve read or heard….

    Inflation – 90%
    Supersymmetry – 50%
    String theory – 50%
    Some form of Higgs boson – 90%
    Large extra dimensions – 33%
    WIMP dark matter – 66%
    Any non-cosmological-constant explanation for cosmic acceleration – 25%

  14. Sean says:

    Interesting answers so far — it’s helpful if you give a bit of background on where you’re coming from (physicist or not, experimentalist/theorist, intelligent AI, etc.).

  15. Joseph Smidt says:

    1. Inflation – 80-90% (People said 25% at Caltech! Come on, we have now several experimentally verified reasons to believe in inflation.)
    Supersymmetry – 30%
    String theory = 25% (Got this because I am ~90% sure it is correct if SUSY is.)
    Some form of Higgs boson – 70%
    Large extra dimensions – 20 %
    WIMP dark matter – 75%
    Any non-cosmological-constant explanation for cosmic acceleration. 20%

    I am a graduate student doing cosmology. (Cooray’s student for those who know him.)

  16. Jeff says:

    Excellent question! As a young experimentalist who has worked on a couple of these (WIMP detection and B-modes)…

    1. Inflation
    50%. Inflation is a great idea to explain how our present universe could arise from our current idea of a generic initial condition. I’m not sure that I could convince a skeptical person that our idea of a generic initial condition is right, however, and I could easily imagine our ideas changing there.

    2. Supersymmetry
    40%. This idea has an enormous amount of theoretical beauty, but there just isn’t enough experimental evidence for me to be surprised if it turned out to be wrong.

    3. String theory
    20%. Again, a beautiful idea with even less experimental support. Then again, it’s not so much a single idea as an entire framework, so some aspect of current string-ish research could easily be part of the right answer.

    4. Some form of Higgs boson
    90%, especially for a sufficiently loose definition of “some form”. Something sure seems to break electroweak symmetry, so it’s reasonable to call whatever that degree of freedom is a Higgs boson…

    5. Large extra dimensions

    6. WIMP dark matter
    This is a tough one, so I’ll answer it two ways:

    – Particle dark matter: 95%. I would be very surprised if dark matter didn’t turn out to be real “stuff” which is not part of the Standard Model.

    – WIMPs in particular: 55%. It’s a fantastic idea, but I’d be less fundamentally shocked if thermal production from weak-scale interactions turned out to be wrong.

    7. Any non-cosmological-constant explanation for cosmic acceleration
    – 60%.

  17. Onchocerciasis says:

    1) Inflation 98% Perfectly reasonable given CMB and consistency of opposite parts of the visible universe that cannot possibly communicate. Consider an intensely chiral Big Bang with a pseudoscalar false vacuum that decayed to power inflation. It set the universe chiral in the massed sector at all scales. The trace dilute pseudoscalar vacuum background remnant is differentially detectable over 90 days’ observation in existing apparatus using opposite chirality probes: a vacuum left foot tested by left and right shoes (and inert to masseless photons and achiral massed processes).
    Two parity Eotvos experiments
    So easy a physicist could do it, with pictures.

    2) Supersymmetry 0% Already dead. Super-Kamiokande is 50 kilotonnes of water. Protons did not decay, SUSY recalculated a few more decimal places, and all was well. IceCube is a cubic kilometer of ice. That is

    [(1000 m)(100 cm/m)]^3(0.9 g/cm^3)(1 kilotonne/10^9 g) = 900,000 kilotonnes or 18,000 Super-Ks. SUSY is pookie pookie.

    3) String theory 0% Mathematically rigorous, physically sterile. Calabi-Yau manifolds are mirror symmetric while the universe is chiral at all scales. Socks – plus manually inserted symmetry breakings – are being used to describe shoes.

    4) Some form of Higgs boson 0% No Higgs. The Standard Model is defective. The Higgs is a curve fitting parameter amidst 25 more,

    5) Large extra dimensions 10% Unsupported by observation; curve fitting of elegant, rigorous, facile, defective theory.

    6) WIMP dark matter 20%
    Section 2 – nice sine wave!
    Nice sine wave!

    If the two sine waves are correlated, we already have a strong candidate by observation – vacuum background (re arxiv:1004.1761; beta decay is chiral, alpha-decay is achiral). Test with three small solid balls gilded with superconductor, cooled then Meissner effect levitated in hard vacuum at 45 latitude, observed for a year. Amorphous fused silica ball; enantiomorphic space group P3(1)21 single crystal alpha-quartz ball; enantiomorphic space group P3(2)21 single crystal alpha-quartz ball. If there is an interactive vacuum background of any kind, the fused silica ball will be rotationally inert over time. The quartz balls will spontaneously rotate in opposite directions, stop, do it mirror-image, stop, do it mirror-mirror-image, over a year’s observation.
    3.5 megabytes; pdf pp. 25-27, calculation of the chiral case.

    The Bullet Cluster! Concentrations of “dark matter” are separable and isolable from galaxies. Perhaps dark matter is not matter at all, but rather an extended finely knotted geometry of spacetime that coexists with black hole event horizon scavenging.

    7) Any non-cosmological-constant explanation for cosmic acceleration 50% Presence of a selective vacuum background is testable. There is room for multiple sources. Theory is always preferred to experiment because virtual mud always packs tighter than real gems.

  18. John says:

    I’m a conservatibve physicist:

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

    And yes, I have other solutions for each problem that these answers imply.

  19. Eric says:

    1. Inflation – 50%
    2. Supersymmetry – 75%
    3. String theory – 60%
    4. Some form of Higgs boson – 99%
    5. Large extra dimensions – 1%
    6. WIMP dark matter – 50%
    7. Any non-cosmological-constant explanation for cosmic acceleration – 50%

    I feel pretty certain that something like inflation occurred, in the sense that there was a short period in which the universe expanded at an incredible rate. I’m just not sure if any of the current models of what caused this incredible rate of expansion are correct. Supersymmetry is the most likely solution to the hierarchy problem, although the exact SUSY spectrum may be heavier than previously thought. There may be more to the issue of electroweak symmetry breaking than the simple Higgs mechanism, but the Higgs undoubtedly exists in some form. If there is TeV-scale SUSY, then the dark matter likely has at least some WIMPy component. The dark energy question is a complete mystery.

  20. Joseph Smidt says:

    Wow… People putting string theory as *more* likely than inflation given inflation’s experimental successes and string theories… Just wow!

    Reminds me of the time someone criticized me for saying we have good evidence for dark matter and when I asked what he was researching said (and I kid you not) some D7-D3 brane something. I about died.

  21. Ted Bunn says:

    Inflation — 75%
    Supersymmetry — 10%
    String theory — 1%
    Some form of Higgs boson — 90%
    Large extra dimensions — 0.1%
    WIMP dark matter — 50%
    Any non-cosmological-constant explanation for cosmic acceleration — 70%

  22. Newtonian Bias says:

    I see a lot of low numbers for the last item, which brings me another question:

    How sure are you that the expansion of the universe really is accelerating?

    Dare you write down any probabilities?

    I’m a mechanical engineer who loves astronomy, and kinda hope for the acceleration to just go away…

  23. Ted Bunn says:

    @John — Saying 0% is just as far from “conservative” as saying 100%. The truly conservative position is to acknowledge ignorance.

  24. Oliver says:

    1) 50%
    2) 40%
    3) 85%
    4) 70%
    5) 15%
    6) 97%
    7) 50%

  25. Ethan Siegel says:


    This is an excellent poll! My own speculations are:

    1) 90%
    2) 0.5%
    3) 0.01%
    4) 98%
    5) 0.000001%
    6) 10%
    7) 0.5%

    Clearly your readers are much more optimistic about many of these ideas than I am…