BICEP2 Updates

Here are the main results on gravitational waves/B-modes from the CMB, as reported by the BICEP2 experiment. For background see my previous post. All of the BICEP2 results and plots are here.

First, the best fit to r, the ratio of gravitational waves to density perturbations:


Rumors were right, and r = 0.2 is the best fit (with errors plus .07, minus .05). Here is the power spectrum (amplitude as a function of angular scale on the sky):


And here we have the contours in r/nS space, analogous to the Planck constraints we showed yesterday. Note that this plot crucially allows for “running” of the spectrum of density perturbations signal — the size of the fluctuations changes in a more complicated way than as a power law in wavelength. If running weren’t included, the different constraints would seem more incompatible.


Comparison with other limits (BICEP2 results are black dots at bottom):


Finally, here is a map of the actual B-mode part of the signal:


Overall, an amazing result (if it holds up!). It implies that the energy scale of inflation is about 2×1016 GeV — pretty close to the Planck scale (2×1018 GeV). An unprecedented view of the earliest moments in the history of the universe.

[From earlier.] Here is an email from BICEP2 PI John Kovac:

Dear friends and colleagues,

We invite you to join us tomorrow (Monday, 17 March) for a special webcast presenting the first results from the BICEP2 CMB telescope. The webcast will begin with a presentation for scientists 10:45-11:30 EDT, followed by a news conference 12:00-1:00 EDT.

You can join the webcast from the link at

Papers and data products will be available at 10:45 EDT from

We apologize for any duplicate copies of this notice, and would be grateful if you would help share this beyond our limited lists to any colleagues who may be interested within our CMB and broader science communities.

thank you,
John Kovac, Clem Pryke, Jamie Bock, Chao-Lin Kuo

on behalf of
The BICEP2 Collaboration

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56 Responses to BICEP2 Updates

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  3. Another mail just arrived from the CfA, clearly sounding like a significant positive detection:

    “The Harvard-Smithsonian Center for Astrophysics (CfA) will host a press conference at 12:00 noon EDT (16:00 UTC) today to present the first results from the BICEP2 experiment, which measures B-mode polarization of the cosmic microwave background. This finding has major implications for our understanding of the first moments of the universe.

    — John Kovac, associate professor of astronomy at Harvard University
    — Clem Pryke, associate professor at the University of Minnesota
    — Jamie Bock, professor of physics at Caltech/JPL
    — Chao-Lin Kuo, assistant professor at Stanford University/SLAC
    — Marc Kamionkowski, professor of physics and astronomy at Johns Hopkins University”

  4. Adam H says:

    Toast again to Dr. Einstein. Good to see science back in action confirming its theory through experiment and not more theory. 😉

  5. Ben Goren says:

    I don’t have anything substantive to contribute…I’m just posting to get the email notifications of updates. But over on Jerry Coyne’s Web site, I just compared this (if it holds up) to the discovery of DNA, which I hope isn’t too hyperbolic….


  6. Steve DeLong says:

    Thanks for this and, especially, yesterday’s post. Yes, their Fig. 13 (the 3rd one you had above) is the perfect comparison to your r/nS figure from yesterday.

    They also nicely explained and qualified their choice of the “running” model (last two paragraphs of Section 11, right before Conclusions, bottom of p. 16 of the manuscript). But I had to laugh at the last sentence: “We anticipate a broad range of possibilities will be explored.” Guess they think some theorists will read it.

  7. Olivier Minazzoli says:

    Do figures 10 and 13 mean that Starobinsky’s R^2 inflation is in trouble? Higgs inflation with strong non-minimal coupling to gravity (xi >>1) as well?

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  9. Allan says:

    I watched this a few days ago. ‘How did the universe begin’ from UC Berkley. It covers a lot of the background. Lecturer also mentioned a project to directly detect gravitational waves. The detector measures distance to an accuracy equivalent to measuring the distance to the nearest star to a nanometre ( or was it a millimetre? ).

  10. West says:

    Skimming the results paper, the tension between the BICEP2 estimate of r_0.002 and the published Planck results hinges on the question of whether one needs a “running spectral index” in the model. What is it, and why should we not expect index to be flat?

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  12. Bob F. says:

    The LIGO experiment has been searching for gravity waves for years, but has come up empty. Any ideas of what’s to become of it, now?

  13. Sean Carroll says:

    Bob– LIGO is looking at a completely different wavelength, so there’s really very little comparison. And of course LIGO would be a direct detection of gravitational waves.

  14. Archil says:

    Another twist of this discovery — Standard model vacuum is unstable! —

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  16. Looks like you are using approximations at the Planck length to allow for inflation. Or, as I would put it, Uncertainty at less than the Planck length. What causes inflation, assuming it can squeeze into less than a Planck length to effectively separate mass? Rest mass conversion to energy?

    Isn’t it better to use a mechanical cause rather than geometrical inflation for pure Isotropic expansion? The FLRW Metric is preserved by inflation, with G.R. operating around it in expanding spacetime. That means we live on a curved two-dimensional surface – which is improbable except by using a mathematical interpretation of reality.

    Inventions using math are of no consequence except to the extent they are based on actual measurement. G.R. is useful in some ways and not others – mathematically useful but conceptually skewed. Rework G.R. and ad hoc inflation of unknown cause, and find a graviton mechanism. That might help.

  17. Jeremy says:

    What does this mean for the primordial Omega_GW today?

    Are the GWs imprinted in the CMB B modes still in the universe today?

  18. Steve says:

    Question: Is this finding significant at the 5-sigma level? Ethan Seigel says no, it’s at the 2.7-signs level, so we should be more cautious with our conclusions. But the BICEP FAQ web page says its significant at the 5-sigma level? Anyone care to clarify this for me?

  19. Joe says:

    Two questions: Does this mean nobels for Kovak and Guth? Does this mean that the LISA experiment is no longer needed?

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  22. OMF says:

    I demand a post from the blogosphere explaining the theory and especially practice of sigma bounds in physics. Mostly because I don’t understand whether this result is a) too low to be indicative, b) high enough to be indicative but too low to be certain, or c) actually high enough to be certain but physicists are being too demanding.

    In short, I would like to Grok sigma levels.

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  24. Nick Hart says:

    This is probably a stupid question, but if an infinite number if photons were constrained in an infinitely small space, what would happen if that restrain was removed?

  25. grepo says:

    Hi Sean,

    I am not a physicist so would you please explain how an error of >= 25% in r is a remarkable value, and how the measured energy scale of inflation, which is 1/100th the Planck scale value, is “pretty close” to it?

  26. J.J. Green says:

    Does anyone know where I could find the data for the E/B maps (Figure 3). I Couldn’t find it on the project website.

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  28. JollyJoker says:


    The scale is pretty close to the Planck scale if you view it on an exponential scale and compare to the stuff we see at the LHC, which is 12 orders of magnitude lower.

    I’m not sure the 25% error in itself has been called remarkable; what is new is that there’s a measurement of a value where there were only upper bounds before. Some not-so-realistic values were previously ruled out but the value of r could have been arbitrarily low. This is the first time we have a sharp enough picture to see that there’s something there. It’s still quite fuzzy though.

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  31. Hassan says:

    Thank you for the post. Do you know what the frequency of the gravitational wave that BICEP detected its signiture is ? 10^-18 Hz ? I looked at their paper and couldn’t find out. I need this for my research. I will quote you if you let me know about it :)

    Thank you

  32. Tristan says:

    @Steve: I took a look at Ethan Seigel’s post, and I think he’s looking at the wrong part of the paper.

    According to the actual BICEP2 paper,, the 2.7 sigma figure refers to “detection of lensing in the BICEP2 BB autospectrum.” This is not the important result of the paper.

    BICEP2 was specifically not looking for evidence of gravitational lensing. They were looking for evidence of primordial gravitational waves leaving a specific imprint on the CMB polarization. Thus, the relevant and important result in the paper is summed up by the following quote:

    “This excess represents a 5.2 sigma excursion from the base lensed-LambdaCDM model.”

    That means there is a large signal above and beyond what is expected due to lensing, which represents rather strong evidence (5 sigma evidence, which in the particle physics community is enough to label something a “detection”) that something else interesting is going on. The most likely explanation is tensor perturbations from gravitational waves created at around the time of the big bang itself.

  33. Tom Renbarger says:

    I was about to post something very similar to what Tristan just posted, but I think he is right that the 2.7-sigma significance level applies specifically to BICEP2’s detection of the foreground lensing signal. If you wanted to be a real stickler about statistical details, you could say that the BICEP2 detection from 40 < ell < 200 is a 5.2-sigma detection of *something* other than lensing, with the reported r = 0.20 +0.07/-0.05 representing a 4-sigma detection of a non-zero tensor-to-scalar ratio. That's still a considerably more robust result than is claimed in Ethan's post.

  34. Tom Renbarger says:

    I forgot to mention, in the interests of full disclosure, I was a postdoc in Brian Keating’s lab from 2004-11, but I’ve been out of the observational cosmology field for three years now. I think my interpretation above is about as conservative as you can fairly be based on the reported results, though.

  35. grepo says:


    Thanks for the explanation, but I am too ignorant on the subject to understand why a 2 order of magnitude difference is considered “pretty close” even on an exponential scale. It’s better than a 12 order of magnitude difference, but we obviously have different ideas on what “pretty close” means. In all my school and professional experience, a 20 dB difference is laughing-stock territory.

  36. Ray says:

    So should Alan Guth and Andrei Linde get a Nobel now?

  37. Jerome says:

    Do the measurements actually agree with the inflation model? If I look at the plots of the power spectrum I would think that the data does not agree with the model. Right when the model is supposed to turn down the data goes up.

    How strong is this evidence? Should we be sceptical until confirmed with other instruments? The data seem to be outside the bounds set by plank data you previously posted but agree with a “running” signal? What are the implications of a large r?

  38. Ben Goren says:


    I get the impression that the physics community has sorta moved beyond the Nobel. I’m pretty sure every physicist would agree that the most notable achievement before BICEP2 in recent history was the CERN team’s discovery of the Higgs, and I’m also pretty sure that that same set of physicists would give credit as I just did: to the many of thousands of people listed as authors on that paper — to the team. But they’re not going to get a Nobel for it…and if the Nobel prize isn’t going to go to the discoverers of the Higgs, then, as nice as the cash and public recognition is, the Nobel just doesn’t really mean anything.

    But being listed as an author on the paper of the Higgs discovery…now that means something.

    Same thing with so much else of modern physics…it’s really been one huge collaborative effort, with so many people contributing so much but no single individual really responsible for more than a small piece of the puzzle. There’s no single individual you could credit with the discovery of Quantum Mechanics; merely a pantheon. And everybody in the field knows exactly what everybody else has done, so what more reward do you need, save seeing somebody else take your ideas farther than you yourself ever could manage to do?

    I should note: I’m not a physicist; this is entirely based on my own observation. But it’s observations such as that wonderful video of Professor Kuo hand-delivering a bottle of champagne to Professor Linde and Professor Kallosh that drive me to this kind of conclusion. Even if Linde does get a Nobel, I don’t think it’ll mean as much to him as Kuo’s bottle of champagne.



  39. TomS says:

    As far as personal satisfaction, the Nobel may mean less, but as far as prestige in one’s university, it means to the president nearly as much as a football championship.

  40. Daniel Shawen says:

    This is the first (indirect) evidence of any gravity wave, and a first-rate observation.

    However, I still don’t get how they can equate the structure observed to either inflation or the big bang.

    Rainbows are linearly polarized in a radial pattern, for obvious internal reflection reasons. Gravity doesn’t have a “rainbow”, but I know enough about optics, birefringence, circular and elliptical polarization to know that it can take considerable effort to analyze polarization data in a manner that makes sense. Too many of my former physics teachers have misinterpreted polarization phenomena as something else (interference) for me to trust a hasty interpretation of what the first evidence of gravity waves are actually showing.

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  42. CB says:

    This is not the first indirect detection of gravity waves. The first was in the orbital decay of binary pulsars which showed energy loss in agreement with GR’s predictions for gravity waves.

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  44. jmv2009 says:

    Quantum gravity
    Electroweak instability
    Planks scale physics
    Big bang
    String theory
    Dark matter

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  46. A comment on terminology: “gravity wave” means something very different to what
    we’re talking about here. A “gravity wave” is a wave of oscillating matter whose retoring forces are due to gravity & buoyancy. Common examples are the waves one
    sees on oceans and lakes. Gravity waves also figure in meteorology and the theory
    of stellar oscillations.

    In contrast, the things we’re talking about here are called “gravitational waves”
    or “gravitational radiation” (those two phrases are synonyms).

  47. Daniel Shawen says:

    Yes, you are right, CB! Thanks for reminding me of that GW detection. So, BICEP will be the second (or perhaps third, if tides count).

    Some of the other BICEP diagrams I found elsewhere are a little more convincing and intriguing. They seem to be saying that the polarization is an artifact of past interaction between density waves and some other force carrier (early photons?). Yes, I agree, that makes perfect sense.

    How awesome it is we can still observe some artifact of the ancient universe that cold, from so long ago. I now seem to remember at least two similar instance of modern forensic audio techniques almost as surprising. The line frequencies of AC power in the US are recorded, along with any adjustments the utility company may have made. By filtering all but the line buzz from the audio and carefully comparing the frequencies recorded by the utility with any time stamp info available with the original recording, the audio content can be rather easily authenticated. It’s not perfect assurance the audio wasn’t modified, but as I understand it, it’s pretty good.

    Nice of the inflation / big bang era to have made such a record for us to authenticate!

  48. CB says:

    Good point Jonathan, gravity waves had been detected long ago. :)