BICEP2: Is the Signal Cosmological?

I have a short gap in my schedule today so I thought I would use it to post a short note about the BICEP2 results announced to great excitement on Monday.

There has been a great deal of coverage in the popular media about a “Spectacular Cosmic Discovery” and this is mirrored by excitement at a more technical level about the theoretical implications of the BICEP2 results. Having taken a bit of time out last night to go through the discovery paper, I think I should say that I think all this excitement is very premature. In that respect I agree with the result of my straw poll.

First of all let me make it clear that the BICEP2 experiment is absolutely superb. It was designed and built by top-class scientists and has clearly functioned brilliantly to improve its sensitivity so much that it has gone so far ahead of so many rivals:

Polarization detections

Notice that the only other detection of the elusive B-mode signal is by POLARBEAR, but that is actually accounted for by gravitational lensing effects rather than being evidence of a primordial gravitational wave contribution.

The B-mode signal is so weak that it is to mind absolutely amazing that an experiment can get anywhere near measuring it. There’s no denying the fact that BICEP2 team have done heroic work.

But – and it’s a big “but” – we have to ask the question “How confident can we be that the signal detected by BICEP2 is, in fact, the imprint of primordial gravitational waves on the cosmic microwave background that cosmologists were hoping for?”

The answer to this question will depend on the individual, but I would say that to convince me the absolute minimum would be a detection of the signal in more than one frequency band. A primordial signal should not vary as a function of frequency, whereas foreground emission (likely to be from dust) would be frequency dependent.

Now BICEP2 only operates at one frequency, 150GHz, so the experiment on its own can’t satisfy this criterion but it could through cross-correlation with the original BICEP1 instrument which worked at 100 GHz and 150 GHz. In the discovery paper we find the

Additionally, cross-correlating BICEP2 against 100GHz maps from the BICEP1 experiment, the excess signal is confirmed with 3sigma significance and its spectral index is found to be consistent with that of the CMB.

Here is the relevant plot, Figure 7 from the paper,

Xcor_BICEP

Well, the correct though the statement in the paper might be,  it is clear from this (rather ratty) cross-correlation that there is actually no firm detection of the B-modes at all at 100GHz. In other words, the 100 GHz BICEP1 data may be consistent with BICEP2 but they are also consistent with zero. (NOTE ADDED: I am ready to rescind this statement when I see a full analysis of these cross-correlations; at face value the scatter looks strange and certainly consistent with a null detection). In any case a positive cross-correlation does not exclude the possibility that the signal in common across the two channels is dust. If we only have a detection at one frequency we have no compelling evidence at all that the signal is cosmological.

When asked on Tuesday about this by Physics World I stated that I wasn’t convinced:

It seems to me though that there’s a significant possibility of some of the polarization signal in E and B [modes] not being cosmological. This is a very interesting result, but I’d prefer to reserve judgement until it is confirmed by other experiments. If it is genuine, then the spectrum is a bit strange and may indicate something added to the normal inflationary recipe.

My scepticism was then derived primarily from the distribution of the points around l=200 in the first figure: they look too high compared to the expected gravitational lensing contribution (which seems to have been pinned down by the POLARBEAR measurements to the right of the plot):

My concern: the three data points circles in blue are all higher than they should be, by about 0.01, which is the same height as the points to their left.  But the prediction of gravitational waves from inflation, circles in green, is that there should be very little contribution here --- which is why these points should lie closer to the solid red "lensing" prediction.  So the model of lensing for the right-hand part of the data + gravitational waves from inflation for the left-hand part of the data does not seem to be a very convincing fit.

I’ve taken this plot from the post I reblogged yesterday. The errors in the measurements ringed in blue are probably correlated so the fact that all three lie well above the red curve may not be as significant as it first seems, but note that the vertical scale is logarithmic. If some sort of systematic error has indeed bumped these points up then the amount of power involved could easily account for all the signal in the points to the left; the fit to the primordial B-mode (red dashed) part of the curve could then be fortuitous.

One possible systematic, apart from foreground contamination by dust, is leakage between E and B modes in the spherical harmonic decomposition. This arises because the spherical harmonic modes are only orthogonal over a complete sphere; BICEP2 does not map the whole sky, so the modes get mixed and separating them becomes extremely messy. Since the E-mode signal is so much larger, the worry is that some of it might leak into the B-mode.

UPDATE: 20/3/2014

I noticed a post on the BICEP2 Facebook Page from Hans Kristian Eriksen pointing another oddity:

PTE

The above plot is one of many showing jackknife estimates relating to various aspects of the polarization signal. What is strange is that all the blue dots lie so close to zero. Statistically speaking this is extremely unlikely and it may suggest that the noise levels have been over-estimated underestimated; roughly one in three data points should be further away than one sigma from zero if sigma is estimated correctly.

Taking all this together I have to say that I stick to the point of view I took when I first saw the results. They are very  interesting, but it is far too earlier to even claim that they are cosmological, let alone to start talking about providing evidence for or against particular models of the early Universe. No doubt I’ll be criticized for trying to put a wet blanket over the whole affair, but this is a measurement of such potential importance that I think we have to set the bar very high indeed when it comes to evidence. If I were running a book on this, I would put it at no better than even money that this is a cosmological signal.

Of course the rush to embrace these results as “definitive proof” of something is a product of human nature and the general level of excitement this amazing experiment has generated. That’s entirely understandable and basically a very good thing. It reminds those of us working in cosmology how lucky we are that we work in a field in which such momentous discoveries do actually happen. This is no doubt why so many budding scientists are drawn into cosmology in the first place. Let’s not forget, however, that there is a thing called the scientific method and often after years of hard work there remain more questions than answers. For the time being, that’s where we are with gravitational waves.

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48 Responses to “BICEP2: Is the Signal Cosmological?”

  1. As luck would have it, the Moriond cosmology meeting is next week. I’m sure I’ll here some more takes on this between skiing and eating.

    Last time I was there, it was the Higgs bosun which was the big buzz. What will it be two years from now?

  2. Lloyd Knox Says:

    The cross correlation with Keck is a very important part of their story. I agree it will be very reassuring to get even better constraints on the possibility of significant galactic contamination. At least I expect better constraints will be reassuring! We will see.

  3. […] a great parallel (if the BICEP2 result holds up!) between Monday’s evidence for inflation and the Higgs discovery back in 2012. When talking […]

    • THE SMOKING GUN

      http://at37.wordpress.com/2013/10/14/aaa-day-in-the-life-of-the-137-ss-mystic-august-3-2013-nun/

      The 37 responses before mine are by mostly ‘scientists’.
      But what else do you believe in?

      You see the problem is that you fellas don’t know that science has become a religion too.
      And the best way to illustrate that is to point out that religion anticipated science finding those B-mode and E-mode gravitational waves, as that link I offered clearly points out.

      Pandora’s Box was just reopened.
      Far bigger things are at stake here than what your religion called science wants to establish as a dogma.

      selah V

  4. The paper claims the excess at l~200-300 is only 3-sigma off the model, though it is about the same scale as the possible TT->BB mixing, as you pointed out the other day – though it depends how well they understand their beam shapes – particularly their cross-polar beams. I haven’t had a chance to study the instrument paper in detail. However, if something like that were the culprit, then I don’t think it would have an impact on smaller scales.

    The polarisation sensitivity of BICEP1 (NEQ) is about 10x worse than BICEP2, largely because of the much smaller number of detectors, but the discrepancy between the agreement at 100 and 150 GHz is a little concerning.

  5. Is it possible to extract from data some hint about the spectral index of tensor modes? Is it possible that they fitted data assuming the spectral index predicted by inflation, while the last point you mention prefer a harder spectral index?

  6. Anton Garrett Says:

    Having informed commentary like this so quickly is brilliant. Long live the internet and thanks to Peter and others.

    • telescoper Says:

      One fascinating aspect of this is the role social media are playing. A facebook group:

      https://www.facebook.com/groups/574544055974988/

      was set up in advance of Monday’s announcement and live discussion started immediately. It now has over 700 members. This is the first time this has happened and it may be a pattern followed in the future..

      • At the DPG school on inflation and the CMB last summer, where Peter (as a lecturer) and I (as part of the audience) were, some of the younger folks set up a Facebook page for it and co-ordinated their pub-crawling activities etc in this fashion. I didn’t see much of a point since everyone was staying and eating in the same building, but I’m told that some people communicate via social media even when in the same room. :-|

      • Anton Garrett Says:

        The book Liar’s Poker which took the lid off the financial yuppies in the early 1990s mentioned that it was normal for traders in the same room or even in adjacent (soundproof) cubicles to phone each other, so as to maintain confidentiality from colleagues even of the fact that they were talking to each other, let alone about they were discussing.

        I was once in a restaurant with a friend and she said she would check her txts and reply if necessary, as something urgent was expected. I said fine, I’ll use the time to do the same – and txt’d her across the table. (“Boo!”)

    • telescoper Says:

      The blog has been getting amazing levels of traffic over the last few days, with more hits per day than it usually gets in a week.

      I think it will need a lie down shortly…

  7. Gravitational waves have already been detected.

    ‘Hubble Finds Ghostly Ring of Dark Matter’
    http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html

    “Astronomers using NASA’s Hubble Space Telescope got a first-hand view of how dark matter behaves during a titanic collision between two galaxy clusters. The wreck created a ripple of dark mater, which is somewhat similar to a ripple formed in a pond when a rock hits the water.”

    The ‘pond’ consists of aether.

    The ripple is an aether displacement wave.

    The ripple is a gravitational wave.

    ‘Galactic Pile-Up May Point to Mysterious New Dark Force in the Universe’
    http://www.wired.com/wiredscience/2013/01/musket-ball-dark-force/

    “The reason this is strange is that dark matter is thought to barely interact with itself. The dark matter should just coast through itself and move at the same speed as the hardly interacting galaxies. Instead, it looks like the dark matter is crashing into something — perhaps itself – and slowing down faster than the galaxies are. But this would require the dark matter to be able to interact with itself in a completely new an unexpected way, a “dark force” that affects only dark matter.”

    It’s not a new force. It’s the aether displaced by each of the galaxy clusters interacting analogous to the bow waves of two boats which pass by each other.

    “The word ‘ether’ has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. . . . Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry. [..] It turns out that such matter exists. About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with ‘stuff’ that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo.” – Robert B. Laughlin, Nobel Laureate in Physics, endowed chair in physics, Stanford University

    Matter, a piece of window glass and stuff have mass.

    When galaxy clusters collide it is the *stuff* which waves.

    • Your Laughlin quote is OK, but the two URLs you mention have nothing to do with gravitational waves as they are normally understood in cosmology. Don’t play with words. There are also gravity waves in the ocean, but these are not what Kip Thorne is interested in (at least not while doing GR).

      • In the following Einstein is describing what occurs physically in nature. What Einstein is describing can be allied to the ripple created when galaxy clusters collide which is a gravitational wave.

        ‘Albert Einstein’s ‘First Paper'; a letter to his Uncle’
        http://www.straco.ch/papers/Einstein%20First%20Paper.pdf

        “The velocity of a wave is proportional to the square root of the elastic forces which cause [its] propagation, and inversely proportional to the mass of the aether moved by these forces.”

        Einstein is referring to the state of displacement of the aether.

        The velocity of a wave is proportional to the square root of the elastic forces which cause its propagation, and inversely proportional to the mass of the aether displaced by these forces.

        The first quote in my original post is referring to a gravitational wave. The ripple created when galaxy clusters collide is a gravitational wave.

        The second quote in my original post is describing the state of the aether as determined by its connections with the galaxy clusters and the state of the aether in neighboring places which is the state of displacement of the aether.

        Aether has mass. Aether is displaced by the particles of matter which exist in it and move through it.

        There is no such thing as non-baryonic dark matter anchored to matter. Matter moves through and displaces the aether.

        What is referred to as the Milky Way’s halo is the state of displacement of the aether.

    • and i thought that “feature” in 0024 is an edge-effect in the mass reconstruction in the small field… (http://adsabs.harvard.edu/abs/2011A%26A…535A.119P). hasn’t stopped the jee paper getting ~100 citations.

      • “The blue streaks near the center of another Hubble image of the cluster are the distorted shapes of more distant galaxies, whose light was bent and magnified by the powerful gravity of Cl 0024+17. … The collision between the two galaxy clusters, the astronomers explained, created a ripple of dark matter that left distinct footprints in the shapes of the background galaxies.”

        ‘[1305.5759] Comment on higher derivative Lagrangians in relativistic theory’
        http://arxiv.org/abs/1305.5759

        “The relativistic theory of an Aether was discussed several time, see for e.g. [8], [9]. In this paper, our hypothesis is different and gives a relativistic theory of the deformation of continuous media (for which the geometry is described by the metric field).”

        The Milky Way’s halo is the deformation of continuous media.

        The Milky Way’s halo is curved spacetime.

        The Milky Way’s halo is evidence of the correctness of relativity.

        The Milky Way’s halo is the state of displacement of the aether.

        The ripple created when the galaxy clusters collided also represents the state of displacement of the aether.

        Aether has mass and is displaced by the particles of matter which exist in it and move through it.

        What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the aether.

        Displaced aether pushing back and exerting inward pressure toward matter is gravity.

        The state of displacement of the aether *is* gravity.

      • The ADS URLs don’t embed automatically in WordPress. One has to explicitly create a link and add some non-link text after it.
        So, I think that a link like this: http://adsabs.harvard.edu/abs/2011A%26A…535A.119P should work.

        I can’t check until submitting, though, since comments can’t be edited. :-(

      • The second try seems to work. Can anyone see why?

      • ‘Offset between dark matter and ordinary matter: evidence from a sample of 38 lensing clusters of galaxies’
        http://arxiv.org/PS_cache/arxiv/pdf/1004/1004.1475v1.pdf

        “Our data strongly support the idea that the gravitational potential in clusters is mainly due to a non-baryonic fluid, and any exotic field in gravitational theory must resemble that of CDM fields very closely.”

        The offset is due to the galaxy clusters moving through the aether. The analogy is a submarine moving through the water. You are under water. Two miles away from you are many lights. Moving between you and the lights one mile away is a submarine. The submarine displaces the water. The state of displacement of the water causes the center of the lensing of the light propagating through the water to be offset from the center of the submarine itself. The offset between the center of the lensing of the light propagating through the water displaced by the submarine and the center of the submarine itself is going to remain the same as the submarine moves through the water. The submarine continually displaces different regions of the water. The state of the water connected to and neighboring the submarine remains the same as the submarine moves through the water even though it is not the same water the submarine continually displaces. This is what is occurring as the galaxy clusters move through and displace the aether.

  8. […] words of caution pushed around. On the blogosphere this is most notable from Matt Strassler and Peter Coles. I endorse their point of view and I think it is fair to say that every scientist we have heard […]

  9. Benjamin Weiner Says:

    Please explain further your comment that the jackknife difference being too close to zero suggests the errors have been underestimated. Typically this suggests that the statistical errors have been _over_ estimated. Maybe it is possible to construct a scenario where the signal is all a systematic and so jackknifing the sample in halves yields zero difference, (ie the _systematic_ errors have been underestimated). But it’s still not clear to me that this would actually explain why the difference would be closer to zero than expected from the statistical errors.

  10. […] Andra mätningar av den kosmiska bakgrundsstrålningen planeras både från rymden och från marken, och de behövs för att reda ut tveksamheter i analysen. Vid Universitetet i Oslo, ett starkt fäste för forskning om den kosmiska bakgrundstrålningen, ställer man frågor om BICEP2-forskarna korrekt uppskattat bakgrundsbruset i sina mätningar: läs Jostein Riiser Kristiansens summering på norska och Phil Bulls kommentarer på engelska. Och från England ser kosmologen Peter Coles flera tecken på saker kan vara fel. […]

  11. […] measurements . The next few months should be very interesting indeed for cosmologists…(see here for a rigorous discussion of the BICEP2 data by Peter […]

  12. […] in der Kosmologie statt, sei es in Blogs mit guten Zusammenfassungen der Grenzen wie Konsequenzen hier, hier und hier, in einer High-End-FB-Gruppe, auf Kolloquien z.B. bei Stanford (Screenshots vom […]

  13. […] am too unlettered in this field to have an opinion about his critiques, except to note Coles saying he would place only an even-money bet that what BICEP2 has found is in fact a cosmological signal. He commends the researchers and likes the fact that they have posted their paper and data for all […]

  14. […] physics-verse has been abuzz this week with news of the BICEP2 experiment’s observations of B-mode polarization in the Cosmic Microwave […]

  15. […] giornalista scientifico con un debole per la provocazione. La seconda, più puntuale, arriva dal blog di Peter Coles, fisico teorico dell’Università di […]

  16. Will Sutherland Says:

    (Sorry, previous comment got mangled again, please delete).

    Clearly, BICEP2 looks like a superbly designed experiment and very good analysis, but there is another reason for caution here… if r is actually close to 0.2, then you can’t remove the tension with previous results just by postulating “Planck has a systematic”…

    I looked around on LAMBDA, and the combination WMAP9+ACT+SPT+BAO fitted to LambdaCDM + r gave
    a limit r less than 0.12 at 95 percent c.l.

    So, if r is near 0.2, then either *two* of the above datasets have a significant systematic, or there is an additional “parameter 8″ such as running spectral index which reconciles these… a moderate coincidence required.

    If r is near 0.1 , then all datasets can agree, but then one can worry that if foregrounds etc can do the other 0.1, why not more ?

    I’m hoping that r is above 0.1 since clearly it’s a fantastic window on inflation if real, but I agree with Peter’s note of caution on this.

  17. Shantanu Says:

    Peter or anyone else: could someone point me to the first paper (or any textbook) where the theoretical spectrum (As a function of l)
    of GWs due to inflation (basically the one shown in Bicep paper)
    is derived.
    Thanks

  18. […] BICEP2: Is the Signal Cosmological? | In the Dark […]

  19. David Brown Says:

    The space roar might be as important as the BICEP2 results. Is there any feedback on the following?
    http://arxiv.org/pdf/1305.7060.pdf “Is there an unaccounted excess Extragalactic Cosmic Radio Background?”, 2013 by Subrahmanyan and Cowsik

  20. […] B-mode experiment, Clover, could have detected what BICEP2 may  have found; I’m still not convinced, by the way. If you’re interested in Clover’s capabilities you can find a nice summary […]

  21. […] analysis of what it means and why we shouldn’t get TOO excited about it yet, see here and here. Now, on with the […]

  22. Readers here might wish to view Clem Pryke’s contribution on this topic at the recent Moriond cosmology meeting.

    Clem’s talk was scheduled quite late, as he didn’t know before registration closed whether he would have finished with the analysis and thus have time to come. As such, it was toward the end of the afternoon break (with 7 hours of talks per day for 6 days, despite the break Moriond meetings have substantially more talks than the typical conference—which is perhaps possible because of the break); nevertheless, almost everyone was there, as well as some people attending a parallel conference on QCD.

    I was fortunate enough not only to be there at his talk, but also to sit next to Clem for dinner on Friday night. The talks (the slides of which, these days, one can find on the web, as here—sometimes even audio or video capture is provided) are only one reason to go to a conference. Others (besides skiing and food) are the questions and discussions after the talks and the opportunity for general face-to-face conversation. (I can dig out a famous quote about a modern conference being similar to a mediaeval pilgrimage.)

    Yes, the internet is good for many things (it’s happened to me more than once that someone recognized me due to my comments here!) and of course the social-network discussion of the BICEP2 results was itself a topic of discussion. However, quite a large number of people (by no means Luddites) were somewhat sceptical of this. At least as much scepticism should be applied to pundits who see a graph and after a couple of minutes start posting on it as is applied to the BICEP2 result itself. Sometimes the impression arises that the BICEP2 team just blindly published a result without thinking about it, then sat back to let the armchair cosmologists weigh in. Nothing could be further from the truth. Probably almost everything other people have thought of was also discussed within the team.

  23. […] I seem to have established myself as an arch-sceptic concerning the cosmological interpretation of the the BICEP2 measurement of the polarization of the […]

  24. […] we already have (there is “tension” between the results, in physics-speak), and some of us still have reserved skepticsm.  The field has been thrown into a big mess, and now we have to figure out what happened. It’s […]

  25. […] il multiverso, ma non penso che sia quello dove viviamo.” Innanzitutto ci sono fisici che hanno fatto le pulci ai dati di BICEP2, indicando vari motivi per cui è bene tenersi cauti e aspettare conferme. Ma per molti fisici il […]

  26. […] il multiverso, ma non penso che sia quello dove viviamo.” Innanzitutto ci sono fisici che hanno fatto le pulci ai dati di BICEP2, indicando vari motivi per cui è bene tenersi cauti e aspettare conferme. Ma per molti fisici il […]

  27. […] sober discussion has been provided by Professor Peter Coles in his ‘In the dark’ blog, who regards confirmation at another frequency as a minimum requirement, and points out some […]

  28. […] sober discussion has been provided by Professor Peter Coles in his ‘In the dark’ blog, who regards confirmation at another frequency as a minimum requirement, and points out some […]

  29. […] il multiverso, ma non penso che sia quello dove viviamo.” Innanzitutto ci sono fisici che hanno fatto le pulci ai dati di BICEP2, indicando vari motivi per cui è bene tenersi cauti e aspettare conferme. Ma per molti fisici […]

  30. […] may recall my scepticism about the recent announcement from the BICEP2 experiment about evidence from polarized microwave […]

  31. […] So there’s been another twist in the story of BICEP2 and whether or not it has actually detected primordial gravitational waves. […]

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