Weekend plot: BICEP limits on tensor modes

 The insurgency gathers pace. This weekend we contemplate a plot from the recent paper of Michael Mortonson and Uroš Seljak:

It shows the parameter space of inflationary models in the plane of the spectral index ns vs. the tensor-to-scalar ratio r. The yellow region is derived from Planck CMB temperature and WMAP polarization data, while the purple regions combine those with the BICEP2 data. Including BICEP gives a stronger constraint on the tensor modes, rather than a detection of r≠0.

The limits on r from Planck temperature data are dominated by large angular scales (low l) data which themselves display an anomaly, so  they should be taken with a grain of salt. The interesting claim here is that BICEP alone does not hint at r≠0, after using the most up-to-date information on galactic foregrounds and marginalizing over current uncertainties. In this respect, the paper by Michael and Uroš reaches similar conclusions as the analysis of Raphael Flauger and collaborators. The BICEP collaboration originally found that the galactic dust foreground can account for at most 25% of their signal. However, judging from scavenged Planck polarization data, it appears that BICEP underestimated the dust polarization fraction by roughly a factor 2. As this enters in square in the B-mode correlation spectrum, dust can easily account for all the signal observed in BICEP2. The new paper adds a few interesting details to the story. One is that not only the normalization but also the shape of the BICEP spectrum can be reasonably explained by dust if it scales as l^-2.3, as suggested by Planck data. Another is the importance of gravitational lensing effects (neglected by BICEP) in extracting the signal of tensor modes.  Although lensing dominates at high l, it also helps to fit the low l BICEP2 data with r=0. Finally, the paper suggests that it is not at all certain that the forthcoming Planck data will clean up the situation. If the uncertainty on the dust foreground in the BICEP patch is of order 20%, which look like a reasonable figure, the improvement over the current sensitivity to tensor modes may be marginal. So, BICEP may remain a Schrödinger cat for a little while longer.

Follow up on BICEP

The BICEP2 collaboration claims the discovery of the primordial B-mode in the CMB at a very high confidence level.  Résonaances recently reported on the chinese whispers that cast doubts about the statistical significance of that result.  They were based in part on the work of Raphael Flauger and Colin Hill, rumors of which were spreading through email and coffee time discussions. Today Raphael gave a public seminar describing this analysis, see the slides and the video.

The familiar number r=0.2 for the CMB tensor-to-scalar ratio is based on the assumption of zero foreground contribution in the region of the sky observed by BICEP. To argue that foregrounds should not be a big effect, the BICEP paper studied several models to estimate the galactic dust emission. Of those, only the data driven models DDM1 and DDM2 were based actual polarization data inadvertently shared by Planck. However, even these models suggest that foregrounds are not completely negligible. For example, subtracting the foregrounds estimated via DDM2 brings the central value of r down to 0.16 or 0.12 depending how the model is used (cross-correlation vs. auto-correlation). If, instead,  the cross-correlated  BICEP2 and Keck Array data are used as an input, the tensor-to-scalar ratio can easily be below 0.1, in agreement with the existing bounds from Planck and WMAP.

Raphael's message is that, according to his analysis, the foreground emissions are larger than estimated by BICEP, and that systematic uncertainties on that estimate (due to incomplete information, modeling uncertainties, and scraping numbers from pdf slides) are also large. If that is true, the statistical significance of the primordial B-mode  detection is much weaker than what is being claimed by BICEP.

In his talk, Raphael described an independent and what is the most complete to date attempt to extract the foregrounds from existing data. Apart from using the same Planck's polarization fraction map as BICEP, he also included the Q and U all-sky map (the letters refer to how polarization is parameterized), and models of polarized dust emission based on  HI maps (21cm hydrogen line emission is supposed to track the galactic dust).  One reason for the discrepancy with the BICEP estimates could be that the effect of the Cosmic Infrared Background - mostly unpolarized emission from faraway galaxies - is non-negligible. The green band in the plot shows the polarized dust emission obtained from the  CIB corrected DDM2 model, and compares it to the original BICEP estimate (blue dashed line).

The analysis then goes on to extract the foregrounds starting from several different premises. All available datasets (polarization reconstructed via HI maps, the information scraped from existing Planck's polarization maps) seem to say a similar story: galactic foregrounds can be large in the region of interest and uncertainties are large.  The money plot is this one:

Recall that the primordial B-mode signal should show up at moderate angular scales with l∼100 (the high-l end is dominated by non-primordial B-modes from gravitational lensing). Given the current uncertainties, the foreground emission may easily account for the entire BICEP2 signal in that region. Again, this does not prove that tensor mode cannot be there. The story may still reach a happy ending, much like the one of  the discovery of accelerated expansion (where serious doubts about systematic uncertainties also were raised after the initial announcement). But the ball is on the BICEP side to convincingly demonstrate that foregrounds are under control.

Until that happens, I think their result does not stand.

Is BICEP wrong?

The BICEP claim of detecting the primordial B-mode in the polarization of the Cosmic Microwave Background was a huge news. If confirmed, it would be an evidence of gravity waves produced during cosmic inflation, and open a window on physics at an incredibly high energy scale of order 10^16 GeV. Possible implications were described in detail in some 300 papers triggered by the BICEP announcement.  But, among this understandable excitement, we have been aware that the signal has to be confirmed by other experiments before the discovery is established. Back then, Résonaances precisely estimated the chances of the signal being true at `fifty-fifty'.  It appears it's the latter fifty that's gaining an upper hand...

Barring a loose cable, the biggest worry about the BICEP signal is that the collaboration may have underestimated the galactic foreground emission. BICEP2 performed the observations at only one frequency of 150 GHz which is very well suited to study the CMB, but less so for polarized dust or synchrotron emission. As for the latter, more can be learned by going to higher frequencies, while combining maps at different frequencies allows one to separate the galactic and the CMB component. Although the patch of the sky studied by BICEP is well away from the galactic plane, the recently published 353 GHz polarized map from Planck demonstrates that there may be significant emission from these parts of the sky (in that paper the BICEP patch is conveniently masked, so one cannot draw any quantitative conclusions). Once the dust from the BICEP announcement had settled, all eyes were thus on precision measurements of the galactic  foreground. The rumors that have been arriving from the Planck camp were not encouraging, as they were not able to confirm the primordial B-mode signal. It seems that experts now put a finger on what exactly went wrong in BICEP.

To estimate polarized emission from the galactic dust, BICEP digitized an unpublished 353 GHz map shown by the Planck collaboration at a conference.  However, it seems they misinterpreted the Planck results: that map shows the polarization fraction for all foregrounds, not for the galactic dust only (see the "not CIB subtracted" caveat in the slide). Once you correct for that and rescale the Planck results appropriately, some experts claim that the polarized galactic dust emission can account for most of the BICEP signal. The rumor is that the BICEP team has now admitted to the mistake [Update: this last statement is disputed and outwardly denied].

Note that we should not conclude that there is no observable tensor modes in the CMB. Indeed, the tensor to scalar ratio of order 0.1 is probably consistent with the existing experimental data, and may be responsible for a part of the B-mode signal detected by BICEP. New data from Planck, POLARBEAR, ACTpole, and Keck Array should clarify the situation within a year from now.  However, at this point, there seems to be no statistically significant evidence for the primordial B-modes of inflationary origin in the CMB.

April Fools'14: 100 TeV collider in the US

Another  good news for high-energy physics: the United States intend to build a new circular proton-proton collider. The plan is to use the existing tunnel in Waxahachie, Texas that was constructed for the SSC collider project canceled in 1993. Thanks to the recent progress in superconducting magnet technology, the new machine, dubbed the NSSC, will be able to reach a much larger center-of-mass energy than originally planned for the SSC. The pre-TDR document released today quotes 80 TeV collision energy with possible upgrades to 100 TeV, which would put it on par with the similar project at CERN. The decision  was first announced at the HEPAP meeting two weeks ago  and today the news article was posted on the DOE web site. The article quotes Ernest Moniz, the US Secretary of Energy,  "We have all we need: the technology, the know-how, and even the tunnel, so it's only natural that we're going to do it".  

This move may be surprising, given the recent funding cuts on fundamental research.  One can speculate that some big politics must be involved. The decision seems to be a response to the quickly advancing plans of building a new high energy collider in China. Apparently, losing the scientific leadership to China would be a too bitter pill to swallow.

Update: This post is obviously an April Fools' joke: the SSC won't be back no more. But the Chinese thingy may well  be for real. The way the wind is blowing, we should start learning Mandarin... 

Weekend Plot: Rorschach Test

This weekend you are invited to contemplate a plot which, for obvious reasons, is not available on arXiv. Here is a plot by the PVDIS collaboration recently published, quite suitably, in the Nature magazine:

To understand what is depicted here we need to make a slight detour. The Standard Model of particle physics is being tested not only in high-energy colliders, but also via precision measurements at low energies. One important class of precision experiments  goes under the name parity violating electron scattering, PVES in short.  This enterprise consists in shooting a beam of polarized  electrons at various targets:  hydrogen, deuteron, spiders on the wall, etc. In the language of effective theory, the process can be described by 4-fermion operators with 2 electron and 2 quarks fields. For PVES experiments, the relevant operators are those that violate the parity symmetry, for example

There is nothing exotic here: these 4-fermion operators  are predicted by the Standard Model, as they are effectively induced by the Z boson exchange between electrons and quarks. Their effect is that left- and right-handed polarized electrons interact differently with quarks. Thus, they lead to  different  cross sections for left- and right-hand electron scattering on atoms. The left-right asymmetry  -- the difference of these two cross section normalized to the sum -- is a convenient observable to measure in a experiment. In fact, the asymmetry was experimentally observed already back in 1978 at SLAC, and at the time it was an important confirmation of the Standard Model structure of weak interactions.  Nevertheless, it makes sense to measure the asymmetry (and therefore the parameters Cq) more precisely  so as to test the Standard Model predictions. There are two ways new physics could affect this observable. On one hand, the couplings of the Z boson to electrons and/or quarks could be modified, and then the Z boson exchange diagrams would lead to different coefficients Cq than the ones predicted by the Standard Model. On the other hand, new heavy vector boson with mass M interacting with  electrons and quarks with strength g would induce contact interactions between the two that would effectively shift the parameter Cq by the amount  of order g^2 v^2/M^2.

Now the parameters C1u and C1d have been measured quite precisely, with 3% accuracy, and they agree well with the Standard Model predictions (see here for the most recent update). This can be translated as order 1% constrains on the Z boson couplings  to quarks and leptons. It is worth stressing that for the couplings to quarks these are the most stringent constraints to date (better than the ones from LEP) so these experiments are really exploring new territories.
   
The parameters C2u and C2d are more tricky because, in order to access them in scattering experiments, one needs to resolve the internal structure of the target atom.  Here enters the PVDIS experiment at Jlab. They studied the left-right asymmetry of  deep-inelastic polarized electron scattering on deuterons.  They were able to demonstrate, for the first time, that a particular combination 2 C2u - C2d is non-zero, in agreement with the Standard Model. The resulting constraints on the Z-boson couplings are weaker than those coming from previous experiments. However, since they probe different operators, the provide non-trivial constraints on some new physics, in particular those that  produces only C2q and not C1q. The plot reproduced at the top of this post is a more graphic presentation of their results.  In the plot, the combined constraints from several low-energy experiments  are recast in terms of the scale Λ=Mass/coupling of new physics that contributes to the C1q and C2q coefficients. The yellow and red regions show the new physics reach before and after including the PVDIS results. The exciting thing about the plot is  that it explicitly shows  these experiments probe the mass scales of order 10 TeV, which is well  beyond the reach of the LHC.  

Complementary constraints from ATLAS can be found here.

Weekend Plot: axing axions

Assuming the B-modes observed by BICEP2 originate from the metric field fluctuating during inflation, implications are profound. Obviously, inflationary models are now constrained to reproduce the energy scale 10^16 GeV driving the expansion (equivalently, the Hubble scale during inflation 10^14 GeV). But the consequences extend to more general new physics scenarios that a-priori have nothing to do with inflation. One prominent example is high-scale axions. Axions are hypothetical particles that pop up in many theories: as a solution to the strong-CP problem in QCD, as a dark matter candidate, as a prediction of string theory, etc. Here is a snapshot of the axion parameter space after BICEP2:

The plot shows the maximum fraction of dark matter that high-scale axions can account for versus the axion mass. If the axion scale f is larger than 10^14 GeV then the axion field can wildly fluctuate during inflation. Axion fluctuations are uncorrelated with those of the inflaton field, and give rise to the so-called isocurvature perturbations where density fluctuations in radiation and matter add up to zero. The isocurvature perturbations, in turn,  are severely constrained by Planck's CMB measurements: their power has to be less than 4% of the adiabatic perturbations produced by the inflaton field.  Hence the severe constraint on the axion: its initial displacement angle  has to be small enough as to suppress the oscillation amplitude. But the same displacement angle gives rise to the production of axion dark matter. Combining these two inputs, we learn from BICEP2 that high-scale axions alone cannot account for the observed abundance of dark matter in the universe. Moreover, even giving up on axion dark matter, one has to fine-tune the initial displacement of the axion fields to a tiny value (of order 10^-8 for f = 10^16 GeV).      

Of course, like for any model that crashed after the BICEP2 announcement, Microsoft-style patches are already available. In this case, one solution is simply to take f < 10^14 GeV, that is the global symmetry whose breaking gives rise to the axion particle gets broken only after the inflation ends  (although this scenario has problems of its own). Another  possible fix is to ensure the axion acquires a large mass (>10^14 GeV) during inflation, or that the isocurvature modes were diluted by late-time entropy production. Nevertheless, high-scale axions are clearly less motivated than they were a week ago.

Erratum: 
A commenter pointed out that the results in this plot disagree with other literature on the 

subject. The point is that, for the axion scale larger than the Hubble scale during inflation,  the minimum displacement angle is of order H/f. For the relevant scales this always produces too much isocurvature perturbations.  Thus, the conclusions from BICEP2 are stronger than what I wrote above: high scale axions are excluded (up to the caveats in the previous paragraph) irrespectively of any assumptions about the initial displacement angle. The plot on the right visualizes the situation for the QCD axion.   The yellow region is excluded by astrophysical and CMB constraints, while the green  region  corresponds to the BICEP2 measurement of the Hubble scale during inflation. The QCD axion is now constrained to a narrow window of 10^9 ≤ f ≤ 10^11 GeV. At the top of this window it accounts for all dark matter in the universe. 

Curly impressions

Today the BICEP2 experiment announced a significant detection of the primordial B-model in the CMB power spectrum (the excess at lower multipoles in the plot). From that one can infer that the tensor-to-scalar ratio of primordial fluctuations is somewhere between 0.1 and 0.2. Cosmologists are strongly represented in the blogosphere, so for the description of technical aspects of the BICEP results and their impact on the models of inflation better see elsewhere, for example here or here or here.  In this post, let me throw in a few random impressions from a particle physicist.  

• If this holds up, it's huge, comparable in magnitude to the discovery of the Higgs boson. Probably even more exciting because of the surprise element.  
• "If this holds up" is the central question now. This sort of  experiments is subject to pesky instrumental effects and systematic effects due to foreground emission.  It's not impossible that BICEP screwed up;  in fact, experts point out some worrying aspects of the data, for example the excess in the BB spectrum at high multipoles. So I would say at this point it's fifty-fifty.  Fortunately, there are many experiments  out there with similar sensitivity (Planck, ACTPole, SPT, POLARBEAR) that should be able to  confirm or refute the claim in the near future. In particular, the release of Planck polarization data this year should straighten many things out. For the sake of this post I'll drop the conditional, but there really should be "If this holds up" in front of every sentence below. 
• The big thing here is not gravitational waves (we observed them before), and not an evidence for inflation (we've already had a few from the CMB alone:  the temperature isotropy across the sky, the scalar fluctuations, the spectral index). The point is that the amplitude of the primordial tensor modes is directly related to the energy density during inflation. This turns out to be (2*10^16 GeV)^4 -- a whopping energy scale unavailable to particle accelerators in this century. Consequently, the Hubble scale during inflation has also been nailed down, and it's 10^14 GeV. While cosmologist  study the universe when it was being born, particle physicists are getting a glimpse of  physics at a very high-energy scale. 
• The proximity of the inflation scale to the unification scale in minimal supersymmetry is certainly intriguing. It may be a numerical accident.... but I'm sure that there will soon be models on arXiv  "predicting" this coincidence. 
• The plot shows the basic parameters of inflation as of today. 
  
   If you think that the Planck region is not what you remember, that's right, it's not the one usually shown. The red region in this plot is the Planck constraint when the spectral index ns is allowed to run, that is to say, to depend on the distance scale. It is a challenge for  inflationary models to get the required amount of running. In the more likely scenario with small running the BICEP result is in about 3 sigma tension with the Planck constraints.   
• There's more challenge for inflation model building. In the single-field inflation one can relate how much the inflaton field was displaced during inflation to the value of r and the number of e-folds (the so-called Lyth bound). For r~0.2 one finds that the displacement is larger than the Planck scale. For particle physicists, who generally like effective field theories,  the focus now will be on identifying all exceptions from this rule. But from another (e.g. string-theoretical) point of view this is an opportunity: may be inflation can be our probe of transplanckian physics? In the coming week there will surely  be an arXiv flood on both  of these fronts.  
• Speaking about model building, Higgs inflation is ruled out, at least in the current version. A robust prediction of Higgs inflation is no tensor modes at an observable level. In other words, we have a new evidence for new physics beyond the Standard Model. 
• It is worth remembering that the gravitational waves during inflation is the most plausible but not the unique explanation of BICEP results. For example, an early  phase transition or decay of massive particles during inflation may also  generate tensor perturbations. That's another model building direction worth following in the coming weeks.   
• If you hear a sledgehammer in the corridor of your lab, that may be your local Planck member banging his head on the wall. Yeah,  apart from many noble aspects, science also has this lowly competition side. A billion dollar experiment that misses a Nobel-prize-worth low-hanging fruit... I wouldn't wish  to be in their skin if BICEP is right.  
• One more thing we learned from the BICEP announcement: mankind can study the universe moments after the bing bang, but setting up an internet connection is a totally different story ;)  

The BICEP2 paper is here, for more material see this page.