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Has BICEP2 bitten the dust?

Posted in The Universe and Stuff with tags , , , , , , , , , , on June 5, 2014 by telescoper

Time for yet another update on twists and turns of the ongoing saga of  BICEP2 and in particular the growing suspicion that the measurements could be accounted for by Galactic dust rather than primordial gravitational waves; see various posts on this blog.

First there is a Nature News and Views article by Paul Steinhardt with the title Big Bang blunder bursts the multiverse bubble. As the title suggests, this piece is pretty scathing about the whole affair, for two main reasons. The first is to do with the manner of the release of the result via a press conference before the results had been subjected to peer review. Steinhardt argues that future announcements of “discoveries” in this area

should be made after submission to journals and vetting by expert referees. If there must be a press conference, hopefully the scientific community and the media will demand that it is accompanied by a complete set of documents, including details of the systematic analysis and sufficient data to enable objective verification.

I also have reservations about the way the communication of this result was handled but I wouldn’t go as far as Steinhardt did. I think it’s quite clear that the BICEP2 team have detected something and that they published their findings in good faith. The fact that the media pushed the result as being a definitive detection of primordial gravitational waves wasn’t entirely their fault; most of the hype was probably down to other cosmologists (especially theorists) who got a bit over-excited.

It is true that if it turns out that the BICEP2 signal is due to dust rather than primordial gravitational waves then the cosmology community will have a certain amount of egg on its face. On the other hand, this is actually what happens in science all the time. If we scientists want the general public to understand better how science actually works we should not pretend that it is about absolute certainties but that it is a process, and because it is a process operated by human beings it is sometimes rather messy. The lesson to be learned is not about hiding the mess from the public but about communicating the uncertainties more accurately and more honestly.

Steinhardt’s other main point is one with which I disagree very strongly. Here is the core of his argument about inflation:

The common view is that it is a highly predictive theory. If that was the case and the detection of gravitational waves was the ‘smoking gun’ proof of inflation, one would think that non-detection means that the theory fails. Such is the nature of normal science. Yet some proponents of inflation who celebrated the BICEP2 announcement already insist that the theory is equally valid whether or not gravitational waves are detected. How is this possible?

The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests.

This is extremely disingenuous. There’s a real difference between a theory that is “immune to experimental and observational tests” and one which is just very difficult to test in that way. For a start, the failure of a given experiment to detect gravitational waves  does not prove that gravitational waves don’t exist at some level; a more sensitive experiment might be needed. More generally, the inflationary paradigm is not completely specified as a theory; it is a complex entity which contains a number of free parameters that can be adjusted in the light of empirical data. The same is also true, for example, of the standard model of particle physics. The presence of these adjustable degrees of freedom makes it much harder to test the hypothesis than would be the case if there were no such wiggle room. Normal science often proceeds via the progressive tightening of the theoretical slack until there is no more room for manoeuvre. This process can take some time.

Inflation will probably be very difficult to test, but then there’s no reason why we should expect a definitive theoretical understanding of the very early Universe to come easily to us. Indeed, there is almost certainly a limit to the extent that we can understand the Universe with “normal science” but I don’t think we’ve reached it yet. We need to be more patient. So what if we can’t test inflation with our current technology? That doesn’t mean that the idea is unscientific. It just means that the Universe is playing hard to get.

Steinhardt continues with an argument about the multiverse. He states that inflation

almost inevitably leads to a multiverse with an infinite number of bubbles, in which the cosmic and physical properties vary from bubble to bubble. The part of the multiverse that we observe corresponds to a piece of just one such bubble. Scanning over all possible bubbles in the multi­verse, every­thing that can physically happen does happen an infinite number of times. No experiment can rule out a theory that allows for all possible outcomes. Hence, the paradigm of inflation is unfalsifiable.

This may seem confusing given the hundreds of theoretical papers on the predictions of this or that inflationary model. What these papers typically fail to acknowledge is that they ignore the multiverse and that, even with this unjustified choice, there exists a spectrum of other models which produce all manner of diverse cosmological outcomes. Taking this into account, it is clear that the inflationary paradigm is fundamentally untestable, and hence scientifically meaningless.

I don’t accept the argument that “inflation almost inevitably leads to a multiverse” but even if you do the rest of the argument is false. Infinitely many outcomes may be possible, but are they equally probable? There is a well-defined Bayesian framework within which one could answer this question, with sufficient understanding of the underlying physics. I don’t think we know how to do this yet but that doesn’t mean that it can’t be done in principle.

For similar discussion of this issue see Ted Bunn’s Blog.

Steinhardt’s diatribe was accompanied  yesterday by a sceptical news piece in the Grauniad entitled Gravitational waves turn to dust after claims of flawed analysis. This piece is basically a rehash of the argument that the BICEP2 results may be accounted for by dust rather than primordial gravitational waves, which definitely a possibility, and that the BICEP2 analysis involved a fairly dubious analysis of the foregrounds. In my opinion it’s an unnecessarily aggressive piece, but mentioning it here gives me the excuse to post the following screen grab from the science section of today’s Guardian website:

BICEP_thenandnow

Aficionados of Private Eye will probably think of the Just Fancy That section!

Where do I stand? I can hear you all asking that question so I’ll make it clear that my view hasn’t really changed at all since March. I wouldn’t offer any more than even money on a bet that BICEP2 has detected primordial gravitational waves at all and I’d offer good odds that, if the detection does stand, the value of the tensor-to-scalar ratio is significantly lower than the value of 0.2 claimed by BICEP2.  In other words, I don’t know. Sometimes that’s the only really accurate statement a scientist can make.

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BICEP2: The Dust Thickens…

Posted in The Universe and Stuff with tags , , , , , , on May 29, 2014 by telescoper

Off to a day-long staff training event today so just time to post a quick update on the BICEP2 saga (see various posts on this blog). There’s a new paper on the arXiv today by Flauger, Hill and Spergel. The first part of its rather lengthy abstract reads:

BICEP2 has reported the detection of a degree-scale B-mode polarization pattern in the Cosmic Microwave Background (CMB) and has interpreted the measurement as evidence for primordial gravitational waves. Motivated by the profound importance of the discovery of gravitational waves from the early Universe, we examine to what extent a combination of Galactic foregrounds and lensed E-modes could be responsible for the signal. We reanalyze the BICEP2 results and show that the 100×150 GHz and 150×150 GHz data are consistent with a cosmology with r=0.2 and negligible foregrounds, but also with a cosmology with r=0 and a significant dust polarization signal. We give independent estimates of the dust polarization signal in the BICEP2 region using four different approaches. While these approaches are consistent with each other, the expected amplitude of the dust polarization power spectrum remains uncertain by about a factor of three. The lower end of the prediction leaves room for a primordial contribution, but at the higher end the dust in combination with the standard CMB lensing signal could account for the BICEP2 observations, without requiring the existence of primordial gravitational waves. By measuring the cross-correlations between the pre-Planck templates used in the BICEP2 analysis and between different versions of a data-based template, we emphasize that cross-correlations between models are very sensitive to noise in the polarization angles and that measured cross-correlations are likely underestimates of the contribution of foregrounds to the map. These results suggest that BICEP1 and BICEP2 data alone cannot distinguish between foregrounds and a primordial gravitational wave signal, and that future Keck Array observations at 100 GHz and Planck observations at higher frequencies will be crucial to determine whether the signal is of primordial origin. (abridged)

The foreground analysis done in this paper seems to me to be much more convincing that that presented in the original BICEP2 paper and it confirms that the data as presented can not discriminate between B-modes arising from a polarized foreground component and from the presence of primordial gravitational waves. As I’ve said before (several times now), the press hype surrounding this discovery was a bit premature and we have to wait for observations at other frequencies before a clearer picture emerges through the dust.

UPDATE: A new Nature News and Views Article contains a strong statement by David Spergel to the effect that BICEP2 provides no evidence either for or against the existence of primordial gravitational waves.

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Sakharov Oscillations in Cosmology

Posted in The Universe and Stuff with tags , , , , on May 21, 2014 by telescoper

No time for much of a post today, but I couldn’t resist commenting on something I picked up from Twitter just now. Today is the 93rd anniversary of the birth of the nuclear physicist and dissident Andrei Dmitrievich Sakharov who died in 1989. Sakharov is probably more famous for his political campaigning and the award of the Nobel Peace Prize in 1975 than for his work in physics, but I couldn’t resist mentioning a classic paper by him which was first published in Russian in 1965.

Here is the abstract:

Sakharov

The importance of this remarkable paper for modern cosmology can’t be overstated, although many modern cosmologists have either forgotten it or were never aware of it in the first place. The details are a bit out of date, but the idea that density perturbations that grew by a process of gravitational instability to form galaxies and the large-scale structure of the Universe has survived almost fifty years, and plays a central role in the standard cosmological model. Moreover, the Sakharov Oscillations predicted in this paper manifest themselves in the temperature fluctuations of the cosmic microwave background as measured by, e.g., the Planck experiment:

Planck_power_spectrum_orig

The wiggles in the power spectrum plotted above appear because these fluctuations, generated in the modern theory during an episode of cosmic inflation, are set up in phase and thus reach the epoch of scattering at different phases of their oscillation and hence with different amplitudes. The detailed behaviour of the spectrum displayed above tells us a huge amount about the composition and evolution of the Universe.

When Francesco Lucchin and I were writing the first edition of our cosmology textbook (second edition here) we were careful to acknowledge Sakharov’s role in the development of cosmological theory, which wasn’t generally reflected in texts written outside Russia. I particularly recall the late Leonid Grischuk banging on about Sakharov’s work at many conferences in order to ensure he got proper credit and some books, e.g. Zel’dovich and Novikov’s two-volume Relativistic Astrophysics, do acknowledge him correctly. Somehow, however, the CMB wiggles never acquired the name of Sakharov; the peaks in the spectrum are often still called Doppler Peaks or Acoustic Peaks, when surely they should be Sakharov Peaks. It’s probably too late to change the nomenclature now, but there you go.

Anyway, I’ve now realized that I was working on the First Edition of Coles & Lucchin in 1994 which is now twenty years ago so before I get too depressed about the passage of time I’ll stop writing and get on with something else!

Anyway, I’ve now realized that

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That BICEP Rumour…

Posted in The Universe and Stuff with tags , , , on May 14, 2014 by telescoper

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

This time it is a blog post on a site called Résonaances by Adam Falkowski who alleges that the BICEP2 team has made a pretty astounding error in their analysis of the data. This suggestion has been picked up by a couple of fairly influential science news sites (here and here). The BICEP2 team deny having made any such error and are quoted in the news stories defending their results in robust terms.

Before I continue let me make it clear that I stand by the scepticism I have expressed on this blog about this result (which, in fact, is shared by many of my colleagues in the cosmology fraternity1). The problem is that the measurement is made at a single frequency (150 GHz) and it is by no means clear on that basis whether it has the black-body spectrum that would characterize it as being associated with the cosmic microwave background rather than some sort of foreground emission. At 150 GHz the major worry is that polarized emission from galactic dust might contribute significantly to the signal, and might even swamp any primordial contribution.

Anyway, the blog post states that:

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.

Here’s the map concerned as it appeared in the conference talk as presented on the blog post:

culprit

The point about this is that dust emission increases with frequency, so that at 353 GHz it would be expected to dominate the primordial cosmic microwave component. However, if one can measure the polarized component of this emission at high frequency (where it is larger and consequently easier to measure) then one could try to estimate the polarized contribution at the lower frequency measured at 150 GHz by BICEP2 by assuming it has a similar polarized fraction. This is actually just about the only way to estimate the foreground contribution.

Unfortunately in this map there is an additional unpolarized foreground arising from the Cosmic Infrared Background (CIB) which comprised integration dust emission from extragalactic sources. Including this component makes the polarized fraction look lower than it would if it were separated out and only the more highly polarized Galactic contribution considered. In other words including the CIB leads to an underestimate of the polarized fraction and consequently an underestimate of the foreground contamination at 150 GHz.

So now there are three issues:

  1. Did BICEP2 actually use this digitized image to estimate the polarized foreground for their experiment?
  2. Did they make the error of which they have been accused?
  3. Does this invalidate the BICEP2 announcement?

The answer to (1) is that I don’t know for sure but it’s certainly possible that they did. It sounds a pretty ropey approach, but the Planck data are not publicly available so they had to improvise. Even if (1) is the case, I am not at all sure that (2) is true. They may have, but in their responses to the suggestion they have denied it. It seems such a silly error that I’d be surprised, but that doesn’t in itself make it untrue.

However, even if (1) and (2) are the case that doesn’t mean that (3) is true, i.e. it does not imply that the entire analysis presented by BICEP2 is wrong. They have several different estimates of the foreground contribution using other methods so the entire result clearly does not stand or fall on the basis of the use of this particular map in a particular way.

I repeat what I’ve said before in response to the BICEP2 analysis, namely that the discussion of foregrounds in their paper is disappointing. I’d also say that I think the foreground emission at these frequencies is so complicated that none of the simple approaches that were available to the BICEP2 team are reliable enough to be convincing. My opinion on the analysis hasn’t therefore changed at all as a result of this rumour. I think BICEP2 has definitely detected something at 150 GHz but we simply have no firm evidence at the moment that it is primordial. That will change shortly, with the possibility of other experiments (specifically Planck, but also possibly SPTPol) supplying the missing evidence.

I’m not particularly keen on the rumour-mongering that has gone on, but then I’m not very keen either on the way the BICEP2 result has been presented in some quarters as being beyond reasonable doubt when it clearly doesn’t have that status. Yet.

Rational scepticism is a very good thing. It’s one of the things that makes science what it is. But it all too easily turns into mudslinging.

Note: 1 I use the word “fraternity” in the sense given in the Chambers Dictionary as “any set of people with something in common” rather than as “an all-male N American college association”. Cosmology is neither “all-male” nor exclusively American and I did not mean to imply either by my use of English.

 

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A Plug for Some Research…

Posted in The Universe and Stuff with tags , , , , on May 12, 2014 by telescoper

Very busy today so I just thought I’d give a bit of publicity to a paper that’s just been accepted for publication. I’m actually one of the authors, but the other guys (Dipak Munshi of Sussex, Bin Hu of Leiden, Alessandro Renzi of Rome, and Alan Heavens of South Kensington Technical Imperial College) did all the work! I’m posting it mainly to remind myself that there is a world outside of administration. If it weren’t for my inestimable (STFC-funded) postdoc, Dipak Munshi, I don’t know where my research would be!

Here is the abstract:

We use the optimised skew-spectrum as well as the skew-spectra associated with the Minkowski Functionals (MFs) to test the possibility of using the cross-correlation of the Integrated Sachs-Wolfe effect (ISW) and lensing of the cosmic microwave background (CMB) radiation to detect deviations in the theory of gravity away from General Relativity (GR). We find that the although both statistics can put constraints on modified gravity, the optimised skew-spectra are especially sensitive to the parameter B0   that denotes the the Compton wavelength of the scalaron at the present epoch. We investigate three modified gravity theories, namely: the Post-Parametrised Friedmanian (PPF) formalism; the Hu-Sawicki (HS) model; and the Bertschinger-Zukin (BZ) formalism. Employing a likelihood analysis for an experimental setup similar to ESA’s Planck mission, we find that, assuming GR to be the correct model, we expect the constraints from the first two skew-spectra, S(0)ℓ   and S(1)ℓ, to be the same: B0 <0.45  at 95  confidence level (CL), and B0 <0.67  at 99  CL in the BZ model. The third skew-spectrum does not give any meaningful constraint. We find that the optimal skew-spectrum provides much more powerful constraint, giving B0 <0.071  at 95  CL and B0 <0.15  at 99  CL, which is essentially identical to what can be achieved using the full bispectrum.

It’s part of a long sequence of papers emanating from work done by Dipak (with various combinations of co-authors, including myself) which have been aimed at optimising the use of statistical techniques for detecting and quantifying possible departures from the standard model of cosmology using various kinds of data; in this case the paper is entitled Probing Modified Gravity Theories with ISW and CMB Lensing; `ISW means the Integrated Sachs-Wolfe Effect and CMB is the cosmic microwave background. This kind of work doesn’t have the glamour of some cosmological research – I don’t think we’ll be writing a press release when it gets published! – but it is the kind of preparatory analysis that is essential if cosmologists are to make the most of present and forthcoming observational data, which is why we keep plugging away…

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Illustris, Cosmology, and Simulation…

Posted in The Universe and Stuff with tags , , , , , , on May 8, 2014 by telescoper

There’s been quite a lot of news coverage over the last day or two emanating from a paper just out in the journal Nature by Vogelsberger et al. which describes a set of cosmological simulations called Illustris; see for example here and here.

The excitement revolves around the fact that Illustris represents a bit of a landmark, in that it’s the first hydrodynamical simulation with sufficient dynamical range that it is able to fully resolve the formation and evolution of  individual galaxies within the cosmic web of large-scale structure.

The simulations obviously represent a tremendous piece or work; they were run on supercomputers in France, Germany, and the USA; the largest of them was run on no less than 8,192 computer cores and took 19 million CPU hours. A single state-of-the-art desktop computer would require more than 2000 years to perform this calculation!

There’s even a video to accompany it (shame about the music):

The use of the word “simulation” always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

So it’s only the third entry that gives the meaning intended to be conveyed by the usage in the context of cosmological simulations. This is worth bearing in mind if you prefer old-fashioned analytical theory and want to wind up a simulationist! In football, of course, you can even get sent off for simulation…

Reproducing a reasonable likeness of something in a computer is not the same as understanding it, but that is not to say that these simulations aren’t incredibly useful and powerful, not just for making lovely pictures and videos but for helping to plan large scale survey programmes that can go and map cosmological structures on the same scale. Simulations of this scale are needed to help design observational and data analysis strategies for, e.g., the  forthcoming Euclid mission.

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Planck versus BICEP2: Round One!

Posted in The Universe and Stuff with tags , , , , , , , on May 6, 2014 by telescoper

You may recall my scepticism about the recent announcement from the BICEP2 experiment about evidence from polarized microwave emission for the existence of primordial gravitational waves generated during a period of cosmic inflation.

Well, in between a couple of meetings this morning, I realised that there’s a paper just out onto the arXiv from the Planck Collaboration. Here’s the abstract:

This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium. We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

There’s also a press release from the European Space Agency which includes this nice picture:

Milky_Way_s_magnetic_fingerprint_large

This study is at 353 GHz, compared to the 150 GHz of the BICEP2 measurements. Galactic dust emission increases with frequency so one would expect more of an effect in this Planck map than in BICEP2, but the fact that polarized foreground emission is so strong at these frequencies does give one pause for thought. The Planck data actually cover the whole sky, so the above map has clearly been censored; below you can see the actual region of the sky covered by BICEP2, so there is little or no direct overlap with what’s been released by Planck:

bicep2_loops

We’ll have to wait until later this year to see what’s going on in the masked regions (i.e. far above and below the Galactic Plane, where the dust emission is presumably weaker) and indeed at the 7 other frequencies measured by Planck. It’s all a bit of a tease so far!

Here’s what the press release says about BICEP2

In March 2014, scientists from the BICEP2 collaboration claimed the first detection of such a signal in data collected using a ground-based telescope observing a patch of the sky at a single microwave frequency. Critically, the claim relies on the assumption that foreground polarised emissions are almost negligible in this region.

Later this year, scientists from the Planck collaboration will release data based on Planck’s observations of polarised light covering the entire sky at seven different frequencies. The multiple frequency data should allow astronomers to separate with great confidence any possible foreground contamination from the tenuous primordial polarised signal.

P.S.  It’s gratifying to see the Planck Collaboration have used extragalactic Faraday Rotation measures to probe the Galactic Magnetic field as I suggested on this blog not long ago. The article that first advocated doing this with CMB maps can be found here.

 

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Galactic Loops as Sources of Polarized Emission

Posted in The Universe and Stuff with tags , , , , , on April 8, 2014 by telescoper

Since I seem to have established myself as an arch-sceptic concerning the cosmological interpretation of the the BICEP2 measurement of the polarization of the cosmic microwave background (CMB), I couldn’t resist posting a link to an interesting paper by Liu et al. that has just appeared on the arXiv.

The abstract is:

We investigate possible imprints of galactic foreground structures such as the `radio loops’ in the derived maps of the cosmic microwave background. Surprisingly there is evidence for these not only at radio frequencies through their synchrotron radiation, but also at microwave frequencies where emission by dust dominates. This suggests the mechanism is magnetic dipole radiation from dust grains enriched by metallic iron, or ferrimagnetic molecules. This new foreground we have identified is present at high galactic latitudes, and potentially dominates over the expected B-mode polarisation signal due to primordial gravitational waves from inflation.

The authors argue that foreground emission from our own Galaxy has not been fully subtracted from maps of the cosmic microwave background. This emission could result in significant contamination of the CMB polarization if it is associated with dust grains aligned with the Galaxy’s magnetic field.

I’m grateful to one of the authors of the paper, Philip Mertsch, for sending me this map of the Galactic Loops with the BICEP2 region superimposed on it, demonstrating that there is potential for a contribution…

bicep2_loops

 

 

This paper is likely to provoke quite a discussion, so I thought I’d suggest one possible way of testing it, namely by updating the analysis presented by myself and Patrick Dineen in 2003 with new data. Here’s the abstract of our old paper:

We present a diagnostic test of possible Galactic contamination of cosmic microwave background sky maps designed to provide an independent check on the methods used to compile these maps. The method involves a non-parametric measurement of cross-correlation between the Faraday rotation measure (RM) of extragalactic sources and the measured microwave signal at the same angular position. We argue that statistical properties of the observed distribution of rotation measures are consistent with a Galactic origin, an argument reinforced by a direct measurement of cross-correlation between dust, free-free and synchrotron foreground maps and RM values with the strongest correlation being for dust and free-free. We do not find any statistically compelling evidence for correlations between the RM values and the COBE DMR maps at any frequency, so there is no evidence of residual contamination in these CMB maps. On the other hand, there is a statistically significant correlation of RM with the preliminary WMAP individual frequency maps which remains significant in the Tegmark et al. Wiener-filtered map but not in the Internal Linear Combination map produced by the WMAP team.

The idea is that cross-correlating the CMB pattern with Faraday rotation measures should provide an independent diagnostic of the effect of magnetic fields. Our analysis was based on old CMB data, so there’s an interesting project to be done updating it with, e.g., Planck CMB data and a larger set of rotation measures. See the comment below for a reference to more recent work along these lines, but still not including Planck.

Anyway, this all goes to show that there’s one question you can always ask about an astrophysics result: have you considered the possible role of magnetic fields?

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BICEP2: Is the Signal Cosmological?

Posted in The Universe and Stuff with tags , , , , , on March 19, 2014 by telescoper

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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BICEP2DAY

Posted in Astrohype, The Universe and Stuff with tags , , , , on March 17, 2014 by telescoper

Well, it’s official that this afternoon’s announcement of a “major discovery” is going to be from the BICEP team, and it specifically concerns the BICEP2 CMB telescope experiment. I’ve just got back to Sussex (after a weekend in Cardiff) and will be following the events in among other things I have to do before going off to give a lecture at 5pm GMT.

The schedule of events is as follows: there will be 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 http://www.cfa.harvard.edu/news/news_conferences.html

Papers and data products will be available at 10:45 EDT from http://bicepkeck.org/

EDT is four hours behind Greenwich Mean Time so the webcast will begin at 14:45 GMT, i.e. in about half an hour.

In the mean time, for those of you wondering what these BICEPS are all about, here is a useful graphic in which a Harvard astrophysicist demonstrates the possibilities:

biceps

LIVE BLOG:

14:36 The press conference server has gone down. There’s no truth in the rumour that ex-members of the Clover collaboration have sabotaged it.

14:42 There’s a grave danger that this press conference will run into tea time.

14:45 The BICEP2 papers are now live at http://bicepkeck.org/

14:48 Straight to the headline: R=0.2 (+0.07, -0.05) with R=0 rejected at about 7 sigma, if you like things stated in such terms…

14:53 Here’s the crucial graph. Results a bit higher than the expected  signal at l in range 200-300?

Bi7-IpDCEAA7frU

15:06 The news avalanche has started, e.g. here at the BBC, but there is some concern about the shape of the spectrum.

15:10 I’m not getting anything from the press conference, so may have missed important details. It seems to me though that there’s a significant possibility of some of the polarization signal in E and B not being cosmological. This is a very interesting result, but I’d prefer to reserve judgement until it is confirmed by other experiments.

15:35 Despite the press hype there’s still some scepticism among cosmologists arising from the strange-looking shape of the spectrum. I’m not convinced myself. Anyway, I have to sign off now in order to prepare a lecture..

16:20 Back-of-the-envelope time: if the result is correct then the inflationary energy scale is about 2×1016 GeV. That’s just two orders of magnitude below the Planck scale…

18:19 Returned from my 5pm Theoretical Physics lecture. Couldn’t resist spending 30 minutes talking about BICEP2, though I did tell them it’s not in the examination.

18:25 Main points of controversy:

  1. there seems to be evidence of leakage of temperature into polarization (lines in Fig. 5);
  2. there’s an excess in the B-B spectrum at l~250 shown above;
  3. there’s an excess at low l in the E-E spectrum
  4. there’s a deficit at low l in the cross-correlation with Keck

There may be a connection between 1. and 2.-4. If 2.-4 are real then they may be evidence of something interesting that requires more than a straightforward modification of inflation (such as might include just a running of the spectral index).

18:35 Other controversy: why has this result been announced before the paper has been published or even peer-reviewed?

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