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Copenhagen, Cosmology and Coleman Hawkins

Posted in Biographical, Jazz, The Universe and Stuff with tags , , on August 18, 2014 by telescoper

Now that I’ve finally checked into my hotel in the wonderful city of Copenhagen I thought I’d briefly check in on the old blog as well. I’m here once again for a meeting, this time as an invited speaker at the 2nd NBIA-APCTP Workshop on Cosmology and Astroparticle Physics; NBIA being the Niels Bohr International Academy (based in Denmark) and APTCTP being the Asia Pacific Centre for Theoretical Physics (based in Korea). This is the kind of meeting I actually like, with relatively few participants and lots of time for discussion; as a welcome gesture for the first day there was also free beer!

I decided for some reason to try an experimental route getting here. There wasn’t a flight at a convenient date and  time from Gatwick, the nearest airport to my Brighton residence, so I decided to get an early morning flight from Heathrow instead. The departure time of 06:40, however, left me with the difficulty of getting there in time by public transport as the relevant trains don’t run overnight. I toyed with the idea of booking an airport hotel for the night, but decided that would be extravagant so instead opted to get a coach from Brighton; this was cheap and comfortable – only a handful of other passengers got on the bus – and got me there right on schedule. The downside was that I had to catch the 01:40 from Brighton Coach Station, which arrived at about 4am at Heathrow Terminal 3. It was quite interesting finding the normally busy terminal almost deserted but although I did a self-service check-in straight away the bag drops didn’t open until almost 5am. None of the cafes in the check-in area were open, so I had to hang around for an hour before finally getting rid of my luggage and passing through to the airside whereupon I nabbed some coffee and a bite to eat.

The flight was almost uneventful. Unfortunately, however, as we came in to land at Copenhagen’s Kastrup airport, a young person sitting behind me vomited uncontrollably and at considerable length, producing a steady flow both of chunder and unpleasant noises. The aftermath was quite unpleasant, so I was quick out of the blocks when the plane finally came to a stop at the gate. An aisle seat turned out to have been a wise choice.

Assuming it would be too early to check into the hotel that had been booked for me, I decided to go straight to the meeting but got to the Niels Bohr Institute’s famous Auditorium A near the end of the first talk, about the Imprint of Radio Loops on the CMB (a subject I’ve blogged about), which is a shame because (a) its interesting and (b) some of my own work was apparently discussed. That happens so rarely these days I’m sorry I missed it.

I was a bit tetchy as a result of my sleepless night, though I limited the expression of this to a  couple of rants about frequentist statistics during the discussions.

After the free beer I finally made my way to the hotel and checked in. It’s not bad, actually. There can’t be that many hotel rooms that have a picture of the great tenor saxophonist Coleman Hawkins on the wall:

SAMSUNG DIGITAL CAMERA

Anyway, I was due to give the conference summary on Friday but I’ve been moved forward to Wednesday so I’d better think of something to say. Maybe in the morning though, I could do with an early night…

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BICEP2 Redux: How the Sausage is Made

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

I came across this (rather lengthy, but very good) discussion of the BICEP2 story so far so thought I would share it here. There’s a particularly useful collection of articles at the end for those who would like to read more.

I’ll also take this opportunity to refer you to a recent BBC News story which states that the BICEP2 and Planck teams are now in discussions about sharing data. About time, if you ask me. Still, it will take a considerable time to work out the ordering of the authors if they ever do write a paper!

Glen Mark Martin's avatarWhiskey...Tango...Foxtrot?

An ongoing problem with communicating science to the general public is the existence of widely-held misconceptions among the public regarding how science actually works. A case in point is the March 17 announcement by the BICEP2 Collaboration regarding the detection of B-Mode polarization in the Cosmic Microwave Background and the events which have unfolded since then.

All too often, news stories and blog posts will trumpet some announcement with sensational headlines like “Scientists Say Cheap, Efficient Solar Cells Just Around the Corner”, or “Scientists Close in on Cure for Cancer.” Many people take such announcements at face value and consider the case closed. The work has been done.  The reality of the situation, however, is that the initial announcement of a discovery or breakthrough is just the beginning of the hard work, breathlessly hyped headlines notwithstanding.

How Science Actually Works (or at least how it is supposed to work)

Once…

View original post 2,843 more words

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The Zel’dovich Universe – Days 5 & 6 Summary

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

Well, it’s Sunday morning and it’s raining in Tallinn. I’ve got a few hours to kill but fortunately don’t have to check out of the hotel until noon so I thought I’d briefly summarize Days Five and Six of IAU Symposium No. 308, The Zel’dovich Universe just to complete the story.

Day Five (Friday) began with a talk by Jaan Einasto, recent winner of the Gruber Prize for Cosmology. As you can see from this picture I took before his talk commenced,  the topic was Yakov Zel’dovich and the Comic Sans Cosmic Web Paradigm:

IMG-20140627-00354

The following talk was by the ebullient Rashid Sunyaev, whose name is associated with Zel’dovich in so many contexts, including the Sunyaev-Zeld’ovich effect. Sunyaev is such a big personality that he is unconstrained by the banal notions of time, and his talk set the schedule back for the rest of the morning. Among the things I remember from his contribution was a discussion of the Berkeley-Nagoya distortion. This was a hot topic during the time I was a graduate student, as it was a measurement that suggested the spectrum of the cosmic microwave background departed significantly from a black-body (Planck) curve in the Wien part of the spectrum; this is now usually known as a y-distortion. Anyway, lots of theorists wrote papers explaining the measured excess in terms of this that and the other and then it was shown to be an error; the excess emission came not from the Big Bang but from the exhaust of the rocket carrying the measurement. The thing I remember most strongly about this was that as soon as the error was identified it ceased to be the Berkeley-Nagoya distortion and became instead the Nagoya-Berkeley distortion…

Rashid Sunyaev was himself a winner of the Gruber prize some years ago, as indeed were Dick Bond and Brent Tully who spoke erlier in the conference, so the organizers decided to form a Gruber-panel to discuss various topics suggested by the audience. Here is Sunyaev, hogging the microphone:

IMG-20140627-00355

Carlos Frenk is also a Gruber prize winner, but he only arrived after lunch so wasn’t part of this discussion. The afternoon was all about cosmological simulations of various aspects of the Cosmic Web. This gives me an opportunity to repeat 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.

In the World Cup players can even get sent off for simulation, although regrettably they seldom are.

Anyway, Friday evening found us at the famous House of Blackheads (aptly on Pikk Street) for an evening of very long speeches punctuated by small amounts of food and wine (and of course some very lovely music as I described yesterday). When the party was over a group of us adjourned to a local bar, from which I returned to my hotel at about 2am.

Day Six was a half-day, with some very interesting talks about gravitational len-sing in the first session and “superstructures” in the cosmic web. Then we were into the final furlong as it were. Nick Kaiser was put in Session (No. 21) all of his own. As usual, given how annoyingly brilliant he is, Nick gave  fabulously interesting talk full of insights and ideas. The organizers had definitely saved the best for second-to-last.

Then, after five-and-a-half days and almost 100 talks, it was down to me to give the conference summary. Obviously I couldn’t really summarize all that such I just picked up a few things that occurred to me during the course of the conference (some of which I’ve written about over the last week or so on this blog) and made a few jokes, primarily at the expense of Carlos Frenk. I was interested to see that signs like this had been put up around Tallinn advertising my talk:

IMG-20140628-00356

The OMG and WOW are self-explanatory, but I was a bit confused about the SAH so I googled it and found that it means the Society of Architectural Historians. I’ve never heard it put quite like that before, but I guess that’s what we cosmologists are: trying to understanding the origins and time evolution of the architecture of the Universe.

A number of speakers at this conference referred to a conference in Hungary in 1987 at which they had met Zel’dovich (who died later that year). I was a graduate student (at Sussex) at that time and owing the shortage of travel funds I wasn’t able to go; I went to a meeting in Cambridge called The Post-Recombination Universe instead. If memory serves that’s when I gave my first conference talk. Anyway, Carlos Frenk gave a talk at that meeting in Hungary which he decribed in his talk at this conference on Friday afternoon. Somebody back in 1987 had written a series of limericks to describe that meeting, so I was challenged to come up with one to conclude this one. Here’s my effort, which is admittedly pretty feeble, but at least the sentiments behind it are genuine..

In Tallinn (IAU 308)
The sessions invariably ran late
But despite being tired
We still much inspired
By Yakov Zel’dovich (the Great).

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The Power Spectrum and the Cosmic Web

Posted in Bad Statistics, The Universe and Stuff with tags , , , , , , on June 24, 2014 by telescoper

One of the things that makes this conference different from most cosmology meetings is that it is focussing on the large-scale structure of the Universe in itself as a topic rather a source of statistical information about, e.g. cosmological parameters. This means that we’ve been hearing about a set of statistical methods that is somewhat different from those usually used in the field (which are primarily based on second-order quantities).

One of the challenges cosmologists face is how to quantify the patterns we see in galaxy redshift surveys. In the relatively recent past the small size of the available data sets meant that only relatively crude descriptors could be used; anything sophisticated would be rendered useless by noise. For that reason, statistical analysis of galaxy clustering tended to be limited to the measurement of autocorrelation functions, usually constructed in Fourier space in the form of power spectra; you can find a nice review here.

Because it is so robust and contains a great deal of important information, the power spectrum has become ubiquitous in cosmology. But I think it’s important to realise its limitations.

Take a look at these two N-body computer simulations of large-scale structure:

The one on the left is a proper simulation of the “cosmic web” which is at least qualitatively realistic, in that in contains filaments, clusters and voids pretty much like what is observed in galaxy surveys.

To make the picture on the right I first  took the Fourier transform of the original  simulation. This approach follows the best advice I ever got from my thesis supervisor: “if you can’t think of anything else to do, try Fourier-transforming everything.”

Anyway each Fourier mode is complex and can therefore be characterized by an amplitude and a phase (the modulus and argument of the complex quantity). What I did next was to randomly reshuffle all the phases while leaving the amplitudes alone. I then performed the inverse Fourier transform to construct the image shown on the right.

What this procedure does is to produce a new image which has exactly the same power spectrum as the first. You might be surprised by how little the pattern on the right resembles that on the left, given that they share this property; the distribution on the right is much fuzzier. In fact, the sharply delineated features  are produced by mode-mode correlations and are therefore not well described by the power spectrum, which involves only the amplitude of each separate mode. In effect, the power spectrum is insensitive to the part of the Fourier description of the pattern that is responsible for delineating the cosmic web.

If you’re confused by this, consider the Fourier transforms of (a) white noise and (b) a Dirac delta-function. Both produce flat power-spectra, but they look very different in real space because in (b) all the Fourier modes are correlated in such away that they are in phase at the one location where the pattern is not zero; everywhere else they interfere destructively. In (a) the phases are distributed randomly.

The moral of this is that there is much more to the pattern of galaxy clustering than meets the power spectrum…

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Published BICEP2 paper admits “Unquantifiable Uncertainty”..

Posted in Bad Statistics, The Universe and Stuff with tags , , , , , , on June 20, 2014 by telescoper

Just a quick post to pass on the news that the BICEP2 results that excited so much press coverage earlier this year have now been published in Physical Review Letters. A free PDF version of the piece can be found here.  The published version incorporates a couple of important caveats that have arisen since the original release of the results prior to peer review. In particular, in the abstract (discussing models of the dust foreground emission:

However, these models are not sufficiently constrained by external public data to exclude the possibility of dust emission bright enough to explain the entire excess signal. Cross correlating BICEP2 against 100 GHz maps from the BICEP1 experiment, the excess signal is confirmed with 3σ significance and its spectral index is found to be consistent with that of the CMB, disfavoring dust at 1.7 σ.

Since the primary question-mark over the original result was whether the signal was due to dust or CMB, this corresponds to an admission that the detection is really at very low significance. I’ll set aside my objection to the frequentist language used in this statement!

There is an interesting comment in the footnotes too:

In the preprint version of this paper an additional DDM2 model was included based on information taken from Planck conference talks. We noted the large uncertainties on this and the other dust models presented. In the Planck dust polarization paper [96] which has since appeared the maps have been masked to include only regions “where the systematic uncertainties are small, and where the dust signal dominates total emission.” This mask excludes our field. We have concluded the information used for the DDM2 model has unquantifiable uncertainty. We look forward to performing a cross-correlation analysis against the Planck 353 GHz polarized maps in a future publication.

The emphasis is mine. The phrase made me think of this:

hazards

The paper concludes:

More data are clearly required to resolve the situation. We note that cross-correlation of our maps with the Planck 353 GHz maps will be more powerful than use of those maps alone in our field. Additional data are also expected from many other experiments, including Keck Array observations at 100 GHz in the 2014 season.

In other words, what I’ve been saying from the outset.

 

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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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