I do not know what dust is.
I do not know where it comes from.
I only know that it settles on things.
I cannot see it in the air or watch it fall.
Sometimes I’m home all day
But I never see it sliding about looking for a place to rest when my back is turned.
Does it wait ’til I go out?
Or does it happen in the night when I go to sleep?
Dust is not fussy about the places it chooses
Though it seems to prefer still objects.
Sometimes, out of kindness, I let it lie for weeks.
On some places it will lie forever
However, dust holds no grudges and once removed
It will always return in a friendly way.
Archive for dust
I do not know what dust is.
I think it’s time to break the worst-kept secret in cosmology, concerning the claimed detection of primordial gravitational waves by the BICEP2 collaboration that caused so much excitement last year; see this blog, passim. If you recall, the biggest uncertainty in this result derived from the fact that it was made at a single frequency, 150 GHz, so it was impossible to determine the spectrum of the signal. Since dust in our own galaxy emits polarized light in the far-infrared there was no direct evidence to refute the possibility that this is what BICEP2 had detected. The indirect arguments presented by the BICEP2 team (that there should be very little dust emission in the region of the sky they studied) were challenged, but the need for further measurements was clear.
Over the rest of last year, the BICEP2 team collaborated with the consortium working on the Planck satellite, which has measurements over the whole sky at a wide range of frequencies. Of particular relevance to the BICEP2 controversy are the Planck mesurements at such high frequency that they are known to be dominated by dust emission, specifically the 353 GHz channel. Cross-correlating these data with the BICEP2 measurements (and also data from the Keck Array which is run by the same team) should allow the identification of that part of the BICEP2 signal that is due to dust emission to be isolated and subtracted. What’s left would be the bit that’s interesting for cosmology. This is the work that has been going on, the results of which will officially hit the arXiv next week.
However, news has been leaking out over the last few weeks about what the paper will say. Being the soul of discretion I decided not to blog about these rumours. However, yesterday I saw the killer graph had been posted so I’ve decided to share it here:
The black dots with error bars show the original BICEP/Keck “detection” of B-mode polarization which they assumed was due to primordial gravitational waves. The blue dots with error bars show the results after subtracting the correlated dust component. There is clearly a detection of B-mode polarization. However, the red curve shows the B-mode polarization that’s expected to be generated not by primordial gravitational waves but by gravitational lensing; this signal is already known. There’s a slight hint of an excess over the red curve at multipoles of order 200, but it is not statistically significant. Note that the error bars are larger when proper uncertainties are folded in.
Here’s a quasi-official statement of the result (orginall issued in French) that has been floating around on Twitter:
To be blunt, therefore, the BICEP2 measurement is a null result for primordial gravitational waves. It’s by no means a proof that there are no gravitational waves at all, but it isn’t a detection. In fact, for the experts, the upper limit on the tensor-to-scalar ratio R from this analysis is R<0.13 at 95% confidences there’s actually till room for a sizeable contribution from gravitational waves, but we haven’t found it yet.
The search goes on…
UPDATE: As noted below in the comments, the actual paper has now been posted online here along with supplementary materials. I’m not surprised as the cat is already well and truly out of the bag, with considerable press interest, some of it driving traffic here!Follow @telescoper
Well, it’s come about three weeks later than I suggested – you should know that you can never trust anything you read in a blog – but the long-awaited Planck analysis of polarized dust emission from our Galaxy has now hit the arXiv. Here is the abstract, which you can click on to make it larger:
My twitter feed was already alive with reactions to the paper when I woke up at 6am, so I’m already a bit late on the story, but I couldn’t resist a quick comment or two.
The bottom line is of course that the polarized emission from Galactic dust is much larger in the BICEP2 field than had been anticipated in the BICEP2 analysis of their data (now published in Physical Review Letters after being refereed). Indeed, as the abstract states, the actual dust contamination in the BICEP2 field is subject to considerable statistical and systematic uncertainties, but seems to be around the same level as BICEP2’s claimed detection. In other words the Planck analysis shows that the BICEP2 result is completely consistent with what is now known about polarized dust emission. To put it bluntly, the Planck analysis shows that the claim that primordial gravitational waves had been detected was premature, to say the least. I remind you that the original BICEP2 result was spun as a ‘7σ’ detection of a primordial polarization signal associated with gravitational waves. This level of confidence is now known to have been false. I’m going to resist (for the time being) another rant about p-values…
Although it is consistent with being entirely dust, the Planck analysis does not entirely kill off the idea that there might be a primordial contribution to the BICEP2 measurement, which could be of similar amplitude to the dust signal. However, identifying and extracting that signal will require the much more sophisticated joint analysis alluded to in the final sentence of the abstract above. Planck and BICEP2 have differing strengths and weaknesses and a joint analysis will benefit from considerable complementarity. Planck has wider spectral coverage, and has mapped the entire sky; BICEP2 is more sensitive, but works at only one frequency and covers only a relatively small field of view. Between them they may be able to identify an excess source of polarization over and above the foreground, so it is not impossible that there may a gravitational wave component may be isolated. That will be a tough job, however, and there’s by no means any guarantee that it will work. We will just have to wait and see.
In the mean time let’s see how big an effect this paper has on my poll:
Note also that the abstract states:
We show that even in the faintest dust-emitting regions there are no “clean” windows where primordial CMB B-mode polarization could be measured without subtraction of dust emission.
It is as I always thought. Our Galaxy is a rather grubby place to live. Even the windows are filthy. It’s far too dusty for fussy cosmologists, who need to have everything just so, but probably fine for astrophysicists who generally like mucking about and getting their hands dirty…
This discussion suggests that a confident detection of B-modes from primordial gravitational waves (if there is one to detect) may have to wait for a sensitive all-sky experiment, which would have to be done in space. On the other hand, Planck has identified some regions which appear to be significantly less contaminated than the BICEP2 field (which is outlined in black):
Could it be possible to direct some of the ongoing ground- or balloon-based CMB polarization experiments towards the cleaner (dark blue area in the right-hand panel) just south of the BICEP2 field?
From a theorist’s perspective, I think this result means that all the models of the early Universe that we thought were dead because they couldn’t produce the high level of primordial gravitational waves detected by BICEP2 have no come back to life, and those that came to life to explain the BICEP2 result may soon be read the last rites if the signal turns out to be predominantly dust.
Another important thing that remains to be seen is the extent to which the extraordinary media hype surrounding the announcement back in March will affect the credibility of the BICEP2 team itself and indeed the cosmological community as a whole. On the one hand, there’s nothing wrong with what has happened from a scientific point of view: results get scrutinized, tested, and sometimes refuted. To that extent all this episode demonstrates is that science works. On the other hand most of this stuff usually goes on behind the scenes as far as the public are concerned. The BICEP2 team decided to announce their results by press conference before they had been subjected to proper peer review. I’m sure they made that decision because they were confident in their results, but it now looks like it may have backfired rather badly. I think the public needs to understand more about how science functions as a process, often very messily, but how much of this mess should be out in the open?
UPDATE: Here’s a piece by Jonathan Amos on the BBC Website about the story.
ANOTHER UPDATE: Here’s the Physics World take on the story.
ANOTHER OTHER UPDATE: A National Geographic storyFollow @telescoper
One of the advantages of informal workshops like this one I’m attending in Copenhagen right now is that there’s a lot of time for discussions and picking up various bits of gossip. Some of the intelligence gathered in this way is unreliable but often it represents knowledge that’s widely known in the cosmological community but which I’ve missed because I don’t spend as much time on the conference circuit these days.
Anyway, those of you with more than a passing interest in cosmology will remember the results from the BICEP2 experiment announced with a great fanfare of publicity in March this year. A significant number of eminent cosmologists immediately seized on the detection of B-mode correlations in the polarized cosmic microwave background as definitive proof of the existence of primordial gravitational waves. Some went even further, in fact, and claimed that the BICEP2 results prove all kinds of other things too.
As time passed, however, and folks had time to digest some of the details presented by the BICEP2 team, there has been a growing unease about the possibility that the measurements may have been misinterpreted. The problem – the Achilles Heel of BICEP, so to speak – is that it operates at a single frequency, 150 GHz. That means that it is not possible for this experiment on its own to determine the spectrum of the detected signal. This is important because it is not only the cosmic microwave background that is capable of producing polarized radiation at a frequency of 150 GHz, foreground dust inside our own Galaxy being the prime suspect as an alternative source. It should be possible to distinguish between dust and CMB using measurements at different frequencies because the microwave background has a black-body spectrum whereas dust does not. However, BICEP2 maps only a small part of the sky and at the time of the announcement there were no other measurements covering the same region, so a convincing test has not so far been possible.
The initial BICEP2 announcement included a discussion of foregrounds that concluded that these were expected to be much lower than their detected signal in the area mapped, but serious doubts have emerged about the accuracy of this claim. Have a look at my BICEP2 folder to see more discussion.
More recently, in July, it was announced that the BICEP2 team would collaborate with the large consortium working on the analysis of data from the Planck experiment to try to resolve these difficulties. Planck not only covers the whole sky but also has detectors making measurements over a wide range of frequencies (all the way up to 857 GHz). This should provide a definitive measurement of the contribution of Galactic dust to the BICEP2 field and at last give us a strong experimental basis on which to decided whether the BICEP2 signal is primordial or not. The result of my informal poll on BICEP2 was a clear majority (~62%) in favour of the statement that it was “too early to say” what the BICEP2 signal actually represents.
Anyway, I have it on very good authority that Planck’s analysis of the Galactic foregrounds in the BICEP2 region will be published (on the arXiv) on or around September 1st 2014. That’s just about 10 days from now. Maybe then this tantalizing wait will be over. I’ll try my best to post about the results when it comes out. In the meantime, I thought I’d do something completely unscientific and try to gauge what how current opinion stands on this issue by means of a poll of the total unrepresentative readership of this blog. Suppose you had to bet on whether the BICEP2 result is due to (a) primordial gravitational waves or (b) Galactic foregrounds, which would you go for?
Of course, those working on this project probably know the answer already so they’ll have to decide for themselves whether they wish to vote!Follow @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!
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)
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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:
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.
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.