Back in the office after the Christmas and New Year break, with a mountain of stuff to work through..
Anyway, I saw this paper on the arXiv yesterday and thoought I’d share it here. It’s from a paper by Wu et al. entitled Initial Performance of BICEP3: A Degree Angular Scale 95 GHz Band Polarimeter. The abstract follows:
BICEP3 is a 550mm aperture telescope with cold, on-axis, refractive optics designed to observe at the 95GHz band from the South Pole. It is the newest member of the BICEP/Keck family of inflationary probes specifically designed to measure the polarization of the cosmic microwave background (CMB) at degree-angular scales. BICEP3 is designed to house 1280 dual-polarization pixels, which, when fully-populated, totals to ∼9× the number of pixels in a single Keck 95GHz receiver, thus further advancing the BICEP/Keck program’s 95GHz mapping speed. BICEP3 was deployed during the austral summer of 2014-2015 with 9 detector tiles, to be increased to its full capacity of 20 in the second season. After instrument characterization measurements were taken, CMB observation commenced in April 2015. Together with multi-frequency observation data from Planck, BICEP2, and the Keck Array, BICEP3 is projected to set upper limits on the tensor-to-scalar ratio to r≲0.03 at 95% C.L..
It all looks very promising, with science results likely to appear later this year, but who will win the race to find those elusive primordial B-modes?
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!
After yesterday’s frivolity, I return to community service mode today with a short post before a series of end-of-term meetings.
You may recall that not long ago I posted an item about a meeting in Ferrara which started on 1st December and which concerned results from the Planck satellite. Well, although the number of new results was disappointingly limited, all the talks given at that meeting are now available online here. Not all of the talks are about new Planck results, and some of those that do are merely tasters of things that will be more completely divulged in due course, but there is still a lot of interesting material there so I recommend cosmology types have a good look through. Any comments would be welcome through the usual channel below.
I’ll take this opportunity to pass on another couple of related items. First is that there is another meeting on Planck, in Paris next week. Coincidentally, I will be in Paris on Monday and Tuesday for a completely unrelated matter (of which more anon) but I will try to keep up with the cosmology business via Twitter etc and pass on whatever I can pick up.
The other bit of news is that there is to be a press conference on December 22nd at which I’m led to believe the outcome of the joint analysis of CMB polarization by Planck and BICEP2 will be unveiled. Now that will be interesting, so stay tuned!
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?
So here I am, then, sitting in my hotel room in Copenhagen and drinking coffee, filling in time before I check out and travel to the airport for the journey home. I don’t have to be there until this afternoon so today is going to be a bit more leisurely than the rest of the week has been. It’s nice to get a couple of hours to myself.
It was an interesting little workshop, with lots of time for discussions, but lurking in the background of course was the question mark over BICEP2. Many theorists have clearly been beavering away on models which assume that BICEP2 has measured primordial gravitational waves and I suspect most of them really want the result to be correct. When I posted a message on Twitter about this, Ian Harrison posted this homage to a famous poster for the TV series The X-files. There’s more than a little truth in the comparison!
Whatever the truth about the BICEP2 measurements there’s no question that it’s a brilliant experiment, with exquisite sensitivity. There is no question that it has detected something so faint that it boggles the mind. Here is a slide from Phil Lubin’s talk at the meeting, which shows the unbelievably rapid improvement in sensitivity of microwave detectors:
I don’t think cosmologists ever pay enough credit to the people behind these technological developments, as it is really they who have driven the subject forward. In the case of BICEP2 the only issue is whether it has picked up a cosmological signal or something from our own Galaxy. Whatever it is, it’s an achievement that deserves to be recognized.
And as for the claims of the person responsible for the post I reblogged yesterday that the cosmic microwave background is a fraud, well I can assure you it is not. Any scientific result is open to discussion and debate, but the ultimate arbiter is experimental test. Several independent teams are working in competition on CMB physics and any fraud would be easily exposed. The cosmic microwave background is out there.
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 measured spectrum of the cosmic microwave background. It’s indistinguishable from the theoretical black-body curve shown as a solid line
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!
The views presented here are personal and not necessarily those of my employer (or anyone else for that matter).
Feel free to comment on any of the posts on this blog but comments may be moderated; anonymous comments and any considered by me to be abusive will not be accepted. I do not necessarily endorse, support, sanction, encourage, verify or agree with the opinions or statements of any information or other content in the comments on this site and do not in any way guarantee their accuracy or reliability.