Archive for B-modes

BICEP2 bites the dust.. or does it?

Posted in Bad Statistics, Open Access, Science Politics, The Universe and Stuff with tags , , , , , , , , on September 22, 2014 by 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:

PlanckvBICEP2

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):

Quieter dust

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 story

Advertisements

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.

Clover: What Might Have Been

Posted in Science Politics, The Universe and Stuff with tags , , , on March 23, 2014 by telescoper

Quite a few people have been asking me whether the UK’s cancelled B-mode experiment, Clover, could have detected what BICEP2 may  have found; I’m still not convinced, by the way. It therefore seemed apt to do a quick post in order to direct you to relevant sources of information. If you’re interested in Clover’s capabilities you can find a nice summary on the ArXiv here. The abstract reads:

We describe the objectives, design and predicted performance of Clover, which is a ground-based experiment to measure the faint “B-mode” polarisation pattern in the cosmic microwave background (CMB). To achieve this goal, clover will make polarimetric observations of approximately 1000 deg^2 of the sky in spectral bands centred on 97, 150 and 225 GHz. The observations will be made with a two-mirror compact range antenna fed by profiled corrugated horns. The telescope beam sizes for each band are 7.5, 5.5 and 5.5 arcmin, respectively. The polarisation of the sky will be measured with a rotating half-wave plate and stationary analyser, which will be an orthomode transducer. The sky coverage combined with the angular resolution will allow us to measure the angular power spectra between 20 < l < 1000. Each frequency band will employ 192 single polarisation, photon noise limited TES bolometers cooled to 100 mK. The background-limited sensitivity of these detector arrays will allow us to constrain the tensor-to-scalar ratio to 0.026 at 3sigma, assuming any polarised foreground signals can be subtracted with minimal degradation to the 150 GHz sensitivity. Systematic errors will be mitigated by modulating the polarisation of the sky signals with the rotating half-wave plate, fast azimuth scans and periodic telescope rotations about its boresight. The three spectral bands will be divided into two separate but nearly identical instruments – one for 97 GHz and another for 150 and 225 GHz. The two instruments will be sited on identical three-axis mounts in the Atacama Desert in Chile near Pampa la Bola. Observations are expected to begin in late 2009.

The following points, gleaned from a very quick skimming of the above paper, are worth noting (but please note the important corrections and clarifications in the comments below from the first author of the above paper and also bear in mind that the Clover numbers are estimated rather than based on actual measurements):

  1. The sky coverage of Clover would have been 1000 square degrees, compared with 380 square degrees of BICEP2
  2. Clover measurements would have been made at three frequencies, 97 GHz, 150 GHz and 225 GHz. This would have enabled the possibility of foreground contamination to be rejected with much greater confidence than in BICEP2 (which only operates at 150 GHz)
  3. The sensitivity of Clover at 150 GHz (the frequency at which BICEP2 operates) would have been about 1.4 times better than BICEP2
  4. Had it gone ahead, Clover would have started taking data at around the same time as BICEP2 (perhaps even a bit earlier).
  5. Clover was originally intended to be positioned at the South Pole, where observing conditions are better than in Chile and where BICEP2 is placed, but this was in the middle of STFC’s financial crisis and running costs would have been much higher than the alternative location in Chile. This might have had a negative impact on its sensitivity.

Here’s a plot from the above paper showing a the anticipated measurement if the tensor scalar ratio had been 0.1; BICEP2 detection (if real) corresponds to a signal twice this amplitude:

Clover_sensitivity

In other words, we don’t know whether Clover would have hit its target sensitivity and there are many other imponderables, but  it’s a very great shame it never got the chance to try…