Archive for Primordial Gravitational Waves

LiteBIRD Newsflash

Posted in The Universe and Stuff with tags , , , , on May 23, 2019 by telescoper

Just a quick post to pass on the news that the space mission LiteBIRD has been selected as the next major mission by the Japanese Space Agency JAXA and  Institute for Space and Astronautical Science (ISAS).

LiteBIRD (which stands for `Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection’) is a planned space observatory that aims to detect the footprint of the primordial gravitational waves on the cosmic microwave background (CMB) in a form of a B-mode polarization pattern. This is the signal that BICEP2 claimed to have detected five years ago to much excitement, but was later shown to be a caused by galactic dust.

It’s great news for a lot of CMB people all round the world that this mission has been selected – include some old friends from Cardiff University. Congratulations to all of them!

I’m not sure when the launch date will be, but the mission will last three years and will be at Earth-Sun Lagrange point known as L2.It will be a very difficult task to extract the B-mode signal from foregrounds and instrumental artifacts so although there’s joy that it has been selected, the real work starts now!

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BICEP2: Is the Signal Cosmological?

Posted in Astrohype, The Universe and Stuff with tags , , on March 28, 2019 by telescoper

An article in Physics Today just reminded me just now that I have missed the fifth anniversary of the BICEP2 announcement of `the detection of primordial gravitational waves’. I know I’m a week but I thought I’d reblog the post I wrote on March 19th 2014.You will see that I was sceptical…

..and it subsequently turned out that I was right to be so.

In the Dark

I have a short gap in my schedule today so I thought I would use it to post a short note about the BICEP2 results announced to great excitement on Monday.

There has been a great deal of coverage in the popular media about a “Spectacular Cosmic Discovery” and this is mirrored by excitement at a more technical level about the theoretical implications of the BICEP2 results. Having taken a bit of time out last night to go through the discovery paper, I think I should say that I think all this excitement is very premature. In that respect I agree with the result of my straw poll.

First of all let me make it clear that the BICEP2 experiment is absolutely superb. It was designed and built by top-class scientists and has clearly functioned brilliantly to improve its sensitivity so much that it has gone so…

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The BICEP2 Bubble Bursts…

Posted in The Universe and Stuff with tags , , , , on January 30, 2015 by telescoper

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:

cross-correlation

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:

BICEP_null

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!

UPDATE TO THE UPDATE: There’s a news item in Physics World and another in Nature News about this, both with comments from me and others.

Launch!

Posted in The Universe and Stuff with tags , , , on January 3, 2015 by telescoper

Meanwhile, in Antarctica, the search for signatures of primordial gravitational waves in the polarization of the cosmic microwave background goes on. Here’s a fascinating blog by a member of the SPIDER team, whose balloon-borne experiment was recently launched and is currently circling the South Pole taking data. Here’s hoping it works out as planned!

SPIDER on the Ice

This is surreal.

I have been working on SPIDER for three and a half years, and much of the rest of the collaboration has been working for many years beyond that. We have all gone through intense times of stress and disappointment, victories and defeats. The personal sacrifice on the part of every individual on the team to get SPIDER to the point of flight readiness has been a weight on all of our shoulders as we prepared to launch our hopes and dreams on a balloon.

Ballooning is incredibly risky. Everything can work flawlessly on the ground, and then one thing can break during launch, or freeze or overheat at float altitude, and no amount of commanding from afar can bring it back to life. This happens so often in ballooning, and all you can do is obsess over every aspect of the experiment, have redundancy where possible, and…

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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.

 

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.