Monday, July 14, 2014

Short news items

Over the past two months I have been on a two-week seminar tour of the UK, taken a short holiday, attended a conference in Estonia and spent a week visiting collaborators in Spain. Posting on the blog has unfortunately suffered as a result: my apologies. Here are some items of interest that have appeared in the meantime:
  • The BICEP and Planck teams are to share their data — here's the BBC report of this news. The information I have from Planck sources is that Planck will put out a paper with new data very soon (about a week ago I heard it would be "maybe in two weeks", so let's say two or three weeks from today). This new data will then be shared with the BICEP team, and the two teams will work together to analyse its implications for the BICEP result. From the timescales involved my guess is that what Planck will be making available is a measurement of the polarised dust foreground in the BICEP sky region, and the joint publication will involve cross-correlating this map with the B-mode map measured by BICEP. A significant cross-correlation would indicate that most (or all) of the signal BICEP detected was due to dust.
  • What Planck will not be releasing in the next couple of weeks is their own measurement of the polarization of the CMB, in particular their own estimate of the value of $r$. The timetable for this release is still October: this is a deadline imposed by the fact that ESA requires Planck to release the data by December, but another major ESA mission (I forget which) is due to be launched in November and ESA don't like scheduling "competing" press conferences in the same month because there's only so much science news Joe Public can absorb at a time. From what I've heard, getting the full polarization data ready for October is a bit of a rush as it is, so it's fairly certain that's not what they're releasing soon.
  • By the way, I think I've recently understood a little better how a collaboration as enormous as Planck manage to remain so disciplined and avoid leaking rumours: it's because most of the people in the collaboration don't know the full details of the results either! That is to say, the collaboration is split into small sub-groups with specified responsibilities, and these sub-groups don't share results with each other. So if you ask a randomly chosen Planck member what the preliminary polarization results are looking like, chances are they don't know any better than you. (Though this may not stop them from saying "Well, I've seen some very interesting plots ..." and smiling enigmatically!)
  • The conference I attended in Estonia was the IAU symposium in honour of the 100th birth anniversary of the great Ya. B. Zel'dovich, on the general topic of large-scale structure and the cosmic web. I'll try to write a little about my general impressions of the conference next time. In the meantime all the talks are available for download from the website here.
  • A science news story you may have seen recently is "Biggest void in universe may explain cosmic cold spot": this is a claim that a recently detected region with a relative deficit of galaxies (the "supervoid") explains the existence of the unusual Cold Spot that has been seen in the CMB, without the need to invoke any unusual new physics. The claim of the explanation is based on this paper. Unfortunately this claim is wrong, and the paper itself has several problems. My collaborators and I are in the process of writing a paper of our own discussing why, and when we are done I will try to explain the issues on here as well. In the meantime, you heard it here first: a supervoid does not explain the Cold Spot!
Update: It has been pointed out to me that last week Julien Lesgourgues gave a talk about Planck and particle physics at the Strong and Electroweak Matter (SEWM14) symposium, in which he also discussed the timeline of forthcoming Planck and BICEP papers. You can see this on page 12 of his talk (pdf) and it is roughly the same as what I wrote above (except that there's a typo in the year — it should be 2014 not 2015!).

6 comments:

  1. Dear Sesh,
    Could you specify your concerns about our supervoid explanation? What do you think about simply the detection of the supervoid in alignment with the Cold Spot?
    Thanks

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  2. Hi Andras,

    I'm afraid I can't provide details for a little while (a week or so) as I am still writing them and I write slowly. But I don't have any strong opinion either way about the detection of the void itself, because a void of that size and density contrast is not actually particularly unusual. In fact that's a sign of the problem: for instance if you look at http://arxiv.org/abs/arXiv:1310.2791 (Figure 6) you will see earlier examples of SDSS voids which are as large as or larger than your "supervoid" and are more underdense. But these voids are in the northern hemisphere, where we don't see any cold spots.

    Of course we'll provide a more quantitative argument soon as well, but hopefully this is enough to see there is a problem.

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  3. Though I don't have the competence to assess the correctness of the claims made in the paper linked to below, its arguments questioning the Cold Spot's existence seem at least to be worthy of consideration.

    http://arxiv.org/abs/1405.1844

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    Replies
    1. You're right that they are worthy of consideration! I've certainly always had a suspicion that most of the "large scale anomalies" in the CMB are related to masking issues. But I don't understand how masking could make much of a difference to the Cold Spot - I should read this paper more carefully to try to follow their argument. (There have also been debates about the significance of the Cold Spot on other grounds in the past, e.g. http://arxiv.org/abs/0908.3988.)

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  4. Sesh,
    I could find the slides for the Zel'dovich talks, but no videos. Where the talks filmed?
    Thanks

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    Replies
    1. No, I'm afraid not. Recording talks on film and live streaming don't seem to be a very common thing at conferences I've been to (in fact I've never been to a conference where it happened).

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