Monday, 1 December 2014

Why we shouldn't try to have our (Planck) cake and eat it too...

Like many cosmologists, I am eagerly awaiting the next update from the Planck mission.

The previous release of Planck result had me all aflutter, and the release of results from BICEP had the whole community talking/discussing for ages.

But we were told recently that the papers will be released on the 22nd of December 2014. While a conference devoted to the announcement of results (rather than the release of data and papers) is being held this week in Ferrara.

So this morning, with no official press conference and a press release from the team (in French) the only meat to go on was bits of information on Twitter from those at the conference.

So what do we know*?

        *from Twitter/discussions and from the press releases here

Well, first of all there are the beautiful spectra in both temperature and polarisation on small scales (the larger scale measurement is in flux at the moment as the team work hard on systematics).

And some spectacular other images of (parts of) the maps in 353 GHz polarisation
with the colours indicating the dust and the relief showing the galactic magnetic field. Pretty, isn't it?

There are still some interesting tensions with the amplitude of clustering from Planck relative to other measurements of galaxy lensing (from the CFHTLens collaboration). The matter density is Omega_m=0.316 +/- 0.009.

Some important parameters like the scalar spectral index, n_s shift around by around 1 sigma. The optical depth, tau, shifts downwards - which is important for how we understand how the universe reionised (tau  is in the range of 0.71-0.79 from Planck lensing and low-multipole LFI measurements.) The errorbar is still converging, so I look forward to the results in December for the final number on this.

The level of non-Gaussianity detected by Planck including polarisation is consistent with zero.

The number of effective degrees of freedom (Neff) is consistent with the simple picture, Neff = 3 +- 0.2, a number consistent with the WMAP9+ACT numbers we presented a while back before Planck.

Planck have a very robust detection of a non-zero lensing deflection power spectrum (I don't think the term 40sigma really means much when we get to such high confidence!) and detected polarisation B-modes from lensing too.

And Planck have improved the constraints (over WMAP) on dark matter models significantly, ruling out previous results (look for the grey rectangle between the yellow WMAP exclusion limit and the blue exclusion limit).

Also, very importantly, the Planck team have also gotten a great handle on their calibration, which brings the calibration into alignment with WMAP at the 0.3% level.

So this is good news! Some have been commenting that the results aren't 'exciting' enough - but I actually think this is great news. The Planck team should be commended for waiting/checking/testing/understanding their data for as long as possible before releasing the papers: they are being cautious. Also, we as a community are taking small, tough and important steps to narrowing parameter space. Being in the large-data limit makes things hard, and they've been under tremendous pressure too to release the data, but I think we should take our hats off for a moment and congratulate the team on their (amazingly) hard work. The scientific method is gruelling, and we can't always get the great upsets that make for great tweets.

I would have loved more of a press release, live stream or presentation of the results publicly. But I'd rather have a cautious late-December gift of papers than a early rush that they aren't happy with.

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