Monday 18 March 2013

'Twas the week before Planckmas...

This week will see cosmologists excitedly waiting for, and celebrating, the upcoming results from ESA's Planck satellite. We've been waiting for this day since the launch of Planck in 2009 (in fact, most people having been waiting for this day since the late 1990s, when the satellite was proposed, initially called COBRAS/SAMBA). This multi-national collaboration has already released some data and results a year ago (on subjects such as point sources and clusters detected through their Sunyaev-Zel'dovich signature), but the first large suite of cosmology results will be announced on Thursday the 21st of March 2013, at a large press event.
Here at Princeton Astrophysics, we are having our own Planck Party at 5 am, and event which will no doubt have as much excitement as the pre-dawn Higgs party we had at the Institute for Advanced Study last summer.

So what is all the excitement about?

Until the Planck release, the tightest constraints at multipoles less than 1000 have come from NASA's WMAP satellite, which was recently awarded the Gruber Foundation Cosmology Prize. WMAP operated for nine years and really helped to pin down the cosmological model on the largest scales.

The plot above shows the power on the y-axis as a function of multipole (x-axis). Multipoles are inversely related to angle, that is, large angles correspond to small values of the multipole, while small scales are large values of l.

On smaller scales (i.e. to the right of this graph) two experiments have dominated the game recently, The Atacama Cosmology Telescope (based in the Chilean desert, and the collaboration I'm a part of) and the South Pole Telescope (no prizes for guessing where this telescope is!)

The gold points are the same as the black points in the top plot, but with a logarithmic scale on the y-axis. From this plot, it is clear to see how ACT and SPT provide all the signal at small scales - the WMAP data points end around l=1000. Combining the data from WMAP with these experiments helps us put tight limits on our cosmological model and on non-standard physics in the early universe.

Planck will improve on this picture by making the error bars much smaller on all scales. On large scales we are looking to see if any of the WMAP anomolies are present, and on intermediate scales Planck will also greatly reduce the error bars (on multipoles of 800 - 2000), where the WMAP error bars are large or unconstrained (see the linear scale plot at the top of the page). 

This is particularly interesting for a parameter of recent interest, namely the effective number of relativistic species, or Neff. If we had three neutrino species (which is the standard picture) - Neff would be 3.046 (this number is not exactly three due to electron-positron annihilations in the early universe). It helps to think of the number in terms of extra neutrinos, but what Neff actually measures is if there was any extra (or less) energy from such a relativistic species. It doesn't specify what that species should be, and many authors have proposed some interesting candidates, from sterile neutrinos to `dark radiation'. If there was more relativistic energy when the CMB was formed, this would lead to a few interesting effects, the most obvious being the decrease in amplitude of the small scale Silk damping tail - the intrinsic CMB spectrum which drops in power as l increases. Of course, there are many degeneracies between Neff and other parameters, which is why better data (and independent data) help us tease apart the degeneracy.

All three experiments (WMAP, ACT and SPT) recently released their constraints on cosmological parameters including Neff (they are here, here and here).
The three experiments have some mild tension the best-fit values of Neff (we discuss the consistency between them in a recent paper) - the plot above shows this. In both cases the ACT and SPT data are combined with the latest WMAP9 results. The left-most panel shows the one-dimensional contours for Neff, while the two right panels show an error ellipse. Dark ellipses shows models which are consistent with the data at 68% confidence, while the lighter ellipses show models consistent at 95% confidence. Any model outside of the ellipses is less than 5% likely to fit the data. The red lines/curves are for WMAP9 and ACT, the green for WMAP9 and SPT and the black curves/contours show the combination of all three experiments together. While SPT sees a higher value of Neff than 3.046 at Neff = 3.74 +/ 0.47, and ACT a slightly lower value with Neff = 2.90 +/ 0.53, the combined data are completely consistent with the standard picture: Neff =  3.37 +/ 0.42 (which may dismay or delight you, depending on your camp of interest!).

By improving the constraints on the power at intermediate scales, Planck should tell us more in a few days. This is particularly interesting because while ACT and SPT look at different regions of the sky (on smaller patches), Planck will release results based on the full sky - another independent measurement of the same underlying physics.

[There is a great post by Jester on RĂ©esonances about Neff (posted just before the ACT constraints were released) written for those with a particle physics interest.]

Planck will also measure the weak lensing of the CMB by gravitational structures - an extremely subtle effect which moves power around on the maps of the CMB temperature on arcminute scales, but coherently over degrees. ACT  and SPT have measured this deflection - and Planck will improve the errors on this measurement by a great deal on all scales. The deflection power spectrum is a strong probe of structure, and things which would wash out that structure, such as a massive neutrino.

Another key constraint that will come from Planck is one on the non-Gaussianity of the initial conditions of the universe, which is a strong test of the various inflationary models out there today.

[There is an awesome TEDx talk by Ed Copeland on CMB physics and inflation which provides a nice summary of the link between the CMB and the early universe.]

One way to think of non-Gaussianity is by imagining a distribution with some level of skewness and kurtosis (so, a normal distribution that has been distorted). A simple picture for how to produce a two-dimensional temperature map from the power spectrum above, is to generate a Gaussian realisation of the power spectrum - at each angular scale (defined by the multipole), use the power to define the variance in temperature on that scale. However, if the temperature field is non-Gaussian, then the full map is not described by the two-point function, or power spectrum: we need to use higher order statistics to characterise the initial conditions if they are non-Gaussian! That is typically why we use the bispectrum (the three point function) and higher order statistical correlation functions to measure non-Gaussianity.

 The WMAP bound is consistent with zero fNL (the parameter describing the level of non-Gaussianity, a quantity we expect to be vanishingly small in the simplest single field models of inflation) with -3< fNL < 77 at 95% confidence. However, the expected errors on fNL from Planck should go from the errors on fNL of about 20 to errors of a few! If the central value of fNL = 37.2 found by WMAP remains while the errors decrease we will put some serious pressure on many inflationary models - it is always a theoretical treat to find you aren't living in a `vanilla' universe.

These are only a few of the presents we are expecting on Thursday. Make sure to tune in to hear the results, and enjoy the flurry of papers on the latest cosmological bounds using the temperature of the CMB. For the polarisation measurements, you will still have a little wait before Planck (and ACTPol and SPTPol) entice you with more results - as it is an even more delicate procedure to tease out polarisation from these sensitive instruments.

Until then, we wait to boldy constrain where only a few experiments have constrained before...

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