Bon Voyage, Planck

The rumors of new imminent delays at the LHC imply that this year we need to look for action elsewhere. Fortunately, experimental astroparticle physics is truly enjoyable these days. FERMI and PAMELA are probing high energy cosmic rays, and there is still a plenty of hope that telltale signals of dark matter will be uncovered. That celebrity couple may soon be outshined by another competitor at shorter wavelengths - the Planck cosmic microwave background observatory. Contrary to my fears, the Ariane rocket which carried Planck into space was not shot down by an Imperial cruiser. Planck is now on its way to the L2 point and in one and half year or so we will bath in a plenty of new precise cosmological data.

Planck is the third in a row, after COBE and WMAP, to chase after small anisotropies of the CMB. At first sight the mission comes close to Lord Kelvin's nightmare: Planck will measure what its predecessors have measured, but more precisely, with better resolution, and in more colors. From the propaganda plot on the right one can see that one practical virtue of Planck is the access to higher multipoles of the CMB temperature anisotropy. Probing more acoustic peaks and the damping tail will allow us to precisely determine the cosmological parameters and put the currently ruling Lambda-CDM cosmological model to a thorough test.

Doesn't sound too exciting? Of course, there is always a good chance that something unexpected will emerge from the data. However, I'm going to argue that even confirming the boring cosmological standard model may provide us with extremely interesting pieces of information. In particular, there is one important question to which Planck, with a little bit of luck, may provide an answer: what is the scale of inflation?

The past missions have collected some shreds and pieces of information about inflation. First of all, we know that the highly primitive model of inflation - a single scalar field slowly rolling down its potential - perfectly describes all available data. That is to say, the power spectrum of the primordial density fluctuations that ultimately produced the CMB temperature fluctuations can be explained by quantum fluctuations of that scalar field. Furthermore, we know something about the potential that provides for vacuum energy driving the accelerated expansion during inflation. In particular, the overall scale of the potential can be inferred from the amplitude of the temperature fluctuations observed by COBE and WMAP. This yields
$(V/\epsilon)^{1/4} \sim 3 \cdot 10^{16}$ GeV
where $\epsilon = (V'/V)^2/2M_{Pl}^2$ is one of the so-called slow-roll parameters. The slow-roll parameters must be small during inflation, of order 0.01 or less, which sets the upper bound on the scale of inflation. But, in principle, there's no lower limit on $\epsilon$, and at this point we cannot make a definitive statement about the magnitude of V.

Planck has a good chance to ultimately pinpoint the scale of inflation. The hopes are based on Planck's fantastic ability to measure the CMB polarization. Thomson scattering at the last scattering surface results in linear polarization of the CMB photons. The polarization can be decomposed into the E-mode (gradient) and the B-mode (curl), each of which is then decomposed into multipoles, much as the temperature fluctuations. The lower multipoles of the E-mode have already been detected by WMAP; the B-mode is more tricky and it is waiting for Planck.

The importance of the B-mode follows from the fact that, at the linear level, it is not produced by scalar density perturbations, but only by tensor perturbations, that is by the primordial gravity waves. The amount of tensor perturbations is directly related to the scale of the inflationary potential. The larger V, the higher is the ratio of tensor to scalar primoridial perturbations. As an example, Planck's sensitivity to the primordial B-mode for the tensor-to-scalar ratio = .1 is plotted on the right. If the tensor-to-scalar ratio is high enough for Planck to detect the primordial B-mode, then we will have the first evidence of the existence of a very high-energy scale in particle physics. (who said neutrinos? it's not certain if they're really Majorana, and besides who cares about neutrinos anyway).

But of course the tensor-to-scalar ratio can be too small for Planck to measure. In the worst case scenario Planck may share the tragic fate of LEP: a successful experiment without much success. Let's cross our fingers.

More info in Planck Bluebook.