Countdown to Planck

These days the CERN Theory Institute program is focused on the interplay between cosmology and LHC phenomenology. Throughout July you should expect overrepresentation of cosmology in this blog. Last Wednesday, Julien Lesgourgues talked about the Planck satellite. Julien is worth listening to. First of all, because of his cute French accent. Also, because his talks are always clear and often damn interesting. Here is what he said this time.

Planck is a satellite experiment to measure the Cosmic Microwave Background. It is the next step after the succesful COBE and WMAP missions. Although it looks like any modern vacuum cleaner, the instruments offer 2*10^(-6) resolution of temperature fluctuations (factor 10 better than WMAP) and 5' angular resolution (factor 3 better than WMAP). Thanks to that, Planck will be able to measure more precisely the angular correlations of the CMB temperature fluctuations, especially at higher multipoles (smaller angular scales). This is illustrated on this propaganda picture:

Even more dramatic is the improvement in measuring the CMB polarization. In this context, one splits the polarization into the E-mode and the B-mode (the divergence and the curl). The E-mode can be seeded by scalar gravitational density perturbations which are responsible for at least half of the already observed amplitude of temperature fluctuations. For large angular scales, the E-mode has already been observed by WMAP. The B-mode, on the other hand, must originate from tensor perturbations, that is from gravity waves in the early universe. These gravity waves can be produced by inflation. Planck will measure the E-mode very precisely, while the B-mode is a chalenge. Observing the latter requires quite some luck, since many models of inflation predict the B-mode well below the Planck sensitivity.

Planck is often described as the ultimate CMB temperature measerument. That is because its angular resolution corresponds to the minimal one at which temperature fluctuations of cosmological origin may exist at all. At scales smaller than 5' the cosmological imprint in the CMB is suppresed by the so-called Silk damping. 5' corresponds roughly to the photon mean free path in the early univere so that fluctuations at smaller scales get washed out. However, there is still room for future missions to improve the polarization measurements.

All these precision measurements will serve the noble cause of precision cosmology, that is a precise determination of the cosmological parameters. Currently, the CMB and other data are well described by the Lambda-CDM model, which has become the standard model of cosmology. Lambda-CDM has 6 adjustable parameters. One is the Hubble constant. The other two are the cold (non-relativistic) dark matter and the baryonic matter densities. In this model matter is supplemented by the cosmological constant, so as to end up in the spatially flat universe. Another two parameters describe the spectrum of gravitational perturbations (the scalar amplitude and the spectral index). The last one is the optical depth to reionization. Currently, we know these parametes with a remarkable 10% accuracy. Planck will further improve the accuracy by a factor 2-3, in most cases.

Of course, Planck may find some deviations from the Lambda-CDM model. There exist, in fact, many reasonable extensions that do not require any exotic physics. For example, there may be the already mentioned tensor perturbations, non-gaussianities or the running of the spectral index, which are predictions of certain models of inflation. Planck could find the trace of the hot (relativistic) component of the dark matter. Such contribution might come from the neutrinos, if the sum of their masses is at least 0.2 eV. Furthermore, Planck will accurately test the spatial flatness assumption. The most exciting discovery would be to see that the equation of state of dark energy differs from w=-1 (the cosmological constant). This would point to some dynamical field as the agent responsible for the vacuum energy.

Finally, the Planck will test models of inflation. Although it is unlikely that the measurement will favour one particular model, it may exclude large classes of models. There are two parameters that appear most interesting in this context. One is the spectral index nS. Inflation predicts small departures from the scale invariant Harrison-Zeldovich spectrum corresponding to nS=1. It would be nice to see this departure beyond all doubt, as it would further strengthen the inflation paradigm. The currently favoured value is nS = 0.95, three sigma away from 1. The other interesting parameter is the ratio r of the tensor to scalar perturbations. The current limit is r < 0.5, while Planck is sensitive down to r = 0.1. If the inflation takes place at energies close to the GUT scale, tensor perturbations might be produced at the observable rate. If nothing is observed, large-field inflation models will be disfavoured.

Planck is going to launch in July 2008. This coincides with the first scheduled collisions at the LHC. Let's hope at least one of us will see something beyond the standard model.

No slided as usual.