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Cosmology from CMB experiments

CMB means Cosmic Microwave Background

CMB polarisation bears information on the structure of the universe on large scales and the universe reionisation.

The CMB radiation propagates towards us along geodesic paths which are distorted in response to the curvature of space. Light rays are bent towards matter over-densities and away from matter under-densities in a way analogous to the effects of convex or concave optical lenses. Differential light deflection distorts the polarisation pattern of the CMB by solid-angle magnification and demagnification, thus leading to specific changes in the polarisation of the lensed compared to the unlensed CMB. After lensing, a primary CMB polarisation pattern of pure E-type exhibits B-type modes.

The epoch of reionisation is a largely unexplored frontier which motivates a range of observations. CMB polarisation on large angular scales complement observations of high redshift galaxies and redshifted H I in constraining physical scenarios. When did reionisation start? How long did it last? What is the nature of the ionising sources? Those questions remain essentially unanswered today. At the redshift of reionisation, additional CMB polarisation is generated on large angular scales, especially in the E-mode. But at the same time, the B-mode signal associated with primordial gravitational waves gains power, in the very same way the primary B-mode polarisation is generated.

Our component separation method will be used to quantify the E- and B-mode CMB polarisations with the aim at deducing as much cosmological information as possible. Firstly, we will exploit the refined results we will have obtained for the extraction of constraints on lensing and on reionisation from the polarisation data. Second, this will naturally lead us to search for the primordial B-mode signal associated with cosmic inflation. A positive measurement of the primordial signal would be an invaluable discovery, which would be compelling evidence that inflation took place, and would determine the energy scale of inflation. Significant upper limits would already allow us to exclude several classes of theoretically motivated inflationary models. We will concentrate our effort on the Planck data but the final analysis will include a cross-correlation between the BICEP2, EBEX and SPIDER experiments in collaboration with the instrument teams.

The impact of our project would be the highest, if we do detect the signatures of inflation on the B modes, but the signal may be too weak to be detected by the present generation of CMB experiments. This does not imply that our project bears high risks as to the content and significance of the work to be done, because the proposed research is required if the CMB community is going to extract the most out of the current generation of CMB polarisation experiments.

Stages in the evolution of the Universe. According to the cosmological standard model, inflation stretches microscopic quantum fluctuations into astronomical density fluctuations that leave an imprint on the cosmic microwave background (CMB), and then grow into the present day galaxy distribution. Figure borrowed from Bock et al. (2006, astro-ph/0604101).

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