Statistics of the CMB polarised anisotropies : unveiling the primordial universe

Abstract : A deep understanding of the first instants of the Universe would not only complete our description of the cosmic history but also enable an exploration of new fundamental phsyics at energy scales unexplored on Earth laboratories and colliders. The most favoured scenario which describes these first instants is the cosmic inflation, an ephemeral period of accelerated expansion shortly after the big bang. Some hints are in favour of this scenario which is however still waiting for a smoking-gun observational signature. The cosmic microwave background (CMB) B modes would be generated at large angular scales by primordial gravitational waves produced during the cosmic inflation. In this frame, the primordial CMB B modes are the aim of various ongoing or being-deployed experiments, as well as being-planned satellite mission. However, unavoidable instrumental and astrophysical features makes its detection difficult. More specifically, a partial sky coverage of the CMB polarisation (inherent to any CMB measurements) leads to the E-to-B leakage, a major issue on the estimation of the CMB B modes power spectrum. This effect can prevent from a detection of the primordial B modes even if the polarisation maps are perfectly cleaned, since the (much more intense) leaked E-modes mask the B-modes. Various methods have been proposed in the literature offering a B modes estimation theoretically free from any leakage. However, when applied to real data, they are no longer completely leakage-free and remove part of the information on B-modes. These methods consequently need to be validate in the frame of real data analysis. In this purpose, I have worked on the implementation and numerical developments of three typical pseudospectrum methods. Afterwards, I have tested each of them in the case of two fiducial experimental set ups, typical of current balloon-borne or ground based experiments and of potential satellite mission. I have therefore stated on the efficiency and necessity of one of them: the so-called pure method. I have also shown that the case of nearly full sky coverage is not trivial because of the intricate shape of the contours of the point-sources and galactic mask. As a result this method is also required for an optimal B modes pseudospectrum estimation in the context of a satellite mission. With this powerful method, I performed realistic forecasts on the constraints that a CMB polarisation detection could set on the physics of the primordial universe. First of all, I have studied the detectability of the tensor-to-scalar ratio r, amounting the amplitude of primordial gravity waves and directly related to the energy scale of inflation, in the case of current suborbital experiments, a potential array of telescopes and a potential satellite mission. I have shown that a satellite-like experiment dedicated to the CMB polarisation detection will enable us to measure a tensor-to-scalar ratio of about 0.001, thus allowing for distinguishing between large and small field models of inflation. Moreover, in extension of the standard model of cosmology, the CMB EB and TB correlations can be generated. In particular, I have forecast the constraints that one could set on a parity violation in the gravitational waves during the primordial universe from observations on a small and a large part of the sky. Our results have shown that a satellite-like experiment is mandatory to set constraints on a range of parity violation models. I finally address the problematic of the detectability of observational signature of a primordial magnetic field.
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Submitted on : Friday, March 6, 2015 - 10:46:37 PM
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Agnès Ferté. Statistics of the CMB polarised anisotropies : unveiling the primordial universe. Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]. Université Paris Sud - Paris XI, 2014. English. ⟨NNT : 2014PA112223⟩. ⟨tel-01126907⟩

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