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Context. One of the main questions in modern physics concerns the origin of the Universe and the mechanisms at work in the very first moments after the Big Bang. Thanks to the considerable progress made over the past decade, largely driven by the European-led Planck satellite, the stage is set to begin to address this question formally and precisely. In this context, the most promising observational probe is the cosmic microwave background (CMB) and in particular its polarization. Indeed, many theories about the beginnings of the Universe predict that gravitational waves were generated in the very first moments of the Universe. These models typically invoke new physics beyond the Standard Model, which determines the properties of primordial gravitational waves and which, therefore, carries clues to the nature of these new physical laws. The detection of primordial gravitational waves could therefore have a revolutionary impact on our understanding of cosmology and fundamental physics, while explaining the origin of structures observed in the Universe. This detection constitutes the scientific goal of our team at the APC, supported in part by the European project SciPol (scipol.in2p3.fr).
The most promising way to characterize the early Universe is to detect divergence-free polarization patterns in CMB emission, called "B-modes" which, to first order, can only be generated by primordial gravitational waves. Although recent experiments have established strict constraints on their amplitude, there is to date no convincing detection of such a signal. However, the most advanced efforts, such as the Simons Observatory, should begin to achieve sufficient sensitivities for detection of this signal, as many theories predict. The next few years will therefore be particularly promising, exciting and fruitful for this area of research, with evidence in favor of or against some of these models finally becoming available.
The impact of CMB polarization is also broader than that. It will also shed new light on many other key questions in modern physics, such as those concerning the nature of dark matter and dark energy, the total mass of neutrinos and their mass hierarchy, or the presence of species of unknown relativistic particles.
In all these cases, the information sought is contained in tiny anisotropies of the CMB polarization, which must be recovered in the enormous volumes of data collected by current and future experiments; then differentiated from other unwanted signals; and finally characterized with unprecedented precision and robustness. The cosmological signal is dominated by parasitic signals which correspond in particular to systematic instrumental and astrophysical effects. The method followed by our team is to characterize the polarization of the CMB and the cosmological signals contained therein via modeling, characterization and subtraction of these undesirable instrumental and astrophysical effects. Thus, the success and importance of CMB polarization as one of the key probes of modern cosmology depends on the success of data analysis which has emerged, over the last decade as one of the main technologies allowing, alongside and in the same way as instrumental progress, the success of this scientific enterprise.
Thesis. The proposed thesis is part of the field of CMB data analysis and proposes to contribute to the international effort to exploit the scientific potential of CMB polarization by developing new methods and techniques adapted to a statistical characterization of CMB polarization signals. These tools and methods will then be applied to the analysis of data from the Simons Observatory, and tested on simulations of future CMB-S4 and LiteBIRD projects.
The Simons Observatory is an international project whose scientific campaign is expected to start in the summer of 2024. The APC CMB group is heavily involved and plays an important coordination role. Located at 5200m in the Atacama Desert, this state-of-the-art experiment will bring unprecedented volumes of data and achieve a sensitivity that will significantly advance the current state of the art in this field, achieving several of the objectives scientists mentioned above, in particular by providing strong constraints on the amplitude of primordial gravitational waves. As part of the proposed thesis work, the student will develop methods for analyzing the “Small Aperture Telescopes” of the Simons Observatory, and will participate in the tasks of characterization, simulations and monitoring of operations specific to large instruments.
The weakness of B-mode polarization relative to other signals makes data analysis difficult, but even partial progress in this area will have a huge impact on our ability to fully exploit the scientific potential of data sets of the CMB and, therefore, of the entire field. Mastery of cutting-edge digital tools, complex statistical concepts and access to quality data will make the student a true leader in this scientific field.
SciPol, the Simons Observatory collaboration and the team at the APC. Modern CMB data analysis is a multidisciplinary effort, drawing on statistical methods, signal processing, machine learning, high-performance scientific computing, as well as physics and cosmology. The student will have the opportunity to collaborate with experts in all these fields, capitalizing on the active research collaborations developed by our group in France and internationally. This will enable the student to develop broad and engaging research experience and scientific training, thus preparing them for a future career in diverse contexts. In addition, the student will have the opportunity to participate concretely in the observation campaigns of the Simons Observatory, and to carry out long-term visits to our collaborators in Berkeley, Princeton, Tokyo, Milan or even Santiago de Chile.
The student will have the opportunity to become a full member of three international collaborations, Simons Observatory, LiteBIRD and CMB-S4, which bring together some of the best experts in the field.
The current SciPol team includes five doctoral students (2 in the first year, 1 in the second year and 2 in the third year), an computer scientist and a postdoctoral student. Additional members will be recruited in 2024 and 2025, providing a stimulating and collaborative research environment.
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