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Evolution of the large structures of the Universe and the counting of galaxy clusters with LSST

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It is proposed to evaluate in terms of cosmological constraints the contribution of the statistics of galaxy clusters collected by the LSST telescope (Large Synoptic Survey Telescope) which is one of the projects that aims at a systematic and very deep mapping of the distribution of galaxies in the Universe.

From 2022 the LSST project will conduct a survey for ten years of the whole sky from a site in the southern hemisphere using a large-field telescope equipped with a camera of more than 3 billion "pixels". It will take 3 nights to complete the observation of the accessible sky in 6 wavelengths. After 10 years, objects up to magnitude 27 will be detected. The first validation data will be available from 2020-21.

One of the objectives of this program is to study the growth of structures and understand better the nature of dark energy. This systematic mapping of objects up to a redshift between 1 and 2, will result in a catalogue of over 3 billion galaxies and several tens of thousands of galaxy clusters.

These clusters, which are composed of hundreds to thousands of galaxies bathed in hot gas, are the largest structures in the Universe. The mass of these objects is divided between galaxies (2%), hot intergalactic gas (13%) and dark matter (85%). The statistics collected will enable us to extract unrivalled information on the history of the formation of these objects and thereby giving us the most accurately measured rate of growth of linear fluctuations under the action of gravity. This growth rate is sensitive to the model used to describe gravitation (general relativity or modified gravitation model) as well as the mass of neutrinos. This measurement provides complementary and independent information on geometric measurements obtained with supernovae, baryon oscillations or the cosmic background of radiation.

The purpose of the proposed internship is to determine the mass and redshift distribution of the clusters observable with the LSST and to extract the constraints on the cosmological parameters. The characteristics of the observed objects, the effects of the atmosphere and the performance of the detector will be modelled with parametrizations. It will also be interesting to compare the performances of the different galaxy surveys (EUCLID, DESI and LSST) and evaluate their complementarities. This work requires basic knowledge in cosmology and a mastery of computer languages Python and C.

Supervisor : Yves Zolnierowki (

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