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Cosmology is today a fullyfledged datadriven science. Forthcoming instruments like PFS, 4MOST, DESI, LSST, Euclid, WFIRST, eROSITA or SKA will produce massive amount of data allowing for precise and accurate studies of galaxy clustering, gravitational lensing, supernovae Ia, and galaxy clusters. Most of the cosmological observing programs, which are primarily intended to investigate the socalled dark sector, will eventually allow for the study of the nonlinear generalrelativistic effects.
Since the early 2000's a large debate emerged in the cosmology community around the socalled «averaging problem» (Buchert 2000; Ellis & Buchert 2005), a question introduced on a more general ground in general relativity by G.F.R. Ellis in 1962. Because on the nonlinearity of the Einstein equations, a nontrivial backreaction effect of the smallscale matter inhomogeneities is expected on the average largescale dynamics of the spacetime. The occurrence of the latetime accelerated expansion of the universe at the same epoch when structure formation becomes nonlinear is a tempting coincidence encouraging the backreaction conjecture against the real need of a dark energy component, which is needed instead in a FriedmannLemaîtreRobertsonWalker setting. Besides, the same backreaction mechanism also accounts for an energy source dynamically equivalent to the dark matter. These arguments are often supported by toy models but not demonstrated in general, both from a theoretical and observational pointofview. Simplified inhomogeneous models have been proposed to interpret the nontrivial dynamics of structures and the light propagation on cosmological scales. Only recently cosmological fully generalrelativistic Nbody simulations have been realised, offering a valuable ground to investigate the geometry of a realistic, "lumpy" universe.
Indeed, the actual amplitude of the backreaction effect, whether it requires a fully nonlinear general relativistic treatment or whether a perturbative approach is sufficient, the impact of the gauge choice, of coarsegraining, and of averaging procedures in defining the observables are still open problems. On the observational side, the reliability and accuracy of the estimators of backreaction  and more generally of observables that take into account generalrelativistic effects in the latetime universe  are also matter of an animated debate; issues like cosmic variance, kinematic bias, or Malmquist bias can potentially mask or could be confused with the genuine generalrelativistic effects. Finally the correct statistical interpretation of observational data and cosmological parameters in a given background is a delicate concern.
CosmoBack is the opportunity to bring together two communities who rarely meet: theoretical cosmologists who actively work on inhomogeneous models and general relativity, possibly open to the backreaction conjecture, and physical cosmologists and astronomers including the leaders of the major past, ongoing, and future observational projects, which standardly use the FLRW model to interpret the data. In order to foster stimulating interactions the contributed talk will be 2025 minutes long, every day will be concluded by a keynote session led by a senior scientist, and the attendance will be limited to about 50 people. The closing discussion of the workshop will be introduced by a review by G.F.R. Ellis (University of Cape Town). Inspiring discussions about the actual opportunity to revisit the cosmological standard model are expected!
In order to assure a broad audience and a rare opportunity for Ph.D. students and curious researchers, this workshop will be prepared by a series of six introductory lectures for a total of about ten hours: Measures in General Relativity (T. Schucker, CPT Marseille), Relativistic cosmology: From perturbation theory to Nbody simulations (R. Durrer, Geneva), From the averaging problem to backreaction (T. Buchert, CRAL Lyon), Cosmic variance and other fluctuations (N. Kaiser, IfA Honolulu & U. ParisSorbonne), Cosmological models beyond ΛCDM (P. Brax, IPhTCEA Saclay), Bayesian inference in cosmology: statistical evidence against Lambda? (A. Heavens, Imperial College, London).
No fees, lunch provided!
