Cosmology is today a fully-fledged data-driven 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 so-called dark sector, will eventually allow for the study of the non-linear general-relativistic effects.

Since the early 2000's a large debate emerged in the cosmology community around the so-called «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 non-linearity of the Einstein equations, a non-trivial backreaction effect of the small-scale matter inhomogeneities is expected on the average large-scale dynamics of the spacetime. The occurrence of the late-time accelerated expansion of the universe at the same epoch when structure formation becomes non-linear is a tempting coincidence encouraging the backreaction conjecture against the real need of a dark energy component, which is needed instead in a Friedmann-Lemaître-Robertson-Walker 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 point-of-view. Simplified inhomogeneous models have been proposed to interpret the non-trivial dynamics of structures and the light propagation on cosmological scales. Only recently cosmological fully general-relativistic N-body 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 coarse-graining, 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 general-relativistic effects in the late-time 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 general-relativistic 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 20-25 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 N-body simulations (R. Durrer, Geneva), From the averaging problem to backreaction (T. Buchert, CRAL Lyon), Cosmic variance and other fluctuations (N. Kaiser, IfA Honolulu & U. Paris-Sorbonne), Cosmological models beyond ΛCDM (P. Brax, IPhT-CEA Saclay), Bayesian inference in cosmology: statistical evidence against Lambda? (A. Heavens, Imperial College, London).

No fees, lunch provided!

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