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21st IAP Colloquium
Mass Profiles and Shapes of Cosmological Structures

The new century has brought two fundamental advances in astrophysics that are the discoveries of the accelerated expansion of the Universe with type Ia supernovae, as well as of the flatness of the Universe with the analysis of the angular fluctuations of the cosmic microwave background. These two discoveries, together with old results on the internal kinematics of structures and new results on the statistics of the large-scale distribution and kinematics of galaxies and weak gravitational lensing effects on galaxy shapes have enabled cosmologists to converge towards a concordance model, called ΛCDM, which describes with surprising success the large-scale properties of the Universe. However, the ΛCDM Universe is mainly built upon two entities of which we do not know the physical nature: dark matter and dark energy. Moreover, there seems to be significant discrepancies with several smaller-scale observations: in particular, in comparison to observational data, structures arising in very large N-body simulations of the ΛCDM Universe appear to be too cuspy in their centres and contain too many sub-structures. These difficulties have generated a lot of interest in theories that modify the behaviour of gravity on large scales, e.g. MOND is able to explain with remarkable success the majority of observational constraints on mass profiles, without recourse to any dark matter.

Is it then time to reconsider ΛCDM because of discrepancies with observations on small scales? Several of these discrepancies are lifted in very recent modellings of mass profiles and shapes of cosmological structures using various techniques, such as the internal kinematics of structures assumed to be in dynamical equilibrium, a similar hydrostatic analysis of X-ray emitting hot gas, as well as the modelling of gravitational lenses. Moreover, several recent cosmological N-body simulations converge towards structures that may be less cuspy in their centres and contain less sub-structure. Conversely, alternative theories of gravitation manage to reproduce increasingly well the observational constraints, and there has been recent progress on building a theoretical foundation for MOND. Morever, there is a growing interest, in the theoretical physics community, for theories where gravity is modified at large distance, with the aim at addressing the dark matter and dark energy puzzles.

We propose a synthesis of ongoing research on the distribution of matter in galaxies and clusters of galaxies. Many questions need answers: Have the cosmological simulations converged or must one increase much more the number of simulated particles? Is there agreement on the mass profiles obtained through different modelling techniques and with the predictions of ΛCDM simulations? Is mass distribution in quasi-spherical manner or is it highly flattened? What fraction of mass is in diffuse form, in between structures? What are the effects of including gas physics on the mass profiles? Can alternative theories explain the full set of observational data and how robust are their theoretical general relativistic foundations? What new key observations need to be realized to constrain much better these alternative theories?

In this context where ideas are rapidly evolving, it seems most useful to bring together simulators, observation modellers and gravitational theorists, with the aim of mixing these three communities to converge in our understanding of the mass distribution in cosmic structures. Our objective is to know whether the concordance ΛCDM model remains realistic or whether the laws of gravity need to be modified to account for the existence of dark matter (and possibly explain at the same time the need for a cosmological constant).