The Lambda-CDM cosmological model is succesful at reproducing various independent sets of observations concerning the large-scale Universe. This model is however currently, and actually in principle, unable to predict the gravitational field of a gal
axy from it observed baryons alone. Indeed the gravitational field should depend on the relative contribution of the particle dark matter distribution to the baryonic one, itself depending on the individual assembly history and environment of the galaxy, including a lot of complex feedback mechanisms. However, for the last thirty years, Milgroms formula, at the heart of the MOND paradigm, has been consistently succesful at predicting rotation curves from baryons alone, and has been resilient to all sorts of observational tests on galaxy scales. We show that the few individual galaxy rotation curves that have been claimed to be highly problematic for the predictions of Milgroms formula, such as Holmberg II or NGC 3109, are actually false alarms. We argue that the fact that it is actually possible to predict the gravitational field of galaxies from baryons alone presents a challenge to the current Lambda-CDM model, and may indicate a breakdown of our understanding of gravitation and dynamics, and/or that the actual lagrangian of the dark sector is very different and richer than currently assumed. On the other hand, it is obvious that any alternative must also, in fine, reproduce the successes of the Lambda-CDM model on large scales, where this model is so well-tested that it presents by itself a challenge to any such alternative.
Gaia is an ambitious ESA space mission which will provide photometric and astrometric measurements with the accuracies needed to produce a kinematic census of almost one billion stars in our Galaxy. These data will revolutionize our understanding of
the dynamics of the Milky Way, and our knowledge of its detailed gravitational potential and mass distribution, including the putative dark matter component and the non-axisymmetric features such as spiral arms. The Gaia mission will help to answer various currently unsettled questions by using kinematic information on both disk and halo stellar populations. Among many others: what does the rotation curve of the outer Galaxy look like? How far from axisymmetry and equilibrium is the Galaxy? What are the respective roles of hierarchical formation and secular evolution in shaping the Galaxy and its various components? Are the properties of the Galaxy in accordance with expectations from the standard model of cosmology?
We summarize the status of Modified Newtonian Dynamics (MOND) in galaxy clusters. The observed acceleration is typically larger than the acceleration threshold of MOND in the central regions, implying that some dark matter is necessary to explain the
mass discrepancy there. A plausible resolution of this issue is that the unseen mass in MOND is in the form of ordinary neutrinos with masses just below the experimentally detectable limit. In particular, we show that the lensing mass reconstructions of the clusters 1E0657-56 (the bullet cluster) and Cl0024+17 (the ring) do not pose a new challenge to this scenario. However, the mass discrepancy for cool X-ray emitting groups, in which neutrinos cannot cluster, pose a more serious problem, meaning that dark baryons could present a more satisfactory solution to the problem of unseen mass in MOND clusters.
