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Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe

Inspired by the statistical mechanics of an ensemble of interacting particles (BBGKY hierarchy), we propose to account for small-scale inhomogeneities in self-gravitating astrophysical fluids by deriving a non-ideal Virial theorem and non-ideal Navier-Stokes equations using a decomposition of the gravitational force into a near- and far-field component. These equations involve the pair radial distribution function (similar to the two-point correlation function), similarly to the interaction energy and equation of state in liquids. Small-scale correlations lead to a non-ideal amplification of the gravitational interaction energy, whose omission leads to a missing mass problem, e.g., in galaxies and galaxy clusters. We also propose an extension of the Friedmann equations in the non-ideal regime. We estimate the non-ideal amplification factor of the gravitational interaction energy of the baryons to lie between 5 and 20, potentially explaining the observed value of the Hubble parameter. Within this framework, the acceleration of the expansion emerges naturally because of the increasing number of sub-structures induced by gravitational collapse, which increases their contribution to the total gravitational energy. A simple estimate predicts a non-ideal deceleration parameter qni~-1;this is potentially the first determination of the observed value based on an intuitively physical argument. We suggest that correlations and gravitational interactions could produce a transition to a viscous regime that can lead to flat rotation curves. This transition could also explain the dichotomy between (Keplerian) LSB elliptical galaxy and (non-Keplerian) spiral galaxy rotation profiles. Overall, our results demonstrate that non-ideal effects induced by inhomogeneities must be taken into account in order to properly determine the gravitational dynamics of galaxies and the larger scale universe.