Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion

Aims. This work wants to show how standard General Relativity is able to explain galactic rotation curves without the need for dark matter, this starting from the idea that when Einstein’s equations are applied to the dynamics of a galaxy embedded in an expanding universe they do not reduce to Poisson’s equation but a generalisation of it taking cosmological expansion into account. Methods. A non-linear scheme to perturb Einstein’s field equations around the Robertson-Walker metric is devised in order to find their non-relativistic limit without losing their characteristic non-linearities. The resulting equation is used to numerically study the gravitational potential of a cosmological perturbation and applied to a simple galactic model with an exponentially decreasing baryonic matter distribution. Results. The non-relativistic limit of General Relativity in a Robertson-Walker space-time produces a generalised Poisson equation for the gravitational potential which is non-linear, parabolic and heat-like. It is shown how its non-linearities generate an effective "dark matter " distribution caused by both cosmological expansion and the dynamics of the perturbation’s gravitational potential. It is also shown how this dynamical effect gets completely lost during a linearisation of Einstein’s equations. The equation is then used to successfully fit real galactic rotation curves numerically using a matter distribution following the shape of a simple Sérsic luminosity profile, common to most galaxies, thus without recourse to dark matter. A relation for the dark to luminous matter ratio is found, explaining the domination of dark matter in low-mass galaxies. A few rotation curves with a faster than Newtonian decrease are also presented and successfully fitted, opening the way to a new possible interpretation of these phenomena in terms of an effective "anti-gravitational " dark matter distribution, purely geometrical in origin. Key words. Dark Matter – Gravitation – Galaxies: Kinematics and Dynamics – Galaxies: Halos From my point of view one cannot arrive, by way of theory, at any at least somewhat reliable results in the field of cosmology, if one makes no use of the principle of relativity. —Albert Einstein (Miscellaneous 1970) 1