Galactic rotation curves in modified gravity with nonminimal coupling between matter and geometry

Abstract

We investigate the possibility that the behavior of the rotational velocities of test particles gravitating around galaxies can be explained in the framework of modified gravity models with nonminimal matter-geometry coupling. Generally, the dynamics of test particles around galaxies, as well as the corresponding mass deficit, is explained by postulating the existence of dark matter. The extra terms in the gravitational field equations with geometry-matter coupling modify the equations of motion of test particles and induce a supplementary gravitational interaction. Starting from the variational principle describing the particle motion in the presence of the nonminimal coupling, the expression of the tangential velocity of a test particle, moving in the vacuum on a stable circular orbit in a spherically symmetric geometry, is derived. The tangential velocity depends on the metric tensor components, as well as on the coupling function between matter and geometry. The Doppler velocity shifts are also obtained in terms of the coupling function. If the tangential velocity profile is known, the coupling term between matter and geometry can be obtained explicitly in an analytical form. The functional form of this function is obtained in two cases, for a constant tangential velocity and for an empirical velocity profile obtained from astronomical observations, respectively. All the physical and geometrical quantities in the modified gravity model with nonminimal coupling between matter and geometry can be expressed in terms of observable/measurable parameters, like the tangential velocity, the baryonic mass of the galaxy, and the Doppler frequency shifts. Therefore, these results open the possibility of directly testing the modified gravity models with nonminimal coupling between matter and geometry by using direct astronomical and astrophysical observations at the galactic or extragalactic scale. © 2010 The American Physical Society.