Using distant globular clusters as a test for gravitational theories

Abstract

We propose to determine the stellar velocity dispersions of globular clusters in the outer halo of the Milky Way in order to decide whether the dynamics of the universe on large scales is governed by dark matter or modified Newtonian dynamics (MOND). We show that for a number of galactic globular clusters, both the internal and the external accelerations are significantly below the critical acceleration parameter $a_0$ of MOND. This leads to velocity dispersions in case of MOND which exceed their Newtonian counterparts by up to a factor of 3, providing a stringent test for MOND. Alternatively, in case high velocity dispersions are found, these would provide the first evidence that globular clusters are dark matter dominated.