Machian MOND: a variable $a_0$ in galaxy clusters

Abstract

Modified Newtonian Dynamics (MOND) generally resolves the need for dark matter in galaxy rotation curves introducing a single new constant of acceleration $a_0$. It is well known that increasing $a_0$ by a factor of a few can alleviate the residual mass discrepancies that MOND leaves in galaxy clusters. Within a parameter-free Machian interpretation of MOND, in which $a_0\sim G{M}_u/R_u^2$ arises from the scalar sum of inverse-square distance gravitational mass contributions in the universe, we promote $a_0$ to a variable influenced by mass external to a locally enclosed region in the spherically symmetric case. Instead of a boost of $a_0$ in terms of gravitational potentials as in EMOND, we show that a boost in terms of this directionless inverse-square field roughly amounts to the boost needed to accommodate the mass discrepancies of MOND in galaxy clusters. We conclude by beginning to generalize the proposed formulation beyond spherical symmetry.

18 pages, 2 figures, 4 graphs, 4 tables, published version in International Journal of Modern Physics D