Interposing a Varying Gravitational Constant Between Modified Newtonian Dynamics and Weak Weyl Gravity

Abstract

The Newtonian gravitational constant $G$ obeys the dimensional relation $[G] [M] [a] = [v]^4$, where $M$, $a$, and $v$ denote mass, acceleration, and speed, respectively. Since the baryonic Tully-Fisher (BTF) and Faber-Jackson (BFJ) relations are observed facts, this relation implies that $G\, a = {\rm constant}$. This result cannot be obtained in Newtonian dynamics which cannot explain the origin of the BTF and BFJ relations. An alternative, modified Newtonian dynamics (MOND) assumes that $G=G_0$ is constant in space and derives naturally a characteristic constant acceleration $a=a_0$, as well as the BTF and BFJ relations. This is overkill and it comes with a penalty: MOND cannot explain the origin of $a_0$. A solid physical resolution of this issue is that $G \propto a^{-1}$, which implies that in lower-acceleration environments the gravitational force is boosted relative to its Newtonian value because $G$ increases. This eliminates all problems related to MOND's empirical cutoff $a_0$ and yields a quantitative method for mapping the detailed variations of $G(a)$ across each individual galaxy as well as on larger and smaller scales. On the opposite end, the large accelerations produced by $G(a)$ appear to be linked to the weak-field limit of the fourth-order theory of conformal Weyl gravity.