Dynamics with a non-standard inertia-acceleration relation: an alternative to dark matter

Abstract

We investigate particle laws of motion derived from nonstandard kinetic actions of a special form. We are guided by a phenomenological scheme--the modified dynamics (MOND)--that imputes the mass discrepancy observed in galactic systems to a departure from Newtonian dynamics below a certain acceleration scale a0. In the limit a0 goes to 0 the theory goes to Newtonian dynamics. In the opposite limit the action becomes proportional to 1/a0. Galilei-invariant such theories must be strongly non-local; this is a blessing, as such theories need not suffer from the illnesses that are endemic to higher-derivative theories. We discuss the possibility that such a modified law of motion is an effective theory resulting from the elimination of degrees of freedom pertaining to the universe at large (the near equality a0=cH0 being a trace of that connection). A virial relation for bounded trajectories is derived. Exact solutions are obtained for circular orbits, which pertain to rotation curves of disk galaxies. We also explore, in passing, theories that depart from the conventional Newtonian dynamics for very low frequencies. The present formulation contrasts, on important accounts, with earlier, modified-gravity formulations.