Comparing different realizations of modified Newtonian dynamics: Virial theorem and elliptical shells

There exists several modified gravity theories designed to reproduce the empirical Milgrom’s formula, modified Newtonian dynamics (MOND). Here we derive analytical results in the context of the static weak-field limit of two of them (bimetric MOND, leading for a given set of parameters to the quasilinear MOND, and Bekenstein’s Tensor-Vector-Scalar). In this limit, these theories are constructed to give the same force field for spherical symmetry, but their predictions generally differ out of it. However, for certain realizations of these theories (characterized by specific choices for their free functions), the binding potential-energy of a system is increased, compared to its Newtonian counterpart, by a constant amount independent of the shape and size of the system. In that case, the virial theorem is exactly the same in these two theories, for the whole gravity regime and even outside of spherical symmetry, although the exact force fields are different. We explicitly show this for the force field generated by the two theories inside an elliptical shell. For more general free functions, the virial theorems are, however, not identical in these two theories. We finally explore the consequences of these analytical results for the two-body force.

Figure 1

Effective density predicted by (deep) MOND inside an elliptical shell (see Sec. 3; the baryonic density is zero inside the dashed-dotted line). The contours are for $|r^2{\rho }_{\mathrm{eff}}|=0$ (black) to $|r^2{\rho }_{\mathrm{eff}}|={10}^{-3}$ (white) in steps of ${10}^{-4}$. The dark band in the middle indicates zero density, the upper grey areas have negative densities, and the lower ones positive. In QUMOND, this effective density is zero inside the shell.