A Test of Dark Matter vs IR Modifications to Gravity with Local Milky Way Observables
Many aspects of the well known observation that rotation curves become flat in the outer regions of rotating galaxies are yet to be explained; a statement which remains true even within the context of Dark Matter (DM) theories. These aspects include observational evidence such as the Baryonic Tully Fisher Relation (BTFR) and the Radial Acceleration Relation (RAR), as well as clues which point to correlations between baryons and observed acceleration on a galaxy by galaxy basis. A number of alternatives to DM, which involve IR modifications to the theory of General Relativity (GR) and attempt to explain these observations, have been proposed over the last decades. These theories often suffer from lack of a consistent UV counterpart to the IR theory, as well as difficulties in describing observations at scales larger than galaxies. However, on galactic scales, IR modifications to gravity remain a viable option and should be tested against observations. Importantly, if such a theory describes galactic dynamics, it should be consistent with all local Milky Way (MW) observables. These include local measurements of the rotation curve, the local stellar and gas surface density and the vertical acceleration field around the Solar position, as well as constraints on the baryonic density profile. The current study tests precisely this requirement on a general class of IR modifications to gravity, focusing on theories which predict isotropic amplifications of gravity, i.e. do not distinguish between projections of the acceleration vector onto any coordinate. Results are compared to the same test for a DM theory. The main finding of this study is that an isotropic IR modification to gravity is in tension with observations of the MW's scale radius and bulge mass and that a DM theory exhibits a better fit to the data. Thus, the theory which governs galactic dynamics is likely not isotropic in nature.
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