Cosmological implications of the Gaia Milky Way declining rotation curve

Although the existence of dark matter has been widely acknowledged in the cosmology community, its nature remains unknown despite decades of research. This calls into questions its very existence. This never-ending search for dark matter has led us to consider alternatives. Since increasing the enclosed mass is the only way to explain the flat appearance of galaxies' rotation curves in a Newtonian framework, modified Newtonian dynamics (MOND) theory proposes a modification to Newton's dynamics when the acceleration of matter at given radius is around or below a threshold value, a_0. Observed rotation curves, generally flat at large distances, are then usually well reproduced by MOND with an a_0 of ∼ 1.2 m/s^2. However, recent Gaia evidence indicates a decline in the Milky Way rotation curve, which a distinct behavior. Therefore, we examine whether MOND can accommodate the Gaia declining rotation curve of the Milky Way. We first use a standard model to describe the Milky Way's baryonic components. Secondly, we show that a Navarro, Frenk, and White model is able to fit the decline, assuming a scale radius (R_s) on the order of $4$ kpc. In a third step, we show that the usual MOND paradigm is not able to reproduce the declining part in a standard baryonic model. Finally, we examine whether the MOND theory can accommodate the declining part of the rotation curve when the characteristics of the baryonic components are relaxed. To do so, we used a Markov chain Monte Carlo method to determine the characteristics of the stellar and text H disks, including their masses, that would result in the best possible fit of the decline. We find that the stellar disk should be massive, on the order of 10^11 M_⊙. The text H disk mass is capped at nearly 1.8 times 10^ M_⊙ but could also be negligible. Finally, a_0 is consistent with 0, with an upper limit of 0.53 m/s^2 (95%), a value much lower than that usually advocated to explain standard flat rotation curves in MOND theory (∼ 1.2 m/s^2).