The line between modified gravity and dark matter particles isn't always a clean one. This approach splits the difference.
The Milky Way's rotation curve with superfluid dark matter
(Submitted on 16 Mar 2020)
Recent studies have shown that dark matter with a superfluid phase in which phonons mediate a long-distance force gives rise to the phenomenologically well-established regularities of Modified Newtonian Dynamics (MOND). Superfluid dark matter, therefore, has emerged as a promising explanation for astrophysical observations by combining the benefits of both particle dark matter and MOND, or its relativistic completions, respectively. We here investigate whether superfluid dark matter can reproduce the observed Milky Way rotation curve forand are able to answer this question in the affirmative. Our analysis demonstrates that superfluid dark matter fits the data well with parameters in reasonable ranges. The most notable difference between superfluid dark matter and MOND is that superfluid dark matter requires about less total baryonic mass (with a suitable interpolation function). Our analysis further allows us to estimate the radius of the Milky Way's superfluid core and the total mass of dark matter in both the superfluid and the normal phase.UPDATE March 24, 2020:
A Critique of Covariant Emergent Gravity
(Submitted on 23 Mar 2020)
I address some problems encountered in the formulation of relativistic models encompassing the MOND phenomenology of radial acceleration. I explore scalar and vector theories with fractional kinetic terms and-type gravity, demanding that the energy density be bounded from below and that superluminal modes be absent, but also that some consistency constraints with observational results hold. I identify configurations whose energy is unbounded from below and formulate some no-go statements for vector field models and modified gravity theories. Finally, I discuss superfluid dark matter as a hybrid theory lying between CDM and modified gravity, highlighting some difficulties present also in this case, which appears preferable to the others.
A visual illustration (from here) of the circumstances under which galaxy dynamics differ from Newtonian gravity without dark matter (which starts to be observable at a Newtonian gravitational acceleration expectation of approximately 1.2 * 10-10 m/s2, which is the MOND constant a0) The quantity D is the amplitude of the discrepancy, basically the ratio of total mass to that which is visible.