A Critique of Covariant Emergent Gravity

Abstract

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 f (R)-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.

On Filaments, Prolate Halos and Rotation Curves

“…However, DM self-interactions tend to favor rounder halo shapes [15], and the MOND framework can only mimic spherical or slightly oblate DM distributions [16]. As a result, ascertaining the actual presence of prolate halos would also provide additional evidence to discriminate among different scenarios, some of which also entail problematic features [17,18].…”