Modified gravity, Dark Energy and MOND

TL;DR

This paper introduces modified gravity models with curvature-dependent actions that can explain galactic rotation curves without dark matter and predict observable deviations from Newtonian gravity at short distances.

Contribution

The paper proposes a class of ghost-free modified gravity actions that incorporate curvature scalars logarithmically, linking late-time cosmic acceleration to local gravitational phenomenology.

Findings

Models recover Newtonian gravity near sources due to scalar mass dependence on curvature.

Predict a long-distance gravity modification and short-distance deviations around 0.1mm on Earth.

Potential to explain galaxy rotation curves without dark matter.

Abstract

We propose a class of actions for the spacetime metric that introduce corrections to the Einstein-Hilbert Lagrangian depending on the logarithm of some curvature scalars. We show that for some choices of these invariants the models are ghost free and modify Newtonian gravity below a characteristic acceleration scale given by a_0 = c\mu, where c is the speed of light and \mu is a parameter of the model that also determines the late-time Hubble constant: H_0 \sim \mu. In these models, besides the massless spin two graviton, there is a scalar excitation of the spacetime metric whose mass depends on the background curvature. This dependence is such that this scalar, although almost massless in vacuum, becomes massive and effectively decouples when one gets close to any source and we recover an acceptable weak field limit at short distances. There is also a (classical) ``running'' of Newton's constant with the distance to the sources and gravity is easily enhanced at large distances by a large ratio. We comment on the possibility of building a model with a MOND-like Newtonian limit that could explain the rotation curves of galaxies without introducing Dark Matter using this kind of actions. We also explore briefly the characteristic gravitational phenomenology that these models imply: besides a long distance modification of gravity they also predict deviations from Newton's law at short distances. This short distance scale depends on the local background curvature of spacetime, and we find that for experiments on the Earth surface it is of order \sim 0.1mm, while this distance would be bigger in space where the local curvature is significantly lower.