# Emergent gravity in galaxies and in the Solar System

## TL;DR

This paper evaluates emergent gravity as an alternative to dark matter, finding it only marginally fits galaxy rotation curves and strongly conflicts with Solar System observations, thus challenging its viability.

## Contribution

It critically tests emergent gravity's predictions against galaxy and Solar System data, revealing significant discrepancies and limitations.

## Key findings

- Emergent gravity produces marginal fits to galaxy rotation curves.
- The theory predicts unobserved features in rotation curves.
- Predictions for Solar System perihelion shifts are vastly inconsistent with observations.

## Abstract

It was recently proposed that the effects usually attributed to particle dark matter on galaxy scales are due to the displacement of dark energy by baryonic matter, a paradigm known as emergent gravity. This formalism leads to predictions similar to Modified Newtonian Dynamics (MOND) in spherical symmetry, but not quite identical. In particular, it leads to a well defined transition between the Newtonian and the modified gravitational regimes, a transition depending on both the Newtonian acceleration and its first derivative with respect to radius. Under the hypothesis of the applicability of this transition to aspherical systems, we investigate whether it can reproduce observed galaxy rotation curves. We conclude that the formula leads to marginally acceptable fits with strikingly low best-fit distances, low stellar mass-to-light ratios, and a low Hubble constant. In particular, some unobserved wiggles are produced in rotation curves because of the dependence of the transition on the derivative of the Newtonian acceleration, leading, even in the most favorable case, to systematically less good fits than MOND. Then, applying the predicted transition from emergent gravity in a regime where it should a priori be applicable, i.e. in spherical symmetry and outside of the bulk of matter, we show that the predictions for the secular advances of Solar System planets' perihelia are discrepant with the data by seven orders of magnitude, ruling out the present emergent gravity weak-field formula with high confidence.