Cotton gravity and 84 galaxy rotation curves

TL;DR

This paper investigates Cotton gravity, a generalization of general relativity, and demonstrates that it can explain galaxy rotation curves without dark matter by fitting a linear potential term to observational data of 84 diverse galaxies.

Contribution

The study derives effective field equations for Cotton gravity and applies them to real galaxy data, showing it can account for rotation curves solely with baryonic matter and a galaxy-specific parameter.

Findings

Cotton gravity can fit rotation curves without dark matter.

The linear potential term becomes significant at large distances.

Galaxy rotation curves are explained by baryonic matter and the parameter mma.

Abstract

Recently, as a generalization of general relativity, a gravity theory has been proposed in which gravitational field equations are described by the Cotton tensor. That theory allows an additional contribution to the gravitational potential of a point mass that rises linearly with radius as $\Phi = -GM/r + \gamma r/2$, where $G$ is the Newton constant. The coefficients $M$ and $\gamma$ are the constants of integration and should be determined individually for each physical system. When applied to galaxies, the coefficient $\gamma$, which has the dimension of acceleration, should be determined for each galaxy. This is the same as having to determine the mass $M$ for each galaxy. If $\gamma$ is small enough, the linear potential term is negligible at short distances, but can become significant at large distances. In fact, it may contribute to the extragalactic systems. In this paper, we derive the effective field equation for Cotton gravity applicable to extragalactic systems. We then use the effective field equation to numerically compute the gravitational potential of a sample of 84 rotating galaxies. The 84 galaxies span a wide range, from stellar disk-dominated spirals to gas-dominated dwarf galaxies. We do not assume the radial density profile of the stellar disk, bulge, or gas; we use only the observed data. We find that the rotation curves of 84 galaxies can be explained by the observed distribution of baryons. This is due to the flexibility of Cotton gravity to allow the integration constant $\gamma$ for each galaxy. In the context of Cotton gravity, "dark matter" is in some sense automatically included as a curvature of spacetime. Consequently, even galaxies that have been assumed to be dominated by dark matter do not need dark matter.