# Gauge Choice in Conformal Gravity

## TL;DR

This paper discusses gauge choices in conformal gravity, showing that the linear term in the Mannheim-Kazanas metric can be eliminated or reversed by gauge transformations, questioning the necessity of Higgs fields for galactic rotation fits.

## Contribution

It demonstrates that the linear term in the Mannheim-Kazanas metric is not essential and can be removed or altered through gauge transformations, challenging previous assumptions about its role in galactic dynamics.

## Key findings

- The linear term can be eliminated without Higgs fields.
- Gauge transformations can reverse the sign of the linear term.
- Effects of the linear term are too small to be observationally significant.

## Abstract

In a recent paper (MNRAS 458, 4122 (2016)) K. Horne examined the effect of a conformally coupled scalar field (referred to as Higgs field) on the Mannheim-Kazanas metric $g_{\mu\nu}$, i.e. the static spherically symmetric metric within the context of conformal gravity (CG), and studied its effect on the rotation curves of galaxies. He showed that for a Higgs field of the form $S(r) = S_0 a/(r + a)$, where $a$ is a radial length scale, the equivalent Higgs-frame Mannheim-Kazanas metric $\tilde{g}_{\mu\nu} = \Omega^2 g_{\mu\nu}$, with $\Omega = S(r)/S_{0}$, lacks the linear $\gamma r$ term, which has been employed in the fitting of the galactic rotation curves without the need to invoke dark matter. In this brief note we point out that the representation of the Mannheim-Kazanas metric in a gauge where it lacks the linear term has already been presented by others, including Mannheim and Kazanas themselves, without the need to introduce a conformally coupled Higgs field. Furthermore, Horne argues that the absence of the linear term resolves the issue of light bending in the wrong direction, i.e. away from the gravitating mass, if $\gamma r > 0$ in the Mannheim-Kazanas metric, a condition necessary to resolve the galactic dynamics in the absence of dark matter. In this case we also point out that the elimination of the linear term is not even required because the sign of the $\gamma r$ term in the metric can be easily reversed by a simple gauge transformation, and also that the effects of this term are indeed too small to be observed.