Gauge Choice in Conformal Gravity

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 µν , 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 metricg µν = Ω 2 g µν , with Ω = S(r)/S 0 , lacks the linear γr 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 γr > 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 γr 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.

Astrophysical gravitational waves in conformal gravity

“…Several authors claimed that light deflection is problematic in CG, but possibly there is a way out [38,[49][50][51][52][53][54][55][56]. The work of Perlick and Xu [57] represented the major criticism on CG for a long time.…”

Gravitational Light Bending in Weyl Gravity and Schwarzschild–de Sitter Spacetime

“…[31] pointed out that this term should be positive, so that it enhances the bending angle, in the same way that it provides an explanation of the flat galactic rotational curves in the absence of dark matter. This issue was settled [53,54] when it was shown that the sign of the linear term γr in the metric ( 6) can be reversed using a conformal transformation ds 2 → Ωds 2 followed by a coordinate transformation…”