Astrophysical tests of conformal gravity

Abstract

Conformal Gravity (CG) is an alternative gravity theory which aims to explain observations without requiring dark matter or a cosmological constant, as in General Relativity (GR). The theory features an action formed from the Weyl tensor and includes a conformal Yukawa cou- pling to the Higgs field, making the total action invariant under conformal transformations. This thesis aims to test CG using astrophysical observations, including cosmology, lensing and clusters. I use SNIa and quasars as standard candles to show that the SNIa provided strong evi- dence, and the full sample provide decisive evidence against the Mannheim CG cosmology, and in favour of ΛCDM. Following the work of Horne (2006), I compare estimates of the dynami- cal mass from X-ray observations in the cluster A2029 to the predictions from the Mannheim potential in CG. I find that CG can fit the cluster without dark matter, but with the Mannheim potential altered by setting γ∗ = 0 and with different values of the Mannheim parameters than in galaxies. Considering lensing, we present the derivation of the bend angle for a point mass in CG, as derived in Horne et al. (in prep). We find that γ makes no 1st-order contribution to the deflection angle, requiring κ < 0 for CG to produce extra bending by galaxies and clusters without dark matter. However, this contradicts the galaxy and cluster fits that have κ > 0. Using this deflection angle, I fit to the SLACS and SL2S samples of galaxy lenses and to the lensing cluster MS2137.3-2353 to determine κ. I find that for SLACS is κ = −7.2 × 10⁻⁴⁹ m⁻² and for SL2S is κ = −3.3 × 10⁻⁴⁹ m⁻²; The tangential and radial arcs MS2137.3-2353 imply κ = −9.6 × 10⁻⁵¹ m⁻² and κ = −8.4 × 10⁻⁵¹ m⁻², respectively. I find evidence that CG cannot fit both images in MS2137.3-2353 simultaneously with the point mass approximation. From these tests I conclude that CG cannot yet be considered a viable alternative theory of gravity to GR.

Date of Award	27 Jul 2020
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Keith Horne (Supervisor)