Tests of \(\Lambda\)CDM and Conformal Gravity using GRB and Quasars as Standard Candles out to \(z \sim 8\)

Abstract

We compare the cosmology of conformal gravity (CG), (Mannheim 2006), to \(\Lambda\)CDM. CG cosmology has repulsive matter and radiation on cosmological scales, while retaining attractive gravity at local scales. Mannheim (2003) finds that CG agrees with \(\Lambda\)CDM for supernova data at redshifts \(z<1\). We use GRBs and quasars as standard candles to contrast these models in the redshift range \(0<z<8\). We find CG deviates significantly from \(\Lambda\)CDM at high redshift and that \(\Lambda\)CDM is favoured by the data with \(\Delta\chi^2=48\). Mannheim's model has a bounded dark energy contribution, but we identify a \(\lambda\) fine-tuning problem and a cosmic coincidence problem.