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We compare the cosmology of conformal gravity (CG), (Mannheim 2006), to$\Lambda$CDM. CG cosmology has repulsive matter and radiation on cosmologicalscales, 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 theredshift range $0<z<8$. We find CG deviates significantly from $\Lambda$CDM athigh redshift and that $\Lambda$CDM is favoured by the data with$\Delta\chi^2=48$. Mannheim's model has a bounded dark energy contribution, butwe identify a $\lambda$ fine-tuning problem and a cosmic coincidence problem.

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