Model-independent Test of the Cosmic Distance Duality Relation

A validation of the cosmic distance duality (CDD) relation, , coupling the luminosity (d _L) and angular-diameter (d _A) distances, is crucial because its violation would require exotic new physics. We present a model-independent test of the CDD, based on strong lensing and a reconstruction of the H ii galaxy Hubble diagram using Gaussian processes, to confirm the validity of the CDD at a very high level of confidence. Using parameterizations and , our best-fit results are , and and , respectively. In spite of these strong constraints, however, we also point out that the analysis of strong lensing using a simplified single isothermal sphere (SIS) model for the lens produces some irreducible scatter in the inferred CDD data. The use of an extended SIS approximation, with a power-law density structure, yields very similar results, but does not lessen the scatter due to its larger number of free parameters, which weakens the best-fit constraints. Future work with these strong lenses should therefore be based on more detailed ray-tracing calculations to determine the mass distribution more precisely.