Model-independent Test of the Cosmic Distance Duality Relation (1808.09331v1)
Abstract: A validation of the cosmic distance duality (CDD) relation, eta(z)=(1+z)2 d_A(z)/d_L(z)=1, 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 HII galaxy Hubble diagram using Gaussian Processes, to confirm the validity of the CDD at a very high level of confidence. Using parameterizations eta(z) = 1 + eta_0 z and eta(z) = 1 + eta_1 z + eta_2 z2, our best-fit results are eta_0 = 0.0147{+0.056}_{-0.066}, and eta_1 = 0.1091{+0.1680}_{-0.1568} and eta_2 = -0.0603{+0.0999}_{-0.0988}, 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.