Model-independent Test of the Cosmic Distance Duality Relation
  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 z^2,
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.
