Model-independent Test of the Cosmic Distance Duality Relation

TL;DR

This paper presents a model-independent test of the cosmic distance duality relation using strong lensing and HII galaxy data, confirming its validity with high confidence and discussing the impact of lens modeling uncertainties.

Contribution

It introduces a novel, model-independent method combining strong lensing and Gaussian Process reconstruction to test the cosmic distance duality relation.

Findings

The CDD relation is confirmed at high confidence levels.

Simplified lens models introduce scatter, affecting the constraints.

Extended lens models do not significantly reduce scatter, highlighting the need for detailed mass modeling.

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 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.