Testing cosmology with double source lensing

Abstract Double source lensing provides a dimensionless ratio of distance ratios, “remote viewing” cosmology through distances relative to the gravitational lens, beyond observer. We use this test cosmological framework, particularly with respect spatial curvature and duality relation. derive consistency equation for constant curvature, allowing not only investigation flat vs curved but Friedmann-Lemaître-Robertson-Walker framework itself. For duality, we demonstrate that evolution lens mass profile slope must be controlled ≳ 5 times tighter fractional precision than claimed violation. Using LensPop forecasts double systems in Euclid LSST surveys also explore constraints on dark energy state parameters any slope.