Peter Schneider, Dominique Sluse
The light travel time differences in strong gravitational lensing systems allows an independent determination of the Hubble constant. This method has been successfully applied to several lens systems. The formally most precise measurements are, however, in tension with the recent determination of H_0 from the Planck satellite for a spatially flat six-parameters $\Lambda CDM$ cosmology. We reconsider the uncertainties of the method, concerning the mass profile of the lens galaxies, and show that the formal precision relies on the assumption that the mass profile is a global power law. Simple analytical and numerical experiments reveal that mass-sheet like transformations yield significant freedom in choosing the mass profile, even when exquisite Einstein rings are observed. Furthermore, the characterization of the environment of the lens will not break that degeneracy which is not physically linked to extrinsic convergence. We present an illustrative example where the multiple imaging properties of a composite (baryons + dark matter) lens can be extremely well reproduced by a power-law model, but with predictions for the Hubble constant that deviate by about 20\%. Hence we conclude that the impact of degeneracies between parametrized models might have been underestimated in current H_0 measurements from lensing, and need to be carefully reconsidered.
View original:
http://arxiv.org/abs/1306.0901
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