Noise in Strong Lensing Cosmography

Giant arcs in strong lensing galaxy clusters can provide a purely geometric determination of cosmological parameters, such as the dark energy density and equation of state. We investigate sources of noise in cosmography with giant arcs, focusing in particular on errors induced by density fluctuations along the line-of-sight, and errors caused by modeling uncertainties. We estimate parameter errors in two independent ways, first by developing a Fisher matrix formalism for strong lensing parameters, and next by directly ray-tracing through N-body simulations using a multi-plane lensing code. We show that for reasonable power spectra, density fluctuations from large-scale structure produce > 100% errors in cosmological parameters derived from any single sightline, precluding the use of individual clusters or “golden lenses” to derive accurate cosmological constraints. Modeling uncertainties similarly can lead to large errors, and we show that the use of parametrized mass models in fitting strong lensing clusters can significantly bias the inferred cosmological parameters. We lastly speculate on means by which these errors may be corrected. Subject headings: gravitational lensing — dark matter — cosmological parameters