2004MNRAS.354.1255S

We discuss a new method that is potentially capable of constraining cosmological parameters using observations of giant luminous arcs in rich X-ray clusters of galaxies. The mass profile and the mass normalization of the lenses are determined from X-ray measurements. The method also allows us to probe the amount and equation of state of the dark energy in the universe. The analysis of a preliminary sample of six luminous, relatively relaxed clusters of galaxies strongly favours an accelerating expansion of the universe. Under the assumption that the dark energy is in the form of a cosmological constant, the data provide an estimate of Ω_Λ0= 1.1 with a statistical error of ±0.2. Including the constraint of a flat universe and an equation of state for the dark energy w_X≥-1, we obtain Ω_M0= 0.10±0.10 and w_X=-0.84±0.14. Relaxing the prior on w_X, we find that the null energy condition (NEC) is satisfied at the 3σ confidence level.