Gravitational lenses as cosmic rulers: Ωm, ΩΛ from time delays and velocity dispersions

We show that a cosmic standard ruler can be constructed from the joint measurement of the time delay, Δτ, between gravitationally lensed quasar images and the velocity dispersion, σ, of the lensing galaxy. This is specifically shown, for a singular isothermal sphere lens, D OL ∝ Δτ/σ 2 , where D OL is the angular diameter distance to the lens. Using MCMC simulations we illustrate the constraints set in the Ω m , - Ω Λ plane from future observations.