Gravitational lenses as cosmic rulers: density of dark matter and dark
  energy from time delays and velocity dispersions
  We show that a cosmic standard ruler can be constructed from the joint
measurement of the time delay (dt) between gravitationally lensed quasar images
and the velocity dispersion (sigma^2) of the lensing galaxy. This is
specifically shown, for a singular isothermal sphere lens, where the angular
diameter distance to the lens is proportional to dt/sigma^2. Using MCMC
simulations we illustrate the constraints set in the density of dark matter and
dark energy plane from future observations.
