Letter to the Editor
Gravitational lenses as cosmic rulers: 
, 
from time delays and velocity dispersions
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: danutas@astro.ku.dk
Received:
16
September
2009
Accepted:
28
October
2009
We show that a cosmic standard ruler can be constructed from the joint measurement
of the time delay, 
, between gravitationally lensed quasar images and the
velocity dispersion, σ, of the lensing galaxy. This is specifically shown, for a singular isothermal sphere lens, 
, where DOL is the angular diameter distance to the lens. Using MCMC simulations we illustrate the constraints set in the 
plane from future observations.
Key words: gravitational lensing / Galaxy: kinematics and dynamics / cosmology: theory / cosmology: dark matter
© ESO, 2009