Constraining the dark energy equation of state with double source plane
  strong lenses
  We investigate the possibility of constraining the dark energy equation of
state by measuring the ratio of Einstein radii in a strong gravitational lens
system with two source planes. This quantity is independent of the Hubble
parameter and directly measures the growth of angular diameter distances as a
function of redshift. We investigate the prospects for a single double source
plane system and for a forecast population of systems discovered by
re-observing a population of single source lenses already known from a
photometrically selected catalogue such as CASSOWARY or from a
spectroscopically selected catalogue such as SLACS. We find that constraints
comparable to current data-sets (15% uncertainty on the dark equation of state
at 68%CL) are possible with a handful of double source plane systems. We also
find that the method's degeneracy between Omega_M and w is almost orthogonal to
that of CMB and BAO measurements, making this method highly complimentary to
current probes.
