Using clusters in SZE + x-ray surveys as an ensemble of rulers to
  constrain cosmology
  Ongoing and upcoming surveys in x-rays and SZE are expected to jointly detect
many clusters due to the large overlap in sky coverage. We show that, these
clusters can be used as an ensemble of rulers to estimate the angular diameter
distance, d_A(z). This comes at no extra observational cost, as these clusters
form a subset of a much larger sample, assembled to build cluster number counts
dn/dz. On using this d_A(z), the dark energy constraints can be improved by
factors of 1.5 - 4, over those from just dn/dn. Even in the presence of a mass
follow-up of 100 clusters (done for mass calibration), the dark energy
constraints can be further tightened by factors of 2 - 3 . Adding d_A(z) from
clusters is similar to adding d_L(z), from the SNe observations; for eg., dn/dn
(from ACT/SPT) plus d_A(z) is comparable to dn/dz plus d_L(z) in constraining
Omega_m and sigma_8.
