Practical Cosmology with Lenses
  Surveys with submillimetre telescopes are revealing large numbers of
gravitationally lensed high-redshift sources. I describe how, in practice,
these lensed systems could be simultaneously used to estimate the values of
cosmological parameters, test models for the evolution of the distribution of
dark-matter halos and investigate the properties of the source population. Even
the existing sample of lenses found with the Hershcel Space Observatory is
enough to formally rule out the standard models of the evolving population of
dark-matter halos, with the likely explanation a combination of baryon physics
and the perturbation by infalling baryons of the density distribution of dark
matter at the centres of the halos. Independently of the evolution of the
halos, observations of a sample of 100 lensed systems would be enough to
estimate Omega_Lambda with a precision of 5% and observations of 1000 lenses
would be enough to estimate w, the parameter in the equation-of-state of dark
energy, with a precision similar to that obtained from the Planck observations
of the cosmic microwave background. While the fraction of submillimetre sources
that are lensed depends weakly on the specific halo mass function that is used
in the model, it depends very strongly on the evolution of the submillimetre
luminosity function of the source population. Therefore measurements of the
lensing fraction could be used to investigate galaxy evolution in a way that is
independent of the properties of the intervening halos.
