Measuring the growth of galaxy clusters
  We suggest how we can use the mass profile of galaxy clusters beyond their
virial radius to measure their mass accretion rate, a key prediction of
structure formation models. The mass profile can be estimated by applying the
caustic technique to dense redshift surveys of clusters and their outskirts,
where dynamical equilibrium does not necessarily hold. An additional probe of
the mass growth of clusters is their mass fraction in substructures. We show
that the caustic technique, that identifies cluster substructures as a
by-product, returns catalogs of substructures with mass larger than a few
$10^{13}h^{-1}M_\odot$ that are between 60% and 80% complete, depending on the
density of the redshift survey.
