Confronting the sound speed of dark energy with future cluster surveys
  Future cluster surveys will observe galaxy clusters numbering in the hundred
thousands. We consider this work how these surveys can be used to constrain
dark energy parameters: in particular, the equation of state parameter w and
the non-adiabatic sound speed c_s^2. We demonstrate that, in combination with
Cosmic Microwave Background (CMB) observations from Planck, cluster surveys
such as that in the ESA Euclid project will be able to determine a
time-independent w with subpercent precision. Likewise, if the dark energy
sound horizon falls within the length scales probed by the cluster survey, then
c_s^2 can be pinned down to within an order of magnitude. In the course of this
work, we also investigate the process of dark energy virialisation in the
presence of an arbitrary sound speed. We find that dark energy clustering and
virialisation can lead to dark energy contributing to the total cluster mass at
approximately the 0.1% level at maximum.
