Gauge conditions in combined dark energy and dark matter systems
  When analysing a system consisting of both dark matter and dark energy, an
often used practice in the literature is to neglect the perturbations in the
dark energy component. However, it has recently been argued, through the use of
numerical simulations, that one cannot do so. In this work we show that by
neglecting such perturbations one is implicitly making a choice of gauge. As
such, one no longer has the freedom to choose, for example, a gauge comoving
with the dark matter -- in fact doing so will give erroneous, gauge dependent
results. We obtain results consistent with the numerical simulations by using
the formalism of cosmological perturbation theory, and thus without resorting
to involved numerical calculations.
