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.