Parameterizing the Effect of Dark Energy Perturbations on the Growth of Structures

We present an analytical fit to the growth function of the dark matter perturbations when dark energy perturbations are present. The growth index depends upon the dark energy equation of state w, the speed of sound of the dark energy fluctuations, the dark matter abundance and the observed comoving scale. The growth index changes by ~5% for small speed of sound and large deviations of w from -1 with respect to its value in the limit of no dark energy perturbations.