The nonlinear power spectrum in clustering quintessence cosmologies
  We study the nonlinear evolution of density perturbations in cosmologies where the late-time accelerated expansion is driven by a quintessence field with vanishing speed of sound. For these models matter and quintessence perturbations are comoving and it is possible to write a single continuity equation for the total density fluctuations given by a weighted sum of the two components. Including the Euler equation for the common velocity field we solve the evolution equations for the nonlinear, total density power spectrum in the Time-Renormalization Group approach. In fact any cosmological observable is directly related by gravity only to the total density perturbations, with the two components being individually unobservable. We show that the clustering of quintessence perturbations induces small corrections with respect to the nonlinear evolution of power spectrum in smooth quintessence models described by the same equation of state. Such small corrections, however, contrast with the large effect of a vanishing speed of sound on the linear growth function at low redshift. For this reason, models with the same normalization of the linear density power spectrum can present significantly different nonlinear corrections depending on the value of the sound speed. Although such differences vanish in the w \rightarrow -1, {\Lambda}CDM limit, we argue that the relation between linear and nonlinear growth of structures should be properly taken into account in constraining models with inhomogeneous dark energy. 
Guido D'Amico, Emiliano Sefusatti
astro-ph.CO gr-qc hep-th