The mass power spectrum in quintessence cosmological models

We present simple analytic approximations for the linear and fully evolved nonlinear mass power spectrum of matter density fluctuations for spatially flat cold dark matter (CDM) cosmological models with quintessence (Q). Quin tessence is a time-evolving, spatially inhomogeneous energy component with negative pressure and an equation of state w(Q) < 0. It clusters gravitatio nally on large length scales but remains smooth like the cosmological const ant on small length scales. We show that the clustering scale is determined by the Compton wavelength of the Q-field and derive a shape parameter, Gam ma(Q), to characterize the linear mass power spectrum, The growth of linear perturbations as functions of redshift, w(Q), and matter density, Omega(m) , is also quantified. Calibrating to N-body simulations, we construct a sim ple extension of Ma's 1998 formula that closely approximates the nonlinear power spectrum for a range of plausible QCDM models.