Sound speed of scalar field dark energy: Weak effects and large uncertainties

Abstract

The possibility of a reconstruction of the Lagrangian for the scalar field dark energy with constant effective sound speed c_s is analyzed. It is found that such a reconstruction can be made with accuracy up to an arbitrary constant. The value of c_s is estimated together with other dark energy parameters (Ω_{d e} , w₀, and c_a²) and main cosmological ones on the basis of data including Planck-2013 results on cosmic microwave background (CMB) anisotropy, baryon acoustic oscillation distance ratios from recent galaxy surveys, the galaxy power spectrum from WiggleZ, magnitude-redshift relations for distant SNe Ia from SNLS3 and Union2.1 compilations, and the Hubble Space Telescope determination of the Hubble constant. It is shown that no value of c_s from the range [0,1] is preferred by the used data because of the very weak influence of dark energy perturbations on the large scale structure formation and CMB temperature fluctuations.