Abstract

We study the imprints of an effective dark energy fluid in the large scale structure of the universe through the observed angular power spectrum of galaxies in the relativistic regime. We adopt the phenomenological approach that introduces two parameters {Q, eta} at the level of linear perturbations and allow to take into account the modified clustering (or effective gravitational constant) and anisotropic stress appearing in models beyond Lambda CDM. We characterize the effective dark energy fluid by an equation of state parameter w = -0.95 and various sound speed cases in the range 10(-6) <= c(s)(2) <= 1, thus covering K-essence and quintessence cosmologies. We calculate the angular power spectra of standard and relativistic effects for these scenarios under the {Q, eta} parametrization, and we compare these relative to a fiducial Lambda CDM cosmology. We find that, overall, deviations relative to Lambda CDM are stronger at low redshift since the behavior of the dark energy fluid can mimic the cosmological constant during matter domination era but departs during dark energy domination. In particular, at z = 0.1 the matter density fluctuations are suppressed by up to similar to 3% for the quintessence-like case, while redshift-space distortions and Doppler effect can be enhanced by similar to 15% at large scales for the lowest sound speed scenario. On the other hand, at z = 2 we find deviations of up to similar to 5% in gravitational lensing, whereas the Integrated Sachs-Wolfe effect can deviate up to similar to 17%. Furthermore, when considering an imperfect dark energy fluid scenario, we find that all effects are insensitive to the presence of anisotropic stress at low redshift, and only the Integrated Sachs-Wolfe effect can detect this feature at z = 2 and very large scales.