  We explore the signatures of quintessence and modified gravity theories in
the relativistic description of galaxy clustering within a parametrized
post-Friedmann framework. For this purpose, we develop a calibration method to
consistently account for horizon-scale effects in the linear parametrized
Post-Friedmann perturbations of minimally and nonminimally coupled
scalar-tensor theories and test it against the full model-specific
fluctuations. We further study the relativistic effects in galaxy clustering
for the normal and self-accelerating branches of the Dvali-Gabadadze-Porrati
braneworld model as well as for phenomenological modifications of gravity. We
quantify the impact of modified gravity and dark energy models on galaxy
clustering by computing the velocity-to-matter density ratio F, the velocity
contribution R, and the potential contribution P and give an estimate of their
detectability in future galaxy surveys. Our results show that, in general, the
relativistic correction contains additional information on gravity and dark
energy, which needs to be taken into account in consistent horizon-scale tests
of departures from LCDM using the galaxy-density field.
