Theoretical systematics in galaxy clustering in LCDM and beyond

Theoretical systematics are known assumptions that we use in cosmological observations, often with unknown consequences. Two possible solutions are (i) drop the assumptions and quantify their systematic biases or (ii) construct assumption-independent observables. We will explore both methods in different contexts: (i) We study the impact of neglecting lensing magnification in galaxy clustering analyses for future galaxy surveys, in the ΛCDM model and two extensions: massive neutrinos and modified gravity. We show that the information of the lensing convergence is significant for large-scale test of GR and its modifications, and it concerns both the cosmological parameters constraints and the biases in their best-fit value. We perform a comprehensive investigation of these two effects for the upcoming photometric and spectroscopic galaxy surveys Euclid and SKA for different redshift binning configurations. (ii) We consider the cosmological dynamics on sub-horizon scales and within the quasi-static approximation allowing for a modification of scalar perturbations with respect to the ΛCDM model. From the galaxy and shear lensing bispectra, and all their cross-correlation, we construct quantities that are observable without imposing any specific parametrization for the perturbations. We also find a constraint equation for the linear structure growth rate expressed in terms of model-independent observables.