Forecasts for f(R) gravity with Euclid's primary probes

1 and 2σ joint marginal error contours on the cosmological parameters for a flat f (R) model with | fR0 | = 5 × 10−6 in the optimistic scenario. Purple is for GCsp , blue for WL, orange for the combination GCsp +WL, and yellow for all the photometric probes including their cross-correlation XCph , combined with GCsp , namely GCsp +WL+GCph +XCph . While the WL probe is unable to properly constrain the Hubble parameter h and the primordial slope ns on its own, the orthogonality of the contours for GCsp and WL in the subspaces involving h and ns , helps to lift degeneracies and further improves the fully-marginalized constraints on log10 | fR0|, when probe combinations are used.