Effect of the effective number of relativistic degrees of freedom N_eff = 3.044 + ΔN_eff on the linear total matter power spectrum, presented as ratios with respect to the ΛCDM spectrum with N_eff = 3.044. Top: Density and Hubble parameters {Ω_m, 0, Ω_b, 0, h} fixed only. Bottom: Quantities best constrained by CMB data, that is, {z_eq, Ω_b, 0h², θ_s}, are fixed to show what is left for Euclid to measure. Here, we assume only massless neutrinos.