The Copernican principle in light of the latest cosmological data

We pursue a program to confront observations with inhomogeneous extensions of the FLRW metric. The main idea is to test the Copernican principle (CP) rather than assuming it a priori. We consider the ΛCDM model endowed with a spherical ΛLTB inhomogeneity around us, that is, we assume isotropy and test the hypothesis of homogeneity. We confront the ΛLTB model with the latest available data from cosmic microwave background, BAO, type Ia supernovae, local H₀, cosmic chronometers, Compton y-distortion, and kinetic Sunyaev-Zeldovich effect. We find that these data can constrain tightly this extra inhomogeneity, almost to the cosmic variance level: on scales ≳ 100 Mpc structures can have a small non-Copernican effective contrast of just δ_L ~ 0.01. Furthermore, the constraints on the standard ΛCDM parameters are not weakened after marginalizing over the parameters that model the local structure, to which we assign ignorance priors. In other words, dropping the CP assumption does not imply worse constraints on the cosmological parameters. This positive result confirms that the present and future data can be meaningfully analyzed within the framework of inhomogeneous cosmology.