Cosmic variance of H0 in light of forthcoming high-redshift surveys

Forthcoming surveys will extend the understanding of cosmological large scale structures up to unprecedented redshift. According to this perspective, we present a fully relativistic framework to evaluate the impact of stochastic inhomogeneities on the determination of the Hubble constant. To this aim, we work within linear perturbation theory and relate the fluctuations of the luminosity distance-redshift relation, in the cosmic concordance model, to the intrinsic uncertainty associated with the measurement of H₀ from high-redshift surveys (0.15 ≤z ≤3.85 ). We first present the detailed derivation of the luminosity distance-redshift relation two-point correlation function and then provide analytical results for all the involved relativistic effects, such as peculiar velocity, lensing, time delay, and (integrated) Sachs-Wolfe, and their angular spectra. Hence, we apply our analytical results to the study of the high-redshift Hubble diagram, according to what has been recently claimed in the literature. Following the specifics of Euclid Deep Survey and LSST, we conclude that the cosmic variance associated with the measurement of the Hubble constant is at most of 0.1%. Our work extends the analysis already done in literature for closer sources, where only peculiar velocity has been taken into account. We then conclude that deep surveys will provide an estimation of the H₀ which will be more precise than the one obtained from local sources, at least in regard of the intrinsic uncertainty related to a stochastic distribution of inhomogeneities.