One of the most debated issues in modern astrophysics is the current tension between the measurements of the Universe expansion rate based on Early- and Late-Universe probes. Indeed, there is a discrepancy in the value of the Hubble constant, H0, as derived from measurements of the anisotropy of the cosmic microwave background (CMB) and as measured from a series of distance indicators in the local Universe (Planck Collaboration+2020).  Currently, there is a 5σ disagreement between Early-vs-Late Universe measurements (see Riess et al. 2022, ApJ, 934, L7). This could point to a serious measurement error, or the standard model of particle physics/standard cosmological model to be revised. It is, therefore, crucial to improve the accuracy of the cosmic distance ladder and quantify the H0 tension and the associated systematics.

This project aims to face this problem from the point of view of stellar distance indicators,  tackling uncertainties and systematics affecting the classical cepheids (DCEPs) which are the main stellar standard candles adopted in the cosmic distance scale. Since their discovery, the Period-Luminosity (PL) and Period-Wesenheit (PW) relations for DCEPs represent the fundamental tool at the basis of the extra-galactic distance scale (e.g. Leavitt & Pickering 1912; Madore 1982; Caputo+2000, Riess+2016). In this context, one of the residual sources of uncertainty in the cosmic distance ladder is represented by the metallicity dependence of the PL/PW relations used to calibrate the secondary distance indicators such as Supernovae Ia. Such dependence must be taken into account to avoid systematic effects in the calibration of the extragalactic distance scale (e.g. Romaniello+2008).