Data analysis of Euclid’s NISP instrument: from the first data to the study of cosmological parameters

Over the past two decades, the standard cosmological model, the so-called ΛCDM model, has achieved remarkable success. We have been able to accurately measure the power spectrum of the cosmic microwave background (CMB) as observed by the Planck experiment. Additionally, the galaxy clustering measurements from Large Scale Structure (LSS) surveys are compatible with the standard model. Despite ΛCDM’s fit to our observations, tensions arise in measurements between the different datasets. These tensions could be attributed to either new physics or to unknown systematical effects, opening new scientific objectives for future experiments. Extensions to ΛCDM model could provide more insights into the true nature dark energy. Unraveling the mysteries of this dark sector stands as the central question for cosmology in the coming decades. One of the prominent analyses in this direction is to correlate the data from CMB and LSS experiments, such as Euclid. The idea, as it will be discussed in this thesis, is to exploit the additional information provided by this cross-correlation in order to reach more stringent constraints on the nature of the dark sector. In particular, the work discussed in this thesis is based on the analysis of the Integrated Sachs-Wolfe effect. This effect arises at late time from the interaction between the CMB photons and an LSS tracer, e.g. the distribution of galaxies. The iSW, which is imprinted in the CMB spectrum at very large scales, encodes information on the nature of dark energy. Unfortunately, the iSW signal is difficult to detected from the CMB aloe, as it is subdominant with respect to the other temperature anisotropies. We can exploit the correlation with the distribution of galaxies. In particular, this thesis aims to forecast the detection significance of the iSW effect from the cross-correlation of the Euclid galaxy distribution, detected by the Near Infrared Spectrometer and Photometer (NISP) instrument, with the CMB temperature perturbations. In this work, we first develop a promising estimator of the temperature-galaxies cross-correlation observable, based on a type of spherical wavelets, and we compare it to the widely used harmonics estimator. Second, we validate the likelihood approximation that will be implemented on the official Euclid analysis for the estimation of cosmological parameters by means of the cross-correlation signal. This thesis aims to prove the robustness of the cross-correlation analysis between the large-scale CMB temperature anisotropies and the Euclid galaxy distribution, in preparation for the first data arriving in the next years.