DAVIDE SCIOTTI

Dottore di ricerca

ciclo: XXXVI


co-supervisore: R. Maoli, R.Scaramella, V.F. Cardone

Titolo della tesi: Deriving Cosmological Parameters from the Euclid mission

The Euclid mission will substantially advance our comprehension of Cosmology thanks to the unprecedented constraining power. However, such a precise experiment requires a careful modelling of subtle, previously neglected effects. In particular, deviations from Gaussianity in the distribution of the fields probed by large-scale structure surveys generate additional terms in the data covariance matrix, increasing the uncertainties in the measurement of the cosmological parameters. Super-sample covariance (SSC) is among the largest of these non-Gaussian contributions, with the potential to significantly degrade constraints on some of the parameters of the cosmological model under study -- especially for weak lensing cosmic shear. We compute and validate the impact of SSC on the forecast uncertainties on the cosmological parameters for the Euclid photometric survey, obtained with a Fisher matrix analysis, both considering the Gaussian covariance alone and adding the SSC term. We find the SSC impact to be non-negligible -- almost halving the Figure of Merit (FoM) of the dark energy parameters in the 3x2pt survey and substantially increasing the uncertainties on the best-constrained parameters for cosmic shear; photometric galaxy clustering, on the other hand, is less affected due to the lower probe response. The relative impact of SSC does not show significant changes under variations of the redshift binning scheme, while it is smaller for weak lensing when marginalising over the multiplicative shear bias nuisance parameters, which also leads to poorer constraints on the cosmological parameters. Finally, we explore how the use of prior information on the shear and galaxy bias changes the SSC impact. Improving shear bias priors does not have a significant impact, while galaxy bias must be calibrated to sub-percent level to increase the Figure of Merit by the large amount needed to achieve the value when SSC is not included. Lastly, we include angular, redshift-dependent scale cuts in the 3x2pt analysis through the use of the BNT transform, and characterize the dependence of the FoM and the other cosmological parameters as a function of the maximum scale included. This exercise allows us to conclude that, to meet the requirements for the mission in terms of FoM, scales at which the different nonlinear models start to substantially disagree must be included.

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