Galaxy Clustering

Principal Investigator: Emiliano Sefusatti


  • Astroparticle Physics
  • Early Universe
  • Structures in the Universe and Emergent Phenomena
  • Theory and Phenomenology of Gravity

Abstract: This line of research is devoted to the statistical analysis of the large-scale distribution of galaxies as observed in spectroscopic and photometric redshift surveys. It entails the theoretical modelling of matter and halo/galaxy correlation functions with analytical predictions from cosmological perturbation theory and their comparison against results from numerical N-body simulations. The goal is the optimal estimation of the cosmological parameters describing the Standard Model, including neutrino masses, as well as extensions due to modified theories of gravity, dark energy, general relativistic effects and non-Gaussian features in the early Universe.

Status of project and perspectives: The group is currently involved in several projects ranging from addressing observational and data analysis problems (also with direct applications to the Euclid mission) to more speculative investigations on the modelling of the large-scale structure in galaxy and weak lensing surveys and intensity mapping measurements. One project of the group consists in developing a pipeline for the joint analysis of the power spectrum and higher-order correlation functions, such as the bispectrum, of the galaxy distribution. A likelihood code capable of analysing simulated catalogs in redshift space is about to be made public and has been used for testing models in perturbation theory in several papers. In the near future we will be working on more technical aspects related to the analysis of actual data, such as the estimation of galaxy correlation functions, the effects due to the window function of the survey and the covariance properties of large-scale structure observations. The problem of a robust estimate of the covariance matrix for Fourier-space correlation function, in particular, is being explored both in terms of numerical measurements based on a large-set of mocks as well as in terms of analytical predictions based on perturbation theory. In particular, the measurement of galaxy clustering is subject to several systematics that must be controlled to percent level, while the residual uncertainty must be propagated to the covariance matrix used for parameter inference. This involves the definition of the spectroscopic sample, redshift measurements, the estimation of the clustering statistics and the models and simulation themselves that induce a so-called theory error. We are developing a pipeline to predict the effect of specific systematics on the measurement of the power spectrum of a set of mock galaxy catalogs, with the aim of quantifying the impact of each systematics and propagate the uncertainty of its mitigation to parameter estimation. Specific projects investigate the modelling of nonlinearities at medium and small scales in massive neutrino cosmologies, including the development of massive neutrino halo models, and of baryonic corrections to the matter power spectrum by means of hydrodynamical simulations of structure formation. At the same time, we explore departures from the standard cosmology, testing alternatives to the CDM paradigm such as mixed Warm+Cold & Warm Dark Matter or Fuzzy Dark Matter, using dedicated N-body simulations and tailored HOD to determine effects on the galaxy distribution. The large-scale structure can also provide important tests on the early Universe, discriminating among alternative models of inflation with constraints on a possible non-Gaussianity of primordial origin. We explore different techniques to achieve this goal from the traditional analysis of summary statistics to innovative topological tools. Almost all members of the group are actively involved, to various degrees, in the Euclid Mission, bringing their expertise to different tasks from the correlation function estimation pipeline, observational systematic effects, galaxy clustering likelihood analysis and beyond standard model constraints. The group has a regular meeting at IFPU every Thursday serving in the first place as a progress report and occasionally as a Journal Club or hosting talks by external visitors. We sponsored a few IFPU activities, including a meeting of the Euclid Galaxy Clustering WP in October 2019, a meeting on recent developments in Large-Scale Structure modelling in Perturbation Theory in July 2022 and a meeting of the Euclid Observational Systematics WP in September 2022. Additional informal workshops on more technical aspects of the preparatory work for Euclid also have taken place at IFPU.