Large-Scale Structure

Principal Investigator: Emiliano Sefusatti


  • Structures in the Universe and Emergent Phenomena

Abstract: This line of research is devoted to cosmological investigations of the statistical properties of large-scale matter perturbations as probed by the galaxy distribution in spectroscopic and photometric redshift surveys, by galaxy cluster abundance, clustering properties and density profile observed at different wavelengths, by 21cm intensity mapping observations and, at smaller scales, the Lyman-alpha forest. These studies involve the theoretical modeling of dark matter, galaxy and gas properties and correlation functions based on both analytical approaches in cosmological perturbation theory and numerical simulations. The goal is the optimal estimation of the cosmological parameters describing the Standard Cosmological 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 research carried out by our members focuses on different observational probes tracing the underlying dark matter distribution with the common goal of determining the statistical properties of matter perturbation and extracting the information they contain on the cosmological model. We often face similar problems and share many of the tools, such as correlation function estimators, analytical and numerical theoretical predictions estimators, error estimation and the analysis pipeline.

Galaxy Clustering: Much work has been devoted in recent years in the development of a full pipeline for the joint analysis of the galaxy power spectrum and bispectrum in spectroscopic surveys. These efforts lead to the PBJ code, about to be made public, already applied to several tests of theoretical models against numerical simulations as well as explorations of beyond LCDM models in the analysis of BOSS data. The same tools have been extensively applied to the preparation for the analysis of the data from the Euclid satellite. Almost all of us are part of the collaboration and many have relevant roles in the Euclid Galaxy Clustering SWG as well as in the software development within the Science Ground Segment. An important, specific goal of our group is the extension of the LCDM model predictions for galaxy clustering to interesting and promising alternative scenarios in the context of well controlled systematic errors, both on the theory and well as observational side. The possibility of probing neutrino masses with large-scale structure observation is an interesting and challenging test case, at the center of our current interests.

Cosmology with galaxy clusters: Arising from the high-density peaks of the initial matter distribution, galaxy clusters bear the imprints of the statistical properties of the matter density field and its evolution, making them an independent and competitive probe of many key cosmological parameters. Our research activity focuses on the cosmological exploitation of galaxy clusters through the analysis of their abundance and spatial distribution over cosmic time. Our team is involved in different activities for the definition of the cosmological pipeline, starting from raw data analysis, leading to the identification of clusters, modeling and calibration of observables and scaling relations, and inferring cosmological constraints. The members of our group are active in various ongoing cluster surveys – e.g. Euclid, LSST, DES, SPT, Chex-Mate – which provide us with a state-of-the-art multi-wavelength view of the galaxy cluster population. In particular, most of the project participants are active members of the Euclid Consortium (EC), involved in the analysis of the photometric cluster survey carried out by the Euclid mission. Within the EC several members of this project hold coordination roles in projects that define the different steps of the cosmological pipeline.

Astrophysics of galaxy clusters: Galaxy clusters are the largest gravitationally bound systems in the Universe, made up of stars, gas, and dark matter. As such they provide a unique laboratory to study gravitational structure formation, galaxy evolution, thermodynamics of the intergalactic medium and plasma physics. In our group we develop and carry out advanced cosmological hydrodynamical simulations of galaxy clusters with the purpose of studying the main processes shaping the observational properties of galaxy clusters at different wavelengths and how they impact on the evolution of their different components: intra-cluster medium, cluster galaxies and the dark matter component. Predictions from simulations are compared with observational data with the purpose of constraining models of galaxy formation in the extreme environment represented by clusters. Understanding galaxy clusters as astrophysical objects is instrumental also for their application as tools to follow cosmic evolution and, ultimately, to constrain cosmological parameters.

21cm Intensity Mapping: The mapping of the 21cm line emission from neutral hydrogen is an optimal tracer of the galaxy and, in turn, matter density perturbations in the Universe. This technique can quickly scan large sky areas preserving at the same time the high-resolution redshift information of the 21cm spectral line providing a rich tomographic picture of the Universe. Our work focuses both on exploring the potentiality of the promising observations (and particularly those of the SKAO facility) as well as tackling the challenges of foreground and instrumental systematics in the ongoing analysis of MeerKAT/MeerKLASS data.

Lyman-α forest: The Lyman-α forest is the main manifestation of the high redshift intergalactic cosmic-web. It is visible in the spectra of quasars (QSOs) and produced by the scattering of the background photons with the neutral hydrogen atoms along the line-of-sight. The Lyman-α forest is a unique probe of geometry and the dynamical state of the Universe, probing diffuse matter around galaxies and in the intergalactic medium (IGM) in regimes which are not covered by other observables, both in terms of redshifts and scales. In our work, we make use of these measurements to constrain dark matter free-streaming and warm dark matter models (sterile neutrinos, gravitinos), fuzzy dark matter, dark photon dark matter, primordial black holes, isocurvature models, interacting dark matter-dark energy scenarios, early dark energy, mass of active neutrinos. In general, any cosmological model which has an impact on the linear matter power spectrum at small 0.1-100 Mpc scales and z=2-6 can be constrained by the forest.

Beyond the Standard LCDM model: One of the major scientific goals of current and forthcoming surveys is to understand the nature of Dark Energy. In our group, there is a devoted effort on this topic, which lead to several analyses and the release of hi_class, a publicly available Einstein-Bolztmann solver for extended theories of gravity. It has been extensively used and validated, and it is the primary tool to test gravity using cosmological data on linear scales. It covers a broad class of models, i.e. the majority of the proposed competitors to the standard LCDM. The next step, also at the center of our work, is going beyond the “linear scales” limitation without restricting to particular and well studied models. This will allow us to take full advantage of the non-linear scales data, to place more stringent cosmological constraints on the theory of gravity. In the context of cluster cosmology studies, we are developing and calibrating theoretical models for the cluster counts, correlation function and density profile to constrain models beyond the LCDM, such as evolving DE (w0 wa), f(R) gravity, and test DM particle properties.

Numerical tools: The investigations described above require several numerical tools, all facing the challenge of increasing accuracy requirements due to the constant improvements in the statistical power of cosmological probes. We have been working on the estimation of correlation functions in Fourier space, providing the implementation and validation of the official processing functions for the Euclid collaboration spectroscopic sample. Along this effort, we also carried out analyses for the calibration of the halo mass function, cluster real-space correlation function and covariance matrices by means of synthetic catalogs. Another important line of work is the estimate of covariance properties among LSS observables following both analytical and numerical approaches.

Activities at IFPU

Researchers afferent to the Large-Scale Structure line of research animate two weekly meetings at IFPU.

A Large-Scale Structure meeting covering galaxy clustering, intensity mapping and Lyman-alpha topics takes place regularly every Thursday at 11:00 at IFPU. It consists in the first place of a progress reports on local activities, but often including contributions from visitors. This group proposed and organised over the years seven Focus Week programs, in addition to hosting meetings such as the recent New Physics from Galaxy Clustering II workshop. More information are available on

A second regular weekly Journal Club on Galaxy Clusters, which has smoothly and successfully continued by remote during the pandemic, is now taking place on Fridays at 2:30pm in a hybrid mode. This JC activity is organized by alternating critical discussions of papers from the arXiv, internal reports on ongoing projects and papers in which IFPU members are involved, and seminars on specific arguments from colleagues external to IFPU. In July the clusters group organized a IFPU Focus Week on Dissecting Cluster cosmology. The primary objective of this workshop was to bring together a select group of about 20 leading experts in various key aspects of cluster cosmology to define a clear path towards fully exploiting the overwhelming quantity and quality of data that will become available in the next decade from cluster surveys conducted at different wavelengths.