Dissecting Cluster Cosmology - Stage 0

Tuesday 1 Sept 2020, 15:30 → 19:30 Europe/Rome

Institute for the Fundamental Physics of the Universe (IFPU – Trieste)

In the last decade, astronomical data-sets have started to be widely and quantitatively used by the scientific community to address important physical questions such as: the nature of the dark matter and dark energy components and their evolution; the physical properties of the baryonic matter; the sum of neutrino masses; the nature of gravity over megaparsec scales and over cosmic times.

Most of these results are based on well-established geometrical cosmological probes (e.g., galaxies, supernovae, cosmic microwave background). Galaxy clusters provide a complementary and necessary approach, as their distribution as a function of time and observables is sensitive to both the geometrical and the dynamical evolution of the Universe, driven by the growth of structures.

Galaxy clusters trace in fact the extreme peaks in the matter density field on Mpc scales. The abundance of these peaks as a function of mass and redshift is highly sensitive to the matter density and the growth of structure and, under the assumption that their abundance can be accurately predicted for a given cosmology, the measurements of cluster abundance can yield powerful cosmological constraints.

Current cosmological constraints from galaxy clusters have demonstrated to provide a consistency test of the DE paradigm and the validity of general relativity (GR).
The ability to cleanly select clusters out to the redshift at which cosmic acceleration takes over is particularly important to disentangle between the DE paradigm e possible modifications of GR. Ideally, a cluster survey must be able to provide a sample of clusters which:

• extends to high-redshift

• presents a clean selection-function

• has associated observables tightly connected to the cluster mass.

While the dominant systematics in current cluster cosmological studies is associated to the capability of accurate and precisely constrain the mass-observable relation for cluster samples, the extremely large statistics of samples widely extending both the mass and redshift range of upcoming cluster surveys (e.g., SPT-3G, eRosita, Euclid and LSST) will stress every single aspect associated to cluster cosmology.

In particular a careful and detailed characterization of all the relevant processes will be needed, including:

• the identification of the cluster samples in multiwavelength data-sets

• the accurate characterization of the associated selection function

• the accurate and precise process of calibration of the mass-observable relation

• the accurate and precise calibration of the theoretical uncertainties associated to the halo-mass function and of the halo bias

• the correct definition of the likelihood

This workshop aims at bringing together leading experts in all the above-mentioned relevant topics, from the very first steps of the definition of a cluster survey to the final cosmological posteriors, in order to define a path toward fully exploiting the overwhelming quantity and quality of data available in the next decade from cosmological surveys at different wavelengths.

Proposed activities
The workshop will be organized in thematic discussions, each focusing on a specific topic that needs to be addressed to firmly establish clusters as credible cosmological tools in the era of large surveys. Each of such discussions will be opened by a limited number of talks (1-2 of about 30 mins each), followed by an open and frank discussion, that will be chaired by a participant expert of the specific topic of the discussion.

List of thematic discussions

Cluster identification and selection function:

• optical/near-IR photometry;
• X-rays;
• Sunyaev-Zeldovich effect.

Calibration of halo masses:

• weak lensing;
• dynamical methods;
• hydrostatic equilibrium;
• scaling relations.

Likelihood definition:

• likelihood for number counts;
• likelihood for clustering;
• definition of (co-)variances.

Theoretical systematics:

• calibrations of halo mass function;
• calibration of the halo bias
• calibration of cosmic (co-)variances.

 

Talks and discussion of Stage 0