The history of the Universe as told by Euclid clusters of galaxies

Euclid galaxy clusters will set powerful constraints on the evolution of the Universe. This is the result obtained by an international team that recently completed a study focused on the role of clusters to be discovered by Euclid. This is an approved satellite mission by the European Space Agency (ESA), scheduled for launch in 2020. The main aim of this mission is to explain the origin of the accelerated expansion of the Universe.

What causes the accelerated expansion of the Universe? This is one of the main questions of current cosmology. The ESA Euclid mission has been designed to answer this question. An international team of scientists, led by B. Sartoris from the University of Trieste, has investigated the role that clusters will play in this mission. Their analysis highlights the strong potential of Euclid clusters in placing constraints on the evolutionary history of the Universe.

Galaxy clusters are the largest structures of the Universe in gravitational equilibrium. Since they are relatively rare objects, their number density depends strongly on some cosmological parameters. This property makes them powerful probes of the evolutionary history of cosmic structures.

The study of Sartoris and collaborators shows that the Euclid satellite will identify photometrically over a million  clusters (see Figure 1), half million at redshifts larger than 1 and 2000 with redshift larger than 2 (corresponding to a distance of 50 billion light years). This huge statistical sample (two orders of magnitude larger than those  currently available) will allow us to study the characteristics of clusters with unprecedented detail, over a wide redshift range reaching close to the epoch of their formation.

Figure 1 - Number density of galaxy clusters identified by the Euclid satellite as a function of redshift, for two different values of the detection threshold.

The use of clusters as cosmological probes requires an estimate of their mass. Since this quantity cannot be observed directly, it is necessary to calibrate scaling relations between mass and an observable cluster property. The new study points out that, given the characteristics of Euclid satellite, it will be possible to calibrate these elationships internally and self-consistently. Using the gravitational lensing effect, the number and the total luminosity of cluster galaxies in clusters, and their velocity distribution, one can calibrate the relationship with the mass with an excellent precision (~10%) up to redshift ~1.5.

With the forthcoming large cosmological data-sets it will be possible to constrain the parameters of the standard cosmological model (LambdaCDM) and its evolution, but also to identify deviations from this model. The accelerated expansion of the Universe could be due to some form of dark energy, or to a deviation from general relativity on large scales (Figure 2). The analysis of Sartoris and collaborators also points out that it will be possible to use the distribution of clusters to estimate the amplitude of possible non-Gaussian fluctuations of the primordial density field.

Figure 2 - Constraints on cosmological parameters that determine the growth of the structures: sigma_8 indicates the structure formation time (structures form earlier for higher values of sigma_8) and gamma. In the standard cosmological model, gamma=0.55; different values indicate deviations from general relativity.

This paper has been recently published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS, 459, 1764-2016, "Next generation cosmology: constraints from the Euclid galaxy cluster survey "). Beside B. Sartoris, the collaboration includes two other astrophysicists of the Astronomical Observatory of Trieste, A. Biviano and S. Borgani.

The results of this study will be crucial for the preparation and optimization of the Euclid cluster survey, and will occupy a central role in the data analysis process.