Probing Dark Energy and Modified Gravity with Galaxy Clusters with Euclid

The primary objective of cosmological research in the coming decade is to understand the accelerated expansion of the Universe.  Is it caused by a new cosmic component commonly called dark energy or a modification to General Relativity?  The abundance of galaxy clusters as a function of their mass and its evolution with redshift is a sensitive probe of the expansion rate and of structure formation.  Detecting and counting clusters is therefore an observational method probe of both dark energy and of possible modifications to General Relativity.

The thesis topic is the development of methods to use clusters detected by the European Euclid space mission as a dark sector probe.  Euclid will detect clusters as overdensities of galaxies through visible and infrared imaging of the extragalactic sky.  The thesis research will focus on directly incorporating cluster mass determinations from gravitational lensing into the cosmological analysis of cluster abundance. This is a critical aspect of cluster cosmology and the current limiting factor in its application.  Thanks to its high-resolution imaging, Euclid will enable significant advance.  The work will involve propagating mass uncertainties from photometric redshift and shape measurement uncertainties forward to the cosmological analysis.  The student will work in the context of the Euclid consortium and will use simulated Euclid data. 

The APC laboratory is heavily involved in preparation of the Euclid mission and shares responsibility of the Cluster Science Working Group within the consortium.

Contact: James Bartlett ( and Cyrille Rosset (


James Bartlett and Cyrille Rosset