Organisateur : Simona Mei (GEPI)

Galaxy clusters are one of our most valuable laboratories for studying cosmology and
structure formation. Their abundance and its evolution with redshift is recognized as a
powerful probe of the low redshift universe that, when combined with CMB and other
constraints, can strongly test the cosmological model and possible extensions, such as
evolving dark energy and neutrino mass, or even modifications to standard gravity.
Moreover, high redshift clusters are windows onto the mechanisms driving galaxy formation,
and whose very existence, if sufficiently massive, can pose serious challenge to the standard
model.
In the last year Sunyaev-Zeldovich surveys (SZ) cluster surveys from Planck, the South Pole
Telescope (SPT) and the Atacama Cosmology Telescope (ACT) produced a wealth of
exciting new cosmological results ; most notably, the tantalizing tension with the primary
CMB constraints from Planck : clusters and other large-scale structure measures prefer lower
values for the matter density and amplitude of the density perturbations. This may indicate
new Physics, such as heavier-than-expected neutrinos, new cluster physics, such as heavierthan-
expected clusters, or both. This has been one of the most intriguing cosmological results
in that past year, emphasizing the importance of clusters as an observational tool. In lock step
with the questions posed by this result, the past year has also seen the release of awaited mass
measurement results from lensing experiments, such as CLASH and the Weighing the Giants
program.
The potential of cluster cosmology has never been so clear, nor the precise nature of the
challenges that we now face. These challenges include more accurate determination of multiwavelength
cluster scaling relations (across the electromagnetic spectrum), of cluster masses,
and of survey selection functions. These are key questions for upcoming cluster surveys : the
next generation ground-based SZ surveys SPTpol and ACTpol with first light in 2012 and
2013, respectively, and the next data release from Planck in 2014 ; Spitzer deep and wide
surveys (SERVS, SSDF) ; the recently started Dark Energy Survey (DES), the completed The
Next Generation Virgo Cluster Survey (NGVS) et le DAFT/FADA ; results from the XXL
survey and the approaching launch of eROSITA ; and the rapidly advancing gravitational
lensing observations of clusters (CLASH, LOCUSS, etc). To this we must add preparation of
the large optical/NIR surveys by Euclid, the Large Synoptic Survey Telescope (LSST) and
WFIRST.
These surveys will also be pushing the redshift frontier with large samples of objects at
redshifts beyond unity, into epochs crucial in the history of the baryonic mass assembly and
galaxy formation in massive structures. In these epochs, we witness the assembly of galaxy
clusters and the transformation of their galaxy population.
The year 2014 lies at the pinnacle of the international effort in this dynamic field, a year that
will see new results from SPTpol, ACTpol, Planck, with their follow-up, from DES, SERVS
and SSDF, and from CLASH and other lensing efforts. It will be a decisive moment for
consolidating the wealth of new information and for formulating the next steps that will
define the way we use clusters as probes of both cosmology and structure formation. These
first steps are key for the preparation of our future large scale surveys, such as Euclid, LSST,
eRosita, WFIRST.