  As the nodes of the cosmic web, clusters of galaxies trace the large-scale
distribution of matter in the Universe. They are thus privileged sites in which
to investigate the complex physics of structure formation. However, the
complete story of how these structures grow, and how they dissipate the
gravitational and non-thermal components of their energy budget over cosmic
time, is still beyond our grasp. Fundamental questions such as How do hot
diffuse baryons accrete and dynamically evolve in dark matter potentials? How
and when was the energy that we observe in the ICM generated and distributed?
Where and when are heavy elements produced and how are they circulated? are
still unanswered. Most of the cluster baryons exists in the form of a diffuse,
hot, metal-enriched plasma that radiates primarily in the X-ray band (the
intracluster medium, ICM), allowing the X-ray observations of the evolving
cluster population to provide a unique opportunity to address these topics.
Athena+ with its large collecting area and unprecedented combination of high
spectral and angular resolution offers the only way to make major advances in
answering these questions. Athena+ will show how the baryonic gas evolves in
the dark matter potential wells by studying the motions and turbulence in the
ICM. Athena+ will be able to resolve the accreting region both spatially and
spectroscopically, probing the true nature and physical state of the X-ray
emitting plasma. Athena+ has the capabilities to permit a definitive
understanding of the formation and evolution of large-scale cosmic structure
through the study of the cluster population.
