Main:

Observations of distant supernova, including 1997ff, suggest that over the past few billion years, a mysterious substance called dark energy has caused gravity, at its largest scale, to become repulsive. When the universe was smaller and the density of matter therefore higher, dark energy would have had a negligible effect. Gravity would have exerted its familiar universal attraction, slowing cosmic expansion.
Z. Levay/Space Telescope Science Institute.

 

 
 

The expansion of the universe is accelerating, apparently as the result of a new form of energy, termed ``dark energy.'' An understanding of the nature of dark energy is of fundamental importance in cosmology and physics, possibly even for the unification of the two most important physical theories, general relativity and quantum theory. Galaxy clusters, the largest building blocks of the universe, represent ideal test objects for the study of dark energy because their evolution depends on the nature of dark energy. Such studies require a detailed understanding of the physics of the intracluster gas and knowledge about the distribution of clusters in space and time as a function of cluster mass.