The Dark Energy Survey - Galaxy Cluster Counts

Galaxy Cluster Counts

An important experimental probe of dark enery comes from counting galaxy clusters. The idea is simple: we can use cosmological models (theories) calculate how many galaxy clusters should be visible, then we can use telescopes to count how many galaxy clusters we actually see. Thus comparing how galaxy clusters are distributed in mass and redshift to predictions made by cosmological models, we can test these cosmological models.

The number of galaxy clusters that form depends on dark energy in two ways. First, dark energy influences how the universe expands, so it affects how the volume grows over time. Second, the formation of a galaxy cluster depends upon the interplay between gravity and dark energy. By measuring how the number of clusters grows over time, DES will probe the relative strengths of these two forces. In this method, DES scientists will measure the abundance of clusters at different times in the past by measuring the number of clusters with a particular redshift. DES will count some galaxy clusters so far away that the light the DES camera sees from them today left the clusters when the universe was less than half its current size! (z~1.3)

Unlike the supernovae and BAO methods, which are only sensitive to cosmic distances and thereby to the expansion rate, galaxy clusters probe both distances and the rate of growth of structure in the universe. By comparing results between these two different classes of probes, cosmologists can determine whether the current theory of gravity, Einstein's General Relativity Theory, is sufficient to explain cosmic acceleration.