The Dark Energy Survey

Galaxy Cluster Counts

Counting galaxy clusters sounds rather simple. The concept is to count the number of galaxy clusters of a given mass within a given volume of the universe and determine how this quantity has changed over time. This quantity is dependent 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)

This method relies on the fact that with powerful computer simulations DES scientists can precisely predict the number of clusters of different mass that should form over time. The observational challenge is that, since astronomers can't plop them down on a bathroom scale, they cannot measure the masses of galaxy clusters directly. However, they have several pieces of information about each cluster that provide strong clues about its mass. DES astronomers will be able to infer approximate cluster masses through their weak gravitational lensing effects on the apparent shapes of more distant galaxies: light from a galaxy behind a cluster is bent by the cluster’s gravitational field, leading to a characteristic warping of the galaxy image. In addition, the number of luminous, red galaxies that a cluster contains provides an indicator of its mass. Finally, clusters are filled with hot gas that shines in X-rays and also scatters the photons in the cosmic microwave background. DES will join forces with the South Pole Telescope, which will measure the photon-scattering effect for several thousand of the clusters DES will detect.

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.

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