Chandra Release - May 18, 2004

Visual Description: Galaxy Clusters and Dark Energy

These Chandra images show 3 galaxy clusters - from left to right - of Abell 2029, MS 2137.3-2353, and MS 1137.5+6624, seen as they looked 1 billion, 3.5 billion, and 6.7 billion years ago, respectively. The background of the image is black, with textured orange blobs in the centers.  These colors represent the intensity of the X-ray emissions from the galaxy clusters, with brighter areas appearing in lighter orange, and dimmer areas in darker orange.  The three clusters are a mixture of circular and elongated shapes, with the first cluster at left the largest and most elliptical, and the second two clusters much smaller and more circular.  These images of galaxy clusters as well as others not shown were used to study the rate of expansion of the Universe. A galaxy cluster is comprised of hundreds of galaxies embedded in a cloud of extremely hot gas, shown in the X-ray images, and invisible dark matter, which is needed to hold the cluster together. X-ray observations were used to determine the ratio of hot gas to dark matter in the clusters. Because galaxy clusters are assumed to be a fair sample of the matter in the Universe, the ratio of hot gas and dark matter should be the same for every cluster. This assumption can be used to make an independent determination of the distances to the clusters. These distances show that the expansion of the universe stopped slowing down about 6 billion years ago and began to accelerate, indicating that the repulsive effects of dark energy are now dominant.