Free Astronomy Magazine May-June 2015

SPACE CHRONICLES lensing display. The supernova may have appeared as a single image some 20 years ago elsewhere in the cluster field, and it is expected to re- appear once more within the next five years. This prediction is based on computer models of the cluster, which describe the various paths the supernova light is taking through the maze of clumpy dark matter in the galactic group- ing. Each im- age takes a dif- ferent route through the cluster and ar- rives at a dif- ferent time, due, in part, to traveling at the same speed and bound for the same location. Each train, however, takes a different route, and the distance for each route is not the same. Some trains travel over hills. Others go through valleys, and still others chug around mountains. Because the trains travel over different track lengths across different terrain, they do not arrive at their destination at the same time. Similarly, the supernova images do not appear at the same time because some of the light is delayed by travel- ing around bends created by the gravity of dense dark matter in the intervening galaxy cluster. "Our model for the dark matter in the cluster gives us the prediction of when the next image will appear be- cause it tells us how long each train track is, which correlates with time," said Steve Rodney of the Johns Hop- kins University in Baltimore, Mary- land, leader of the FrontierSN team. "We already missed one that we think appeared about 20 years ago, and we found these four images after they had already appeared. The prediction of this future image is the one that is most exciting be- cause we might be able to catch it. We hope to come back to this field with Hubble, and we'll keep look- ing to see when that expected next image appears." Measuring the time delays between images offers clues to the type of warped-space terrain the superno- va's light had to cover and will help differences in the length of the path- ways the light follows to reach Earth. The four supernova images captured by Hubble, for example, appeared within a few days or weeks of each other. The supernova's various light paths are analogous to several trains that leave a station at the same time, all T his image shows four different images of the same supernova whose light has been distorted and magnified by the huge galaxy cluster MACS J1149.6+2223 in front of it. The huge mass of the cluster and one of the gal- axies within it is bending the light from a supernova behind them and creat- ing four separate images of the supernova. The light has been magnified and distorted due to gravitational lensing and as a result the images are arranged around the elliptical galaxy in a formation known as an Einstein cross. [NASA, ESA, S. Rodney (John Hopkins University, USA) and the FrontierSN team; T. Treu (University of California Los Angeles, USA), P. Kelly (University of California Berkeley, USA) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI)] T his animation shows how the huge galaxy cluster MACS J1149.6+2223 — whose light took over 5 billion years to reach us — bends the light from a supernova lying behind the cluster so that Hubble captures four im- ages of the supernova. The huge mass of the cluster and one of the galaxies within it is bending the light from a supernova behind them and creating four separate im- ages of the supernova. The light has been magnified and distorted due to gravitational lensing and as a result the images are arranged around the elliptical galaxy in a for- mation known as an Einstein cross. [NASA & ESA]