<b>High Energy-Astrophysics</b><br><i>Using Galaxy Cluster Sizes as a Standard Ruler for Testing Cosmic Isotropy</i>
Speaker: Michael Longo (UM Physics)
According to the cosmological principle, galaxy cluster sizes, when averaged over sufficiently large volumes of space, are expected to be constant everywhere except for a slow variation with redshift (look-back time). Thus, average cluster sizes can serve as a new measure of cosmic length scales and provide a means of testing the standard cosmological model that is almost free of selection biases. Using 269,000 galaxies from the SDSS DR7 survey, Professor Longo shows that the average cluster size, averaged over many clusters, remains constant to <10% from small redshifts out to redshifts ~0.25. An analysis of the COSMOS survey shows that this technique can be extended to redshifts on the order of 2 with similar results. Applying the same analysis to the Millennium Simulation, Professor Longo shows that the variation of cluster size with look-back time is expected to be small out to redshifts ~ 2. This technique allows sensitive tests of the concordance LCDM and for large-scale inhomogeneities. Professor Longo shows that regions of space separated by ~1 Gpc have the same average cluster size and density to about 10%. This appears to contradict models that attribute evidence for dark energy to our location in a void or due to a local under-density caused by an expanding wave.