| Abstract: |
Wojtak, Hansen and Hjorth and others have measured the long-predicted gravitational redshift of light escaping from galaxy clusters using Sloan Digital Sky Survey data. The effect is very small, corresponding to a velocity shift of only ~10 km/s in clusters with internal random motions of order 600 km/s, but the result appears to be fairly robust and seemed to be in good agreement with general relativity predictions and possibly in conflict with some alternative theories. The effect was initially imagined to be a simple astronomical analogue of the famous terrestrial Pound and Rebka experiment that verified Einstein's theoretical prediction. However, it was soon realised that the physics of this effect is considerably more complex. As I shall describe, there are actually three other contributions to the measured signal that need to be taken into account. I shall describe recent attempts to model these effects using N-body experiments. I shall also relate the cluster studies to measurements of front-back asymmetry in galaxy clustering on larger scales; the so-called "relativistic" extension to the lowest order redshift space distortion effect, and how these measurements can be used to test of theories of gravity. |
