Nick Kaiser (Hawaii) "The Physics of Gravitational Redshifts in Clusters of Galaxies"

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 robust and is in good agreement with general relativity predictions and possibly in conflict with some alternative theories. It was soon realised that the interpretation of this measurement is more complex than initially thought as one needs to allow for the transverse Doppler (TD) redshift. In this talk I will describe how there are actually two more rather subtle and unexpected physical effects that need to be considered in interpreting these observations; there is a `light cone’ effect that augments the TD shift, and there is a competing effect that reverses the sign of the transverse Doppler effect so that we actually observe a transverse Doppler blue-shift. I will discuss how these observations constrain gravitation theory, and along the way discuss some issues concerning the interpretation of astronomical redshifts in a broader context.