Fredagskollokvium: The Physics of Gravitational Redshifts in Clusters of Galaxies

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.