Gravitational redshifts in cosmology

University of Hawaii
Higgs Centre Seminar Room, JCMB
Friday, August 8, 2014
13:00 to 14:00


Wojtak, Hansen and Hjorth (Nature, 2011) have measured the long-predicted
gravitational redshifts in 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 ~600 km/s, but
is in good agreement with general relativity predictions and possibly in
conflict with some alternative gravity theories.  Zhao, Peacock and Li
(2012) showed that the measured shift includes a competing special relativistic
effect - the transverse Doppler (TD) redshift - which is of similar magnitude.
In this talk I will describe how there are two more kinematic effects
that need to be considered in interpreting these observations; a `light
cone' effect that augments the TD shift and a competing effect caused by
modulation of the surface brightness of galaxies by relativistic beaming.
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.  I will also mention how
the physics here echoes an interesting debate that took place in
the 60's regarding the transformation of thermodynamic variables
under Lorentz boosts.