Gravitational Redshifts in Clusters of Galaxies

by Prof. Nick Kaiser (IfA, Hawaii)

Thursday Sep 26, 2013, 3:30 PM → 4:30 PM Pacific/Honolulu

112 (Watanabe Hall)

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 modified gravity theories. Zhao, Peacock and Li (2012) showed there should also be a competing special relativistic effect — the transverse Doppler (TD) effect — of similar magnitude. It turns out there are two more kinematic effects. One is a ‘light cone’ effect that augments the TD shift. The TD effect is also a little subtle: while randomly moving objects are red-shifted on average the photons are, on average, blue-shifted. Accounting for this, including detector response, shows this is the dominant kinematic effect in this experiment. I will discuss how these observations constrain gravitation theory, though perhaps this subject is more interesting as a playground in which to explore special relativity. The analysis also echoes a controversy over the relativistic transformation laws for thermodynamic quantities that raged in the 1960s.