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Gravitational redshifts in cosmology



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.

Gravitational redshifts in cosmology


Higgs Centre Seminar Room, JCMB (Find us on campus maps)
The Higgs Centre for Theoretical Physics
School of Physics and Astronomy
James Clerk Maxwell Building, 4305
Peter Guthrie Tait Road