Université de GenèveDépartement de Physique ThéoriqueCAP Genève

What is the distance to the CMB?

22. September 2014
Cite as: 
Chris Clarkson, Obinna Umeh, Roy Maartens and Ruth Durrer, arXiv:1405.7860
Members involved: 

The success of precision cosmology depends not only on accurate observations, but also on the theoretical model -- which must be understood to at least the same level of precision. Subtle relativistic effects can lead to biased measurements if they are neglected.  
One such effect gives a systematic shift in the distance-redshift relation away from its background value, due to the non-linear relativistic conservation of total photon flux. We also show directly how this shiftfollows from a fully relativistic analysis of the geodesic deviation equation.  
We derive the expectation value of the shift using second-order perturbations about a concordance background, and show that
the distance to last scattering is increased by \%. Neglecting this shift leads to significant bias in the background cosmological parameters.
Correcting the CMB analysis for this shift removes the tension with local measurements of $H_0$, by raising the $H_0$ value inferred from the CMB by 5%. Other CMB parameters which depend on the distance are shifted

by ~1sigma when combined with local $H_0$ data.Give a non-specialist summary of your paper.


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