What is the distance to the CMB?
- Astrophysics, Cosmology and Gravity Centre, University of Cape Town, Cape Town 7701 (South Africa)
- Physics Department, University of the Western Cape, Cape Town 7535 (South Africa)
- Département de Physique Théorique and Center for Astroparticle Physics, Université de Genève, 24, Quai E. Ansermet, CH-1211 Genève 4 (Switzerland)
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 shift follows 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 1%. We argue that neglecting this shift could lead to a significant bias in the background cosmological parameters, because it alters the meaning of the background model. A naive adjustment of CMB parameter estimation if this shift is really a correction to the background would raise the H{sub 0} value inferred from the CMB by 5%, potentially removing the tension with local measurements of H{sub 0}. Other CMB parameters which depend on the distance would also be shifted by ∼ 1σ when combined with local H{sub 0} data. While our estimations rely on a simplistic analysis, they nevertheless illustrate that accurately defining the background model in terms of the expectation values of observables is critical when we aim to determine the model parameters at the sub-percent level.
- OSTI ID:
- 22375735
- Journal Information:
- Journal of Cosmology and Astroparticle Physics, Vol. 2014, Issue 11; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 1475-7516
- Country of Publication:
- United States
- Language:
- English
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