A precise and accurate determination of the cosmic microwave background temperature at z = 0.89

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: mullers@chalmers.se
² Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France
³ Sydney Institute for Astronomy, School of Physics, The University of Sydney, 2006 NSW, Australia
⁴ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia
⁵ Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
⁶ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, 38406 Saint-Martin d’Hères, France
⁷ École Normale Supérieure/LERMA, 24 rue Lhomond, 75005 Paris, France
⁸ Max-Planck-Institut für Radioastonomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Astron. Dept., King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia

Received: 22 October 2012
Accepted: 21 December 2012

Abstract

Context. According to the Big Bang theory and as a consequence of adiabatic expansion of the Universe, the temperature of the cosmic microwave background (CMB) increases linearly with redshift. This relation is, however, poorly explored, and detection of any deviation would directly lead to (astro-)physics beyond the standard model.

Aims. We aim to measure the temperature of the CMB with an accuracy of a few percent at z = 0.89 toward the molecular absorber in the galaxy lensing the quasar PKS 1830−211.

Methods. We adopted a Monte-Carlo Markov chain approach, coupled with predictions from the non-LTE radiative transfer code RADEX, to solve the excitation conditions of a set of various molecular species directly from their spectra.

Results. We determine T_CMB = 5.08  ±  0.10 K at 68% confidence level. Our measurement is consistent with the value T_CMB = 5.14 K predicted by the standard cosmological model with adiabatic expansion of the Universe. This is the most precise determination of T_CMB at z > 0 to date.