Is there a need and another way to measure the cosmic microwave background temperature more accurately?

Free access article

Issue		A&A Volume 478, Number 2, February I 2008


Page(s)		L27 - L30
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20078200

A&A 478, L27-L30 (2008)
DOI: 10.1051/0004-6361:20078200

Letter

Is there a need and another way to measure the cosmic microwave background temperature more accurately?

J. Chluba¹ and R. A. Sunyaev^{1, 2}

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany
e-mail: jchluba@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

(Received 2 July 2007 / Accepted 4 December 2007)

Abstract
The recombination history of the Universe depends exponentially on the temperature, T₀, of the cosmic microwave background. Therefore tiny changes of T₀ are expected to lead to significant changes in the free electron fraction. Here we show that even the current $1\sigma$ -uncertainty in the value of T₀ results in more than half a percent ambiguity in the ionization history, and more than $0.1\%$ uncertainty in the TT and EE power spectra at small angular scales. We discuss how the value of T₀ affects the highly redshifted cosmological hydrogen recombination spectrum and demonstrate that T₀ could, in principle, be measured by looking at the low frequency distortions of the cosmic microwave background spectrum. For this no absolute measurements are necessary, but sensitivities on the level of ~ $30\,$ nK are required to extract the quasi-periodic frequency-dependent signal with typical $\Delta \nu/\nu\sim 0.1$ coming from cosmological recombination. We also briefly mention the possibility of obtaining additional information on the specific entropy of the Universe, and other cosmological parameters.

Key words: cosmology: cosmic microwave background -- cosmology: theory -- cosmology: cosmological parameters

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