Letter to the Editor

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

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 -uncertainty in the value of T₀ results in more than half a percent ambiguity in the ionization history, and more than  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 ~nK are required to extract the quasi-periodic frequency-dependent signal with typical 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.