Fine Structure Constant and the CMB Damping Scale

E. Menegoni, M. Archidiacono, E. Calabrese, S. Galli, C. J. A. P. Martins, A. Melchiorri

Abstract
The recent measurements of the Cosmic Microwave Background anisotropies at arcminute angular scales performed by the ACT and SPT experiments are probing the damping regime of CMB fluctuations. The analysis of these datasets unexpectedly suggests that the effective number of relativistic degrees of freedom is larger than the standard value of N_eff = 3.04, and inconsistent with it at more than two standard deviations. In this paper we study the role of a mechanism that could affect the shape of the CMB angular fluctuations at those scales, namely a change in the recombination process through variations in the fine structure constant. We show that the new CMB data significantly improve the previous constraints on variations of α, with α/α₀ = 0.984 ± 0.005, i.e. hinting also to a more than two standard deviation from the current, local, value α₀. A significant degeneracy is present between α and N_eff, and when variations in the latter are allowed the constraints on α are relaxed and again consistent with the standard value. Deviations of either parameter from their standard values would imply the presence of new, currently unknown physics.

Notes
This is the value expected in the case of 3 relativistic neutrinos species. The little deviation from N_eff = 3 takes into account effects from the non-instantaneous neutrino decoupling from the primordial photon-baryon plasma (see e.g. [7])

Physical Review D
Volume 85, Page 107301_1
May 2012

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