Metals at the surface of last scatter
Standard big-bang nucleosynthesis (BBN) predicts only a trace abundance of lithium and no heavier elements, but some alternatives predict a nonzero primordial metallicity. Here we explore whether CMB measurements may set useful constraints to the primordial metallicity and/or whether the standard CMB calculations are robust, within the tolerance of forthcoming CMB maps, to the possibility of primordial metals. Metals would affect the recombination history (and thus CMB power spectra) in three ways: (1) Lyα photons can be removed (and recombination thus accelerated) by photoionizing metals; (2) The Bowen resonance-fluorescence mechanism may degrade Lyβ photons and thus enhance the Lyβ escape probability and speed up recombination; (3) Metals could affect the low-redshift tail of the CMB visibility function by providing additional free electrons. The last two of these provide the strongest CMB signal. However, the effects are detectable in the Planck satellite only if the primordial metal abundance is at least a few hundredths of solar for (2) and a few tenths of solar for (3). We thus conclude that Planck will not be able to improve upon current constraints to primordial metallicity, at the level of a thousandth of solar, from the Lyman-α forest and ultra-metal-poor halo stars, and that the CMB power-spectrum predictions for Planck suffer no uncertainty arising from the possibility that there may be primordial metals.
Additional Information© 2011 American Physical Society. Received 7 February 2011; published 12 April 2011. M. K. thanks the support of the Miller Institute for Basic Research in Science at the University of California, Berkeley, where part of this work was completed. This work was supported by DoE Contract No. DE-FG03-92-ER40701, NASA Contract No. NNX10AD04G (M. K.), NSF Contract No. AST-0807337 (Y.A.H. and C. H.), the Gordon and Betty Moore Foundation, the Alfred P. Sloan Foundation (C. H.) and the David & Lucile Packard Foundation (C. H.).
Published - AliHaimoud2011p13649Phys_Rev_D.pdf