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) Lyman-alpha photons can be
removed (and recombination thus accelerated) by photoionizing metals. (2) The
Bowen resonance-fluorescence mechanism may degrade Lyman-beta photons and thus
enhance the Lyman-beta 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-alpha 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.
