Statistics – Computation
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20922406w&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #224.06; Bulletin of the American Astronomical Society,
Statistics
Computation
1
Scientific paper
In this era of high precision cosmology, the theoretical calculation of anisotropies C_l in the cosmic microwave background (CMB) should be well below the 1% level in order to avoid biasing the estimates of cosmological parameters. The dominant uncertainty in computing the anisotropies is the precise recombination calculation, which determines the profile of the last scattering surface. We revisit the multi-level atom calculation of recombination (without any thermal equilibrium assumptions for the excited states) and investigate the relevant forbidden transitions for hydrogen and helium, for example, the two-photon transitions and the semi-forbidden transitions for HeI. We find that the most significant change in ionization fraction x_e comes from the intercombination, spin-forbidden transition (triplet 2P state to singlet 1S ground state) for HeI, which produces a more than 1% change in both x_e and C_l (in the range of l from 2 to 2500) compared with the results from the current version of RECFAST. We also discuss some other effects proposed previously in the literature and show that several such corrections have been over-estimated. Improved calculation of atomic transition rates, as well as increasingly complex multi-level atom computations, are required to achieve sub-precision accuracy on C_ls. In other words: in order to probe 10^15 GeV physics we need to know eV physics extraordinarily well.
Scott Daniel
Wong Wan Yan
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