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**Session 32 - Curriculum in Crisis: Reinventing Stellar Astrophysics for Today's Graduate Student.**

*Oral session, Tuesday, June 11*

*Playcircle, *

## [32.02] A Distant Photosphere: CBR Spectral Distortions from the Recombination Epoch

*G. B. Rybicki (CfA), I. P. Dell'Antonio (Lucent Technologies, Bell Labs)*
The spectral distortions introduced during the hydrogen recombination
epoch in the early universe are features of the CBR that could, in
principle, determine a host of cosmological parameters, such as
Ømega, Ømega_B, and H_0. Previous calculations have indicated
that such distortions will extremely small due the small ratio of
baryons to photons in the universe. However, because of their
potential importance, it seems worthwhile to do a more complete
calculation of the distortions, taking special care to include any
physical effects that might affect their magnitude.

Such a calculation provides an instructive example of how classical
techniques of stellar atmosphere theory can be applied to
significantly different regimes of physical parameters and boundary
conditions. For example, here atomic transition rates are almost
completely dominated by thermal radiative processes induced by the CBR
itself, and the ``photosphere'' exists more in time than in space.

Several improvements in the calculation will be described. An new
formalism is developed for determining the spectral distortions due to
the hydrogen lines, which is based on a perturbation expansion of the
excited level populations away from their thermodynamic equilibrium
values. The temperature difference between the radiation and
electrons is taken into account. A new type of visibility function is
introduced that describes where photons were originally generated by
true absorption processes, rather than where they were last Thomson
scattered. Improved values for the relic ionized fraction, matter
temperature, and the visibility function are found for a range of
cosmological parameters. Despite these improvements, the spectral
deviations found were not significantly different from previous
estimates, and are still several orders of magnitude below the
expected backgrounds, well below detectability for the forseeable
future.

**Program
listing for Tuesday**