# Recombination in the Early Universe

**Refined treatment of H, HeI, and HeII recombination in the Early Universe.
We use multi-level atoms (n=300 for H) and evolve the population of each level
with redshift by including all transitions. The result affects the predicted
power spectrum of microwave anisotropies at the few percent level.**
### CODE:

# RECFAST.C

This code is meant to reproduce the calculations described in the paper
"A new calculation of the recombination epoch",
Seager, S., Sasselov, D. & Scott, D., 1999, ApJ, 523, L1,
which is a fast approximation to the detailed calculations described in
"How exactly did the Universe become neutral?", Seager, S.,
Sasselov, D. & Scott, D., 2000, ApJS, 128, 407-430.
**You can get
RECFAST.C here;
a Fortran version (by D. Scott) is available
here.**

Please consult the postscript
README file for RECFAST.C.

*** You can use RECFAST with
CMBFAST
(version 4.5).

**
**

### PAPERS:

1.
**"How exactly did the Universe become neutral?"**, Seager, S., Sasselov, D.
& Scott, D., 2000, ApJS, 128, 407.
2.
** "A new calculation of the recombination epoch"**, Seager, S., Sasselov, D.
& Scott, D., 1999, ApJ, 523, L1.

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`This page last updated on 06/27/03 by
dsasselov@cfa.harvard.edu.
`