A Caltech Library Service

Molecular hydrogen in the cosmic recombination epoch

Alizadeh, Esfandiar and Hirata, Christopher M. (2011) Molecular hydrogen in the cosmic recombination epoch. Physical Review D, 84 (8). Art. No. 083011. ISSN 1550-7998.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


The advent of precise measurements of the CMB anisotropies has motivated correspondingly precise calculations of the cosmic recombination history. Cosmic recombination proceeds far out of equilibrium because of a “bottleneck” at the n=2 level of hydrogen: atoms can only reach the ground state via slow processes—two-photon decay or Lyman-α resonance escape. However, even a small primordial abundance of molecules could have a large effect on the interline opacity in the recombination epoch and lead to an additional route for hydrogen recombination. Therefore, this paper computes the abundance of the H_2 molecule during the cosmic recombination epoch. Hydrogen molecules in the ground electronic levels X^1Σ_g+ can either form from the excited H_2 electronic levels B^1Σ_u^+ and C^1Π_u or through the charged particles H_2^+, HeH^+, and H^-. We follow the transitions among all of these species, resolving the rotational and vibrational sublevels. Since the energies of the X^1Σ_g^+-B^1Σ_u^+ (Lyman band) and X^1Σ_g^+-C^1Π_u (Werner band) transitions are near the Lyman-α energy, the distortion of the CMB spectrum caused by escaped H Lyman-line photons accelerates both the formation and the destruction of H_2 due to this channel relative to the thermal rates. This causes the populations of H_2 molecules in X^1Σ_g^+ energy levels to deviate from their thermal equilibrium abundances. We find that the resulting H_2 abundance is 10^(-17) at z=1200 and 10^(-13) at z=800, which is too small to have any significant influence on the recombination history.

Item Type:Article
Related URLs:
URLURL TypeDescription
Hirata, Christopher M.0000-0002-2951-4932
Additional Information:© 2011 American Physical Society. Received 21 February 2011; published 27 October 2011. E. A. thanks Ben Wandelt for his support, Laura Book for her constant help, and Yacine Ali-Haїmoud and Dan Grin for useful conversations. He acknowledges financial support from the National Science Foundation (Grant No. AST 07-08849). C. H. is supported by the U.S. Department of Energy (DOE-FG03-92-ER40701), the National Science Foundation (NSF AST-0807337), and the David & Lucile Packard Foundation.
Funding AgencyGrant Number
NSFAST 07-08849
Department of Energy (DOE)DOE-FG03-92-ER40701
David and Lucile Packard FoundationUNSPECIFIED
Issue or Number:8
Classification Code:PACS: 98.70.Vc, 95.30.Ft, 98.62.Ra
Record Number:CaltechAUTHORS:20111219-075610258
Persistent URL:
Official Citation:Molecular hydrogen in the cosmic recombination epoch Esfandiar Alizadeh and Christopher M. Hirata Published 27 October 2011 (14 pages) 083011
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:28507
Deposited By: Ruth Sustaita
Deposited On:19 Dec 2011 18:56
Last Modified:03 Oct 2019 03:33

Repository Staff Only: item control page