Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...521...17a&link_type=abstract
The Astrophysical Journal, Volume 521, Issue 1, pp. 17-29.
Astronomy and Astrophysics
Astronomy
5
Cosmology: Theory, Cosmology: Early Universe, Nuclear Reactions, Nucleosynthesis, Abundances
Scientific paper
This paper treats the energetic and nucleosynthetic history of a Friedmann model with low photon-baryon ratio eta_gamma and positive lepton-baryon ratio eta_L. At early times, such a universe is in equilibrium and is energetically dominated by particle Fermi energies; at later times photon release from elementary particle decays probably lifts degeneracy of nucleons but not of leptons. After outlining the early history I present the results of full nucleosynthesis calculations for nondegenerate baryons and degenerate electrons and neutrinos. The results show that, for eta_gamma<~0.01 (consistent with the particle-decay heating), the neutron-proton ratio depends strongly on eta_L and that production of sufficient helium to match observations creates large primordial metallicity, in agreement with a previous argument by Carr for the eta_gamma=0 case. Excessive metallicity in the cold model is avoided only if eta_L is sufficiently high (eta_L>1.5 for eta_gamma=0 eta_L>~5 for eta_gamma=0.01) to suppress all nucleosynthesis before the Population III epoch. The calculations also reveal certain combinations of eta_gamma>~100 (corresponding to T_0~0.02) and eta_L>~5 that produce acceptable helium yields. This result contradicts the widespread notion that astrophysical observations of helium abundance together with nucleosynthesis calculations within a big bang model predict the current radiation background temperature.
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