Limits on cosmic matter-antimatter domains from big bang nucleosynthesis

We present detailed numerical calculations of the light element abundances synthesized in a universe consisting of matter and antimatter domains, as predicted to arise in some electroweak baryogenesis scenarios. In our simulations all relevant physical effects, such as baryon-antibaryon annihilations, production of secondary particles during annihilations, baryon diffusion, and hydrodynamic processes, are coupled to the nuclear reaction network. We identify two dominant effects, according to the typical spatial dimensions of the domains. Small antimatter domains are dissipated via neutron diffusion prior to ${}^4\mathrm{He}$ synthesis at $T_{{}^4\mathrm{He}}\approx 80\mathrm{keV},$ leading to a suppression of the primordial ${}^4\mathrm{He}$ mass fraction. Larger domains are dissipated below $T_{{}^4\mathrm{He}}$ via a combination of proton diffusion and hydrodynamic expansion. In this case the strongest effects on the elemental abundances are due to $\overline{p}{}^4\mathrm{He}$ annihilations, leading to an overproduction of ${}^3\mathrm{He}$ relative to ${}^2\mathrm{H}$ and to an overproduction of ${}^6\mathrm{Li}$ via nonthermal nuclear reactions. Both effects may result in light element abundances deviating substantially from the standard big bang nucleosynthesis yields and from the observationally inferred values. This allows us to derive stringent constraints on the antimatter parameters. For some combinations of the parameters, one may obtain both low ${}^2\mathrm{H}$ and low ${}^4\mathrm{He}$ at a common value of the cosmic baryon density, a result seemingly favored by current observational data.