We i n v estigate Big Bang nucleosynthesis (hereafter, BBN) in a cosmic environment c haracterised by a distribution of small-scale matter{antimatter domains. Production of antimatter domains in a baryo-asymmetric universe is predicted in some electroweak baryogenesis scenarios. We nd that cosmic antimatter domains of size exceeding the neutron-di usion length at temperature T 1 MeV signi cantly a ect the light-element production. Annihilation of antimatter preferentially occurs on neutrons such that antimatter domains may yield a reduction of the 4 He abundance relative to a standard BBN scenario. In the limiting case, all neutrons will be removed before the onset of light-element production, and a universe with net baryon number but without production of light elements results. In general, antimatter domains spoil agreement b e t w een BBN abundance yields and observationally inferred primordial abundances limits which allows us to derive limits on their presence in the early universe. However, if only small amounts of antimatter are present, BBN with low deuterium and low 4 He, as seemingly favored by current observational data, is possible. 26.35.+c,98.80Cq,25.43.+t Big Bang Nucleosynthesis is one of the furthest back{ reaching cosmological probes available. By means of comparing the predicted and observationally inferred light element abundances the cosmic conditions as early as a few seconds after the Big Bang may be scrutinized. Nucleosynthesis has thus been used to constrain, for example, inhomogeneities in the baryon-to-photon ratio or abundances and properties of decaying particles during the BBN era (for reviews on non-standard BBN see [1]).In this Letter we examine the BBN process in a universe where matter and antimatter are segregated. A segregation of matter and antimatter may a ect BBN abundance yields even if the average segregation scale is very small in terms of characteristic astrophysical scales. Such small scales imply annihilation of antimatter well before the present epoch. We are therefore particularly interested in the case where the universe contains net baryon number, as for example, through an excess of matter domains over antimatter domains. A baryo-asymmetric universe lled with a distribution of small-scale matter{ antimatter domains may arise during an epoch of baryogenesis at the electroweak scale. It has been shown within the minimal supersymmetric standard model, and under the assumption of explicit as well as spontaneous CP violation, that during a rst-order electroweak phase transition the baryogenesis process may result in individual bubbles containing either net baryon number, or net antibaryon number [2]. Recently, it has been argued that pre-existing stochastic (hyper)magnetic elds in the early universe, in conjunction with an era of electroweak baryogenesis, may cause the production of regions containing either matter or antimatter [3]. In general, any segregation of matter and antimatter either produced on scales larger than the neutron di usion length at weak freeze...

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 He synthesis at T4 He ≈ 80 keV, leading to a suppression of the primordial 4 He mass fraction. Larger domains are dissipated below T4 He via a combination of proton diffusion and hydrodynamic expansion. In this case the strongest effects on the elemental abundances are due top 4 He annihilations, leading to an overproduction of 3 He relative to 2 H and to overproduction of 6 Li via non-thermal 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 H and low 4 He, at a common value of the cosmic baryon density, a result seemingly favored by current observational data. 26.35.+c,98.80Cq,25.43.+t

We examine the Big Bang nucleosynthesis (BBN) process in the presence of small-scale baryon inhomogeneities. Primordial abundance yields for D, 4 He, 6 Li, 7 Li, 9 Be, and 11 B are computed for wide ranges of parameters characterizing the inhomogeneities taking account of all relevant diffusive and hydrodynamic processes. These calculations may be of interest due to (a) recent observations of the anisotropies in the cosmic microwave background radiation favoring slightly larger baryonic contribution to the critical density, Ω b , than allowed by a standard BBN scenario and (b) new observational determinations of 6 Li and 9 Be in metal-poor halo stars. We find considerable parameter space in which production of D and 4 He is in agreement with observational constraints even for Ω b h 2 a factor 2-3 larger than the Ω b inferred from standard BBN. Nevertheless, in this parameter space synthesis of 7 Li in excess of the inferred 7 Li abundance on the Spite plateau results. Production of 6 Li, 9 Be, and 11 B in inhomogeneous BBN scenarios is still typically well below the abundance of these isotopes observed in the most metal-poor stars to date thus neither confirming nor rejecting inhomogeneous BBN. In an appendix we summarize results of a reevaluation of baryon diffusion constants entering inhomogeneous BBN calculations.