Antimatter Domains: What a Homogeneous Bounce Must Avoid

Some baryogenesis mechanisms produce the right average asymmetry with the wrong sign in places, leaving antimatter domains. This paper computes two of the constraints such domains face in a bounce cosmology. Diffusion around recombination erases domains smaller than about 6.14e-02 Mpc, with the diffusion length estimated at 1.90e+21 metres from a decoupling time of 1.20e+13 seconds; larger domains survive diffusion but meet the gamma-ray bound, which requires the present domain scale to exceed roughly 20 Mpc if domains exist at all. A bounce that homogenises the sign of the asymmetry avoids the entire problem, and this paper argues that is the only comfortable option: domain-forming mechanisms must either be excluded or pushed to scales so large that the observable universe is effectively single-sign, which is observationally indistinguishable from a homogeneous mechanism. Computed from stated assumptions, not measured.