A Matter‐Antimatter Universe?

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Abstract
We ask whether the universe can be a patchwork consisting of distinct regions of matter and antimatter. We demonstrate that, af- ter recombination, it is impossible to avoid annihilation near regional boundaries. We study the dynamics of this process to estimate two of its signatures: a contribution to the cosmic diffuse -ray background and a distortion of the cosmic microwave background. The former signal exceeds observational limits unless the matter domain we in- habit is virtually the entire visible universe. On general grounds, we conclude that a matter-antimatter symmetric universe is empirically excluded.