Causality of the brane universe.
Causal structure of the brane universe with respect to null geodesics in the bulk spacetime is studied. It is pointed out that apparent causality violation is possible for the brane universe which contains matter energy. It is also shown that there is no "horizon problem" in the Friedmann-Robertson-Walker brane universe.
Ishihara
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Département d'Astrophysique Relativiste et de Cosmologie, Observatoire de Paris, 92195 Meudon, France.