Constraints on dark matter annihilation cross section in the brane-world and quintessence scenarios

We investigate the dark matter annihilation in the brane-world and quintessence scenarios, in which the modified cosmological expansion rate can enhance the thermal relic density of dark matter. According to the observed dark matter abundance, we constrain the thermally averaged annihilation cross section <> in these two scenarios. In addition, the big bang nucleosynthesis and the partial-wave unitarity are also used to place bounds on <>. It is found that both scenarios can lead to a large annihilation cross section, so they can be used to explain the recent PAMELA, ATIC, and PPB-BETS anomalies.