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Orbifold branes in string/M-Theory and their cosmological applications

In this brief report, we summarize our recent studies in brane cosmology in both string theory and M-Theory on $S^{1}/Z_{2}$. In such setups, we find that the radion is stable and its mass, with a very conservative estimation, can be of the order of $0. 1 \sim 0.01$ GeV. The hierarchy problem can be addressed by combining the large extra dimension, warped factor, and tension coupling mechanisms. Gravity is localized on the visible brane, and the spectrum of the gravitational Kaluza-Klein (KK) modes is discrete and can have a mass gap of TeV. The corrections to the 4D Newtonian potential from the higher order gravitational KK modes are exponentially suppressed. Applying such setups to cosmology, we find that a late transient acceleration of the universe seems to be the generic feature of the theory, due to the interaction between branes and bulk. A bouncing early universe is also rather easily realized.