Brane cosmology driven by the rolling tachyon

Brane cosmology driven by the tachyon rolling down to its ground state is investigated. We adopt an effective field theoretical description for the tachyon and Randall-Sundrum type brane world scenario. After formulating basic equations, we show that the standard cosmology with a usual scalar field can mimic the low energy behavior of the system near the tachyon ground state. We also investigate qualitative behavior of the system beyond the low energy regime for positive, negative and vanishing 4-dimensional effective cosmological constant $\Lambda_4=\kappa_5^4V(T_0)^2/12-|\Lambda_5|/2$, where $\kappa_5$ and $\Lambda_5$ are 5-dimensional gravitational coupling constant and (negative) cosmological constant, respectively, and $V(T_0)$ is the (positive) tension of the brane in the tachyon ground state. In particular, for $\Lambda_4<0$ the tachyon never settles down to its potential minimum and the universe eventually hits a big-crunch singularity.