Coupling of Rolling Tachyon to Closed Strings

TL;DR

We address how Sen's rolling tachyon boundary state couples to massive closed-string modes and whether the boundary-state description remains valid at late times. Using exact boundary CFT methods, we extend the boundary state to higher-level Ishibashi states in both bosonic and superstring theories and compute the coefficients with discrete momenta via SU(2) rotations. We find that the couplings to massive closed-string modes grow exponentially with time, leading to a significant back reaction that restricts validity to times when $g_s e^{|t|/l_s}\ll 1$ and suggesting possible decay of tachyon matter into massive closed strings. Additionally, special limits connect the rolling tachyon boundary state to S-brane boundary states, illuminating a link between time-dependent tachyon condensation and spacelike branes. These results imply that massive closed-string dynamics must be incorporated for a complete description of tachyon decay, with potential cosmological implications for rolling tachyon scenarios.

Abstract

We study the late time behavior of the boundary state representing the rolling tachyon constructed by Sen. It is found that the coupling of the rolling tachyon to massive modes of the closed string grows exponentially as the system evolves. We argue that the description of rolling tachyon by a boundary state is valid during the finite time determined by string coupling, and that energy could be dissipated to the bulk beyond this time. We also comment on the relation between the rolling tachyon boundary state and the spacelike D-brane boundary state.