Rolling closed string tachyons and the big crunch

We study the low-energy effective field equations that couple gravity, thedilaton, and the bulk closed string tachyon of bosonic closed string theory. Weestablish that whenever the tachyon induces the rolling process, the stringmetric remains fixed while the dilaton rolls to strong coupling. For negativedefinite potentials we show that this results in an Einstein metric thatcrunches the universe in finite time. This behavior is shown to be rathergeneric even if the potentials are not negative definite. The solutions arereminiscent of those in the collapse stage of a cyclic universe cosmology wherescalar field potentials with negative energies play a central role.Comment: 13 pages, 2 figures, LaTeX. Replaced version: one reference adde