Tachyon Dynamics in Open String Theory

In this review we describe our current understanding of the properties of open string tachyons on an unstable D-brane or brane-antibrane system in string theory. The various string theoretic methods used for this study include techniques of two dimensional conformal field theory, open string field theory, boundary string field theory, non-commutative solitons etc. We also describe various attempts to understand these results using field theoretic methods. These field theory models include toy models like singular potential models and p-adic string theory, as well as more realistic version of the tachyon effective action based on Dirac-Born-Infeld type action. Finally we study closed string background produced by the ‘decaying’ unstable D-branes, both in the critical string theory and in the two dimensional string theory, and describe the open string completeness conjecture that emerges out of this study. According to this conjecture the quantum dynamics of an unstable D-brane system is described by an internally consistent quantum open string field theory without any need to couple the system to closed strings. Each such system can be regarded as a part of the ‘hologram’ describing the full string theory. Based on lectures given at the 2003 and 2004 ICTP Spring School, TASI 2003, 2003 Summer School on Strings, Gravity and Cosmology at Vancouver, 2003 IPM String School at Anzali, Iran, 2003 ICTP Latin American School at Sao Paolo, 2004 Nordic meeting at Groningen and 2004 Onassis Foundation lecture at Crete.