Tachyon Matter

TL;DR

Sen analyzes the classical decay of unstable D-branes in bosonic and superstring theories and demonstrates that tachyon condensation yields a pressureless tachyon matter with nonzero energy density stored in open string fields, even though the tachyon minimum has no open string DOF. The main approach combines boundary-state calculations of the energy-momentum tensor for rolling tachyon configurations with an effective field theory description, showing that in both bosonic and superstring cases T00 tends to a constant while Tij vanishes as t -> infinity, with T00 = K(cos(2 pi tilde_lambda) + 1) and K = Tp/2. In the superstring case, the NS-NS sector boundary state yields explicit f(x0) and g(x0) functions from an SU(2) rotation of the boundary perturbation, e.g. f(x0) = 1/(1+ e^{sqrt2 x0} sin^2(pi tilde_lambda)) + 1/(1+ e^{-sqrt2 x0} sin^2(pi tilde_lambda)) - 1. An effective field theory argument with S ~ -∫ V(T) sqrt{-det A} and A_mu nu = eta_mu nu + ∂_mu T ∂_nu T reproduces the same late-time behavior and links the constant energy density to the tachyon potential minimum. The work suggests tachyon matter as a possible component in cosmology and motivates further exploration of tachyon condensation in other unstable brane systems.

Abstract

It is shown that classical decay of unstable D-branes in bosonic and superstring theories produces pressureless gas with non-zero energy density. The energy density is stored in the open string fields, even though around the minimum of the tachyon potential there are no open string degrees of freedom. We also give a description of this phenomenon in an effective field theory.