Linear dilatons, NS5-branes and holography

We argue that vacua of string theory which asymptote at weak coupling to li near dilaton backgrounds are holographic. The full string theory in such va cua is "dual" to a theory without gravity in fewer dimensions. The dual the ory is generically not a local quantum field theory. Excitations of the str ing vacuum, which can be studied in the weak coupling region using worldshe et methods, give rise to observables in the dual theory. An interesting exa mple is string theory in the near-horizon background of parallel NS5-branes , the CHS model, which is dual to the decoupled NS5-brane theory ("little s tring theory"). This duality can be used to study some of the observables i n this theory and some of their correlation functions. Another interesting example is the "old" matrix model, which gives a holographic description of two dimensional string theory.