Exact expectation values of local fields in the quantum sine-Gordon model

We propose an explicit expression for vacuum expectation values < e^iaϕ> of the exponential fields in the sine-Gordon model. Our expression agrees both with semi-classical results in the sine-Gordon theory and with perturbative calculations in the massive Thirring model. We use this expression to make new predictions about the large-distance asymptotic form of the two-point correlation function in the XXZ spin chain.