NONPERTURBATIVE 2-D GRAVITY, PUNCTURED SPHERES AND THETA VACUA IN STRING THEORIES

We consider a model of 2D gravity with the coefficient of the Euler characteristic having an imaginary part π/2. This is equivalent to introduce a Θ-vacuum structure in the genus expansion whose effect is to convert the expansion into a series of alternating signs, presumably Borel summable. We show that the specific heat of the model has a physical behaviour. It can be represented nonperturbatively as a series in terms of integrals over moduli spaces of punctured spheres and the sum of the series can be rewritten as a unique integral over a suitable moduli spaces of punctured spheres and the an explicit realization à la Friedan-Shenker of 2D quantum gravity. We conjecture that the expansion in terms of punctures and the genus expansion can be derived using the Duistermaat-Heckman theorem. We briefly analyze expansions in terms of punctured spheres also for multicritical models.