Spacetime foam and the cosmological constant

How would we tell if the cosmological constant were large? For a homogeneous universe this is a simple question, but for a universe inhomogeneous at the Planck scale it is more subtle: there is a large class of initial data in which expanding and contracting regions coexist and give an average expansion near zero. Classically, though, such data develop singularities, and we need a quantum description of their evolution. I describe results from a spherically symmetric midisuperspace model -- a model in which most degrees of freedom are frozen out but an infinite number remain -- in which the wave function can become "trapped" for long periods in regions in which the average expansion remains small, effectively hiding a large cosmological constant.