Minisuperspacetime Foam

It has been suggested that Planck-scale "spacetime foam" could have significant macroscopic consequences, perhaps even offering a way to address the cosmological constant problem.  I will describe progress in constructing a locally spherically symmetric minisuperspace model with a positive cosmological constant that may shed light on these questions.  Classically, the model includes spacetimes that contain both expanding and contracting regions.  Quantum mechanically, the Wheeler-DeWitt equation is relatively tractable, and has stationary solutions, although as usual there are interpretational ambiguities. The model may also admit instantons that describe the nucleation of contracting regions in an exponentially expanding spacetime. Results so far are quite incomplete, but the approach seems promising.