2.30 pm Monday March 6 in Curia II.
We argue that nonperturbative vacuum contributions to the effective action of a free quantized scalar field of low mass lead to a cosmological solution in which the scalar curvature has a constant value after a time of roughly half the age of the universe. This solution implies an accelerating universe at the present time, and agrees well with the data from high-redshift Type-Ia supernovae, the present average mass density (including dark matter), and the age of the universe. The solution is spatially flat, and hence is consistent with the small-scale CMBR fluctuation spectrum. The low mass field may be associated with a previously suggested pseudo Nambu-Goldstone boson, or possibly an ultralight graviton. We show that the imaginary part of the same term in the effective action that gives rise to the acceleration also gives a particle production rate that is in agreement with that predicted by other methods.