In recent work, we showed that non-perturbative vacuum effects of a very low mass particle could induce, at a redshift of order 1, a transition from a matter-dominated to an accelerating universe. In that work, we used the simplification of a sudden transition out of the matter-dominated stage and were able to fit the Type Ia supernovae (SNe-Ia) data points with a spatially-open universe. In the present work, we find a more accurate, smooth spatially-flat analytic solution to the quantumcorrected Einstein equations. This solution gives a good fit to the SNe-Ia data with a particle mass parameter m h in the range 6.40 × 10 −33 eV to 7.25 × 10 −33 eV. It follows that the ratio of total matter density (including dark matter) to critical density, Ω0, is in the range 0.58 to 0.15, and the age t0 of the universe is in the range 8.10 h −1 Gyr to 12.2 h −1 Gyr, where h is the present value of the Hubble constant, measured as a fraction of the value 100 km/(s Mpc). This spatially-flat model agrees with estimates of the position of the first acoustic peak in the small angular scale fluctuations of the cosmic background radiation, and with light-element abundances of standard big-bang nucleosynthesis. Our model has only a single free parameter, m h , and does not require that we live at a special time in the evolution of the universe. PACS numbers: 98.80. Cq, 04.62.+v, 98.80.Es