Vacuum effects of an ultralow mass particle account for the recent acceleration of the universe

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 quantum-corrected Einstein equations. This solution gives a good fit to the SNe-Ia data with a particle mass parameter m₇ in the range 6. 4010^-33 eV to 7. 2510^-33 eV. It follows that the ratio of total matter density (including dark matter) to critical density, ₀, is in the range 0. 58 to 0. 15, and the age t₀ of the universe is in the range 8. 10h^-1 to 12. 2h^-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, m₇, and does not require that we live at a special time in the evolution of the universe.