  A semi-classical analysis of vacuum energy in the expanding spacetime
suggests that the cosmological term decays with time, with a concomitant matter
production. For early times we find, in Planck units, $\Lambda \approx H^4$,
where H is the Hubble parameter. The corresponding cosmological solution has no
initial singularity, existing since an infinite past. During an infinitely long
period we have a quasi-de Sitter, inflationary universe, with $H \approx 1$.
However, at a given time, the expansion undertakes a phase transition, with H
and $\Lambda$ decreasing to nearly zero in a few Planck times, producing a huge
amount of radiation. On the other hand, the late-time scenario is similar to
the standard model, with the radiation phase followed by a dust era, which
tends asymptotically to a de Sitter universe, with vacuum dominating again.
