Evolution of the scale factor with a variable cosmological term

The evolution of the scale factor a(t) in Friedmann models (those with zero pressure and a constant cosmological term Lambda) is well understood, and elegantly summarized in the review of Felten and Isaacman [Rev. Mod. Phys. 58, 689 (1986)]. Developments in particle physics and inflationary theory, however, increasingly indicate that Lambda ought to be treated as a dynamical quantity. We reexamine the evolution of the scale factor with a variable Lambda term, and also generalize the treatment to include nonzero pressure. New solutions are obtained and evaluated using a variety of observational criteria. Existing arguments for the inevitability of a big bang (i.e., an initial state with a = 0) are substantially weakened, and can be evaded in some cases with Lambda(0) (the present value of Lambda) well below current experimental limits.