DEFLATIONARY COSMOLOGY WITH DECAYING VACUUM ENERGY DENSITY

A nonsingular cosmological scenario based on a new phenomenological de cay law for the effective A term is proposed. In such a model, the cos mic history began from an instability of the de Sitter spacetime rathe r than a singularity. The process is characterized by an arbitrary tim e scale H(I)-1 giving the largest value of the cosmological constant ( A(I) =3H(I)2) and the initial temperature of the Universe. Subsequentl y, the Universe evolved continuously toward a slightly modified Friedm ann-Robertson-Walker cosmology. Among the ''remnants'' of the primordi al inflationary period are the present values of the cosmological term LAMBDA0 approximately 3betaH-0(2) the density parameter OMEGA0 approx imately 1-beta, and the deceleration parameter q0 approximately (1 - 3 beta)/2, where beta is smaller than unity. In addition, the age parame ter of the vacuum-matter-dominated phase is H-0t0 approximately 2 /3(1 -beta). It is also shown that, if H-I(1) is of the order of the Planck time, the Universe started with the Planck temperature and this model is consistent with the theoretically suggested value LAMBDA(I)/LAMBDA 0 approximately 10(118).