Cosmic acceleration with a positive cosmological constant

We have considered a cosmological model with a phenomenological model for the cosmological constant of the form (lambda) = (beta) R¨/R where (beta) is a constant. For an age parameter consistent with observational data, the universe must be accelerating in the presence of a positive cosmological constant. The minimum age of the universe is H -1 0, where H0 is the present Hubble constant. The cosmological constant is found to decrease as t^-2. Allowing the gravitational constant to change with time leads to an ever-increasing gravitational constant at the present epoch. In the presence of a viscous fluid this decay law for (lambda) is equivalent to the one with (lambda) = 3(alpa)H2 ((alpha) = const) provided (alpha) = (beta)/3((beta)-2). The inflationary solution obtained from this model is that of the de Sitter type.