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)double over dot>/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-0(-1), where H-0 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 = 3alphaH(2) (alpha = const) provided alpha = beta/3(beta - 2). The inflationary solution obtained from this model is that of the de Sitter type.