Cosmological vacuum decay, irreversible thermodynamics and event horizons

We propose a new time evolution law for the cosmological 'constant' Lambda( t) in a spatially flat (k = 0) Friedmann-Lemaitre-Robertson-Walker spacetim e. From a thermodynamic model of the vacuum based upon work of Gibbons, Haw king and Davies, we obtain the law Lambda proportional to r(-2) where r is the proper radius of the cosmological event horizon. From the field equatio ns we can deduce a second-order differential equation for Lambda(t), that w e solve numerically, showing that the cosmological 'constant' problem could be solved phenomenologically. The decay of Lambda takes place during the d eflationary era, and has the effect of creating a perfect fluid of strings. Finally, our model suggests a mechanism of destabilization of the de Sitte r spacetime to explain the exit from inflation and the matter creation.