Cosmological horizons and the generalised second law of thermodynamics

The author investigates the change in event-horizon area in cosmological models that depart slightly from de Sitter space. If the gravitational source is a perfect inviscid fluid satisfying the dominant energy condition, the area is proved to be non-decreasing. If the fluid is viscous, the area can decrease, but the attendant generation of entropy ensures that the generalised second law of thermodynamics remains unviolated.