Dark Energy and Search for the Generalised Second Law

The discovery of accelerated Hubble expansion in the SN Ia data and the observed power spectrum of the microwave background radiation provide an ample support for Dark energy and Dark matter. Except for the so far well known facts that cold dark matter (or simply dark matter) is pressureless, and dark energy has a negative pressure, the nature of these two remains a complete mystery. The mystery facilitates different consideration. In one hand, dark matter and dark energy are assumed as distinct entities, and other interpretation is that both are different manifestation of a common structure, often referred as quartessence. Chaplygin gas, a perfect fluid also favours the second interpretation. Here, we consider modified chaplygin gas as dark energy candidate. Taking into account the existence of the observer’s event horizon in accelerated universe, we find the condition where the generalised second law of gravitational thermodynamics is valid and the positivity of the temperature of the phantom fluid remains intact.