  The discovery of accelerated Hubble expansion in the SNIa 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 still 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 generalized second law of
gravitational thermodynamics is valid and the positivity of the temperature of
the phantom fluid remains intact.
