Thermodynamics of an accelerated expanding universe

We investigate the laws of thermodynamics in an accelerating universe driven by dark energy with a time-dependent equation of state. In the case we consider that the physically relevant part of the Universe is that enveloped by the dynamical apparent horizon, we have shown that both the first law and second law of thermodynamics are satisfied. On the other hand, if the boundary of the Universe is considered to be the cosmological event horizon the thermodynamical description based on the definitions of boundary entropy and temperature breaks down. No parameter redefinition can rescue the thermodynamics laws from such a fate, rendering the cosmological event horizon unphysical from the point of view of the laws of thermodynamics.

Figure 1

The rate changes of the apparent horizon and the event horizon over one Hubble scale with ${\Omega }_{D0}=0.7$ and $c=1$.

Figure 2

The evolution of entropies and ${\omega }_D$ with the parameters $b^2=0.08$ and $c=1$ and the initial conditions ${\Omega }_{D0}=0.7$ and $H^2{S}_0={10}^{-30}$.