Black hole thermodynamics suggests that there is a fundamental connection between gravitation and thermodynamics. Hawking radiation, the proportionality relation between the temperature and surface gravity, and also, the connection between horizon entropy and the area of a black hole support this idea. Jacobson was the first to deduce the Einstein field equations from the Clausius relation together with the fact that the entropy is proportional to the horizon area. Subsequently, this identity between Einstein equations and thermodynamical laws has been applied in the cosmological context considering universe as a thermodynamical system bounded by the apparent horizon.
