Heat Death is a hypothesis about the ultimate state of the universe- one of maximum entropy. Applying the second rule of thermodynamics (total entropy within a closed system must increase irreversably to some extent over time), if the universe is an isolated system, then its entropy must increase with time. The implication is that the universe must ultimately suffer a “heat death” as its entropy progressively increases toward a maximum value and all parts come into thermal equilibrium at a uniform temperature. After that point, no further changes involving the conversion of heat into useful work are possible.