The condition of the universe when entropy is maximized and all large-scale samples of matter are at a uniform temperature. In this condition no energy is available for doing work and the universe is finally unwound. The condition was predicted by Rudolph Clausius, who introduced the concept of entropy. Clausius’s dictum that ‘the energy of the universe is constant, its entropy tends to a maximum’ is a statement of the first two laws of thermodynamics. These laws apply in this sense only to closed systems and, for the predicted heat death to occur, the universe must be a closed system. Whether the heat death of the universe will occur is a contentious issue that has been extensively discussed.