The heat death of the universe is the end state predicted by the Second Law of Thermodynamics. Eventually — in approximately 10^100 years, give or take — the universe will reach thermodynamic equilibrium. All matter will have decayed. All energy will be uniformly distributed. The temperature will be the same everywhere, just above absolute zero. Nothing further can happen. The universe will have dissolved into a uniform, featureless, eternal stillness.
This is dissolution at the cosmic scale. Everything that has ever happened — every star that has burned, every planet that has formed, every life that has lived, every thought that has been thought — is a temporary fluctuation against the background trend toward equilibrium. The universe is using up its order. When the order is gone, nothing remains but the dissolution of dissolution itself — a state in which there is nothing left to dissolve.
The heat death is not a violent end. It is not a collapse or a catastrophe. It is a fading — a gradual, inexorable reduction of difference until no difference remains. It is the logical consequence of the laws that govern the universe. If those laws are correct, the heat death is inevitable. The only question is when.
The heat death has a peculiar emotional register. It is both terrifying and strangely peaceful. There is nothing to be done about it. It cannot be prevented, delayed, or escaped. It is the ultimate context for every other dissolution: all of them, from the death of a single cell to the collapse of a civilization, are local expressions of the same universal trend. Everything dissolves, eventually, completely, finally.
In Fiction
Isaac Asimov's The Last Question (1956) and Olaf Stapledon's Last and First Men (1930) both end with the heat death of the universe. Stapledon's vision is the more haunting: the last humans, evolved beyond recognition, gather on Neptune to witness the dissolution of everything. "The stars were extinguished," he writes. "The planets were frozen." The dissolution is total, but the consciousness that witnesses it — the last consciousness — is itself a kind of triumph. Something saw the end. Something was there.