A hot cup of coffee cools until it matches the room. A ball rolls downhill and stops. Left alone, energy always spreads out, and the measure of that spreading, entropy, only grows. Applied to the whole universe, the same rule suggests an ending: a far future in which energy is spread so evenly that nothing can happen any more. That end is called the heat death of the universe.
A 19th-century idea
The idea comes from the founders of thermodynamics. In the early 1850s Rudolf Clausius in Germany and William Thomson, later Lord Kelvin, in Britain set out the second law of thermodynamics. Thomson wrote of a “universal tendency in nature to the dissipation of mechanical energy” and drew the conclusion that the Earth, and eventually the universe, would run down.
Heat death does not mean the universe becomes hot. It means it reaches the same temperature everywhere. With no temperature differences left, no engine, no star and no living thing can extract useful energy. The more common modern name is the Big Freeze, because in an expanding universe that uniform temperature is close to absolute zero.
The long goodbye
In 1997 Fred Adams and Gregory Laughlin worked out what would happen to the universe over unimaginable spans of time. Their timeline, measured in powers of ten, runs roughly like this:
- By about 10¹⁴ years, a hundred trillion years from the start, the gas for new stars runs out and the last small red dwarfs fade.
- From about 10¹⁵ to 10³⁷ years, the universe holds only the remains of stars: white dwarfs cooling to black dwarfs, neutron stars and black holes. If protons decay, which has never been observed, these remains slowly dissolve.
- From about 10³⁸ to 10¹⁰⁰ years, black holes are the last large objects. Each one slowly loses mass as Hawking radiation and finally evaporates. The largest take longest.
- After that comes a dark era of scattered particles and faint radiation, ever more thinly spread, with nothing left to change.
Why it is the likeliest end
In 1998 two teams studying distant supernovae found that the expansion of the universe is speeding up, a discovery that earned the 2011 Nobel Prize in Physics. If the expansion continues as it is now, galaxies will drift ever further apart, and the universe will cool towards heat death.
That conclusion is not certain. It depends on the nature of dark energy, which is still unknown. If dark energy grows stronger, the universe might end in a Big Rip instead; if our vacuum is unstable, it could end suddenly in vacuum decay. Some physicists also question whether the entropy of an entire universe can be defined at all.