Heat death
The far-future state of maximum entropy: no temperature differences, no usable energy, nothing left to happen — the universe ends in equilibrium, not fire.
Lukasz Szramuk ·
Heat death is the universe's most likely ending: the slow approach to maximum entropy, where every temperature difference has evened out and no energy remains available to do work. The idea is older than cosmology. William Thomson described nature's universal tendency to dissipate mechanical energy in 1852; Helmholtz gave it the German name Wärmetod two years later; Clausius stated it as a law in 1865 — the entropy of the universe tends to a maximum. None of them knew the universe was expanding. That the prediction survived that discovery, and came out stronger, is the surprising part.
The timetable, worked out in detail by Fred Adams and Gregory Laughlin in the 1990s, runs in eras. Star formation exhausts the available gas within about 10^14 years, leaving white dwarfs, neutron stars and black holes. If protons decay — still an open question — the remnants dissolve somewhere between 10^34 and 10^40 years. Black holes evaporate last, the stellar-mass ones by roughly 10^67 years and the largest by 10^100. What remains is a thin, cold bath of photons and stray particles, expanding forever, with nothing left to mark the time.
Two caveats keep this from being a prophecy. It assumes dark energy stays a cosmological constant; if its equation of state drifts, the ending switches to a Big Rip or a Big Crunch instead. And equilibrium is subtler than it sounds in an accelerating universe: de Sitter space has a horizon with a temperature around 10^-30 K, so fluctuations never entirely stop — which is precisely the door the Boltzmann brain problem walks through. Heat death is also not the universe running out of energy. Energy is conserved. What runs out is the difference between one place and another, which was the only part that was ever useful.
On this site, Λ is the dial that decides it. Turn it up and expansion tears structure apart long before entropy levels anything; turn it down and gravity gets the last word instead. Our value gives the slow version — the one where the lights go out one at a time across a hundred orders of magnitude.