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How the universe ends
1852· Open question
Well supportedStrong evidence or math, but not fully tested.
The open question of how the universe's story ends, which depends mainly on how dark energy behaves over billions of years.
See it on the Science map →How it works
If dark energy stays constant, as the simplest model says, expansion speeds up forever. In about 100 billion years, galaxies beyond our own merged group will have faded from view. Stars stop forming, and the last dim ones burn out after about 100 trillion years. The largest black holes evaporate after about 10¹⁰⁰ years, leaving a cold, dark, even spread of faint radiation: the 'heat death'. If dark energy grows, a Big Rip tears everything apart; if it turns negative, a Big Crunch.
Why it matters
It is the long-range forecast for everything, and it hinges on dark energy, the least understood ingredient. It also shows that today's era of stars is brief.
How we know
Accelerating expansion, found in 1998 and confirmed by Planck, favors a heat death if dark energy is constant. DESI's 2024–25 hints that it is weakening, at about 2.8–4.2 sigma, short of the 5-sigma discovery standard, keep other endings open.
Still a mystery
Is dark energy truly constant? Is our vacuum stable? The Higgs mass hints it may be only metastable, though decay would take vastly longer than the universe's age. Could the end seed a new beginning, as Penrose proposes?
Connections (2)
▸Dark energy will decide How the universe endsWell supported
The relationship
Matter thins out as space grows, so the future rides on how dark energy's density changes. If it stays constant, as with a cosmological constant, expansion accelerates forever into a cold, thinning universe. If it grows, it eventually overwhelms every bond, from galaxies to atoms: a Big Rip. If it fades and turns negative, gravity could win and the universe recollapse in a Big Crunch.
How the link was found
Robert Caldwell, Marc Kamionkowski and Nevin Weinberg described the Big Rip in 2003. Recollapse scenarios gained renewed attention after DESI's 2024–25 results.
Is it proven?
Dark energy has outweighed matter for about the last 3.5 billion years, and matter keeps thinning, so in the Friedmann equations dark energy's behavior controls the far future. That logic is solid; which ending follows depends on measurements still in progress.
Open questions and mysteries
Forecasts beyond a few billion years extrapolate a quantity whose nature is unknown. Even the direction of its long-term trend is uncertain.
▸Entropy: disorder always grows points toward heat death in How the universe endsWell supported
The relationship
Total entropy keeps rising. Stars, weather and life all run on flows from hot to cold and from ordered to disordered energy. In an ever-expanding universe those differences eventually run out: stars burn out, black holes evaporate into faint radiation, and everything approaches the same near-zero temperature. With no differences left, nothing more can happen. That end state is the heat death.
How the link was found
William Thomson (later Lord Kelvin) argued in 1852 that the dissipation of energy would eventually leave the world unable to support life. Hermann von Helmholtz extended the idea to the whole universe in an 1854 lecture.
Is it proven?
The second law is among the most tested ideas in physics. Applying it to the whole universe, with gravity and expansion included, is reasonable but uncertain, because gravitating systems behave unusually.
Open questions and mysteries
Could rare random fluctuations in an endless, maximum-entropy universe create new structure, or even new universes? The 'Boltzmann brain' puzzle, about lone minds born from such fluctuations, hinges on this.