The heat death is a possible final state of the universe, in which it has "run down" to a state of no free energy to sustain motion or life. In physical terms, it has reached maximum entropy.

Origins of the idea

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The idea of heat death stems from the second law of thermodynamics, which states that entropy tends to increase in an isolated system.

If the universe lasts for a sufficient time, it will asymptotically approach a state where all energy is evenly distributed. Hermann von Helmholtz is thought to be the first to propose the idea of heat death in 1854, 11 years before Clausius's definitive formulation of the Second law of thermodynamics in terms of entropy (1865). However, observations about the loss of available energy as heat had been formulated by Sadi Carnot as early as 1824.

Temperature of the universe

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Despite the term "heat death", the temperature of the entire universe would be very close to absolute zero in this scenario. Heat death is however not quite the same as "cold death" or the "Big Freeze" in which the universe simply becomes too cold to sustain life due to continued expansion, though the result is quite similar (see: ^* for a more detailed explanation).

Current status

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Recent work in inflationary cosmology has inverted the heat death notion, suggesting that the early universe was in a thermal equilibrium and thus heat death–like state before cosmic expansion. Meanwhile, in an expanding universe, some believe the maximum possible entropy increases far more quickly than the actual entropy with each time increment, pushing the universe continually further away from an equilibrium state despite increasing entropy. Furthermore, the very notion of thermodynamic modelling of the universe has been questioned, since the effects of such factors as gravity and quantum phenomena are very difficult to reconcile with simple thermodynamic models, rendering the utility of such models as predictive systems highly doubtful according to some.

Nonetheless, assuming that the second law of thermodynamics is an appropriate model and the Universe is a closed system, the scientific evidence overwhelmingly points to an eventual heat death.

However, see Final anthropic principle for a discussion of the idea that the second law does not imply life's eventual extinction.

Timeline for Heat Death

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The Degenerate Age - from 10¹⁴ to 10⁴⁰ years

Galaxy and Star Formation Ceases: 10¹⁴ years

Stellar formation stops, leaving matter to decay over a very long period of time. The hydrogen fuel used for fusion by stars will be eventually depleted, leaving all matter in the Universe in a compact state populated by the following objects after all stars burn out:

• Planets and planetoids (this category includes asteroids, comets, brown dwarfs, etc.)
• White dwarfs
• Neutron stars
• Quark stars
• Black Holes

Formerly luminous bodies like stars cool and dim, eventually reaching the same temperature as the Universe's microwave background radiation.

Planets are Flung from Orbits: 10¹⁵ years

Over time, the orbits of planets are kicked into other masses (see above) or scattered throughout the Universe due to gravitational perturbations.

Stars are Flung from Orbits: 10¹⁶ years

The same scattering effect happens to stars and their remnants within galaxies, leaving mostly scattered stellar debris and supermassive black holes

An estimated 1/2 of Protons Decay: 10³⁶ years

If estimates on the half-life of protons are correct, then one-half of all the free-floating matter in the Universe has been converted into gamma radiation through proton decay

All Protons Decay: 10⁴⁰ years

If estimates on the half-life of protons are correct, then these particles (and nucleonic neutrons as well) have now undergone roughly 10,000 half-lives. To put this into perspective: There are an estimated 10⁸⁰ protons in the Universe, and the estimated half-life for protons is 10³⁶ years. That means the proton's numbers have been slashed in half 10,000 times. If one does the math, there are now roughly 10^-3,000 as many protons as there were at the beginning of the Universe. So that means the total number of remaining protons in the Universe at the end of the Degenerate Age would be far less than one (a very tiny fraction something like 3,000 zeroes after the decimal place before the first significant digit). Effectively, all matter is now contained in the only bodies in the Universe immune to proton decay: black holes

Note: This number is based on loose estimates as the exact value for the half-life of protons is an unknown quantity with only a known lower-bound. The end of the Degenerate Era is meant to mark the end of baryonic matter's influence on the Universe, so the estimate for how long this era will last may change if and when the exact value for proton decay is pinned down. The specific numerical values are not meant to be taken literally, and are provided only for demonstration purposes.

The Black Hole Age - from 10⁴⁰ years to 10¹⁰⁰ years

Black Holes Dominate: 10⁴⁰ years

Black holes continue to evaporate via Hawking radiation, but this process is very slow. The first to go are the small ones, then the medium sized ones, and eventually the supermassive black holes too eventually decay into photons.

Black Holes Disintegrate: 10¹⁰⁰ years

Few if any black holes remain; virtually all matter is now converted into photons.

See also 10¹⁹ seconds for times further than 3 billion years into the future.

Ultimate fate

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The Dark Age - from 10¹⁰⁰ years until 10¹⁵⁰ years

All Black Holes now Disintegrated: 10¹⁵⁰ years

The remaining black holes evaporate: first the small ones, and then the supermassive black holes. All matter that used to make up the stars and galaxies has now degenerated into photons

The Photon Age - from 10¹⁵⁰ years until the Distant Future

The Universe Achieves Low-Energy State: 10¹⁰⁰⁰ years and beyond

The Universe now reaches extreme low-energy state. What happens after this is speculative. It's possible a Big Rip event may occur far off into the future, or the Universe may settle into this state forever, achieving true heat death. Extreme low-energy states imply that localized quantum events become major macro-scale phenomenon rather than micro-scale non-events because the smallest pertubations make the biggest difference in this era, so there is no telling what may happen to space or time during this era.

See also

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• Second law of thermodynamics
• Big Rip
• Big Crunch
• Big Freeze
• Big Bang
• Dyson's eternal intelligence
• Ultimate fate of the Universe
• Isaac Asimov's "The Last Question"

External links

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• Entropy and the second law (includes a brief mention regarding heat death)
• Heat death vs. cold death

Physical cosmology | Eschatology | Heat | Entropy

Wärmetod (Physik) | Morte termica dell'Universo | 熱的死 | Тепловая смерть | Теплова смерть