Heat death of the universe
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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.
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[edit] Origins of the idea
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.
[edit] Temperature of the universe
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: [1] for a more detailed explanation).
[edit] Current status
Inflationary cosmology suggests that in the early universe (or, more accurately, the small part of it from which the currently observed universe stemmed) before cosmic expansion the energy was uniformly distributed[1] and thus it was in a state superficially similar to heat death. However, the two states are in fact very different: in the early universe gravity was a very important force, and in a gravitational system if the energy is uniformly distributed the entropy is quite low, compared to a state in which most matter has collapsed into black holes. Thus it is not in thermal equilibrium, and in fact there is no thermal equilibrium for such a system (it is thermodynamically unstable)[2][3]. However, in the heat death scenario the energy density is so low that the system can be thought of as non-gravitational, and a state in which the energy is uniformly distributed is a thermal equilibrium state, i.e. the state of maximal entropy.
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, all models of the universe assume its approximate homogeneity in large scales. Advanced living beings in the far future may in principle be able to change this (presumably by dragging galaxies from one place to another), thus changing the fate of at least a small part of the universe. See Final anthropic principle for a discussion of another perspective of the idea that the second law does not imply life's eventual extinction.
[edit] Timeline for heat death
[edit] The Degenerate Age - from 1014 to 1040 years
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 and leptons through proton decay.
[edit] 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, 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.
[edit] The Black Hole Age - from 1040 years to 10100 years
[edit] Black holes dominate: 1040 years
Black holes continue to evaporate via Hawking radiation, but this process is very slow.
[edit] 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.
[edit] Ultimate fate
[edit] 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 and leptons.
[edit] The Photon Age - from 10150 years until Distant Time
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 macroscopic phenomena rather than negligible microscopic events because the smallest perturbations make the biggest difference in this era, so there is no telling what may happen to space or time.
[edit] See also
- Second law of thermodynamics
- Big Rip
- Big Crunch
- Big Freeze
- Big Bang
- Dyson's eternal intelligence
- Final anthropic principle
- Ultimate fate of the Universe
- Isaac Asimov's "The Last Question"
- Graphical timeline of the Stelliferous Era
- Graphical timeline from Big Bang to Heat Death. This timeline uses the loglog scale for comparison with the graphical timeline included in this article.
[edit] References
- ^ An introduction to cosmological inflation. proceedings of ICTP summer school in high-energy physics, 1998. Retrieved on 2006-09-09.
- ^ Black holes and thermodynamics. Phys. Rev. D 13, 191–197 (1976). Retrieved on 2006-09-09.
- ^ Thermodynamics of black holes in anti-de Sitter space. Comm. Math. Phys. 87, no. 4 (1982), 577–588. Retrieved on 2006-09-09.
