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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 thermodynamic free energy to sustain motion or life. In physical terms, it has reached maximum entropy (because of this, the term "entropy" has often been confused with Heat Death, to the point of entropy being labeled as the "force killing the universe"). The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson (Lord Kelvin) who extrapolated the theory of heat views of mechanical energy loss in nature, as embodied in the first two laws of thermodynamics, to universal operation.

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. In other words, in nature there is a tendency to the dissipation (energy loss) of mechanical energy (motion); hence, by extrapolation, there exists the view that the mechanical movement of the universe will run down in time due to the second law. The idea of heat death was first proposed in loose terms beginning in 1851 by William Thomson, who theorized further on the mechanical energy loss views of Sadi Carnot (1824), James Joule (1843), and Rudolf Clausius (1850). Thomson’s views were then elaborated on more definitively over the next decade by Hermann von Helmholtz and William Rankine.

History

The idea of heat death of the universe derives from discussion of the application of the first two laws of thermodynamics to universal processes. Specifically, in 1851 William Thomson outlined the view, as based on recent experiments on the dynamical theory of heat, that “heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect.” [1]

William Thomson (Lord Kelvin) - originated the idea of universal heat death in 1852.

In 1852, Thomson published his “On a Universal Tendency in Nature to the Dissipation of Mechanical Energy” in which he outlined the rudiments of the second law of thermodynamics summarized by the view that mechanical motion and the energy used to create that motion will tend to dissipate or run down, naturally.[2] The ideas in this paper, in relation to their application to the age of the sun and the dynamics of the universal operation, attracted the likes of William Rankine and Hermann von Helmholtz. The three of them were said to have exchanged ideas on this subject.[3] In 1862, Thomson published the article “On the age of the sun’s heat” in which he reiterated his fundamental beliefs in the indestructibility of energy (the first law) and the universal dissipation of energy (the second law), leading to diffusion of heat, cessation of motion, and exhaustion of potential energy through the material universe while clarifying his view of the consequences for the universe as a whole. The key paragraph is:[4]

The result would inevitably be a state of universal rest and death, if the universe were finite and left to obey existing laws. But it is impossible to conceive a limit to the extent of matter in the universe; and therefore science points rather to an endless progress, through an endless space, of action involving the transformation of potential energy into palpable motion and hence into heat, than to a single finite mechanism, running down like a clock, and stopping for ever.

In the years to follow both Thomson’s 1852 and the 1865 papers, Helmholtz and Rankine both credited Thomson with the idea, but read further into his papers by publishing views stating that Thomson argued that the universe will end in a “heat death” (Helmholtz) which will be the “end of all physical phenomena” (Rankine).[3][5]

Temperature of the universe

In a "heat death", the temperature of the entire universe would be very close to absolute zero. 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.[6] For a "heat death" to occur, proton decay must take place.

Current status

Inflationary cosmology suggests that in the early universe, before cosmic expansion, energy was uniformly distributed[7], and thus the universe 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 energy is uniformly distributed, entropy is quite low, compared to a state in which most matter has collapsed into black holes. Thus, such a state is not in thermal equilibrium, and in fact there is no thermal equilibrium for such a system, as it is thermodynamically unstable.[8][9] However, in the heat death scenario, the energy density is so low that the system can be thought of as non-gravitational, such that a state in which energy is uniformly distributed is a thermal equilibrium state, i.e., the state of maximal entropy.

The final state of the universe depends on the assumptions made about its ultimate fate, and these assumptions have varied considerably over the late 20th century and early 21st century. In a "closed" universe that undergoes recollapse, a heat death is expected to occur, with the universe approaching arbitrarily high temperature and maximal entropy as the end of the collapse approaches. In an "open" or "flat" universe that continues expanding indefinitely, a heat death is also expected to occur, with the universe cooling to approach absolute zero temperature and approaching a state of maximal entropy over a very long time period. There is dispute over whether or not an expanding universe can approach maximal entropy; it has been proposed that in an expanding universe, the value of maximum entropy increases faster than the universe gains entropy, causing the universe to move progressively further away from heat death.[citation needed] Finally, some models of dark energy cause the universe to expand in ways that result in some amount of usable energy always being available, preventing the universe from ever reaching a state of maximum entropy.[citation needed] The expectation of the scientific community as of 2007 is that the universe will continue expanding indefinitely.[citation needed]

Timeline for heat death

The Degenerate Era, 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:

Formerly luminous bodies like stars cool and dim, eventually reaching the same temperature as the Universe's microwave background radiation. Occasionally, brown dwarfs collide with each other and form a new red dwarf star which will survive for trillions of years, which is the only visible light source in the universe.

200 trillion years – the estimated time until low-mass stars cool off. The smallest red dwarf stars are the longest-lived stars, and are believed to have a lifetime of up to 14 trillion years (1.4 x 1013 years). Star formation is expected to cease in galaxies in about 1014 years as galaxies are depleted of the gas clouds they need to form stars. The longest-lived stars formed from the last gas clouds will therefore cool off after about 2 x 1014 years.

Planets fall or are flung from orbits: 1015 years

Over time, the orbits of planets decay due to gravitational radiation or the planets are ejected from their local systems by gravitational perturbations.

Stars fall or 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.

The supermassive black holes are all that remains of galaxies once all protons decay, but even these giants are not immortal.

An estimated half of protons decay: 1036 years

If estimates of 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.

All protons decay: 1040 years

If estimates on the half-life of protons (1036 years) are correct, then protons (and nucleonic neutrons as well) will undergo roughly 10,000 half-lives by the time the universe is 1040 years-old. To put this into perspective, there are an estimated 1080 protons currently in the Universe. This means the proton's numbers will be slashed in half 10,000 times by the time it is 1040 years-old. Hence, there will be roughly ½10,000 (approximately 5 × 10–3011) as many protons remaining as there are today; that is, zero protons remaining in the Universe at the end of the Degenerate Age. Effectively, all matter would be contained within black holes, which are immune to proton decay, and leptons.[10]

The Black Hole Era, 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.

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 317 billion years into the future.

Ultimate fate

The lowly photon is now king of the Universe as the last of the supermassive black holes evaporate.

The Dark Era, 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.

The Photon Era, 10150 years and Beyond

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

Not to be confused with Photon epoch.

The Universe now reaches an extremely 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 even unimaginably small perturbations would make the biggest difference in this era, so there is no telling what may happen to space or time. It is perceived that the laws of "macro-physics" will break down, and the laws of "quantum-physics" will prevail.

Logarithmic scale

See also

References

  1. ^ Thomson, William. (1951). “On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule’s equivalent of a Thermal Unit, and M. Regnault’s Observations on Steam.” Excerpts. [§§1-14 & §§99-100], Transactions of the Royal Society of Edinburgh, March, 1851; and Philosophical Magazine IV. 1852, [from Mathematical and Physical Papers, vol. i, art. XLVIII, pp. 174]
  2. ^ Thomson, William (1952). “On a Universal Tendency in Nature to the Dissipation of Mechanical Energy” Proceedings of the Royal Society of Edinburgh for April 19, 1852, also Philosophical Magazine, Oct. 1852. [This version from Mathematical and Physical Papers, vol. i, art. 59, pp. 511.]
  3. ^ a b Smith, Crosbie & Wise, Matthew Norton. (1989). Energy and Empire: A Biographical Study of Lord Kelvin. (pg. 500). Cambridge University Press.
  4. ^ Thomson, William. (1862). “On the age of the sun’s heat”, Macmillan’s Mag., 5, 288-93; PL, 1, 394-68.
  5. ^ Physics Timeline (Helmholtz and Heat Death, 1854)
  6. ^ see http://www.physlink.com/Education/AskExperts/ae181.cfm for a more detailed explanation
  7. ^ "An introduction to cosmological inflation". proceedings of ICTP summer school in high-energy physics, 1998. Retrieved 2006-09-09. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ "Black holes and thermodynamics". Phys. Rev. D 13, 191–197 (1976). Retrieved 2006-09-09. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ "Thermodynamics of black holes in anti-de Sitter space". Comm. Math. Phys. 87, no. 4 (1982), 577–588. Retrieved 2006-09-09. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ 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.
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