Heat death of the universe

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. 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.

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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]   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]

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]

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 farther 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]}

The fate of sapient life in a universe approaching heat death is speculative, and again depends on the assumptions made. It has been proposed that in a collapsing universe, there is enough usable energy to perform an arbitrarily large amount of computation before the collapse completes. This scenario is called the Omega point, and it would theoretically allow a society that practices mind uploading to experience an arbitrarily large amount of subjective time in simulated realities. In an expanding universe, it has been proposed that some mechanism would exist to allow sapient life to continue processing information indefinitely. This proposal is called the final anthropic principle. However, the expectation among the majority of scientists is that the heat death of the universe will bring an end to the ability to perform information processing, and hence an end to any form of sapient life.^{[citation needed]}

Timeline for heat death

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
• Black 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. Occasionally, brown dwarfs collide with each other and form a new red dwarf star which will survive for billions of years, which is the only visible light source in the universe.

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.  

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 ½^10,000 (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 in the only known bodies in the Universe immune to proton decay: black holes.^[10]

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.

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 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.

The Photon Age - 10¹⁵⁰ years and Beyond

The Universe Achieves Low-Energy State: 10^10³ years and beyond

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 the smallest perturbations 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.

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See also

• Second law of thermodynamics
• Big Rip
• Big Crunch
• Big Bounce
• Big Freeze
• Big Bang
• Cyclic model
• Dyson's eternal intelligence
• Final anthropic principle
• Ultimate fate of the Universe
• 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.

References