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The heat death of the universe, also known as the Big Chill or Big Freeze,[1] is a conjecture on the ultimate fate of the universe, which suggests the universe would evolve to a state of no thermodynamic free energy and would therefore be unable to sustain processes that increase entropy. Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work. In the language of physics, this is when the universe reaches thermodynamic equilibrium (maximum entropy).

If the topology of the universe is open or flat, or if dark energy is a positive cosmological constant (both of which are consistent with current data), the universe will continue expanding forever, and a heat death is expected to occur,[2] with the universe cooling to approach equilibrium at a very low temperature after a very long time period.

The hypothesis of heat death stems from the ideas of Lord Kelvin, who in the 1850s took the theory of heat as mechanical energy loss in nature (as embodied in the first two laws of thermodynamics) and extrapolated it to larger processes on a universal scale.

In the years to follow both Thomson's 1852 and the 1862 papers, Helmholtz and Rankine both credited Thomson with the idea, but read further into his papers by publishing views sta

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.[7]

In the years to follow both Thomson's 1852 and the 1862 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).[6][8][unreliable source?]

Proposals about the final state of the universe depend 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 hypothesized "open" or "flat" universe that continues expanding indefinitely, either a heat death or a Big Rip is expected to eventually occur.[2] If the cosmological constant is zero, the universe will approach absolute zero temperature over a very long timescale. However, if the cosmological constant is positive, as appears to be the case in recent observations, the temperature will asymptote to a non-zero positive value, and the universe will approach a state of maximum entropy in which no further work is possible.[9]

If a Big Rip does not happen long before that and protons, electrons, and neutrons bound to atom's nucleus are stable and never decay, the full "heat death" situation could be avoided if there is a method or mechanism to regenerate hydrogen atoms from radiation, dark matter, If a Big Rip does not happen long before that and protons, electrons, and neutrons bound to atom's nucleus are stable and never decay, the full "heat death" situation could be avoided if there is a method or mechanism to regenerate hydrogen atoms from radiation, dark matter, dark energy, zero-point energy, or other sources, such retrieving matter and energy from black holes or causing black holes to explode so that mass contained in them is released, which can lead to formation of new stars and planets. If so, it is at least possible that star formation and heat transfer can continue, avoiding a gradual running down of the universe due to the conversion of matter into energy and heavier elements in stellar processes, and the absorption of matter by black holes and their subsequent evaporation as Hawking radiation.[10][11]

From the Big Bang through the present day, matter and dark matter in the universe are thought to have been concentrated in stars, galaxies, and galaxy clusters, and are presumed to continue to do so well into the future. Therefore, the universe is not in thermodynamic equilibrium, and objects can do physical work.[12]:§VID The decay time for a supermassive black hole of roughly 1 galaxy mass (1011 solar masses) due to Hawking radiation is on the order of 10100 years,[13] so entropy can be produced until at least that time. Some large black holes in the universe are predicted to continue to grow up to perhaps 1014 M during the collapse of superclusters of galaxies. Even these would evaporate over a timescale of up to 10106 years.[14] After that time, the universe enters the so-called Dark Era and is expected to consist chiefly of a dilute gas of photons and leptons.[12]:§VIA With only very diffuse matter remaining, activity in the universe will have tailed off dramatically, with extremely low energy levels and extremely long timescales. Speculatively, it is possible that the universe may enter a second inflationary epoch, or assuming that the current vacuum state is a false vacuum, the vacuum may decay into a lower-energy state.[12]:§VE It is also possible that entropy production will cease and the universe will reach heat death.[12]:§VID Another universe could possibly be created by random quantum fluctuations or quantum tunneling in roughly years.[15] Over vast periods of time, a spontaneous entropy decrease would eventually occur via the Poincaré recurrence theorem,[16] thermal fluctuations,[17][18][19] and fluctuation theorem.[20][21] Such a scenario, however, has been described as "highly speculative, probably wrong, [and] completely untestable".[22] Sean M. Carroll, originally an advocate of this idea, no longer supports it.[23][24]

Opposing views

Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition.&

Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition.[25][26] More recently, Walter Grandy writes: "It is rather presumptuous to speak of the entropy of a universe about which we still understand so little, and we wonder how one might define thermodynamic entropy for a universe and its major constituents that have never been in equilibrium in their entire existence."[27] According to Tisza: "If an isolated system is not in equilibrium, we cannot associate an entropy with it."[28] Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system".[29] According to Gallavotti: "... there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state."[30] Discussing the question of entropy for non-equilibrium states in general, Lieb and Yngvason express their opinion as follows: "Despite the fact that most physicists believe in such a nonequilibrium entropy, it has so far proved impossible to define it in a clearly satisfactory way."[31] In Landsberg's opinion: "The third misconception is that thermodynamics, and in particular, the concept of entropy, can without further enquiry be applied to the whole universe. ... These questions have a certain fascination, but the answers are speculations, and lie beyond the scope of this book."[32]

A 2010 analysis of entropy states, "The entropy of a general gravitational field is still not known", and "gravitational entropy is difficult to quantify". The analysis considers several possible assumptions that would be needed for estimates and suggests that the obs

A 2010 analysis of entropy states, "The entropy of a general gravitational field is still not known", and "gravitational entropy is difficult to quantify". The analysis considers several possible assumptions that would be needed for estimates and suggests that the observable universe has more entropy than previously thought. This is because the analysis concludes that supermassive black holes are the largest contributor.[33] Lee Smolin goes further: "It has long been known that gravity is important for keeping the universe out of thermal equilibrium. Gravitationally bound systems have negative specific heat—that is, the velocities of their components increase when energy is removed. ... Such a system does not evolve toward a homogeneous equilibrium state. Instead it becomes increasingly structured and heterogeneous as it fragments into subsystems."[34] This point of view is also supported by the fact of a recent experimental discovery of a stable non-equilibrium steady state in a relatively simple closed system. It should be expected that an isolated system fragmented into subsystems does not necessarily come to thermodynamic equilibrium and remain in non-equilibrium steady state. Entropy will be transmitted from one subsystem to another, but its production will be zero, which does not contradict the second law of thermodynamics.[35][36]