The heat death of the universe, also known as the Big Chill or Big Freeze, 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, 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 William Thomson, 1st Baron Kelvin (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.
Origins of the idea
The idea of heat death stems from the second law of thermodynamics, of which one version states that entropy tends to increase in an isolated system. From this, the hypothesis implies that if the universe lasts for a sufficient time, it will asymptotically approach a state where all energy is evenly distributed. In other words, according to this hypothesis, there is a tendency in nature to the dissipation (energy transformation) of mechanical energy (motion) into thermal energy;
hence, by extrapolation, there exists the view that, in time, the
mechanical movement of the universe will run down as work is converted
to heat because of the second law.
The conjecture that all bodies in the universe cool off,
eventually becoming too cold to support life, seems to have been first
put forward by the French astronomer Jean Sylvain Bailly in 1777 in his writings on the history of astronomy and in the ensuing correspondence with Voltaire. In Bailly's view, all planets have an internal heat and are now at some particular stage of cooling. Jupiter, for instance, is still too hot for life to arise there for thousands of years, while the Moon is already too cold. The final state, in this view, is described as one of "equilibrium" in which all motion ceases.
The idea of heat death as a consequence of the laws of
thermodynamics, however, 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:
"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."
Lord Kelvin originated the idea of universal heat death in 1852.
In 1852, Thomson published 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 naturally tend to dissipate or run down. 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.
In 1862, Thomson published "On the age of the Sun’s heat", an article
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 useful motion (work), and exhaustion of potential energy
through the material universe, while clarifying his view of the
consequences for the universe as a whole. In a key paragraph, Thomson
wrote:
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).
Current status
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. 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.
If a Big Rip does not happen long before that, the "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 so that star formation and heat transfer
can continue to avoid 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.
Time frame for heat death
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 be so well into the future. Therefore,
the universe is not in thermodynamic equilibrium, and objects can do
physical work. 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,
so entropy can be produced until at least that time. Some monster 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. 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.
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. It is also possible that entropy production will cease and the universe will reach heat death. Another universe could possibly be created by random quantum fluctuations or quantum tunneling in roughly years. Over vast periods of time, a spontaneous entropy decrease would eventually occur via the Poincaré recurrence theorem, thermal fluctuations, and fluctuation theorem. Such a scenario, however, has been described as "highly speculative, probably wrong, [and] completely untestable". Sean M. Carroll, originally an advocate of this idea, no longer supports it.
Controversies
Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition.
More recently, 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." According to Tisza: "If an isolated system is not in equilibrium, we cannot associate an entropy with it." Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system". According to Gallavotti: "... there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state." 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." 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."
A recent 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. 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."
