This article is about scientific theories of the end of the universe. For religious beliefs and other common beliefs regarding the ultimate fate of the universe, see eschatology. For theories about the end of the planet Earth or human civilization, see End of civilization.
The ultimate fate of our universe is a topic in physical cosmology. Rival scientific theories predict whether the universe will be of finite or infinite duration. Once the notion that the universe started with Big Bang became popular among scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the universe's average density of matter and rate of expansion.
Emerging scientific basis
The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1915 theory of general relativity. General relativity can be employed to describe the universe on the largest possible scale. There are many possible solutions to the equations of general relativity, and each solution implies a possible ultimate fate of the universe. Alexander Friedmann proposed one such solution in 1921. This solution implies that the universe has been expanding from an initial singularity; this is, essentially, the Big Bang.
An important parameter in fate of the universe theory is the density parameter, Omega (Ω), defined as the average matter density of the universe divided by a critical value of that density. This creates three possible ultimate fates of the universe, depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the Flat, Open and Closed universes. These three adjectives refer to the overall geometry of the universe, and not to the local curving of spacetime caused by smaller clumps of mass (for example, galaxies and stars).
Observational evidence was not long in coming. In 1929, Edwin Hubble published his conclusion, based on his observations of Cepheid variable stars in distant galaxies, that the universe was expanding. From then on, the beginning of the universe and its possible end have been the subjects of serious scientific investigation. In 1933, Georges-Henri Lemaître set out a theory that has since come to be called the Big Bang theory of the origin of the universe. In 1948, Fred Hoyle set out his opposing theory of a static universe, called the Steady state theory. These two theories were active contenders until the 1965 discovery, by Arno Penzias and Robert Wilson, of the cosmic microwave background radiation, a fact that is a straightforward prediction of the Big Bang theory, and one that the Steady State theory cannot account for. The Big Bang theory immediately became the most widely held view of the origin of the universe.
When Einstein formulated general relativity, he and his contemporaries believed in a static universe. When Einstein found that his equations could easily be solved in such a way as to allow the universe to be expanding now, and to contract in the far future, he added to those equations what he called a cosmological constant whose role was to offset the effect of gravity on the universe as a whole in such a way that the universe would remain static. After Hubble announced his conclusion that the universe was expanding, Einstein wrote that his cosmological constant was his "greatest blunder".
Starting in 1998, observations of supernovae in distant galaxies have been interpreted as consistent with a universe whose rate of expansion is accelerating. Subsequent cosmological theorizing has been designed so as to allow for this possible acceleration, nearly always by invoking dark energy and dark matter. Hence recent theorizing about the ultimate fate of the universe allows for a nonzero cosmological constant.
Role of the shape of the universe
The current scientific consensus of most cosmologists is that the ultimate fate of the universe depends on its overall shape, and on how much dark energy it contains.
Closed universe
If Ω>1, then the geometry of space is closed like the surface of a sphere. The sum of the angles of a triangle exceeds 180 degrees and there are no parallel lines; all lines eventually meet. The geometry of the universe is, at least on a very large scale, elliptic.
In a closed universe lacking the repulsive effect of dark energy, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the "Big Crunch," by analogy with Big Bang. However, if the universe has a large amount of dark energy (as suggested by recent findings), then the expansion of the universe can continue forever - even if Ω>1.
Open universe
Flat universe
If the average density of the universe exactly equals the critical density so that Ω=1, then the geometry of the universe is flat: as in Euclidian geometry, the sum of the angles of a triangle is 180 degrees and parallel lines never meet.
Absent dark energy, a flat universe expands forever but at a continually decelerating rate, the rate of expansion asymptotically approaching zero. With dark energy, the expansion rate of the universe initially slows down, due to the effect of gravity, but eventually increases. The ultimate fate of the universe is the same as an open universe; either a heat death, Big Freeze or a Big Rip. Most astrophysical data to date is consistent with a flat universe.
Theories about the end of universe
The fate of the universe is determined by the density of the universe. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will not collapse.
Big Freeze or Heat Death
The Big Freeze is a scenario under which continued expansion results in a universe that is too cold to sustain life. It could occur under a flat or hyperbolic geometry, because such geometries are a necessary condition for a universe that expands forever. A related scenario is Heat Death, which states that the universe goes to a state of maximum entropy in which everything is evenly distributed, and there are no gradients — which are needed to sustain information processing, one form of which is life.
Big Rip: infinite time, finite lifespan
In an open universe, general relativity predicts that the universe will have an indefinite future existence, but will approach a state where life as we know it cannot exist. Under this scenario, dark energy causes the rate of expansion of the universe to accelerate. Taken to the extreme, an ever-accelerating expansion means that all material objects in the universe, starting with galaxies and eventually all life forms, no matter how small, will disintegrate into unbound elementary particles. The end state of the universe is then a gas of photons, leptons and protons (or only leptons and photons, if protons decay) growing ever less dense. For a possible timeline based on current physical theories, see 1 E19 s and more.
Big Crunch: finite time and lifespan
The Big Crunch theory is a symmetric view of the ultimate fate of the universe. Just as the Big Bang started a cosmological expansion, this theory postulates that the average density of the universe is enough to stop its expansion and begin contracting. The end result is unknown; a simple extrapolation would have all the matter and space-time in the universe collapse into a dimensionless singularity, but at these scales unknown quantum effects need to be considered.
This scenario allows the Big Bang to have been immediately preceded by the Big Crunch of a preceding universe. If this occurs repeatedly, we have an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, each finite universe ending with a Big Crunch that is also the Big Bang of the next universe. Theoretically, the oscillating universe could not be reconciled with the second law of thermodynamics: entropy would build up from oscillation to oscillation and cause heat death. Other measurements suggested the universe is not closed. These arguments caused cosmologists to abandon the oscillating universe model.
Multiverse: no complete end
The Multiverse (or parallel universe in the singular case) scenario states that while our universe may be of finite duration, it is but one universe among many. Moreover, the physics of the multiverse may permit it to exist indefinitely. In particular, other universes may be subject to physical laws differing from those that apply in our own universe.
False vacuum
If the vacuum is not in its lowest energy state (a false vacuum), it could collapse into a lower energy state. This is called the Vacuum metastability disaster. This would fundamentally alter our universe; the various physical constants could have different values, severely affecting the foundations of matter.
Observational constraints on theories
Choosing among these rival scenarios is done by 'weighing' the universe, i.e. measuring the relative contributions of matter, radiation, dark matter and dark energy to the critical density. More concretely, competing scenarios are evaluated against data on galaxy clustering and distant supernovae, and on the anisotropies in the Cosmic Microwave Background.
Life in a mortal universe
Dyson's eternal intelligence hypothesis proposes that an advanced civilization could survive for an effectively infinite period of time while consuming only a finite amount of energy. Such a civilization would alternate brief periods of activity with ever longer periods of hibernation.
Barrow and Tipler (1986) propose a Final anthropic principle: the emergence of intelligent life is inevitable, and once such life comes into being somewhere in the universe, it will never die out. Barrow and Tipler go even further: the eventual fate of intelligent life is to permeate and control the entire universe in all respects but one: intelligence cannot halt the Big Crunch. Moreover, it will not want to do so because the main source of energy in a universe undergoing a Big Crunch will be a hot spot in the sky arising from an asymmetrical contraction of the universe. They speculate that the required asymmetry will be engineered by some form of intelligent life.
Frank J. Tipler's Omega point scenario (Tipler 1994) concludes that the reverse would be the case for a civilization caught in the final stages of a Big Crunch. Such a civilization would, in effect, experience an infinite amount of "subjective" time during the remaining finite life of the universe, using the enormous energy of the Crunch to accelerate information processing faster than the approach of the final singularity.
Though possible in theory, it is not obvious whether there will ever exist technologies that will make either of these scenarios feasible. Moreover, effective solutions may be indistinguishable from the present state of our universe. In other words, if beings cannot stop the universe from collapsing, at least they can use the energy of the collapse to simulate future universes (roughly remeniscent of the Matrix movies) that resemble the ending universe, but with artificial or compressed time scales.
Recent work in inflationary cosmology, string theory, and quantum mechanics has moved the discussion of the ultimate fate of the universe in directions distinct from the scenarios set out by Dyson and Tipler. Theoretical work by Eric Chaisson and David Layzer finds that an expanding spacetime gives rise to an increasing "entropy gap", casting doubt on the heat death hypothesis. Invoking Ilya Prigogine's work on far-from-equilibrium thermodynamics, their analysis suggests that this entropy gap may contribute to information, and hence to the formation of structure.
Meanwhile, Andrei Linde, Alan Guth, Edward Harrison, and Ernest Sternglass argue that inflationary cosmology strongly suggests the presence of a multiverse, and that it would be practical even with today's knowledge for intelligent beings to generate and transmit de novo information into a distinct universe. Alan Guth has speculated that a civilization at the top of the Kardashev scale might create fine-tuned universes in a continuation of the evolutionary drive to exist, grow, and multiply. Moreover, recent theoretical work on the unresolved quantum gravity problem and the Holographic Principle suggests that traditional physical quantities may possibly themselves be describable in terms of exchanges of information, which in turn raises questions about the applicability of older cosmological models.
