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The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration.
Once the notion that the universe started with a rapid inflation nicknamed the Big Bang became accepted by the majority of scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the physical properties of the mass/energy in the universe, its average density, and the rate of expansion.
The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1916 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 Friedman proposed a number of such solutions in 1922 as did Georges Lemaître in 1927. In some of these the universe has been expanding from an initial singularity; this is, essentially, the Big Bang.
In 1931, 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 1927, Georges 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 steady state theory in which the universe continually expanded but remained statistically unchanged as new matter is constantly created. 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 original Steady State theory could not account for. As a result The Big Bang theory quickly 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, essentially a constant energy density unaffected by any expansion or contraction, 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 "the greatest blunder of my life".
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 selects one of three possible geometries 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). If the primary content of the universe is inert matter, as in the dust models popular for much of the 20th century, there is a particular fate corresponding to each geometry. Hence cosmologists aimed to determine the fate of the universe by measuring Ω, or equivalently the rate at which the expansion was decelerating.
Starting in 1998, observations of supernovae in distant galaxies have been interpreted as consistent with a universe whose expansion is accelerating. Subsequent cosmological theorizing has been designed so as to allow for this possible acceleration, nearly always by invoking dark energy, which in its simplest form is just a positive cosmological constant. In general, dark energy is a catch-all term for any hypothesised field with negative pressure, usually with a density that changes as the universe expands.
The current scientific consensus of most cosmologists is that the ultimate fate of the universe depends on its overall shape, how much dark energy it contains, and on the equation of state which determines how the dark energy density responds to the expansion of the universe. Recent observations have shown that, from 7.5 billion years after the Big Bang onwards, the expansion rate of the universe has actually been increasing, commensurate with the Open Universe theory.
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 significant amount of dark energy that will be used as an infinite force, then the expansion of the universe can continue forever—even if Ω > 1.
If Ω < 1, the geometry of space is open, i.e., negatively curved like the surface of a saddle. The angles of a triangle sum to less than 180 degrees, and lines that do not meet are never equidistant; they have a point of least distance and otherwise grow apart. The geometry of such a universe is hyperbolic.
Even without dark energy, a negatively curved universe expands forever, with gravity barely slowing the rate of expansion. With dark energy, the expansion not only continues but accelerates. The ultimate fate of an open universe is either universal heat death, the "Big Freeze", or the "Big Rip", where the acceleration caused by dark energy eventually becomes so strong that it completely overwhelms the effects of the gravitational, electromagnetic and strong binding forces.
Conversely, a negative cosmological constant, which would correspond to a negative energy density and positive pressure, would cause even an open universe to recollapse to a big crunch. This option has been ruled out by observations.
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 Euclidean geometry, the sum of the angles of a triangle is 180 degrees and parallel lines continuously maintain the same distance.
Absent dark energy, a flat universe expands forever but at a continually decelerating rate, with 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.
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 continue to expand indefinitely, resulting in the "big freeze" scenario below. However, new understandings of the nature of dark matter also suggest its interactions with mass and gravity demonstrate the possibility of an oscillating universe.
The Big Freeze is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature. It could, in the absence of dark energy, occur only under a flat or hyperbolic geometry. With a positive cosmological constant, it could also occur in a closed universe. This scenario is currently the most commonly accepted theory within the scientific community. 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. The heat death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum. 
In the special case of phantom dark energy, which has even more negative pressure than a simple cosmological constant, the density of dark energy increases with time, causing the rate of acceleration to increase, leading to a steady increase in the Hubble constant. As a result, all material objects in the universe, starting with galaxies and eventually (in a finite time) all forms, no matter how small, will disintegrate into unbound elementary particles and radiation, ripped apart by the phantom energy force and shooting apart from each other. The end state of the universe is a singularity, as the dark energy density and expansion rate becomes infinite. For a possible timeline based on current physical theories, see 1 E19 s and more.
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 assumes that the average density of the universe is enough to stop its expansion and begin contracting. The end result is unknown; a simple estimation 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 (see Quantum gravity).
This scenario allows the Big Bang to be immediately after the Big Crunch of a preceding universe. If this occurs repeatedly, we have a cyclic model which is also known as 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 cyclic 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. A somewhat similar idea is embraced by the cyclic model, but this idea evades heat death, because of an expansion of the branes that dilutes entropy accumulated in the previous cycle.
The Big Bounce is a theorized scientific model related to the beginning of the known universe. It derives from the oscillatory universe or cyclic repetition interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.
According to one version of the Big Bang theory of cosmology, in the beginning the universe had infinite density. Such a description seems to be at odds with everything else in physics, and especially quantum mechanics and its uncertainty principle. It is not surprising, therefore, that quantum mechanics has given rise to an alternative version of the Big Bang theory. Also, if the universe is closed, this theory would predict that once this universe collapses it will spawn another universe in an event similar to the Big Bang after a universal singularity is reached or a repulsive quantum force causes re-expansion.
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One multiverse hypothesis states that our observable universe is merely one among an infinite number of expanding regions of "normal" space within a larger volume of inflationary space.
During the early universe, a period of cosmic inflation occurred, where space expanded very rapidly (in a false vacuum state dominated by an "inflationary field"). The conventional model of cosmic inflation assumes that the entire universe changes state from inflationary to non-inflationary state at the same time. The eternal inflation model, by contrast, assumes that different parts of the universe undergo vacuum decay from inflationary to non-inflationary states at different times. The end result is to produce many regions of normal space surrounded by still-expanding regions of inflationary space where the vacuum has not yet decayed.
These regions of normal space cannot contact each other, and so can each be considered separate universes. While any given universe eventually reaches heat death, there are always other regions that haven't, and new universes being produced within the inflationary volume, so the multiverse as a whole never ends.
If the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state. This is called the vacuum metastability event. This has the potential to fundamentally alter our universe; in more audacious scenarios even the various physical constants could have different values, severely affecting the foundations of matter, energy, and spacetime. It is also possible that all structures will be destroyed instantaneously, without any forewarning.
According to the many-worlds interpretation of quantum mechanics, the universe will not end this way. Instead, each time a quantum event happens that causes the universe to decay from a false vacuum to a true vacuum state, the universe splits into several new worlds. In some of the new worlds the universe decays; in some others the universe continues as before.
Each possibility described so far is based on a very simple form for the dark energy equation of state. But as the name is meant to imply, we know almost nothing of the real physics of the dark energy. If the theory of inflation is true, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang; but inflation ended, indicating an equation of state much more complicated than those assumed so far for present-day dark energy. It is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict.
Choosing among these rival scenarios is done by 'weighing' the universe, for example, 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.
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. However Lawrence Krauss and Glenn Starkman have argued that this proposal ignores the power demands of the alarm clock needed to end the hibernation. They also argued that quantum mechanics limits the number of states that a finite system can have, and so prohibits any civilization with access to only a finite amount of energy from having more than a finite number of thoughts. 
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. Theoretical work by Eric Chaisson 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, Ted 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. This has been further developed by the Selfish Biocosm Hypothesis, and by the proposal that the existence of the fundamental physical constants may be subject to a kind of cosmological natural selection. 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.
Scientific speculation about the ultimate fate of life in the universe merges almost seamlessly into science fiction. Many works describe the end of the universe—occasionally purely educational exercises describing theories of the day, more often exploiting its potential as the ultimate sense of wonder plot device, or satirising the pretensions of humanity in general and cosmologists in particular. Science fiction can try to suggest a scientific eschatology that searches for meaning in the face of the new knowledge. Countless sci-fi and fantasy works use the threatened destruction of the universe as their plot device, usually with an evil supervillain and/or the incompetence of humanity as the cause, and usually with human ingenuity saving the day.
The topic of heat death was explored in science fiction as early as 1895 in H. G. Wells' The Time Machine, which includes an evocation of the heat death of the universe as imagined by scientists like Lord Kelvin at that time, consisting of the fading out of the Sun to an exhausted red ember and a vision of Earth as a cold and bland eroded desert, to as recently as 2007 in the Doctor Who episode "Utopia", with the last remnants of society struggling to survive in a universe without stars and few planets still capable of supporting life.
Religion is not wholly excluded from science fiction's explorations of the end of our universe. Olaf Stapledon's 1937 science fiction novel Star Maker describes intelligent life in the far future in each galaxy merging into hive mind-like Galactic Minds which themselves finally merge into a Cosmic Mind which, ascending into hyperspace, encounters God (the Star Maker). The "Star Maker" reveals to the "Cosmic Mind" a vision of the simpler Cosmoses He created in the past and of those more complex Cosmoses He will create in the future. Arthur C. Clarke's 1953 short story "The Nine Billion Names of God" treats non-scientific eschatology seriously. Its famous last line ominously chronicles the end of the universe as observed by mankind: Overhead, without any fuss, the stars were going out.
James Blish's Cities in Flight series of books (1955 and 1962) ends with the disruption of the Universe in accordance with the hypercollision theory. The protagonists are able to 'seed' the resultant new universes with their own bodies (dying in the process) by using technology which isolates them from local space-time at the instant of the collision.
Isaac Asimov's short story, "The Last Question" was published in 1956. The story is broken up into several segments, with each segment revolving around an artificial, evolving supercomputer. In each segment, humans pose the question "Can entropy be reversed?" to the computer, to which the computer responds, "There is insufficient data for a meaningful answer." At the end of the story, humankind has long since succumbed to heat death (the only cosmological end-scenario articulated at the time). The evolving super-computer, now having evolved to the point where it existed without physical form in hyperspace, finally discovers how to reverse the process and with the proclamation "'LET THERE BE LIGHT!' And there was light—" the story ends.
Piers Anthony's soft science fiction novel Ghost, deals with the topic of an energy-poor future humanity struggling to find every energy resource possible, and deals with the eventual dying of a universe. In the novel, near limitless energy is able to be found by effectively cannibalizing dead galaxies from other dead universes. Even though the novel does not necessarily give a plausible scientific answer, it does bring up the question of what happens when a galaxy's central black hole becomes so massive that not even gravity can escape it, and what happens when a black hole's event horizon is so far from the central point that it does not have significant gravitational effects. The title of the book, "Ghost" refers to the remains of such a devoured galaxy.
The Big Crunch as the fate of the Universe was also explored in Poul Anderson's 1970 novel Tau Zero which posits a cyclic universe where the big crunch will be surrounded by a cloud of hydrogen, and that a starship could navigate a course to avoid the singularity and emerge into the new universe after the subsequent big bang.
The end of the universe has been used for satirical and comedic effect. In Douglas Adams's science-fiction series The Hitchhiker's Guide to the Galaxy, the "Restaurant at the End of the Universe" and its patrons are projected through time to the end of the universe, for guests to watch the event as dinner entertainment. Zaphod Beeblebrox, the three-armed, two-headed former President of the Galaxy, scorned the main event, describing it as nothing but a "gnaB giB", or the Big Bang in reverse. The astronomical cost of this exercise is paid for by depositing a small sum in the restaurant's account when the booking is made—by the end of the universe this has become a huge fortune due to the operation of compound interest.
The concept of an end to the universe has inspired some authors to explore the more human-centric topics of fate and free will. In Kurt Vonnegut's classic novel Slaughterhouse Five, the primary character is a war veteran who is contacted by aliens from the planet Tralfamadore who claim that one of their scientists will accidentally destroy the universe while testing a new type of spaceship fuel. Tralfamadorians are aware of this event because they perceive all of time instantaneously, in a similar way to how someone would observe an entire range of mountains in one instant.
Though intended for comedic purposes, the Star Trek: Deep Space Nine episode "Chrysalis" features a trio of genetically-enhanced humans contemplating the end of the universe. They come to agree that the universe is far too massive, and that it will inevitably collapse in on itself; essentially, a Big Crunch. The quandary is depicted in the episode as evidence of the characters' eccentricities.
In the Futurama episode The Late Philip J. Fry, a time machine is invented that can only travel into the future. An accident during the initial test hurdles Fry, Bender and Professor Farnsworth 10,000 years in the future. With no hope of getting back to their own time, the trio decide to kick back and watch the end of the universe together. Eventual Heat Death occurs, which leads to another Big Bang after which all time simply repeats itself indefinitely allowing the main characters to travel forward to their familiar location in the continuum and resume their lives unaffected. Even after making the same mistake again, Professor Farnsworth simply suggest that they "bring her around again". This 3rd universe is 10 feet lower than the old one however.
The setting for Greg Bear's City at the End of Time is one hundred trillion years in the future and explores the cold death of the universe.