Ultimate fate of the universe

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"End of the Universe" redirects here. For the physical location, see Shape of the
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Part of a series on

Physical cosmology

 Big Bang · Universe
 Age of the universe
 Chronology of the universe

Early universe[show]

Expansion · Future[hide]

 Hubble's law · Redshift
 Expansion of the universe
 Accelerating expansion of the universe
 Comoving and proper distances
 FLRW metric · Friedmann equations
 Inhomogeneous cosmology
 Future of an expanding universe
 Ultimate fate of the universe
 Heat death of the universe
 Big Rip
 Big Crunch
  Big Bounce

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Experiments[show]

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The ultimate fate of the universe is a topic in physical cosmology, whose theoretical

restrictions allow possible scenarios for the evolution and ultimate fate of the universe to
be described and evaluated. Based on available observational evidence, deciding the
fate and evolution of the universe has become a valid cosmological question, being
beyond the mostly untestable constraints of mythological or theological beliefs. Several
possible futures have been predicted by different scientific hypotheses, including that
the universe might have existed for a finite and infinite duration, or towards explaining
the manner and circumstances of its beginning.
Observations made by Edwin Hubble during the 1920s–1950s found that galaxies
appeared to be moving away from each other, leading to the currently accepted Big
Bang theory. This suggests that the universe began – very small and very dense –
about 13.82 billion years ago, and it has expanded and (on average) become less
dense ever since.[1] Confirmation of the Big Bang mostly depends on knowing the rate of
expansion, average density of matter, and the physical properties of the mass–
energy in the universe.
There is a strong consensus among cosmologists that the universe is considered "flat"
(see Shape of the universe) and will continue to expand forever.[2][3]
Factors that need to be considered in determining the universe's origin and ultimate fate
include the average motions of galaxies, the shape and structure of the universe, and
the amount of dark matter and dark energy that the universe contains.

Contents

 1Emerging scientific basis

o 1.1Theory
 o 1.2Observation
o 1.3Big Bang and Steady State theories
o 1.4Cosmological constant
o 1.5Density parameter
o 1.6Repulsive force
 2Role of the shape of the universe
o 2.1Closed universe
o 2.2Open universe
o 2.3Flat universe
 3Theories about the end of the universe
o 3.1Big Freeze or heat death
o 3.2Big Rip
o 3.3Big Crunch
o 3.4Big Bounce
o 3.5Big Slurp
o 3.6Cosmic uncertainty
 4Observational constraints on theories
 5See also
 6References
 7Further reading
 8External links

Emerging scientific basis[edit]

See also: Timeline of cosmological theories and Chronology of the universe
Theory[edit]
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 several
possible solutions to the equations of general relativity, and each solution implies a
possible ultimate fate of the universe.
Alexander Friedmann proposed several solutions in 1922, as did Georges Lemaître in
1927.[4] In some of these solutions, the universe has been expanding from an
initial singularity which was, essentially, the Big Bang.
Observation[edit]
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.
Big Bang and Steady State theories[edit]
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.[4] 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.
Cosmological constant[edit]
Einstein and his contemporaries believed in a static universe. When Einstein found that
his general relativity equations could easily be solved in such a way as to allow the
universe to be expanding at the present and contracting 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. However, after Hubble announced his conclusion that the universe was
expanding, Einstein would write that his cosmological constant was "the greatest
blunder of my life."[5]
Density parameter[edit]
An important parameter in fate of the universe theory is the density parameter, omega
(), 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  is
equal to, less than, or greater than . 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 , or equivalently the rate at which the expansion was
decelerating.
Repulsive force[edit]
Starting in 1998, observations of supernovas in distant galaxies have been interpreted
as consistent[6] 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 hypothesized
field with negative pressure, usually with a density that changes as the universe
expands.

Role of the shape of the universe[edit]

See also: Shape of the universe
The ultimate fate of an expanding universe depends on the matter density  and the dark energy density 

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.[3] Recent observations conclude, from 7.5 billion years after
the Big Bang, that the expansion rate of the universe has likely been increasing,
commensurate with the Open Universe theory.[7] However, other recent measurements
by Wilkinson Microwave Anisotropy Probe suggest that the universe is either flat or very
close to flat.[2]
Closed universe[edit]
If , 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, 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", the opposite of the Big Bang. Some new modern theories
assume the universe may have a significant amount of dark energy, whose repulsive
force may be sufficient to cause the expansion of the universe to continue forever—
even if .[8]
Open universe[edit]
If , 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
negligibly 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, a "Big Freeze" (not to be confused with heat death, despite seemingly similar
name interpretation — see §Theories about the end of the universe below), or a "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 re-
collapse to a big crunch. This option has been ruled out by observations. [citation needed]
Flat universe[edit]
If the average density of the universe exactly equals the critical density so that , 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.
Measurements from Wilkinson Microwave Anisotropy Probe have confirmed the
universe is flat within a 0.4% margin of error.[2]
In the absence of 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 effects of gravity, but
eventually increases, and the ultimate fate of the universe becomes the same as that of
an open universe.

Theories about the end of the universe[edit]

The fate of the universe is determined by its density. 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.[9] However, observations are not conclusive, and alternative models are still
possible.[10]
Big Freeze or heat death[edit]
Main articles: Future of an expanding universe and Heat death of the universe
The Big Freeze (or Big Chill) is a scenario under which continued expansion results in a
universe that asymptotically approaches absolute zero temperature.[11] This scenario, in
combination with the Big Rip scenario, is gaining ground as the most important
hypothesis.[12] 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. In this scenario, stars are expected to form normally for 1012 to 1014 (1–
100 trillion) years, but eventually the supply of gas needed for star formation will be
exhausted. As existing stars run out of fuel and cease to shine, the universe will slowly
and inexorably grow darker. Eventually black holes will dominate the universe, which
themselves will disappear over time as they emit Hawking radiation.[13] Over infinite time,
there would be a spontaneous entropy decrease by the Poincaré recurrence
theorem, thermal fluctuations,[14][15] and the fluctuation theorem.[16][17]
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.[18]
Big Rip[edit]
Main article: Big Rip
The current Hubble constant defines a rate of acceleration of the universe not large
enough to destroy local structures like galaxies, which are held together by gravity, but
large enough to increase the space between them. A steady increase in the Hubble
constant to infinity would result in all material objects in the universe, starting with
galaxies and eventually (in a finite time) all forms, no matter how small, disintegrating
into unbound elementary particles, radiation and beyond. As the energy density, scale
factor and expansion rate become infinite the universe ends as what is effectively a
singularity.
In the special case of phantom dark energy, which has supposed negative kinetic
energy that would result in a higher rate of acceleration than other cosmological
constants predict, a more sudden big rip could occur.
Big Crunch