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| The Big Crunch. The
vertical axis can be considered as expansion or
contraction with time. |
Destruction Theories
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. 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.
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. |
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| An animation of the
expected behavior of a Big Crunch. |
Theories about the end of
the universe
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. However, observations are not conclusive, and
alternative models are still possible.
Big Freeze or heat death
The Big Freeze (or Big Chill) is a scenario under which
continued expansion results in a universe that
asymptotically approaches absolute zero temperature.
This scenario, in combination with the Big Rip scenario,
is gaining ground as the most important hypothesis. 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. Over infinite time, there
would be a spontaneous entropy decrease by the Poincaré
recurrence theorem, thermal fluctuations, and the
fluctuation theorem.
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.
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
The Big Crunch hypothesis is a symmetric view of the
ultimate fate of the universe. Just as the Big Bang
started as a cosmological expansion, this theory assumes
that the average density of the universe will be enough
to stop its expansion and the universe will 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
back into how the universe started with the Big Bang,
but at these scales unknown quantum effects need to be
considered (see Quantum gravity). Recent evidence
suggests that this scenario is unlikely but has not been
ruled out, as measurements have been available only over
a short period of time, relatively speaking, and could
reverse in the future.
This scenario allows the Big Bang to occur immediately
after the Big Crunch of a preceding universe. If this
happens repeatedly, it creates a cyclic model, which is
also known as an oscillatory universe. The universe
could then consist of an infinite sequence of finite
universes, with each finite universe ending with a Big
Crunch that is also the Big Bang of the next universe. A
problem with the cyclic universe is that it does not
reconcile with the second law of thermodynamics, as
entropy would build up from oscillation to oscillation
and cause the eventual heat death of the universe.
Current evidence also indicates the universe is not
closed. This has 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.
Big Bounce
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 was infinitely
dense. Such a description seems to be at odds with other
more widely accepted theories, 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.
In simple terms, this theory states that the universe
will continuously repeat the cycle of a Big Bang,
followed up with a Big Crunch.
Big Slurp
This theory posits that the universe currently exists in
a false vacuum and that it could become a true vacuum at
any moment.
In order to best understand the false vacuum collapse
theory, one must first understand the Higgs field which
permeates the universe. Much like an electromagnetic
field, it varies in strength based upon its potential. A
true vacuum exists so long as the universe exists in its
lowest energy state, in which case the false vacuum
theory is irrelevant. However, if the vacuum is not in
its lowest energy state (a false vacuum), it could
tunnel into a lower energy state. This is called vacuum
decay. 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. |
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