The Universe is commonly defined as the
galaxies, the contents of
intergalactic space, the smallest
subatomic particles, and all
Similar terms include the
observable universe is about 46 billion light years
Scientific observation of the Universe has led to
inferences of its earlier stages. These observations
suggest that the Universe has been governed by the same
physical laws and constants throughout most of its
extent and history. The
Big Bang theory is the prevailing cosmological model
that describes the early development of the Universe,
which is calculated to have begun
13.798 ± 0.037
billion years ago.
supernovae have shown that the Universe is expanding
There are many competing theories about the
ultimate fate of the universe. Physicists remain
unsure about what, if anything, preceded the Big Bang.
Many refuse to speculate, doubting that any information
from any such prior state could ever be accessible.
There are various
multiverse hypotheses, in which some physicists have
suggested that the Universe might be one among many
universes that likewise exist.
XDF size compared to the size of
Moon – several thousand
galaxies, each consisting of
stars, are in this small view.
XDF (2012) view – each light
speck is a
galaxy – some of these are as old as
13.2 billion years –
the visible Universe is estimated to
contain 200 billion galaxies.
XDF image shows fully mature
galaxies in the foreground plane –
nearly mature galaxies from 5 to 9
billion years ago –
protogalaxies, blazing with
young stars, beyond 9 billion years.
Throughout recorded history, several
cosmogonies have been proposed to account for
observations of the Universe. The earliest quantitative
geocentric models were developed by the
ancient Greek philosophers. Over the centuries, more
precise observations and improved theories of gravity
heliocentric model and the
Newtonian model of the
Solar System, respectively. Further improvements in
astronomy led to the realization that the Solar System
is embedded in a
galaxy composed of billions of stars, the
Milky Way, and that other galaxies exist outside it,
as far as astronomical instruments can reach. Careful
studies of the distribution of these galaxies and their
spectral lines have led to much of
modern cosmology. Discovery of the
red shift and cosmic
microwave background radiation suggested that the
Universe is expanding and had a beginning.
According to the prevailing scientific model of the
Universe, known as the
Big Bang, the Universe expanded from an extremely
hot, dense phase called the
Planck epoch, in which all the matter and energy of
observable universe was concentrated. Since the
Planck epoch, the Universe has been
expanding to its present form, possibly with a brief
period (less than
10−32 seconds) of
cosmic inflation. Several independent experimental
measurements support this theoretical
expansion and, more generally, the Big Bang theory.
The universe is composed of
ordinary matter (5%) including atoms, stars, and
dark matter (25%) which is a hypothetical particle
that has not yet been detected, and
dark energy (70%), which is a kind of energy density
that seemingly exists even in completely empty space.
Recent observations indicate that this expansion is
accelerating because of dark energy, and that most of
the matter in the Universe may be in a form which cannot
be detected by present instruments, called dark matter.
The common use of the "dark matter" and "dark energy"
placeholder names for the unknown entities purported
to account for about 95% of the
mass-energy density of the Universe demonstrates the
present observational and conceptual shortcomings and
uncertainties concerning the nature and
ultimate fate of the Universe.
On 21 March 2013, the European research team behind
Planck cosmology probe released the mission's
all-sky map of the
cosmic microwave background.
The map suggests the universe is slightly older than
thought. According to the map, subtle fluctuations in
temperature were imprinted on the deep sky when the
cosmos was about 370,000 years old. The imprint
reflects ripples that arose as early, in the existence
of the universe, as the first nonillionth (10−30)
of a second. Apparently, these ripples gave rise to the
cosmic web of
galaxy clusters and dark matter. According to the
team, the universe is 13.798 ± 0.037 billion years old,
and contains 4.9% ordinary matter, 26.8% dark matter and
68.3% dark energy. Also, the
Hubble constant was measured to be 67.80 ± 0.77
An earlier interpretation of
astronomical observations indicated that the
age of the Universe was 13.772 ± 0.059
and that the diameter of the
observable universe is at least 93 billion
light years or 8.80×1026
general relativity, space can expand faster than the
speed of light, although we can view only a small
portion of the Universe due to the limitation imposed by
light speed. Since we cannot observe space beyond the
limitations of light (or any electromagnetic radiation),
it is uncertain whether the size of the Universe is
finite or infinite.
Etymology, synonyms and definitions
The word Universe derives from the
Old French word Univers, which in turn
derives from the
Latin word universum.
The Latin word was used by
Cicero and later Latin authors in many of the same
senses as the modern
English word is used.
The Latin word derives from the poetic contraction
Unvorsum — first used by
Lucretius in Book IV (line 262) of his
De rerum natura (On the Nature of Things) —
which connects un, uni (the combining form of
unus, or "one") with vorsum, versum (a noun
made from the perfect passive participle of vertere,
meaning "something rotated, rolled, changed").
An alternative interpretation of unvorsum is
"everything rotated as one" or "everything rotated by
one". In this sense, it may be considered a translation
of an earlier Greek word for the Universe,
(periforá, "circumambulation"), originally used
to describe a course of a meal, the food being carried
around the circle of dinner guests.
This Greek word refers to
celestial spheres, an early Greek model of the
Universe. Regarding Plato's
Metaphor of the sun,
Aristotle suggests that the rotation of the sphere
fixed stars inspired by the
prime mover, motivates, in turn, terrestrial change
via the Sun. Careful
astronomical and physical measurements (such as the
Foucault pendulum) are required to prove the Earth
rotates on its axis.
A term for "Universe" in ancient Greece was
Pan (mythology)). Related terms were matter, (τὸ
ὅλον, tò ólon, see also
Hyle, lit. wood) and place (τὸ
κενόν, tò kenón).
Other synonyms for the Universe among the ancient Greek
Nature, from which we derive the word
The same synonyms are found in Latin authors (totum,
and survive in modern languages, e.g., the German words
Das All, Weltall, and Natur for
Universe. The same synonyms are found in English, such
as everything (as in the
theory of everything), the cosmos (as in
world (as in the
many-worlds interpretation), and
Nature (as in
natural laws or
Broadest definition: reality and probability
The broadest definition of the Universe is found in
De divisione naturae by the
Johannes Scotus Eriugena, who defined it as simply
everything: everything that is created and everything
that is not created.
Definition as reality
More customarily, the Universe is defined as
everything that exists, (has existed, and will exist)[citation
needed]. According to our current
understanding, the Universe consists of three
spacetime, forms of
matter, and the
physical laws that relate them.
Definition as connected space-time
It is possible to conceive of disconnected
space-times, each existing but unable to interact
with one another. An easily visualized metaphor is a
group of separate
soap bubbles, in which observers living on one soap
bubble cannot interact with those on other soap bubbles,
even in principle. According to one common terminology,
each "soap bubble" of space-time is denoted as a
universe, whereas our particular
space-time is denoted as the Universe, just
as we call our moon the
Moon. The entire collection of these separate
space-times is denoted as the
In principle, the other unconnected universes may have
space-time, different forms of
energy, and different
physical laws and
physical constants, although such possibilities are
Definition as observable reality
According to a still-more-restrictive definition, the
Universe is everything within our connected
space-time that could have a chance to interact with
us and vice versa.[citation
needed] According to the
general theory of relativity, some regions of
space may never interact with ours even in the
lifetime of the Universe due to the finite
speed of light and the ongoing
expansion of space. For example, radio messages sent
from Earth may never reach some regions of space, even
if the Universe would live forever: space may expand
faster than light can traverse it.
Distant regions of space are taken to exist and be
part of reality as much as we are, yet we can never
interact with them. The spatial region within which we
can affect and be affected is the
observable universe. Strictly speaking, the
observable Universe depends on the location of the
observer. By traveling, an observer can come into
contact with a greater region of space-time than an
observer who remains still. Nevertheless, even the most
rapid traveler will not be able to interact with all of
space. Typically, the observable Universe is taken to
mean the Universe observable from our vantage point in
the Milky Way Galaxy.
Size, age, contents, structure, and laws
The size of the Universe is unknown; it may be
infinite. The region visible from Earth (the
observable universe) is a sphere with a radius of
about 46 billion
based on where the
expansion of space has
taken the most distant objects observed. For
comparison, the diameter of a typical
galaxy is 30,000 light-years, and the typical
distance between two neighboring galaxies is 3 million
As an example, the
Milky Way Galaxy is roughly 100,000 light years in
and the nearest sister galaxy to the Milky Way, the
Andromeda Galaxy, is located roughly 2.5 million
light years away.
There are probably more than 100 billion (1011)
galaxies in the observable Universe.
Typical galaxies range from
dwarfs with as few as ten million
stars up to giants with one
(1012) stars, all orbiting the galaxy's
center of mass. A 2010 study by astronomers estimated
that the observable Universe contains 300 sextillion (3×1023)
The Universe is believed to be mostly
dark energy and
dark matter, both of which are poorly
understood at present. Less than 5% of the
Universe is ordinary matter, a relatively
The observable matter is spread homogeneously (uniformly)
throughout the Universe, when averaged over distances
longer than 300 million light-years.
However, on smaller length-scales, matter is observed to
form "clumps", i.e., to cluster hierarchically; many
atoms are condensed into
stars, most stars into galaxies, most galaxies into
clusters, superclusters and, finally, the
largest-scale structures such as the
Great Wall of galaxies. The observable matter of the
Universe is also spread isotropically, meaning
that no direction of observation seems different from
any other; each region of the sky has roughly the same
The Universe is also bathed in a highly
radiation that corresponds to a
blackbody spectrum of roughly 2.725
The hypothesis that the large-scale Universe is
homogeneous and isotropic is known as the
supported by astronomical observations.
The present overall
density of the Universe is very low, roughly 9.9 ×
10−30 grams per cubic centimetre. This
mass-energy appears to consist of 73%
dark energy, 23%
cold dark matter and 4%
ordinary matter. Thus the density of atoms is on the
order of a single hydrogen atom for every four cubic
meters of volume.
The properties of dark energy and dark matter are
largely unknown. Dark matter
gravitates as ordinary matter, and thus works to
expansion of the Universe; by contrast, dark energy
accelerates its expansion.
current estimate of the
Universe's age is 13.798
± 0.037 billion years old.
The Universe has not been the same at all times in its
history; for example, the relative populations of
quasars and galaxies have changed and
space itself appears to have
expanded. This expansion accounts for how
Earth-bound scientists can observe the light from a
galaxy 30 billion light years away, even if that light
has traveled for only 13 billion years; the very space
between them has expanded. This expansion is consistent
with the observation that the light from distant
galaxies has been
photons emitted have been stretched to longer
wavelengths and lower
frequency during their journey. The rate of this
spatial expansion is
accelerating, based on studies of
Type Ia supernovae and corroborated by other data.
relative fractions of different
chemical elements — particularly the lightest atoms
helium — seem to be identical throughout the
Universe and throughout its observable history.
The Universe seems to have much more
antimatter, an asymmetry possibly related to the
The Universe appears to have no net
electric charge, and therefore
gravity appears to be the dominant interaction on
cosmological length scales. The Universe also appears to
have neither net
angular momentum. The absence of net charge and
momentum would follow from accepted physical laws (Gauss's
law and the non-divergence of the
stress-energy-momentum pseudotensor, respectively),
if the Universe were finite.
The Universe appears to have a smooth
space-time continuum consisting of three
dimensions and one temporal (time)
dimension. On the average,
space is observed to be very nearly flat (close to
curvature), meaning that
Euclidean geometry is experimentally true with high
accuracy throughout most of the Universe.
Spacetime also appears to have a
topology, at least on the length-scale of the
observable Universe. However, present observations
cannot exclude the possibilities that the Universe has
more dimensions and that its spacetime may have a
multiply connected global topology, in analogy with the
toroidal topologies of two-dimensional
The Universe appears to behave in a manner that
regularly follows a set of
physical laws and
According to the prevailing
Standard Model of physics, all matter is composed of
three generations of
quarks, both of which are
elementary particles interact via at most three
fundamental interactions: the
electroweak interaction which includes
electromagnetism and the
weak nuclear force; the
strong nuclear force described by
quantum chromodynamics; and
gravity, which is best described at present by
general relativity. The first two interactions can
be described by
quantum field theory, and are mediated by
gauge bosons that correspond to a particular type of
gauge symmetry. A renormalized quantum field theory
of general relativity has not yet been achieved,
although various forms of
string theory seem promising. The theory of
special relativity is believed to hold throughout
the Universe, provided that the spatial and temporal
length scales are sufficiently short; otherwise, the
more general theory of general relativity must be
applied. There is no explanation for the particular
physical constants appear to have throughout our
Universe, such as
Planck's constant h or the
gravitational constant G. Several
conservation laws have been identified, such as the
conservation of charge,
angular momentum and
energy; in many cases, these conservation laws can
be related to
It appears that many of the properties of the
Universe have special values in the sense that a
Universe where these properties differ slightly would
not be able to support intelligent life.
Not all scientists agree that this
In particular, it is not known under what conditions
intelligent life could form and what form or shape that
would take. A relevant observation in this discussion is
that for an observer to exist to observe fine-tuning,
the Universe must be able to support intelligent life.
As such the
conditional probability of observing a Universe that
is fine-tuned to support intelligent life is 1. This
observation is known as the
anthropic principle and is particularly relevant if
the creation of the Universe was probabilistic or if
multiple universes with a variety of properties exist
Many models of the cosmos (cosmologies) and its
origin (cosmogonies) have been proposed, based on the
then-available data and conceptions of the Universe.
Historically, cosmologies and cosmogonies were based on
narratives of gods acting in various ways. Theories of
an impersonal Universe governed by physical laws were
first proposed by the Greeks and Indians. Over the
centuries, improvements in astronomical observations and
theories of motion and gravitation led to ever more
accurate descriptions of the Universe. The modern era of
cosmology began with
Albert Einstein's 1915
general theory of relativity, which made it possible
to quantitatively predict the origin, evolution, and
conclusion of the Universe as a whole. Most modern,
accepted theories of cosmology are based on general
relativity and, more specifically, the predicted
Big Bang; however, still more careful measurements
are required to determine which theory is correct.
Many cultures have
stories describing the origin of the world, which
may be roughly grouped into common types. In one type of
story, the world is born from a
world egg; such stories include the
Chinese story of
Pangu or the
Brahmanda Purana. In related stories, the Universe
is created by a single entity emanating or producing
something by him- or herself, as in the
Tibetan Buddhism concept of
ancient Greek story of
Gaia (Mother Earth), the
Coatlicue myth, the
Atum story, or the
Genesis creation narrative. In another type of
story, the Universe is created from the union of male
and female deities, as in the
Maori story of
Rangi and Papa. In other stories, the Universe is
created by crafting it from pre-existing materials, such
as the corpse of a dead god — as from
Tiamat in the
Enuma Elish or from the giant
Norse mythology – or from chaotic materials, as in
Japanese mythology. In other stories, the Universe
emanates from fundamental principles, such as
creation myth of the
yin and yang of the
From the 6th century BCE, the
pre-Socratic Greek philosophers developed the
earliest known philosophical models of the Universe. The
earliest Greek philosophers noted that appearances can
be deceiving, and sought to understand the underlying
reality behind the appearances. In particular, they
noted the ability of matter to change forms (e.g., ice
to water to steam) and several philosophers proposed
that all the apparently different materials of the world
are different forms of a single primordial material, or
arche. The first to do so was
Thales, who proposed this material is
Water. Thales' student,
Anaximander, proposed that everything came from the
Air on account of its perceived attractive and
repulsive qualities that cause the arche to condense or
dissociate into different forms.
Anaxagoras, proposed the principle of
fire (and spoke of
Empedocles proposed the elements: earth, water, air
and fire. His four element theory became very popular.
Plato believed that all things were composed of
number, with the Empedocles' elements taking the
form of the
Platonic solids. Democritus, and later
Leucippus—proposed that the Universe was composed of
atoms moving through
Aristotle did not believe that was feasible because
air, like water, offers
resistance to motion. Air will immediately rush in
to fill a void, and moreover, without resistance, it
would do so indefinitely fast.
Although Heraclitus argued for eternal change, his
Parmenides made the radical suggestion that all
change is an illusion, that the true underlying reality
is eternally unchanging and of a single nature.
Parmenides denoted this reality as
(The One). Parmenides' theory seemed implausible to many
Greeks, but his student
Zeno of Elea challenged them with several famous
paradoxes. Aristotle responded to these paradoxes by
developing the notion of a potential countable infinity,
as well as the infinitely divisible continuum. Unlike
the eternal and unchanging cycles of time, he believed
the world was bounded by the celestial spheres, and thus
magnitude was only finitely multiplicative.
Kanada, founder of the
Vaisheshika school, developed a theory of
atomism and proposed that
heat were varieties of the same substance.
In the 5th century AD, the
Buddhist atomist philosopher
atoms to be point-sized, durationless, and made of
energy. They denied the existence of substantial matter
and proposed that movement consisted of momentary
flashes of a stream of energy.
The theory of
temporal finitism was inspired by the doctrine of
Creation shared by the three
John Philoponus, presented the philosophical
arguments against the ancient Greek notion of an
infinite past and future. Philoponus' arguments against
an infinite past were used by the
early Muslim philosopher,
Al-Kindi (Alkindus); the
Saadia Gaon (Saadia ben Joseph); and the
Al-Ghazali (Algazel). Borrowing from Aristotle's
Physics and Metaphysics, they employed two
logical arguments against an infinite past, the first
being the "argument from the impossibility of the
existence of an actual infinite", which states:
- "An actual infinite cannot exist."
- "An infinite temporal regress of events is an
An infinite temporal regress of events cannot
The second argument, the "argument from the
impossibility of completing an actual infinite by
successive addition", states:
- "An actual infinite cannot be completed by
- "The temporal series of past events has been
completed by successive addition."
The temporal series of past events cannot be an
Both arguments were adopted by Christian philosophers
and theologians, and the second argument in particular
became more famous after it was adopted by
Immanuel Kant in his thesis of the first
Aristarchus's 3rd century BCE
calculations on the relative sizes of from
left the Sun, Earth and Moon, from a
10th-century AD Greek copy
Astronomical models of the Universe were proposed
astronomy began with the
Babylonian astronomers, who viewed the Universe as a
flat disk floating in the ocean, and this forms the
premise for early Greek maps like those of
Hecataeus of Miletus.
Greek philosophers, observing the motions of the
heavenly bodies, were concerned with developing models
of the Universe based more profoundly on
empirical evidence. The first coherent model was
Eudoxus of Cnidos. According to Aristotle's physical
interpretation of the model,
celestial spheres eternally
rotate with uniform motion around a stationary
matter, is entirely contained within the terrestrial
sphere. This model was also refined by
Callippus and after concentric spheres were
abandoned, it was brought into nearly perfect agreement
with astronomical observations by
Ptolemy. The success of such a model is largely due
to the mathematical fact that any function (such as the
position of a planet) can be decomposed into a set of
circular functions (the
Fourier modes). Other Greek scientists, such as the
Philolaus postulated that at the center of the
Universe was a "central fire" around which the
Planets revolved in uniform circular motion.
Aristarchus of Samos was the first known individual
to propose a
heliocentric model of the Universe. Though the
original text has been lost, a reference in Archimedes'
book The Sand Reckoner describes Aristarchus'
Archimedes wrote: (translated into English)
You King Gelon are aware the 'Universe' is the
name given by most astronomers to the sphere the
center of which is the center of the Earth, while
its radius is equal to the straight line between the
center of the Sun and the center of the Earth. This
is the common account as you have heard from
astronomers. But Aristarchus has brought out a book
consisting of certain hypotheses, wherein it
appears, as a consequence of the assumptions made,
that the Universe is many times greater than the
'Universe' just mentioned. His hypotheses are that
the fixed stars and the Sun remain unmoved, that the
Earth revolves about the Sun on the circumference of
a circle, the Sun lying in the middle of the orbit,
and that the sphere of fixed stars, situated about
the same center as the Sun, is so great that the
circle in which he supposes the Earth to revolve
bears such a proportion to the distance of the fixed
stars as the center of the sphere bears to its
Aristarchus thus believed the stars to be very far
away, and saw this as the reason why there was no
parallax apparent, that is, no observed movement of the
stars relative to each other as the Earth moved around
the Sun. The stars are in fact much farther away than
the distance that was generally assumed in ancient
times, which is why stellar parallax is only detectable
with precision instruments. The geocentric model,
consistent with planetary parallax, was assumed to be an
explanation for the unobservability of the parallel
phenomenon, stellar parallax. The rejection of the
heliocentric view was apparently quite strong, as the
following passage from Plutarch suggests (On the
Apparent Face in the Orb of the Moon):
Cleanthes [a contemporary of Aristarchus and
head of the Stoics] thought it was the duty of the
Greeks to indict Aristarchus of Samos on the charge
of impiety for putting in motion the Hearth of the
Universe [i.e. the earth], . . . supposing the
heaven to remain at rest and the earth to revolve in
an oblique circle, while it rotates, at the same
time, about its own axis. 
The only other astronomer from antiquity known by
name who supported Aristarchus' heliocentric model was
Seleucus of Seleucia, a
Hellenistic astronomer who lived a century after
Plutarch, Seleucus was the first to prove the
heliocentric system through
reasoning, but it is not known what arguments he
used. Seleucus' arguments for a heliocentric theory were
probably related to the phenomenon of
Strabo (1.1.9), Seleucus was the first to state that
tides are due to the attraction of the Moon, and
that the height of the tides depends on the Moon's
position relative to the Sun.
Alternatively, he may have proved the heliocentric
theory by determining the constants of a
geometric model for the heliocentric theory and by
developing methods to compute planetary positions using
this model, like what Nicolaus Copernicus later did in
the 16th century.
Middle Ages, heliocentric models may have also been
proposed by the
and by the
The Aristotelian model was accepted in the
Western world for roughly two millennia, until
Copernicus revived Aristarchus' theory that the
astronomical data could be explained more plausibly if
earth rotated on its axis and if the
sun were placed at the center of the Universe.
In the center rests the sun. For who would place
this lamp of a very beautiful temple in another
or better place than this wherefrom it can
illuminate everything at the same time?
Copernicus, in Chapter 10, Book 1 of De
Revolutionibus Orbium Coelestrum (1543)
As noted by Copernicus himself, the suggestion that
Earth rotates was very old, dating at least to
Philolaus (c. 450 BC),
Heraclides Ponticus (c. 350 BC) and
Ecphantus the Pythagorean. Roughly a century before
Copernicus, Christian scholar
Nicholas of Cusa also proposed that the Earth
rotates on its axis in his book, On Learned Ignorance
Al-Sijzi, also proposed that the Earth rotates on
needed] The first
empirical evidence for the Earth's rotation on its
axis, using the phenomenon of
comets, was given by
Tusi (1201–1274) and
Ali Qushji (1403–1474).[citation
This cosmology was accepted by
Christiaan Huygens and later scientists.
Edmund Halley (1720)
Jean-Philippe de Cheseaux (1744)
noted independently that the assumption of an infinite
space filled uniformly with stars would lead to the
prediction that the nighttime sky would be as bright as
the sun itself; this became known as
Olbers' paradox in the 19th century.
Newton believed that an infinite space uniformly filled
with matter would cause infinite forces and
instabilities causing the matter to be crushed inwards
under its own gravity.
This instability was clarified in 1902 by the
Jeans instability criterion.
One solution to these paradoxes is the
Charlier Universe, in which the matter is arranged
hierarchically (systems of orbiting bodies that are
themselves orbiting in a larger system, ad infinitum)
fractal way such that the Universe has a negligibly
small overall density; such a cosmological model had
also been proposed earlier in 1761 by
Johann Heinrich Lambert.
A significant astronomical advance of the 18th century
was the realization by
Immanuel Kant and others of
The modern era of
physical cosmology began in 1917, when
Albert Einstein first applied his general theory of
relativity to model the structure and dynamics of the
High-precision test of general relativity by
Cassini space probe (artist's
radio signals sent between the Earth and
the probe (green wave) are
delayed by the warping of
space and time (blue lines) due to the
Of the four
gravitation is dominant at cosmological length
scales; that is, the other three forces play a
negligible role in determining structures at the level
of planetary systems, galaxies and larger-scale
structures. Because all matter and energy gravitate,
gravity's effects are cumulative; by contrast, the
effects of positive and negative charges tend to cancel
one another, making electromagnetism relatively
insignificant on cosmological length scales. The
remaining two interactions, the
strong nuclear forces, decline very rapidly with
distance; their effects are confined mainly to
sub-atomic length scales.
General theory of relativity
Given gravitation's predominance in shaping
cosmological structures, accurate predictions of the
Universe's past and future require an accurate theory of
gravitation. The best theory available is
Albert Einstein's general theory of relativity,
which has passed all experimental tests to date.
However, because rigorous experiments have not been
carried out on cosmological length scales, general
relativity could conceivably be inaccurate.
Nevertheless, its cosmological predictions appear to be
consistent with observations, so there is no compelling
reason to adopt another theory.
General relativity provides a set of ten nonlinear
partial differential equations for the
spacetime metric (Einstein's
field equations) that must be solved from the
momentum throughout the Universe. Because these are
unknown in exact detail, cosmological models have been
based on the
cosmological principle, which states that the
Universe is homogeneous and isotropic. In effect, this
principle asserts that the gravitational effects of the
various galaxies making up the Universe are equivalent
to those of a fine
dust distributed uniformly throughout the Universe
with the same average density. The assumption of a
uniform dust makes it easy to solve Einstein's field
equations and predict the past and future of the
Universe on cosmological time scales.
Einstein's field equations include a
cosmological constant (Λ),
that corresponds to an energy density of empty space.
Depending on its sign, the cosmological constant can
either slow (negative Λ) or accelerate (positive
expansion of the Universe. Although many scientists,
including Einstein, had speculated that Λ was
recent astronomical observations of
type Ia supernovae have detected a large amount of "dark
energy" that is accelerating the Universe's
Preliminary studies suggest that this dark energy
corresponds to a positive Λ, although alternative
theories cannot be ruled out as yet.
Zel'dovich suggested that Λ is a measure of
zero-point energy associated with
virtual particles of
quantum field theory, a pervasive
vacuum energy that exists everywhere, even in empty
Evidence for such zero-point energy is observed in the
Special relativity and space-time
Only its length L is intrinsic to the
rod (shown in black); coordinate differences
between its endpoints (such as Δx, Δy or Δξ,
Δη) depend on their frame of reference
(depicted in blue and red, respectively).
The Universe has at least three
spatial and one temporal (time)
dimension. It was long thought that the spatial and
temporal dimensions were different in nature and
independent of one another. However, according to the
special theory of relativity, spatial and temporal
separations are interconvertible (within limits) by
changing one's motion.
To understand this interconversion, it is helpful to
consider the analogous interconversion of spatial
separations along the three spatial dimensions. Consider
the two endpoints of a rod of length L. The
length can be determined from the differences in the
three coordinates Δx, Δy and Δz of the two endpoints in
a given reference frame
Pythagorean theorem. In a rotated reference frame,
the coordinate differences differ, but they give the
Thus, the coordinates differences (Δx, Δy, Δz) and
(Δξ, Δη, Δζ) are not intrinsic to the rod, but merely
reflect the reference frame used to describe it; by
contrast, the length L is an intrinsic property
of the rod. The coordinate differences can be changed
without affecting the rod, by rotating one's reference
The analogy in
spacetime is called the interval between two events;
an event is defined as a point in spacetime, a specific
position in space and a specific moment in time. The
spacetime interval between two events is given by
where c is the speed of light. According to
special relativity, one can change a spatial and
time separation (L1, Δt1)
into another (L2, Δt2)
by changing one's reference frame, as long as the change
maintains the spacetime interval s. Such a change
in reference frame corresponds to changing one's motion;
in a moving frame, lengths and times are different from
their counterparts in a stationary reference frame. The
precise manner in which the coordinate and time
differences change with motion is described by the
Solving Einstein's field equations
The distances between the spinning galaxies increase
with time, but the distances between the stars within
each galaxy stay roughly the same, due to their
gravitational interactions. This animation illustrates a
closed Friedmann Universe with zero
cosmological constant Λ; such a Universe oscillates
Big Bang and a
In non-Cartesian (non-square) or curved coordinate
systems, the Pythagorean theorem holds only on
infinitesimal length scales and must be augmented with a
metric tensor gμν, which can vary
from place to place and which describes the local
geometry in the particular coordinate system. However,
cosmological principle that the Universe is
homogeneous and isotropic everywhere, every point in
space is like every other point; hence, the metric
tensor must be the same everywhere. That leads to a
single form for the metric tensor, called the
where (r, θ, φ) correspond to a
spherical coordinate system. This
metric has only two undetermined parameters: an
overall length scale R that can vary with time,
and a curvature index k that can be only 0, 1 or
−1, corresponding to flat
Euclidean geometry, or spaces of positive or
curvature. In cosmology, solving for the history of
the Universe is done by calculating R as a
function of time, given k and the value of the
cosmological constant Λ, which is a (small)
parameter in Einstein's field equations. The equation
describing how R varies with time is known as the
Friedmann equation, after its inventor,
The solutions for R(t) depend on k and
Λ, but some qualitative features of such
solutions are general. First and most importantly, the
length scale R of the Universe can remain
constant only if the Universe is perfectly
isotropic with positive curvature (k=1) and has
one precise value of density everywhere, as first noted
Albert Einstein. However, this equilibrium is
unstable and because the Universe is known to be
inhomogeneous on smaller scales, R must change,
general relativity. When R changes, all the
spatial distances in the Universe change in tandem;
there is an overall expansion or contraction of space
itself. This accounts for the observation that galaxies
appear to be flying apart; the space between them is
stretching. The stretching of space also accounts for
the apparent paradox that two galaxies can be 40 billion
light years apart, although they started from the same
point 13.8 billion years ago
and never moved faster than the
speed of light.
Second, all solutions suggest that there was a
gravitational singularity in the past, when R
goes to zero and matter and energy became infinitely
dense. It may seem that this conclusion is uncertain
because it is based on the questionable assumptions of
perfect homogeneity and isotropy (the cosmological
principle) and that only the gravitational interaction
is significant. However, the
Penrose–Hawking singularity theorems show that a
singularity should exist for very general conditions.
Hence, according to Einstein's field equations, R
grew rapidly from an unimaginably hot, dense state that
existed immediately following this singularity (when
R had a small, finite value); this is the essence of
Big Bang model of the Universe. A common
misconception is that the Big Bang model predicts that
matter and energy exploded from a single point in space
and time; that is false. Rather, space itself was
created in the Big Bang and imbued with a fixed amount
of energy and matter distributed uniformly throughout;
as space expands (i.e., as R(t) increases), the
density of that matter and energy decreases.
Space has no
boundary – that is empirically more certain than
any external observation. However, that does not
imply that space is infinite... (translated,
Bernhard Riemann (Habilitationsvortrag,
Third, the curvature index k determines the
sign of the mean spatial curvature of
spacetime averaged over length scales greater than a
light years. If k=1, the curvature is
positive and the Universe has a finite volume. Such
universes are often visualized as a
three-dimensional sphere S3 embedded
in a four-dimensional space. Conversely, if k
is zero or negative, the Universe may have
infinite volume, depending on its overall
topology. It may seem counter-intuitive that an
infinite and yet infinitely dense Universe could be
created in a single instant at the Big Bang when R=0,
but exactly that is predicted mathematically when k
does not equal 1. For comparison, an infinite plane has
zero curvature but infinite area, whereas an infinite
cylinder is finite in one direction and a
torus is finite in both. A toroidal Universe could
behave like a normal Universe with
periodic boundary conditions, as seen in
"wrap-around" video games such as
Asteroids; a traveler crossing an outer
"boundary" of space going outwards would reappear
instantly at another point on the boundary moving
Illustration of the Big Bang theory, the
prevailing model of the origin and
spacetime and all that it contains.
In this diagram time increases from left
to right, and one dimension of space is
suppressed, so at any given time the
Universe is represented by a disk-shaped
"slice" of the diagram.
ultimate fate of the Universe is still unknown,
because it depends critically on the curvature index
k and the cosmological constant Λ. If the
Universe is sufficiently dense, k equals +1,
meaning that its average curvature throughout is
positive and the Universe will eventually recollapse in
Big Crunch, possibly starting a new Universe in a
Big Bounce. Conversely, if the Universe is
insufficiently dense, k equals 0 or −1 and the
Universe will expand forever, cooling off and eventually
becoming inhospitable for all life, as the stars die and
all matter coalesces into black holes (the
Big Freeze and the
heat death of the Universe). As noted above, recent
data suggests that the expansion speed of the Universe
is not decreasing as originally expected, but
increasing; if this continues indefinitely, the Universe
will eventually rip itself to shreds (the
Big Rip). Experimentally, the Universe has an
overall density that is very close to the critical value
between recollapse and eternal expansion; more careful
astronomical observations are needed to resolve the
The prevailing Big Bang model accounts for many of
the experimental observations described above, such as
the correlation of distance and
redshift of galaxies, the universal ratio of
hydrogen:helium atoms, and the ubiquitous, isotropic
microwave radiation background. As noted above, the
redshift arises from the
metric expansion of space; as the space itself
expands, the wavelength of a
photon traveling through space likewise increases,
decreasing its energy. The longer a photon has been
traveling, the more expansion it has undergone; hence,
older photons from more distant galaxies are the most
red-shifted. Determining the correlation between
distance and redshift is an important problem in
Other experimental observations can be explained by
combining the overall expansion of space with
atomic physics. As the Universe expands, the energy
density of the
electromagnetic radiation decreases more quickly
than does that of
matter, because the energy of a photon decreases
with its wavelength. Thus, although the energy density
of the Universe is now dominated by matter, it was once
dominated by radiation; poetically speaking, all was
light. As the Universe expanded, its energy density
decreased and it became cooler; as it did so, the
elementary particles of matter could associate
stably into ever larger combinations. Thus, in the early
part of the matter-dominated era, stable
neutrons formed, which then associated into
atomic nuclei. At this stage, the matter in the
Universe was mainly a hot, dense
plasma of negative
neutrinos and positive nuclei.
Nuclear reactions among the nuclei led to the
present abundances of the lighter nuclei, particularly
helium. Eventually, the electrons and nuclei
combined to form stable atoms, which are transparent to
most wavelengths of radiation; at this point, the
radiation decoupled from the matter, forming the
ubiquitous, isotropic background of microwave radiation
Other observations are not answered definitively by
known physics. According to the prevailing theory, a
slight imbalance of
antimatter was present in the Universe's creation,
or developed very shortly thereafter, possibly due to
CP violation that has been observed by
particle physicists. Although the matter and
antimatter mostly annihilated one another, producing
photons, a small residue of matter survived, giving
the present matter-dominated Universe. Several lines of
evidence also suggest that a rapid
cosmic inflation of the Universe occurred very early
in its history (roughly 10−35 seconds after
its creation). Recent observations also suggest that the
cosmological constant (Λ) is not zero and
that the net
mass-energy content of the Universe is dominated by
dark energy and
dark matter that have not been characterized
scientifically. They differ in their gravitational
effects. Dark matter gravitates as ordinary matter does,
and thus slows the expansion of the Universe; by
contrast, dark energy serves to accelerate the
Some speculative theories have proposed that this
Universe is but one of a
set of disconnected universes, collectively denoted
multiverse, challenging or enhancing more limited
definitions of the Universe.
Scientific multiverse theories are distinct from
concepts such as
alternate planes of consciousness and
simulated reality, although the idea of a larger
Universe is not new; for example, Bishop
Étienne Tempier of Paris ruled in 1277 that God
could create as many universes as he saw fit, a question
that was being hotly debated by the French theologians.
Max Tegmark developed a four-part
classification scheme for the different types of
multiverses that scientists have suggested in various
problem domains. An example of such a theory is the
chaotic inflation model of the early Universe.
Another is the
many-worlds interpretation of quantum mechanics.
Parallel worlds are generated in a manner similar to
quantum superposition and
decoherence, with all states of the
wave function being realized in separate worlds.
Effectively, the multiverse evolves as a
universal wavefunction. If the big bang that created
our multiverse created an ensemble of multiverses, the
wave function of the ensemble would be entangled in this
The least controversial category of multiverse in
Tegmark's scheme is
Level I, which describes distant space-time events
"in our own Universe". If space is infinite, or
sufficiently large and uniform, identical instances of
the history of Earth's entire
Hubble volume occur every so often, simply by
chance. Tegmark calculated our nearest so-called
doppelgänger, is 1010115
meters away from us (a
double exponential function larger than a
In principle, it would be impossible to scientifically
verify an identical Hubble volume. However, it does
follow as a fairly straightforward consequence from
otherwise unrelated scientific observations and
theories. Tegmark suggests that statistical analysis
anthropic principle provides an opportunity to test
multiverse theories in some cases. Generally, science
would consider a multiverse theory that posits neither a
common point of causation, nor the possibility of
interaction between universes, to be an idle
Shape of the Universe
The shape or
geometry of the Universe includes both
local geometry in the
observable Universe and
global geometry, which we may or may not be able to
measure. Shape can refer to curvature and
topology. More formally, the subject in practice
3-manifold corresponds to the spatial section in
comoving coordinates of the four-dimensional
space-time of the Universe. Cosmologists normally
work with a given
space-like slice of spacetime called the
comoving coordinates. In terms of observation, the
section of spacetime that can be observed is the
light cone (points within the
cosmic light horizon, given time to reach a given
observer). If the observable Universe is smaller than
the entire Universe (in some models it is many orders of
magnitude smaller), one cannot determine the global
structure by observation: one is limited to a small
Friedmann–Lemaître–Robertson–Walker (FLRW) models,
the presently most popular shape of the Universe found
to fit observational data according to cosmologists is
the infinite flat model,
while other FLRW models include the
Poincaré dodecahedral space
The data fit by these FLRW models of space especially
Wilkinson Microwave Anisotropy Probe (WMAP) and
Planck maps of cosmic background radiation. NASA
released the first WMAP cosmic background radiation data
in February 2003, while a higher resolution map
regarding Planck data was released by ESA in March 2013.
Both probes have found almost perfect agreement with
inflationary models and the standard model of cosmology,
describing a flat, homogeneous universe dominated by
dark matter and dark energy.
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