## English

### Noun

universes- Plural of universe

The Universe is most commonly defined as everything that physically
exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical
laws and constants
that govern them. However, the term "universe" may be used in
slightly different contextual senses, denoting such concepts as the
cosmos, the world
or Nature.

Astronomical
observations indicate that the universe is 13.73
± 0.12 billion years old and at least 93 billion light years
across. The event that started the universe is called the Big Bang. At
this point in time all matter and energy of the observable
universe was concentrated in one point of infinite density. After the Big
Bang the universe started to expand to its present form. Since
special
relativity states that matter cannot exceed the speed of
light in a fixed space-time, it
may seem paradoxical that two galaxies can be separated by 93
billion light years in
13 billion years; however, this separation is a natural consequence
of general
relativity. Stated simply, space can expand with no intrinsic
limit on its rate; thus, two galaxies can separate more quickly
than the speed of light if the space between them grows.
Experimental measurements such as the redshifts and
spatial distribution of distant galaxies, the
cosmic microwave background radiation, and the relative
percentages of the lighter chemical
elements, support this theoretical
expansion and, more generally, the Big Bang theory,
which proposes that space itself was created ex nihilo at a
specific time in the past. Recent observations have shown that this
expansion is accelerating, and that most of the matter and energy
in the universe is fundamentally different from that observed on
Earth and not directly observable (cf. dark energy).
The imprecision of current observations has hindered predictions of
the
ultimate fate of the universe.

Experiments suggest that the universe has been
governed by the same physical laws and constants throughout its
extent and history. The dominant force at cosmological distances is
gravity, and general
relativity is currently the most accurate theory of
gravitation. The remaining three fundamental
forces and the particles on which they act are described by the
Standard
Model. The universe has at least three dimensions of space and one of
time, although extremely
small additional dimensions cannot be ruled out experimentally.
Spacetime
appears to be smoothly
and simply
connected, and space has very
small mean curvature,
so that Euclidean
geometry is accurate on the average throughout the
universe.

According to some speculations, this universe may
be one of many disconnected universes, which are collectively
denoted as the multiverse. In one
theory, there is an infinite variety of universes, each with
different physical
constants. In another
theory, new universes are spawned with every quantum
measurement. By definition, these speculations cannot currently
be tested experimentally.

Throughout recorded history, several cosmologies and cosmogonies have been proposed
to account for observations of the universe. The earliest
quantitative models were developed by the ancient
Greeks, who proposed that the universe possesses infinite space
and has existed eternally, but contains a single set of concentric
spheres of finite size -
corresponding to the fixed stars, the Sun and various
planets - rotating about
a spherical but unmoving Earth. Over the
centuries, more precise observations and improved theories of
gravity led to Copernicus'
heliocentric model
and the Newtonian
model of the solar
system, respectively. Further improvements in astronomy led to
the characterization of the Milky Way, and
the discovery of other galaxies and the microwave background
radiation; careful studies of the distribution of these galaxies
and their spectral
lines have led to much of modern
cosmology.

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 multiverse. In principle, the
other unconnected universes may have different dimensionalities and topologies of space-time,
different forms of matter
and energy, and different
physical
laws and physical
constants, although it is impossible to know for sure. These
multiverses could also exist within other universes, in the same
way that the interior of a black hole is discontinuous with our
world; once something goes in it will never come out.

The observable matter is spread uniformly
(homogeneously) 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 content. The universe is also bathed in a highly isotropic
microwave radiation
that corresponds to a thermal
equilibrium blackbody
spectrum of roughly 2.725 Kelvin. The
hypothesis that the large-scale universe is homogeneous and
isotropic is known as the cosmological
principle, which is 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 slow the
expansion of the universe; by contrast, dark energy accelerates
its expansion.

The universe is old
and evolving. The
most precise estimate of the universe's age is 13.73±0.12
billion years old, based on observations of the
cosmic microwave background radiation. Independent estimates
(based on measurements such as radioactive
dating) agree, although they are less precise, ranging from
11-20 billion years to 13–15 billion years. 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 redshifted; the 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.

The
relative fractions of different chemical
elements — particularly the lightest atoms such as hydrogen, deuterium and helium — seem to be identical
throughout the universe and throughout its observable history. The
universe seems to have much more matter than antimatter, an asymmetry
possibly related to the observations of CP
violation. The universe appears to have no net electric
charge, and therefore gravity appears to be the
dominant interaction on cosmological length scales. The universe
appears to have no net momentum and 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 spacetime continuum consisting of three
spatial dimensions and one temporal
(time) dimension. On the
average, space is observed
to be very nearly flat (close to zero curvature), meaning that
Euclidean
geometry is experimentally true with high accuracy throughout
most of the universe. Spacetime also appears to have a simply
connected 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 cylindrical or toroidal topologies of
two-dimensional spaces.

The universe appears to be governed throughout by
the same physical
laws and physical
constants. According to the prevailing Standard
Model of physics, all matter is composed of three generations
of leptons and quarks, both of which are fermions. These 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
renormalized
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 values that 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, momentum,
angular momentum and energy;
in many cases, these conservation laws can be related to symmetries or mathematical
identities.

Many models of the cosmos (cosmologies) and its
origin (cosmogonies) have been proposed, based on the then
available data and conceptions of the universe. Initially,
cosmologies and cosmogonies were based on narratives of gods acting
in various ways. The Greeks were the first to propose theories of
an impersonal universe governed by physical laws. 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 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.

Although Heraclitus argued for eternal change,
his rough contemporary 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 το εν (The One).
Parmenides' theory seemed implausible to many Greeks, but his
student Zeno of
Elea challenged them with several famous paradoxes.
Aristotle resolved these paradoxes by developing the notion of an
infinitely divisible continuum, and applying it to
space and time.

More practical Greek philosophers were concerned
with developing models of the universe that would account for the
observed motion of the stars and planets. The first coherent model
was proposed by Eudoxus
of Cnidos. According to this model, space and time are infinite
and eternal, the Earth is spherical and stationary, and all other
matter is confined to rotating concentric spheres. This model was
refined by Callippus and
Aristotle, and
brought into nearly perfect agreement with astronomical
observations by Ptolemy. The
success of this 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). However, not all Greek scientists accepted the
geocentric model of the Universe. Aristarchus
of Samos was the first astronomer to propose a heliocentric
model. Though the original text has been lost, a reference in
Archimedes' book The Sand Reckoner describes Aristarchus'
heliocentric theory. 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 surface.

Aristarchus thus believed the stars to be very
far away, and saw this as the reason why there was no visible
parallax, that is, an 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 telescopes. 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. [1]

The only other astronomer from antiquity known by
name who supported Aristarchus' heliocentric model was Seleucus of
Seleucia, a Greek astronomer who lived a century after
Aristarchus.

The Aristotelian model was accepted for roughly
two millennia, until Copernicus
revived Aristarchus' theory that the astronomical data could be
explained more plausibly if the earth rotated on its axis and if
the sun were placed at the
center of the universe

As noted by Copernicus himself, the suggestion
that the 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, Nicholas
of Cusa also proposed that the Earth rotates on its axis in his
book, On Learned Ignorance (1440). Copernicus' heliocentric model allowed
the stars to be placed uniformly through the (infinite) space
surrounding the planets, as first proposed by Thomas
Digges in his Perfit Description of the Caelestiall Orbes
according to the most aunciente doctrine of the Pythagoreans,
latelye revived by Copernicus and by Geometricall Demonstrations
approved (1576). Giordano
Bruno accepted the idea that space was infinite and filled with
solar systems similar to our own; for the publication of this view,
he was burned
at the stake in the Campo dei
Fiori in Rome on 17 February 1600. although it had several
paradoxes that were resolved only with the development of general
relativity. The first of these was that it assumed that space
and time were infinite, and that the stars in the universe had been
burning forever; however, since stars are constantly radiating
energy, a finite star
seems inconsistent with the radiation of infinite energy. Secondly,
Edmund Halley (1720) and 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. Third, Newton himself showed 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. One solution to these latter two 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) in a 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 Thomas
Wright, Immanuel
Kant and others that stars are not distributed uniformly
throughout space; rather, they are grouped into galaxies.

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
universe. This theory and its implications will be discussed in
more detail in the following section.

Of the four fundamental
interactions, gravitation is dominant at
cosmological length scales; that is, the other three forces are
believed to play a negligible role in determining structures at the
level of planets, stars, galaxies and larger-scale structures.
Since 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 weak
and strong
nuclear forces, decline very rapidly with distance; their
effects are confined mainly to sub-atomic length scales.

General relativity provides of a set of ten
nonlinear partial differential equations for the
spacetime metric (Einstein's
field equations) that must be solved from the distribution of
mass-energy and
momentum throughout the
universe. Since 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 Λ) the
expansion of the universe. Although many scientists, including
Einstein,
had speculated that Λ was zero, recent astronomical observations of
type Ia
supernovae have detected a large amount of "dark energy"
that is accelerating the universe's expansion. Preliminary studies
suggest that this dark energy corresponds to a positive Λ, although
alternative theories cannot be ruled out as yet. Russian physicist
Zel'dovich suggested that Λ is a measure of the zero-point
energy associated with virtual
particles of quantum
field theory, a pervasive vacuum
energy that exists everywhere, even in empty space. Evidence
for such zero-point energy is observed in the Casimir
effect.

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

L^ = \Delta x^ + \Delta y^ + \Delta z^

using the Pythagorean
theorem. In a rotated reference frame, the coordinate
differences differ, but they give the same length

L^ = \Delta \xi^ + \Delta \eta^ + \Delta
\zeta^

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
frame.

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

s^ = L_^ - c^ \Delta t_^ = L_^ - c^ \Delta
t_^

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 Lorentz
transformation.

ds^2 = -c^ dt^2 + R(t)^2 \left( \frac + r^2
d\theta^2 + r^2 \sin^2 \theta \, d\phi^2 \right)

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 negative 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, Alexander
Friedmann.

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 by Albert
Einstein. However, this equilibrium is unstable and since the
universe is known to be inhomogeneous on smaller scales, R must
change, according to 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. The 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 13.7 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 since 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 the 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.

Third, the curvature index k determines the sign
of the mean spatial curvature of spacetime averaged over length
scales greater than a billion 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
inwards.

The
ultimate fate of the universe is still unknown, since 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 a 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 of the universe is not decreasing as
originally expected, but accelerating; 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
decide the question.

Other experimental observations can be explained
by combining the overall expansion of space with nuclear
and atomic
physics. As the universe expands, the energy density of the
electromagnetic
radiation decreases more quickly than does that of matter, since 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 protons and
neutrons formed, which
then associated into atomic
nuclei. At this stage, the matter in the universe was mainly a
hot, dense plasma of
negative electrons,
neutral neutrinos and
positive nuclei. Nuclear
reactions among the nuclei led to the present abundances of the
lighter nuclei, particularly hydrogen, deuterium, and 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 observed today.

Other observations are not answered definitively
by known physics. According to the prevailing theory, a slight
imbalance of matter over
antimatter was
present in the universe's creation, or developed very shortly
thereafter, possibly due to the 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 a 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 universe's
expansion.

There are two scientific senses in which multiple
universes can occur. First, disconnected spacetime continua may exist;
presumably, all forms of matter and energy are confined to one
universe and cannot "tunnel" between them. An example of such a
theory is the chaotic
inflation model of the early universe. Second, according to the
many-worlds
hypothesis, a parallel universe is born with every quantum
measurement; the universe "forks" into parallel copies, each
one corresponding to a different outcome of the quantum
measurement. Authors have explored this concept in some fiction,
most notably Jorge
Borges' short story
The Garden of Forking Paths. However, both senses of the term
"multiverse" are speculative and may be considered unscientific;
the fact that universes cannot interact makes it impossible to test
experimentally in this universe whether another universe
exists.

- Abiogenesis
- Anthropic principle
- Big Bang
- Big Crunch
- Cosmic latte
- Cosmology
- Dyson's eternal intelligence
- Esoteric cosmology
- False vacuum
- Final anthropic principle
- Fine-tuned Universe
- Gaia hypothesis
- Heat death of the universe
- Hindu Cycle Of The Universe
- Kardashev scale
- Multiverse (religion)
- Nucleocosmochronology
- Non-standard cosmology
- Omega point
- Omniverse
- Rare Earth hypothesis
- Reality
- Shape of the Universe
- Ultimate fate of the universe
- World development
- World view

- The First Three Minutes: A Modern View of the Origin of the Universe

- Essential Relativity: Special, General, and Cosmological

- The Classical Theory of Fields (Course of Theoretical Physics, Vol. 2)

- Gravitation (See Gravitation (book).)

- Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity

- Age of the Universe at Space.Com
- Cosmology FAQ
- Cosmos - an "illustrated dimensional journey from microcosmos to macrocosmos"
- Illustration comparing the sizes of the planets, the sun, and other stars
- Logarithmic Maps of the Universe
- My So-Called Universe arguments for and against an infinite and parallel universes
- Parallel Universes by Max Tegmark
- The Dark Side and the Bright Side of the Universe Princeton University, Shirley Ho
- Richard Powell: An Atlas of the Universe - images at various scales, with explanations
- Size of the Universe at Space.Com
- Stephen Hawking's Universe - why is the universe the way it is?
- Universe - Space Information Centre by Exploreuniverse.com

universes in Contenese: 宇宙

universes in Arabic: فضاء كوني

universes in Asturian: Universu

universes in Bengali: মহাবিশ্ব

universes in Min Nan: Ú-tiū

universes in Belarusian: Сусвет

universes in Belarusian (Tarashkevitsa):
Сусвет

universes in Bosnian: Svemir

universes in Bulgarian: Вселена

universes in Catalan: Univers

universes in Czech: Vesmír

universes in Welsh: Bydysawd (seryddiaeth)

universes in Danish: Universet

universes in German: Universum

universes in Lower Sorbian: Uniwersum

universes in Estonian: Universum

universes in Modern Greek (1453-): Σύμπαν

universes in Spanish: Universo

universes in Esperanto: Universo

universes in Basque: Unibertso

universes in French: Univers

universes in Western Frisian: Hielal

universes in Galician: Universo

universes in Classical Chinese: 宇宙

universes in Hakka Chinese: Yî-chhiu

universes in Korean: 우주

universes in Hindi: ब्रह्माण्ड

universes in Croatian: Svemir

universes in Ido: Universo

universes in Indonesian: Alam semesta

universes in Icelandic: Alheimurinn

universes in Italian: Universo

universes in Hebrew: היקום

universes in Pampanga: Sikluban

universes in Georgian: სამყარო

universes in Kashubian: Swiatnica

universes in Swahili (macrolanguage):
Ulimwengu

universes in Kurdish: Gerdûn

universes in Latin: Universum

universes in Latvian: Visums

universes in Lithuanian: Visata

universes in Lombard: Ünivers

universes in Hungarian: Világegyetem

universes in Macedonian: Вселена

universes in Malayalam: പ്രപഞ്ചം

universes in Malay (macrolanguage): Alam
semesta

nah:Cemānāhuac
universes in Dutch: Heelal

universes in Japanese: 宇宙

universes in Neapolitan: Annevierzo

universes in Norwegian: Universet

universes in Norwegian Nynorsk: Universet

universes in Narom: Eunivers

universes in Novial: Universe

universes in Occitan (post 1500): Univèrs

universes in Uzbek: Olam

universes in Low German: Weltruum

universes in Polish: Wszechświat

universes in Portuguese: Universo

universes in Kölsch: Weltall

universes in Romanian: Univers

universes in Quechua: Cosmos

universes in Russian: Вселенная

universes in Saterfriesisch: Al

universes in Albanian: Gjithësia

universes in Simple English: Universe

universes in Slovak: Vesmír

universes in Slovenian: Vesolje

universes in Serbian: Свемир

universes in Serbo-Croatian: Svemir

universes in Sundanese: Jagat

universes in Finnish: Maailmankaikkeus

universes in Swedish: Universum

universes in Tamil: அண்டம்

universes in Thai: เอกภพ

universes in Vietnamese: Vũ trụ

universes in Tajik: Коинот

universes in Turkish: Evren

universes in Ukrainian: Всесвіт

universes in Urdu: کائنات

universes in Yiddish: אוניווערס

universes in Samogitian: Vėsata

universes in Chinese: 宇宙

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