Universe is all of space and time[a] and their contents,
including planets, stars, galaxies, and all other forms of matter and
energy. While the spatial size of the entire
Universe is still
unknown, it is possible to measure the observable universe.
The earliest scientific models of the
Universe were developed by
ancient Greek and Indian philosophers and were geocentric, placing
Earth at the centre of the Universe. Over the centuries, more
precise astronomical observations led
Nicolaus Copernicus to develop
the heliocentric model with the
Sun at the centre of the Solar System.
In developing the law of universal gravitation, Sir
Isaac Newton built
upon Copernicus's work as well as observations by
Tycho Brahe and
Johannes Kepler's laws of planetary motion.
Further observational improvements led to the realization that our
Solar System is located in the
Milky Way galaxy, which is one of many
galaxies in the Universe. It is assumed that galaxies are distributed
uniformly and the same in all directions, meaning that the Universe
has neither an edge nor a center. Discoveries in the early 20th
century have suggested that the
Universe had a beginning and that it
is expanding at an increasing rate. Roughly eighty percent of
mass in the
Universe appears to exist in an unknown form called dark
matter which cannot be directly observed.
Big Bang theory is the prevailing cosmological description of the
development of the Universe. Under this theory, space and time emerged
together 7001137990000000000♠13.799±0.021 billion years ago
with a fixed amount of energy and matter that has become less dense as
Universe has expanded. After the initial expansion, the Universe
cooled, allowing the first subatomic particles to form and then simple
atoms. Giant clouds later merged through gravity to form galaxies,
stars, and everything else seen today. It is possible to see objects
that are now further away than 13.799 billion light-years because
space itself has expanded. This means that objects which are now 46
billion light years away can still be seen in their distant past,
because at that time they were much closer to us.
There are many competing hypotheses about the ultimate fate of the
universe and about what, if anything, preceded the Big Bang, while
other physicists and philosophers refuse to speculate, doubting that
information about prior states will ever be accessible. Some
physicists have suggested various multiverse hypotheses, in which the
Universe might be one among many universes that likewise
Part of a series on
Big Bang · Universe
Age of the universe
Chronology of the universe
Grand unification epoch
Big Bang nucleosynthesis
Cosmic background radiation
Cosmic background radiation (CBR)
Gravitational wave background
Gravitational wave background (GWB)
Cosmic microwave background
Cosmic microwave background (CMB) · Cosmic neutrino background
Cosmic infrared background
Cosmic infrared background (INB)
Expansion · Future
Hubble's law · Redshift
Metric expansion of space
FLRW metric · Friedmann equations
Future of an expanding universe
Ultimate fate of the universe
Heat death of the universe
Components · Structure
Cold dark matter
Warm dark matter
Hot dark matter
Shape of the universe
Reionization · Structure formation
Large quasar group
Black Hole Initiative
Black Hole Initiative (BHI)
Cosmic Background Explorer
Cosmic Background Explorer (COBE)
Planck space observatory
Sloan Digital Sky Survey
Sloan Digital Sky Survey (SDSS)
Redshift Survey ("2dF")
Discovery of cosmic microwave
History of the
Big Bang theory
Religious interpretations of
Big Bang theory
Timeline of cosmological theories
3 Chronology and the Big Bang
4 Physical properties
4.1 Size and regions
4.2 Age and expansion
4.5 Support of life
5.1 Dark energy
5.2 Dark matter
5.3 Ordinary Matter
6 Cosmological models
6.1 Model of the
Universe based on general relativity
7 Historical conceptions
7.2 Philosophical models
7.3 Astronomical concepts
8 See also
12 External links
Universe is defined as all of space and time[a]
(collectively referred to as spacetime) and their contents. Such
contents encompass all of energy in its various forms, including
electromagnetic radiation and matter, and therefore planets, moons,
stars, galaxies, and the contents of intergalactic space.
Universe also includes the physical laws that influence energy and
matter, such as conservation laws, classical mechanics, and
Universe is often defined as "the totality of existence", or
everything that exists, everything that has existed, and everything
that will exist. In fact, some philosophers and scientists support
the inclusion of ideas and abstract concepts – such as mathematics
and logic – in the definition of the Universe. The word
universe may also refer to concepts such as the cosmos, the world, and
The word universe derives from the
Old French word univers, which in
turn derives from the
Latin word universum. The
Latin word was
Cicero and later
Latin authors in many of the same senses as
the modern English word is used.
A term for "universe" among the ancient Greek philosophers from
Pythagoras onwards was τὸ πᾶν tò pân ("the all"), defined as
all matter and all space, and τὸ ὅλον tò hólon ("all
things"), which did not necessarily include the void. Another
synonym was ὁ κόσμος ho kósmos (meaning the world, the
cosmos). Synonyms are also found in
Latin authors (totum, mundus,
natura) 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 cosmology), the world (as in the many-worlds
interpretation), and nature (as in natural laws or natural
Chronology and the Big Bang
Big Bang and Chronology of the universe
view • discuss • edit
Earliest universe (−13.80)
Omega Centauri forms
Milky Way Galaxy
spiral arms form
Alpha Centauri forms
Earliest sexual reproduction
Axis scale: billion years
Human timeline and
The prevailing model for the evolution of the
Universe is the Big Bang
Big Bang model states that the earliest state of
Universe was an extremely hot and dense one, and that the Universe
subsequently expanded and cooled. The model is based on general
relativity and on simplifying assumptions such as homogeneity and
isotropy of space. A version of the model with a cosmological constant
(Lambda) and cold dark matter, known as the Lambda-CDM model, is the
simplest model that provides a reasonably good account of various
observations about the Universe. The
Big Bang model accounts for
observations such as the correlation of distance and redshift of
galaxies, the ratio of the number of hydrogen to helium atoms, and the
microwave radiation background.
In this diagram, time passes from left to right, so at any given time,
Universe is represented by a disk-shaped "slice" of the diagram.
The initial hot, dense state is called the Planck epoch, a brief
period extending from time zero to one
Planck time unit of
approximately 10−43 seconds. During the Planck epoch, all types of
matter and all types of energy were concentrated into a dense state,
where gravitation is believed to have been as strong as the other
fundamental forces, and all the forces may have been unified. Since
the Planck epoch, the
Universe has been expanding to its present form,
possibly with a very brief period of cosmic inflation which caused the
Universe to reach a much larger size in less than 10−32 seconds.
Planck epoch and inflation came the quark, hadron, and
lepton epochs. Together, these epochs encompassed less than 10 seconds
of time following the Big Bang. The observed abundance of the elements
can be explained by combining the overall expansion of space with
nuclear and atomic physics. As the
Universe expands, the energy
density of electromagnetic radiation decreases more quickly than does
that of matter because the energy of a photon decreases with its
wavelength. As the
Universe expanded and cooled, elementary particles
associated stably into ever larger combinations. Thus, in the early
part of the matter-dominated era, stable protons and neutrons formed,
which then formed atomic nuclei through nuclear reactions. This
process, known as
Big Bang nucleosynthesis, led to the present
abundances of lighter nuclei, particularly hydrogen, deuterium, and
Big Bang nucleosynthesis ended about 20 minutes after the Big
Bang, when the
Universe had cooled enough so that nuclear fusion could
no longer occur. At this stage, matter in the
Universe was mainly a
hot, dense plasma of negatively charged electrons, neutral neutrinos
and positive nuclei. This era, called the photon epoch, lasted about
380 thousand years.
Eventually, at a time known as recombination, electrons and nuclei
formed stable atoms, which are transparent to most wavelengths of
radiation. With photons decoupled from matter, the
the matter-dominated era.
Light from this era could travel freely, and
it can still be seen in the
Universe as the cosmic microwave
background (CMB). After around 100 million years, the first stars
formed; these were likely very massive, luminous, and responsible for
the reionization of the Universe. Having no elements heavier than
lithium, these stars also produced the first heavy elements through
stellar nucleosynthesis. The
Universe also contains a mysterious
energy called dark energy, the density of which does not change over
time. After about 9.8 billion years, the
Universe had expanded
sufficiently so that the density of matter was less than the density
of dark energy, marking the beginning of the present
dark-energy-dominated era. In this era, the expansion of the
Universe is accelerating due to dark energy.
Main articles: Observable universe, Age of the Universe, and Metric
expansion of space
Of the four fundamental interactions, gravitation is the dominant at
astronomical length scales. Gravity's effects are cumulative; by
contrast, the effects of positive and negative charges tend to cancel
one another, making electromagnetism relatively insignificant on
astronomical 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.
Universe appears to have much more matter than antimatter, an
asymmetry possibly related to the CP violation. This imbalance
between matter and antimatter is partially responsible for the
existence of the
Universe in itself, since matter and antimatter, if
equally produced at the Big Bang, would have completely annihilated
each other and, as a result, the universe would not have
Universe also appears to have neither net
momentum nor angular momentum, which follows accepted physical laws if
Universe is finite. These laws are the
Gauss's law and the
non-divergence of the stress-energy-momentum pseudotensor.
Constituent spatial scales of the observable universe
This diagram shows Earth's location in the Universe.
Size and regions
Observable universe and Observational cosmology
The size of the
Universe is somewhat difficult to define. 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 were to exist forever: space may expand faster
than light can traverse it.
Distant regions of space are assumed to exist and to be part of
reality as much as we are, even though we can never interact with
them. The spatial region that we can affect and be affected by is the
observable universe. The observable universe depends on the location
of the observer. By traveling, an observer can come into contact with
a greater region of spacetime 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 portion of the
Universe that is observable from our
vantage point in the Milky Way.
The proper distance—the distance as would be measured at a specific
time, including the present—between
Earth and the edge of the
observable universe is 46 billion light-years (14 billion
parsecs), making the diameter of the observable universe about
91 billion light-years (28×10^9 pc). The distance the light
from the edge of the observable universe has travelled is very close
to the age of the
Universe times the speed of light, 13.8 billion
light-years (4.2×10^9 pc), but this does not represent the
distance at any given time because the edge of the observable universe
Earth have since moved further apart. For comparison, the
diameter of a typical galaxy is 30,000 light-years (9,198 parsecs),
and the typical distance between two neighboring galaxies is 3 million
light-years (919.8 kiloparsecs). As an example, the
Milky Way is
roughly 100,000–180,000 light years in diameter, and the
nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is
located roughly 2.5 million light years away.
Because we cannot observe space beyond the edge of the observable
universe, it is unknown whether the size of the
Universe in its
totality is finite or infinite. Estimates for the total
size of the universe, if finite, reach as high as
displaystyle 10^ 10^ 10^ 122
megaparsecs, implied by one resolution of the No-Boundary
Age and expansion
Age of the universe
Age of the universe and Metric expansion of space
Astronomers calculate the age of the
Universe by assuming that the
Lambda-CDM model accurately describes the evolution of the Universe
from a very uniform, hot, dense primordial state to its present state
and measuring the cosmological parameters which constitute the
model. This model is well understood theoretically
and supported by recent high-precision astronomical observations such
WMAP and Planck. Commonly, the set of observations
fitted includes the cosmic microwave background anisotropy, the
brightness/redshift relation for Type Ia supernovae, and large-scale
galaxy clustering including the baryon acoustic oscillation
feature. Other observations, such as the Hubble
constant, the abundance of galaxy clusters, weak gravitational lensing
and globular cluster ages, are generally consistent with these,
providing a check of the model, but are less accurately measured at
present. With the prior that the
Lambda-CDM model is
correct, the measurements of the parameters using a variety of
techniques by numerous experiments yield a best value of the age of
Universe as of 2015 of 13.799 ± 0.021 billion years.
Over time, the
Universe and its contents have evolved; for example,
the relative population of quasars and galaxies has changed and
space itself has expanded. Due to this expansion, scientists on Earth
can observe the light from a galaxy 30 billion light years away even
though 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. Analyses of Type Ia supernovae
indicate that the spatial expansion is accelerating.
The more matter there is in the Universe, the stronger the mutual
gravitational pull of the matter. If the
Universe were too dense then
it would re-collapse into a gravitational singularity. However, if the
Universe contained too little matter then the expansion would
accelerate too rapidly for planets and planetary systems to form.
Since the Big Bang, the universe has expanded monotonically. Perhaps
unsurprisingly, our universe has just the right mass density of about
5 protons per cubic meter which has allowed it to expand for the last
13.8 billion years, giving time to form the universe as observed
There are dynamical forces acting on the particles in the Universe
which affect the expansion rate. Before 1998, it was expected that the
rate of increase of the Hubble Constant would be decreasing as time
went on due to the influence of gravitational interactions in the
Universe, and thus there is an additional observable quantity in the
Universe called the deceleration parameter which cosmologists expected
to be directly related to the matter density of the Universe. In 1998,
the deceleration parameter was measured by two different groups to be
consistent with −1 but not zero, which implied that the present-day
rate of increase of the Hubble Constant is increasing over
Spacetime and World line
See also: Lorentz transformation
Spacetimes are the arenas in which all physical events take place. The
basic elements of spacetimes are events. In any given spacetime, an
event is defined as a unique position at a unique time. A spacetime is
the union of all events (in the same way that a line is the union of
all of its points), formally organized into a manifold.
Universe appears to be a smooth spacetime continuum consisting of
three spatial dimensions and one temporal (time) dimension (an event
in the spacetime of the physical
Universe can therefore be identified
by a set of four coordinates: (x, y, z, t) ). On the average, space is
observed to be very nearly flat (with a curvature close to zero),
Euclidean geometry is empirically true with high accuracy
throughout most of the Universe.
Spacetime also appears to have a
simply connected topology, in analogy with a sphere, at least on the
length-scale of the observable Universe. However, present observations
cannot exclude the possibilities that the
Universe has more dimensions
(which is postulated by theories such as the String theory) and that
its spacetime may have a multiply connected global topology, in
analogy with the cylindrical or toroidal topologies of two-dimensional
spaces. The spacetime of the
Universe is usually interpreted
from a Euclidean perspective, with space as consisting of three
dimensions, and time as consisting of one dimension, the "fourth
dimension". By combining space and time into a single manifold
called Minkowski space, physicists have simplified a large number of
physical theories, as well as described in a more uniform way the
workings of the
Universe at both the supergalactic and subatomic
Spacetime events are not absolutely defined spatially and temporally
but rather are known to be relative to the motion of an observer.
Minkowski space approximates the
Universe without gravity; the
pseudo-Riemannian manifolds of general relativity describe spacetime
with matter and gravity.
Main article: Shape of the universe
The three possible options for the shape of the Universe.
General relativity describes how spacetime is curved and bent by mass
and energy (gravity). The topology or geometry of the Universe
includes both local geometry in the observable universe and global
geometry. Cosmologists often work with a given space-like slice of
spacetime called the comoving coordinates. The section of spacetime
which can be observed is the backward light cone, which delimits the
cosmological horizon. The cosmological horizon (also called the
particle horizon or the light horizon) is the maximum distance from
which particles can have traveled to the observer in the age of the
Universe. This horizon represents the boundary between the observable
and the unobservable regions of the Universe. The existence,
properties, and significance of a cosmological horizon depend on the
particular cosmological model.
An important parameter determining the future evolution of the
Universe theory is the density parameter, Omega (Ω), defined as the
average matter density of the universe divided by a critical value of
that density. This selects one of three possible geometries depending
on whether Ω is equal to, less than, or greater than 1. These are
called, respectively, the flat, open and closed universes.
Observations, including the
Cosmic Background Explorer
Cosmic Background Explorer (COBE),
Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe (WMAP), and Planck maps of the
CMB, suggest that the
Universe is infinite in extent with a finite
age, as described by the Friedmann–Lemaître–Robertson–Walker
(FLRW) models. These FLRW models thus support
inflationary models and the standard model of cosmology, describing a
flat, homogeneous universe presently dominated by dark matter and dark
Support of life
Main article: Fine-tuned Universe
Universe may be fine-tuned; the
Fine-tuned Universe hypothesis is
the proposition that the conditions that allow the existence of
observable life in the
Universe can only occur when certain universal
fundamental physical constants lie within a very narrow range of
values, so that if any of several fundamental constants were only
slightly different, the
Universe would have been unlikely to be
conducive to the establishment and development of matter, astronomical
structures, elemental diversity, or life as it is understood. The
proposition is discussed among philosophers, scientists, theologians,
and proponents of creationism.
Galaxy formation and evolution,
Galaxy cluster, Illustris
project, and Nebula
Universe is composed almost completely of dark energy, dark
matter, and ordinary matter. Other contents are electromagnetic
radiation (estimated to constitute from 0.005% to close to 0.01% of
the total mass of the Universe) and antimatter.
The proportions of all types of matter and energy have changed over
the history of the Universe. The total amount of electromagnetic
radiation generated within the universe has decreased by 1/2 in the
past 2 billion years. Today, ordinary matter, which includes
atoms, stars, galaxies, and life, accounts for only 4.9% of the
contents of the Universe. The present overall density of this type
of matter is very low, roughly 4.5 × 10−31 grams per cubic
centimetre, corresponding to a density of the order of only one proton
for every four cubic meters of volume. The nature of both dark
energy and dark matter is unknown. Dark matter, a mysterious form of
matter that has not yet been identified, accounts for 26.8% of the
cosmic contents. Dark energy, which is the energy of empty space and
is causing the expansion of the
Universe to accelerate, accounts for
the remaining 68.3% of the contents.
The formation of clusters and large-scale filaments in the cold dark
matter model with dark energy. The frames show the evolution of
structures in a 43 million parsecs (or 140 million light years) box
from redshift of 30 to the present epoch (upper left z=30 to lower
A map of the superclusters and voids nearest to Earth.
Matter, dark matter, and dark energy are distributed homogeneously
Universe over length scales longer than 300 million
light-years or so. However, over shorter length-scales, matter
tends to clump hierarchically; many atoms are condensed into stars,
most stars into galaxies, most galaxies into clusters, superclusters
and, finally, large-scale galactic filaments. The observable Universe
contains approximately 300 sextillion (3×1023) stars and more
than 100 billion (1011) galaxies. Typical galaxies range from
dwarfs with as few as ten million (107) stars up to giants with
one trillion (1012) stars. Between the larger structures are
voids, which are typically 10–150 Mpc (33 million–490 million ly)
in diameter. The
Milky Way is in the
Local Group of galaxies, which in
turn is in the Laniakea Supercluster. This supercluster spans over
500 million light years, while the
Local Group spans over 10 million
light years. The
Universe also has vast regions of relative
emptiness; the largest known void measures 1.8 billion ly (550 Mpc)
Comparison of the contents of the
Universe today to 380,000 years
Big Bang as measured with 5 year
WMAP data (from 2008).
(Due to rounding errors, the sum of these numbers is not 100%). This
reflects the 2008 limits of WMAP's ability to define dark matter and
Universe is isotropic on scales significantly larger
than superclusters, meaning that the statistical properties of the
Universe are the same in all directions as observed from Earth. The
Universe is bathed in highly isotropic microwave radiation that
corresponds to a thermal equilibrium blackbody spectrum of roughly
2.72548 kelvin. The hypothesis that the large-scale
homogeneous and isotropic is known as the cosmological principle.
Universe that is both homogeneous and isotropic looks the same from
all vantage points and has no center.
Main article: Dark energy
An explanation for why the expansion of the
Universe is accelerating
remains elusive. It is often attributed to "dark energy", an unknown
form of energy that is hypothesized to permeate space. On a
mass–energy equivalence basis, the density of dark energy (~ 7 ×
10−30 g/cm3) is much less than the density of ordinary matter or
dark matter within galaxies. However, in the present dark-energy era,
it dominates the mass–energy of the universe because it is uniform
Two proposed forms for dark energy are the cosmological constant, a
constant energy density filling space homogeneously, and scalar
fields such as quintessence or moduli, dynamic quantities whose energy
density can vary in time and space. Contributions from scalar fields
that are constant in space are usually also included in the
cosmological constant. The cosmological constant can be formulated to
be equivalent to vacuum energy. Scalar fields having only a slight
amount of spatial inhomogeneity would be difficult to distinguish from
a cosmological constant.
Main article: Dark matter
Dark matter is a hypothetical kind of matter that is invisible to the
entire electromagnetic spectrum, but which accounts for most of the
matter in the Universe. The existence and properties of dark matter
are inferred from its gravitational effects on visible matter,
radiation, and the large-scale structure of the Universe. Other than
neutrinos, a form of hot dark matter, dark matter has not been
detected directly, making it one of the greatest mysteries in modern
Dark matter neither emits nor absorbs light or any other
electromagnetic radiation at any significant level.
Dark matter is
estimated to constitute 26.8% of the total mass–energy and 84.5% of
the total matter in the Universe.
Main article: Matter
The remaining 4.9% of the mass–energy of the
Universe is ordinary
matter, that is, atoms, ions, electrons and the objects they form.
This matter includes stars, which produce nearly all of the light we
see from galaxies, as well as interstellar gas in the interstellar and
intergalactic media, planets, and all the objects from everyday life
that we can bump into, touch or squeeze. As a matter of fact, the
great majority of ordinary matter in the universe is unseen, since
visible stars and gas inside galaxies and clusters account for less
than 10 per cent of the ordinary matter contribution to the
mass-energy density of the universe.
Ordinary matter commonly exists in four states (or phases): solid,
liquid, gas, and plasma. However, advances in experimental techniques
have revealed other previously theoretical phases, such as
Bose–Einstein condensates and fermionic condensates.
Ordinary matter is composed of two types of elementary particles:
quarks and leptons. For example, the proton is formed of two up
quarks and one down quark; the neutron is formed of two down quarks
and one up quark; and the electron is a kind of lepton. An atom
consists of an atomic nucleus, made up of protons and neutrons, and
electrons that orbit the nucleus. Because most of the mass of an atom
is concentrated in its nucleus, which is made up of baryons,
astronomers often use the term baryonic matter to describe ordinary
matter, although a small fraction of this "baryonic matter" is
Soon after the Big Bang, primordial protons and neutrons formed from
the quark–gluon plasma of the early
Universe as it cooled below two
trillion degrees. A few minutes later, in a process known as Big Bang
nucleosynthesis, nuclei formed from the primordial protons and
neutrons. This nucleosynthesis formed lighter elements, those with
small atomic numbers up to lithium and beryllium, but the abundance of
heavier elements dropped off sharply with increasing atomic number.
Some boron may have been formed at this time, but the next heavier
element, carbon, was not formed in significant amounts. Big Bang
nucleosynthesis shut down after about 20 minutes due to the rapid drop
in temperature and density of the expanding Universe. Subsequent
formation of heavier elements resulted from stellar nucleosynthesis
and supernova nucleosynthesis.
Standard model of elementary particles: the 12 fundamental fermions
and 4 fundamental bosons. Brown loops indicate which bosons (red)
couple to which fermions (purple and green). Columns are three
generations of matter (fermions) and one of forces (bosons). In the
first three columns, two rows contain quarks and two leptons. The top
two rows' columns contain up (u) and down (d) quarks, charm (c) and
strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and
gluon (g), respectively. The bottom two rows' columns contain electron
neutrino (νe) and electron (e), muon neutrino (νμ) and muon (μ),
tau neutrino (ντ) and tau (τ), and the Z0 and W± carriers of the
weak force. Mass, charge, and spin are listed for each particle.
Main article: Particle physics
Ordinary matter and the forces that act on matter can be described in
terms of elementary particles. These particles are sometimes
described as being fundamental, since they have an unknown
substructure, and it is unknown whether or not they are composed of
smaller and even more fundamental particles. Of central
importance is the Standard Model, a theory that is concerned with
electromagnetic interactions and the weak and strong nuclear
Standard Model is supported by the experimental
confirmation of the existence of particles that compose matter: quarks
and leptons, and their corresponding "antimatter" duals, as well as
the force particles that mediate interactions: the photon, the W and Z
bosons, and the gluon. The
Standard Model predicted the existence
of the recently discovered Higgs boson, a particle that is a
manifestation of a field within the
Universe that can endow particles
with mass. Because of its success in explaining a wide
variety of experimental results, the
Standard Model is sometimes
regarded as a "theory of almost everything". The Standard Model
does not, however, accommodate gravity. A true force-particle "theory
of everything" has not been attained.
Main article: Hadron
A hadron is a composite particle made of quarks held together by the
strong force. Hadrons are categorized into two families: baryons (such
as protons and neutrons) made of three quarks, and mesons (such as
pions) made of one quark and one antiquark. Of the hadrons, protons
are stable, and neutrons bound within atomic nuclei are stable. Other
hadrons are unstable under ordinary conditions and are thus
insignificant constituents of the modern Universe. From approximately
10−6 seconds after the Big Bang, during a period is known as the
hadron epoch, the temperature of the universe had fallen sufficiently
to allow quarks to bind together into hadrons, and the mass of the
Universe was dominated by hadrons. Initially the temperature was high
enough to allow the formation of hadron/anti-hadron pairs, which kept
matter and antimatter in thermal equilibrium. However, as the
temperature of the
Universe continued to fall, hadron/anti-hadron
pairs were no longer produced. Most of the hadrons and anti-hadrons
were then eliminated in particle-antiparticle annihilation reactions,
leaving a small residual of hadrons by the time the
Universe was about
one second old.:244–66
Main article: Lepton
A lepton is an elementary, half-integer spin particle that does not
undergo strong interactions but is subject to the Pauli exclusion
principle; no two leptons of the same species can be in exactly the
same state at the same time. Two main classes of leptons exist:
charged leptons (also known as the electron-like leptons), and neutral
leptons (better known as neutrinos). Electrons are stable and the most
common charged lepton in the Universe, whereas muons and taus are
unstable particle that quickly decay after being produced in high
energy collisions, such as those involving cosmic rays or carried out
in particle accelerators. Charged leptons can combine with
other particles to form various composite particles such as atoms and
positronium. The electron governs nearly all of chemistry, as it is
found in atoms and is directly tied to all chemical properties.
Neutrinos rarely interact with anything, and are consequently rarely
Neutrinos stream throughout the
Universe but rarely interact
with normal matter.
The lepton epoch was the period in the evolution of the early Universe
in which the leptons dominated the mass of the Universe. It started
roughly 1 second after the Big Bang, after the majority of hadrons and
anti-hadrons annihilated each other at the end of the hadron epoch.
During the lepton epoch the temperature of the
Universe was still high
enough to create lepton/anti-lepton pairs, so leptons and anti-leptons
were in thermal equilibrium. Approximately 10 seconds after the Big
Bang, the temperature of the
Universe had fallen to the point where
lepton/anti-lepton pairs were no longer created. Most leptons and
anti-leptons were then eliminated in annihilation reactions, leaving a
small residue of leptons. The mass of the
Universe was then dominated
by photons as it entered the following photon epoch.
See also: Photino
A photon is the quantum of light and all other forms of
electromagnetic radiation. It is the force carrier for the
electromagnetic force, even when static via virtual photons. The
effects of this force are easily observable at the microscopic and at
the macroscopic level because the photon has zero rest mass; this
allows long distance interactions. Like all elementary particles,
photons are currently best explained by quantum mechanics and exhibit
wave–particle duality, exhibiting properties of waves and of
The photon epoch started after most leptons and anti-leptons were
annihilated at the end of the lepton epoch, about 10 seconds after the
Atomic nuclei were created in the process of nucleosynthesis
which occurred during the first few minutes of the photon epoch. For
the remainder of the photon epoch the
Universe contained a hot dense
plasma of nuclei, electrons and photons. About 380,000 years after the
Big Bang, the temperature of the
Universe fell to the point where
nuclei could combine with electrons to create neutral atoms. As a
result, photons no longer interacted frequently with matter and the
Universe became transparent. The highly redshifted photons from this
period form the cosmic microwave background. Tiny variations in
temperature and density detectable in the CMB were the early "seeds"
from which all subsequent structure formation took
Timeline of the Big Bang
Chronology of the universe
Grand unification epoch
Electroweak epoch (Inflationary epoch, Reheating, Baryogenesis)
Photon epoch (
Big Bang nucleosynthesis,
Fate of the universe
Heat death of the universe
Graphical timeline of the Big Bang Cosmology
Model of the
Universe based on general relativity
Main article: Solutions of the Einstein field equations
Big Bang and Ultimate fate of the Universe
General relativity is the geometric theory of gravitation published by
Albert Einstein in 1915 and the current description of gravitation in
modern physics. It is the basis of current cosmological models of the
General relativity generalizes special relativity and
Newton's law of universal gravitation, providing a unified description
of gravity as a geometric property of space and time, or spacetime. In
particular, the curvature of spacetime is directly related to the
energy and momentum of whatever matter and radiation are present. The
relation is specified by the Einstein field equations, a system of
partial differential equations. In general relativity, the
distribution of matter and energy determines the geometry of
spacetime, which in turn describes the acceleration of matter.
Therefore, solutions of the
Einstein field equations
Einstein field equations describe the
evolution of the Universe. Combined with measurements of the amount,
type, and distribution of matter in the Universe, the equations of
general relativity describe the evolution of the
With the assumption of the cosmological principle that the
homogeneous and isotropic everywhere, a specific solution of the field
equations that describes the
Universe is the metric tensor called the
displaystyle ds^ 2 =-c^ 2 dt^ 2 +R(t)^ 2 left( frac dr^ 2
1-kr^ 2 +r^ 2 dtheta ^ 2 +r^ 2 sin ^ 2 theta ,dphi ^ 2 right)
where (r, θ, φ) correspond to a spherical coordinate system. This
metric has only two undetermined parameters. An overall dimensionless
length scale factor R describes the size scale of the
Universe as a
function of time; an increase in R is the expansion of the
Universe. A curvature index k describes the geometry. The index k
is defined so that it can take only one of three values: 0,
corresponding to flat Euclidean geometry; 1, corresponding to a space
of positive curvature; or −1, corresponding to a space of positive
or negative curvature. The value of R as a function of time t
depends upon k and the cosmological constant Λ. The cosmological
constant represents the energy density of the vacuum of space and
could be related to dark energy. 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: because the
Universe is known to be inhomogeneous on smaller scales, R must change
over time. 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 went to zero and matter and energy
were 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 the
Big Bang model of the
Universe. Understanding the singularity of the
Big Bang likely
requires a quantum theory of gravity, which has not yet been
Third, the curvature index k determines the sign of the mean spatial
curvature of spacetime averaged over sufficiently large length
scales (greater than about a billion light years). If k=1, the
curvature is positive and the
Universe has a finite volume. A
Universe with positive curvature is often visualized as a
three-dimensional sphere embedded in a four-dimensional space.
Conversely, if k is zero or negative, the
Universe has an infinite
volume. It may seem counter-intuitive that an infinite and yet
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. By analogy, 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
Universe with periodic boundary conditions.
The ultimate fate of the
Universe is still unknown, because it depends
critically on the curvature index k and the cosmological constant Λ.
Universe were sufficiently dense, k would equal +1, meaning
that its average curvature throughout is positive and the Universe
will eventually recollapse in a Big Crunch, possibly starting a
Universe in a Big Bounce. Conversely, if the
insufficiently dense, k would equal 0 or −1 and the
expand forever, cooling off and eventually reaching the Big Freeze and
the heat death of the Universe. Modern data suggests that the
rate of expansion of the
Universe is not decreasing, as originally
expected, but increasing; if this continues indefinitely, the Universe
may eventually reach a Big Rip. Observationally, the
to be flat (k = 0), with an overall density that is very close to the
critical value between recollapse and eternal expansion.
Main articles: Multiverse, Many-worlds interpretation, Bubble universe
theory, and Parallel universe (fiction)
Depiction of a multiverse of seven "bubble" universes, which are
separate spacetime continua, each having different physical laws,
physical constants, and perhaps even different numbers of dimensions
See also: Eternal inflation
Some speculative theories have proposed that our
Universe is but one
of a set of disconnected universes, collectively denoted as the
multiverse, challenging or enhancing more limited definitions of the
Universe. Scientific multiverse models are distinct from
concepts such as alternate planes of consciousness and simulated
Max Tegmark developed a four-part classification scheme for the
different types of multiverses that scientists have suggested in
response to various
Physics problems. An example of such multiverses
is the one resulting from the chaotic inflation model of the early
universe. Another is the multiverse resulting from the
many-worlds interpretation of quantum mechanics. In this
interpretation, parallel worlds are generated in a manner similar to
quantum superposition and decoherence, with all states of the wave
functions being realized in separate worlds. Effectively, in the
many-worlds interpretation 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 sense.
The least controversial category of multiverse in Tegmark's scheme is
Level I. The multiverses of this level are composed by distant
spacetime events "in our own universe". If space is infinite, or
sufficiently large and uniform, identical instances of the history of
Hubble volume occur every so often, simply by chance.
Tegmark calculated that our nearest so-called doppelgänger, is
1010115 meters away from us (a double exponential function larger than
a googolplex). In principle, it would be impossible to
scientifically verify the existence of an identical Hubble volume.
However, this existence does follow as a fairly straightforward
consequence from otherwise unrelated scientific observations and
theories.[clarification needed (which ones?)]
It is possible to conceive of disconnected spacetimes, each existing
but unable to interact with one another. An easily
visualized metaphor of this concept 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 spacetime is denoted as
a universe, whereas our particular spacetime is denoted as the
Universe, just as we call our moon the Moon. The entire collection
of these separate spacetimes is denoted as the multiverse. With
this terminology, different Universes are not causally connected to
each other. In principle, the other unconnected Universes may have
different dimensionalities and topologies of spacetime, different
forms of matter and energy, and different physical laws and physical
constants, although such possibilities are purely speculative.
Others consider each of several bubbles created as part of chaotic
inflation to be separate Universes, though in this model these
universes all share a causal origin.
See also: Cosmology, Timeline of cosmological theories, Nicolaus
Copernicus § Copernican system, and Philosophiæ Naturalis
Principia Mathematica § Beginnings of the Scientific Revolution
Historically, there have been many ideas of the cosmos (cosmologies)
and its origin (cosmogonies). Theories of an impersonal Universe
governed by physical laws were first proposed by the Greeks and
Indians. Ancient Chinese philosophy encompassed the notion of the
Universe including both all of space and all of time. 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
Main articles: Creation myth, Creator deity, and Religious cosmology
Many cultures have stories describing the origin of the world and
universe. Cultures generally regard these stories as having some
truth. There are however many differing beliefs in how these stories
apply amongst those believing in a supernatural origin, ranging from a
god directly creating the
Universe as it is now to a god just setting
the "wheels in motion" (for example via mechanisms such as the big
bang and evolution).
Ethnologists and anthropologists who study myths have developed
various classification schemes for the various themes that appear in
creation stories. For example, in one type of story, the
world is born from a world egg; such stories include the Finnish epic
poem Kalevala, the Chinese story of
Pangu or the Indian 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 Adi-Buddha, the ancient Greek story of Gaia
(Mother Earth), the Aztec goddess
Coatlicue myth, the ancient Egyptian
Atum story, and the
Genesis creation narrative
Genesis creation narrative in
which the Abrahamic God created the Universe. In another type of
Universe is created from the union of male and female
deities, as in the Maori story of Rangi and Papa. In other stories,
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
Ymir in Norse
mythology – or from chaotic materials, as in
Izanami in Japanese mythology. In other stories, the
from fundamental principles, such as
Brahman and Prakrti, the creation
myth of the Serers, or the yin and yang of the Tao.
Further information: Cosmology
See also: Pre-Socratic philosophy,
Physics (Aristotle), Hindu
cosmology, Islamic cosmology, and
Philosophy of space and time
The pre-Socratic Greek philosophers and Indian philosophers developed
some of the earliest philosophical concepts 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 physical materials in the world are different forms of a
single primordial material, or arche. The first to do so was Thales,
who proposed this material to be water. Thales' student, Anaximander,
proposed that everything came from the limitless apeiron. Anaximenes
proposed the primordial material to be 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
Nous (Mind), while
Heraclitus proposed fire (and spoke of logos).
Empedocles proposed the elements to be earth, water, air and fire. His
four-element model became very popular. Like Pythagoras, Plato
believed that all things were composed of number, with Empedocles'
elements taking the form of the Platonic solids. Democritus, and later
philosophers—most notably Leucippus—proposed that the
composed of indivisible atoms moving through a void (vacuum), although
Aristotle did not believe that to be 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
Heraclitus argued for eternal change, his contemporary
Parmenides made the radical suggestion that all change is an illusion,
that the true underlying reality is eternally unchanging and of a
Parmenides denoted this reality as τὸ ἐν (The
One). Parmenides' idea seemed implausible to many Greeks, but his
Zeno of Elea
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 that the world is bounded by the celestial spheres and that
cumulative stellar magnitude is only finitely multiplicative.
The Indian philosopher Kanada, founder of the
developed a notion of atomism and proposed that light and heat were
varieties of the same substance. In the 5th century AD, the
Buddhist atomist philosopher
Dignāga proposed 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 notion of temporal finitism was inspired by the doctrine of
creation shared by the three Abrahamic religions: Judaism,
Christianity and Islam. The Christian philosopher, 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 Jewish philosopher,
Saadia Gaon (Saadia ben Joseph);
and the Muslim theologian,
History of astronomy
History of astronomy and Timeline of astronomy
Aristarchus's 3rd century BCE calculations on the relative sizes of,
from left to right, the Sun, Earth, and Moon, from a 10th-century AD
Astronomical models of the
Universe were proposed soon after 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
Anaximander and Hecataeus of Miletus.
Later 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
proposed by Eudoxus of Cnidos. According to Aristotle's physical
interpretation of the model, celestial spheres eternally rotate with
uniform motion around a stationary Earth. Normal matter is entirely
contained within the terrestrial sphere.
De Mundo (composed before 250 BC or between 350 and 200 BC), stated,
"Five elements, situated in spheres in five regions, the less being in
each case surrounded by the greater—namely, earth surrounded by
water, water by air, air by fire, and fire by ether—make up the
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
Pythagorean philosopher Philolaus, postulated (according to Stobaeus
account) that at the center of the
Universe was a "central fire"
around which the Earth, Sun,
Moon and Planets revolved in uniform
The Greek astronomer
Aristarchus of Samos
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's heliocentric model.
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
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
Aristarchus thus believed the stars to be very far away, and saw this
as the reason why stellar parallax had not been observed, that is, the
stars had not been observed to move relative 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
Earth to revolve in an oblique circle, while it rotates, at
the same time, about its own axis
Flammarion engraving, Paris 1888.
The only other astronomer from antiquity known by name who supported
Aristarchus's heliocentric model was Seleucus of Seleucia, a
Hellenistic astronomer who lived a century after
Aristarchus. According to 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 cosmology were probably related to the phenomenon of
tides. According to
Strabo (1.1.9), Seleucus was the first to
state that the 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 heliocentricity by
determining the constants of a geometric model for it, and by
developing methods to compute planetary positions using this model,
Nicolaus Copernicus later did in the 16th century.
During the Middle Ages, heliocentric models were also proposed by the
Indian astronomer Aryabhata, and by the Persian astronomers
Albumasar and Al-Sijzi.
Model of the Copernican
Thomas Digges in 1576, with the
amendment that the stars are no longer confined to a sphere, but
spread uniformly throughout the space surrounding the planets.
The Aristotelian model was accepted in the
Western world for roughly
two millennia, until Copernicus revived Aristarchus's perspective 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
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?
— Nicolaus Copernicus, in Chapter 10, Book 1 of De Revolutionibus
Orbium Coelestrum (1543)
As noted by Copernicus himself, the notion that the
Earth rotates is
very old, dating at least to
Philolaus (c. 450 BC), Heraclides
Ponticus (c. 350 BC) and Ecphantus the Pythagorean. Roughly a century
before Copernicus, the Christian scholar
Nicholas of Cusa
Nicholas of Cusa also
proposed that the
Earth rotates on its axis in his book, On Learned
Ignorance (1440). Al-Sijzi also proposed that the Earth
rotates on its axis.
Empirical evidence for the
Earth's rotation on
its axis, using the phenomenon of comets, was given by Tusi
Ali Qushji (1403–1474).
This cosmology was accepted by Isaac Newton,
Christiaan Huygens and
Edmund Halley (1720) and Jean-Philippe de
Chéseaux (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) 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 of nebulae.
In 1919, when
Hooker Telescope was completed, the prevailing view
still was that the
Universe consisted entirely of the Milky Way
Galaxy. Using the Hooker Telescope,
Edwin Hubble identified Cepheid
variables in several spiral nebulae and in 1922–1923 proved
conclusively that Andromeda
Nebula and Triangulum among others, were
entire galaxies outside our own, thus proving that
of multitude of 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.
Chronology of the Universe
Cosmic Calendar (scaled down timeline)
Detailed logarithmic timeline
Earth's location in the Universe
Future of an expanding Universe
Galaxy And Mass Assembly survey
Heat death of the universe
History of the Center of the Universe
Multiverse (set theory) (Hyperverse, Megaverse or Omniverse)
Space and survival
Terasecond and longer
Timeline of the far future
Timeline of the formation of the universe
Timeline of the near future
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intertwined and physically meaningless if taken separately from each
other. See Theory of relativity.
^ Although listed in megaparsecs by the cited source, this number is
so vast that it's digits would remain virtually unchanged for all
intents and purposes regardless of which conventional units it is
listed in, whether it to be nanometres or gigaparsecs, as the
differences would disappear into the error.
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