Universe
The universe is commonly defined as the totality of everything that exists, including all matter and energy, the
planets, stars, galaxies, and the contents of intergalactic space.Definitions and usage vary and similar termsa
include the cosmos, the world and nature. Scientific observation of earlier stages in the development of the universe,
which can be seen at great distances, suggests that the universe has been governed by the same physical laws and
constants throughout most of its extent and history. There are various multiverse theories, in which physicists have
suggested that our universe might be one among many universes that likewise exist.
History
Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations
of the universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers. Over
the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and
the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization
that the Solar System is embedded in a galaxy composed of billions of stars, the Milky Way, and that other galaxies
exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and
their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave
background radiation revealed that the universe is expanding and apparently had a beginning.
According to the prevailing scientific model of
the universe, known as the Big Bang, the
universe expanded from an extremely hot,
dense phase called the Planck epoch, in which
all the matter and energy of the observable
universe was concentrated. Since the Planck
epoch, the universe has been expanding to its
present form, possibly with a brief period (less
than 10−32 seconds) of cosmic inflation.
Several independent experimental
measurements support this theoretical
expansion and, more generally, the Big Bang
theory. Recent observations indicate that this
expansion is accelerating because of dark
energy, and that most of the matter in the
universe may be in a form which cannot be
detected by present instruments, and so is not
accounted for in the present models of the
universe; this has been named dark matter.
The imprecision of current observations has
hindered predictions of the ultimate fate of the
universe.
Current interpretations of astronomical
observations indicate that the age of the universe is 13.75 ± 0.17 billion years,(whereas the decoupling of light
and matter, see CMBR, happened already 380,000 years after the Big Bang), and that the diameter of the observable
universe is at least 93 billion light years or 8.80 × 1026 metres.According to general relativity, space can expand
faster than the speed of light, although we can view only a small portion of the universe due to the limitation
imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic
radiation), it is uncertain whether the size of the universe is finite or infinite.
Etymology, synonyms and definitions
The word universe derives from the Old French word Univers, which in turn derives from the Latin word
universum.The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern
English word is used. The Latin word derives from the poetic contraction Unvorsum — first used by Lucretius in
Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining
form of unus', or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning
"something rotated, rolled, changed").
An alternative interpretation of unvorsum is "everything rotated as
one" or "everything rotated by one". In this sense, it may be considered
a translation of an earlier Greek word for the universe, περιφορά,
(periforá, "circumambulation"), originally used to describe a course of
a meal, the food being carried around the circle of dinner guests.
This Greek word refers to celestial spheres, an early Greek model of
the universe. Regarding Plato's Metaphor of the sun, Aristotle suggests
that the rotation of the sphere of fixed stars inspired by the prime
mover, motivates, in turn, terrestrial change via the Sun. Careful
astronomical and physical measurements (such as the Foucault
pendulum) are required to prove the Earth rotates on its axis.
A term for "universe" in ancient Greece was τὸ πᾶν (tò pán, The All,
Pan (mythology)). Related terms were matter, (τὸ ὅλον, tò ólon, see
also Hyle, lit. wood) and place (τὸ κενόν, tò kenón). Other
synonyms for the universe among the ancient Greek philosophers
included κόσμος (cosmos) and φύσις (meaning Nature, from which we
derive the word physics). The same synonyms are 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
hypothesis), and Nature (as in natural laws or natural philosophy).
Broadest definition: reality and probability
The broadest definition of the universe can be found in De divisione naturae by the medieval philosopher and
theologian Johannes Scotus Eriugena, who defined it as simply everything: everything that is created and everything
that is not created.
Definition as reality
More customarily, the universe is defined as everything that exists, (has existed, and will exist) . According to our
current understanding, the universe consists of three principles: spacetime, forms of energy, including momentum
and matter, and the physical laws that relate them.
Definition as connected space-time
It is possible to conceive of disconnected space-times, each existing but unable to interact with one another. An
easily visualized metaphor is a group of separate soap bubbles, in which observers living on one soap bubble cannot
interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap
bubble" of space-time is denoted as a universe, whereas our particular space-time is denoted as the universe, just as
we call our moon the Moon. The entire collection of these separate space-times is denoted as the 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 such possibilities
are currently speculative.
Definition as observable reality
According to a still-more-restrictive definition, the universe is everything within our connected space-time that could
have a chance to interact with us and vice versa. According to the general theory of relativity, some regions of space
may never interact with ours even in the lifetime of the universe, due to the finite speed of light and the ongoing
expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the
universe would live forever; space may expand faster than light can traverse it. It is worth emphasizing that those
distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with
them. The spatial region within which we can affect and be affected is denoted as the observable universe. Strictly
speaking, the observable universe depends on the location of the observer. By traveling, an observer can come into
contact with a greater region of space-time than an observer who remains still, so that the observable universe for the
former is larger than for the latter. Nevertheless, even the most rapid traveler will not be able to interact with all of
space. Typically, the observable universe is taken to mean the universe observable from our vantage point in the
Milky Way Galaxy.
Size, age, contents, structure, and laws
The universe is immensely large and possibly infinite in volume. The region visible from Earth (the observable
universe) is a sphere with a radius of about 46 billion light years,based on where the expansion of space has
taken the most distant objects observed. For comparison, the diameter of a typical galaxy is only 30,000 light-years,
and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky
Way Galaxy is roughly 100,000 light years in diameter,and our nearest sister galaxy, the Andromeda Galaxy, is
located roughly 2.5 million light years away.There are probably more than 100 billion (1011) galaxies in the
observable universe.Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants
with one trillion (1012) stars, all orbiting the galaxy's center of mass. A 2010 study by astronomers estimated that
the observable universe contains 300 sextillion (3×1023) stars.
The observable matter is spread
homogeneously (uniformly) throughout
the universe, when averaged over
distances longer than 300 million
light-years.[26] 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-kelvins.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.[30] 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 most precise estimate of the universe's age is 13.72±0.12 billion years old, based on observations of the cosmic
microwave background radiation.Independent estimates (based on measurements such as radioactive dating)
agree at 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.[33] 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 also appears to have neither net momentum nor angular
momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and
the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite.
The universe appears to have a smooth
space-time continuum consisting of three
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 behave in a manner
that regularly follows a set of 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.
Fine tuning
It appears that many of the properties of the universe have special values in the sense that a universe where these
properties only differ slightly would not be able to support intelligent life.Not all scientists agree that this
fine-tuning exists. In particular, it is not known under what conditions intelligent life could form and what
form or shape that would take. A relevant observation in this discussion is that for an observer to exist to observe
fine-tuning, the universe must be able to support intelligent life. As such the conditional probability of observing a
universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle and is
particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of
properties exist (see below).
Historical models
Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the
then-available data and conceptions of the universe. Historically, cosmologies and cosmogonies were based on
narratives of gods acting in various ways. Theories of an impersonal universe governed by physical laws were first
proposed by the Greeks and Indians. Over the centuries, improvements in astronomical observations and theories of
motion and gravitation led to ever more accurate descriptions of the universe. The modern era of cosmology began
with Albert Einstein's 1915 general theory of relativity, which made it possible to quantitatively predict the origin,
evolution, and conclusion of the universe as a whole. Most modern, accepted theories of cosmology are based on
general relativity and, more specifically, the predicted Big Bang; however, still more careful measurements are
required to determine which theory is correct.
Creation
Many cultures have stories describing the origin of the world, which may be roughly grouped into common types. In
one type of story, the world is born from a world egg; such stories include the Finnish epic poem Kalevala, the
Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the creation idea is caused 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 god Atum
story, or the Genesis creation narrative. In another type of story, the world is created from the union of male and
female deities, as in the Maori story of Rangi and Papa. In other stories, the universe is created by crafting it from
pre-existing materials, such as the corpse of a dead god — as from Tiamat in the Babylonian epic Enuma Elish or
from the giant Ymir in Norse mythology – or from chaotic materials, as in Izanagi and Izanami in Japanese
mythology. In other stories, the universe emanates from fundamental principles, such as Brahman and Prakrti, or the
yin and yang of the Tao.
Philosophical models
Further information: Cosmology
From the 6th century BCE, the pre-Socratic Greek philosophers developed the earliest known philosophical models
of the universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand
the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice
to water to steam) and several philosophers proposed that all the apparently different materials of the world are
different forms of a single primordial material, or arche. The first to do so was Thales, who proposed this material is
Water. Thales' student, Anaximander, proposed that everything came from the limitless apeiron. Anaximenes
proposed 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). Heraclitus proposed fire (and
spoke of logos). Empedocles proposed the elements: earth, water, air and fire. His four element theory became very
popular. Like Pythagoras, Plato believed that all things were composed of number, with the Empedocles' elements
taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus—proposed that
the universe was composed of indivisible atoms moving through void (vacuum). Aristotle did not believe that was
feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover,
without resistance, it would do so indefinitely fast.
Although Heraclitus argued for eternal change, his quasi-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 τὸ ἐν (The One). Parmenides' theory seemed implausible to many Greeks, but his student
Zeno of Elea challenged them with several famous paradoxes. Aristotle responded to these paradoxes by developing
the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and
unchanging cycles of time, he believed the world was bounded by the celestial spheres, and thus magnitude was only
finitely multiplicative.
The Indian philosopher Kanada, founder of the Vaisheshika school, developed a theory 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 theory 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, Al-Ghazali (Algazel). Borrowing from Aristotle's Physics and Metaphysics,
they employed two logical arguments against an infinite past, the first being the "argument from the impossibility of
the existence of an actual infinite", which states:
"An actual infinite cannot exist."
"An infinite temporal regress of events is an actual infinite."
" An infinite temporal regress of events cannot exist."
The second argument, the "argument from the impossibility of completing an actual infinite by successive addition",
states:
"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
" The temporal series of past events cannot be an actual infinite."
Both arguments were adopted by Christian philosophers and theologians, and the second argument in particular
became more famous after it was adopted by Immanuel Kant in his thesis of the first antinomy concerning time.
Astronomical models
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. 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 that at the center of the universe was a "central fire" around which the Earth, Sun,
Moon and Planets revolved in uniform circular motion.The Greek astronomer 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' 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.
The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was
Seleucus of Seleucia, a Hellenistic astronomer who lived a century after 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 theory were probably related to the phenomenon of
tides.[50] 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 the heliocentric theory by determining the constants of a geometric model for the heliocentric theory
and by developing methods to compute planetary positions using this model, like what Nicolaus Copernicus later did
in the 16th century. During the Middle Ages, heliocentric models may have also been proposed by the Indian
astronomer, Aryabhata, and by the Persian astronomers, Albumasar and Al-Sijzi.
The Aristotelian model was accepted in the Western world for roughly
two millennia, until Copernicus revived Aristarchus' theory that the
astronomical data could be explained more plausibly if the earth
rotated on its axis and if the sun were placed at the center of the
universe.
“In the center rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can
illuminate everything at the same time? ”
—Nicolaus Copernicus, in Chapter 10, Book 1 of De Revolutionibus Orbium Coelestrum (1543)
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,
Christian scholar Nicholas of Cusa also proposed that the Earth rotates on its axis in his book, On Learned Ignorance
(1440). Aryabhata (476–550), Brahmagupta (598–668), Albumasar and Al-Sijzi, also proposed that the Earth
rotates on its axis. The first empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets,
was given by Tusi (1201–1274) and Ali Qushji (1403–1474).
This cosmology was accepted by Isaac Newton, Christiaan Huygens
and later scientists. Edmund Halley (1720)and Jean-Philippe de
Cheseaux (1744)[59] 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.
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.
Theoretical models
Of the four fundamental interactions, gravitation is dominant at
cosmological length scales; that is, the other three forces play a
negligible role in determining structures at the level of planetary
systems, galaxies and larger-scale structures. 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 theory of relativity
Given gravitation's predominance in shaping cosmological structures,
accurate predictions of the universe's past and future require an
accurate theory of gravitation. The best theory available is Albert
Einstein's general theory of relativity, which has passed all
experimental tests hitherto. However, since rigorous experiments have
not been carried out on cosmological length scales, general relativity
could conceivably be inaccurate. Nevertheless, its cosmological
predictions appear to be consistent with observations, so there is no
compelling reason to adopt another theory.
General relativity provides a set of ten nonlinear partial differential equations for the spacetime metric (Einstein's
field equations) that must be solved from the 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),[hat 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.
Special relativity and space-time
The universe has at least three spatial and
one temporal (time) dimension. It was long
thought that the spatial and temporal
dimensions were different in nature and
independent of one another. However,
according to the special theory of relativity,
spatial and temporal separations are
interconvertible (within limits) by changing
one's motion.
To understand this interconversion, it is
helpful to consider the analogous
interconversion of spatial separations along
the three spatial dimensions. Consider the
two endpoints of a rod of length L. The
length can be determined from the
differences in the three coordinates Δx, Δy
and Δz of the two endpoints in a given
reference frame
using the Pythagorean theorem. In a rotated
reference frame, the coordinate differences
differ, but they give the same length
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
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.
Solving Einstein's field equations
The distances between the spinning galaxies increase with time, but the distances between the stars within each
galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann
universe with zero cosmological constant Λ; such a universe oscillates between a Big Bang and a Big Crunch.
In non-Cartesian (non-square) or curved coordinate systems, the Pythagorean theorem holds only on infinitesimal
length scales and must be augmented with a more general metric tensor gμν, which can vary from place to place and
which describes the local geometry in the particular coordinate system. However, assuming the cosmological
principle that the universe is homogeneous and isotropic everywhere, every point in space is like every other point;
hence, the metric tensor must be the same everywhere. That leads to a single form for the metric tensor, called the
Friedmann–Lemaître–Robertson–Walker metric
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. 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 13.7 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.
Space has no boundary – that is empirically more certain than any external observation. However, that does not imply that space is
infinite...(translated, original German)
Bernhard Riemann (Habilitationsvortrag, 1854)
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, because it depends critically on the curvature index k and the
cosmological constant Λ. If the universe is sufficiently dense, k equals +1, meaning that its average curvature
throughout is positive and the universe will eventually recollapse in 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 speed of the universe is not decreasing as originally expected, but increasing; if this continues indefinitely,
the universe will eventually rip itself to shreds (the Big Rip). Experimentally, the universe has an overall density that
is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations
are needed to resolve the question.
Big Bang model
The prevailing Big Bang model accounts for many of the experimental observations described above, such as the
correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous,
isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space; as
the space itself expands, the wavelength of a photon traveling through space likewise increases, decreasing its
energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from
more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an
important problem in experimental physical cosmology.
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.
Multiverse theory
Some speculative theories have proposed that this 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 theories are distinct from
concepts such as alternate planes of consciousness and simulated
reality, although the idea of a larger universe is not new; for example,
Bishop Étienne Tempier of Paris ruled in 1277 that God could create as
many universes as he saw fit, a question that was being hotly debated
by the French theologians.
Max Tegmark developed a four part classification scheme for the
different types of multiverses that scientists have suggested in various
problem domains. An example of such a theory is the chaotic inflation model of the early universe. Another is the
many-worlds interpretation of quantum mechanics. Parallel worlds are generated in a manner similar to quantum
superposition and decoherence, with all states of the wave function being realized in separate worlds. Effectively, the
multiverse evolves as a universal wavefunction. If the big bang that created our multiverse created an ensemble of
multiverses, the wave function of the ensemble would be entangled in this sense.
The least controversial category of multiverse in Tegmark's scheme is Level I, which describes distant space-time
events "in our own universe". If space is infinite, or sufficiently large and uniform, identical instances of the history
of Earth's entire Hubble volume occur every so often, simply by chance. Tegmark calculated our nearest so-called
doppelgänger, is 1010115 meters away from us (a double exponential function larger than a googolplex). In
principle, it would be impossible to scientifically verify an identical Hubble volume. However, it does follow as a
fairly straightforward consequence from otherwise unrelated scientific observations and theories. Tegmark suggests
that statistical analysis exploiting the anthropic principle provides an opportunity to test multiverse theories in some
cases. Generally, science would consider a multiverse theory that posits neither a common point of causation, nor the
possibility of interaction between universes, to be an idle speculation.
Shape of the universe
The shape or geometry of the universe includes both local geometry in the observable universe and global geometry,
which we may or may not be able to measure. Shape can refer to curvature and topology. More formally, the subject
in practice investigates which 3-manifold corresponds to the spatial section in comoving coordinates of the
four-dimensional space-time of the universe. Cosmologists normally work with a given space-like slice of spacetime
called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the
backward light cone (points within the cosmic light horizon, given time to reach a given observer). If the observable
universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot
determine the global structure by observation: one is limited to a small patch.
Among the Friedmann–Lemaître–Robertson–Walker (FLRW) models, the presently most popular shape of the
Universe found to fit observational data according to cosmologists is the infinite flat model,[77] while other FLRW
models include the Poincaré dodecahedral space[78][79] and the Picard horn.[80] The data fit by these FLRW models
of space especially include the Wilkinson Microwave Anisotropy Probe (WMAP) maps of cosmic background
radiation. NASA released the first WMAP cosmic background radiation data in February 2003. In 2009 the Planck
observatory was launched to observe the microwave background at higher resolution than WMAP, possibly
providing more information on the shape of the Universe. The data should be released in late 2012.
The universe is commonly defined as the totality of everything that exists, including all matter and energy, the
planets, stars, galaxies, and the contents of intergalactic space.Definitions and usage vary and similar termsa
include the cosmos, the world and nature. Scientific observation of earlier stages in the development of the universe,
which can be seen at great distances, suggests that the universe has been governed by the same physical laws and
constants throughout most of its extent and history. There are various multiverse theories, in which physicists have
suggested that our universe might be one among many universes that likewise exist.
History
Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations
of the universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers. Over
the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and
the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization
that the Solar System is embedded in a galaxy composed of billions of stars, the Milky Way, and that other galaxies
exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and
their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave
background radiation revealed that the universe is expanding and apparently had a beginning.
According to the prevailing scientific model of
the universe, known as the Big Bang, the
universe expanded from an extremely hot,
dense phase called the Planck epoch, in which
all the matter and energy of the observable
universe was concentrated. Since the Planck
epoch, the universe has been expanding to its
present form, possibly with a brief period (less
than 10−32 seconds) of cosmic inflation.
Several independent experimental
measurements support this theoretical
expansion and, more generally, the Big Bang
theory. Recent observations indicate that this
expansion is accelerating because of dark
energy, and that most of the matter in the
universe may be in a form which cannot be
detected by present instruments, and so is not
accounted for in the present models of the
universe; this has been named dark matter.
The imprecision of current observations has
hindered predictions of the ultimate fate of the
universe.
Current interpretations of astronomical
observations indicate that the age of the universe is 13.75 ± 0.17 billion years,(whereas the decoupling of light
and matter, see CMBR, happened already 380,000 years after the Big Bang), and that the diameter of the observable
universe is at least 93 billion light years or 8.80 × 1026 metres.According to general relativity, space can expand
faster than the speed of light, although we can view only a small portion of the universe due to the limitation
imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic
radiation), it is uncertain whether the size of the universe is finite or infinite.
Etymology, synonyms and definitions
The word universe derives from the Old French word Univers, which in turn derives from the Latin word
universum.The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern
English word is used. The Latin word derives from the poetic contraction Unvorsum — first used by Lucretius in
Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining
form of unus', or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning
"something rotated, rolled, changed").
An alternative interpretation of unvorsum is "everything rotated as
one" or "everything rotated by one". In this sense, it may be considered
a translation of an earlier Greek word for the universe, περιφορά,
(periforá, "circumambulation"), originally used to describe a course of
a meal, the food being carried around the circle of dinner guests.
This Greek word refers to celestial spheres, an early Greek model of
the universe. Regarding Plato's Metaphor of the sun, Aristotle suggests
that the rotation of the sphere of fixed stars inspired by the prime
mover, motivates, in turn, terrestrial change via the Sun. Careful
astronomical and physical measurements (such as the Foucault
pendulum) are required to prove the Earth rotates on its axis.
A term for "universe" in ancient Greece was τὸ πᾶν (tò pán, The All,
Pan (mythology)). Related terms were matter, (τὸ ὅλον, tò ólon, see
also Hyle, lit. wood) and place (τὸ κενόν, tò kenón). Other
synonyms for the universe among the ancient Greek philosophers
included κόσμος (cosmos) and φύσις (meaning Nature, from which we
derive the word physics). The same synonyms are 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
hypothesis), and Nature (as in natural laws or natural philosophy).
Broadest definition: reality and probability
The broadest definition of the universe can be found in De divisione naturae by the medieval philosopher and
theologian Johannes Scotus Eriugena, who defined it as simply everything: everything that is created and everything
that is not created.
Definition as reality
More customarily, the universe is defined as everything that exists, (has existed, and will exist) . According to our
current understanding, the universe consists of three principles: spacetime, forms of energy, including momentum
and matter, and the physical laws that relate them.
Definition as connected space-time
It is possible to conceive of disconnected space-times, each existing but unable to interact with one another. An
easily visualized metaphor is a group of separate soap bubbles, in which observers living on one soap bubble cannot
interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap
bubble" of space-time is denoted as a universe, whereas our particular space-time is denoted as the universe, just as
we call our moon the Moon. The entire collection of these separate space-times is denoted as the 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 such possibilities
are currently speculative.
Definition as observable reality
According to a still-more-restrictive definition, the universe is everything within our connected space-time that could
have a chance to interact with us and vice versa. According to the general theory of relativity, some regions of space
may never interact with ours even in the lifetime of the universe, due to the finite speed of light and the ongoing
expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the
universe would live forever; space may expand faster than light can traverse it. It is worth emphasizing that those
distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with
them. The spatial region within which we can affect and be affected is denoted as the observable universe. Strictly
speaking, the observable universe depends on the location of the observer. By traveling, an observer can come into
contact with a greater region of space-time than an observer who remains still, so that the observable universe for the
former is larger than for the latter. Nevertheless, even the most rapid traveler will not be able to interact with all of
space. Typically, the observable universe is taken to mean the universe observable from our vantage point in the
Milky Way Galaxy.
Size, age, contents, structure, and laws
The universe is immensely large and possibly infinite in volume. The region visible from Earth (the observable
universe) is a sphere with a radius of about 46 billion light years,based on where the expansion of space has
taken the most distant objects observed. For comparison, the diameter of a typical galaxy is only 30,000 light-years,
and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky
Way Galaxy is roughly 100,000 light years in diameter,and our nearest sister galaxy, the Andromeda Galaxy, is
located roughly 2.5 million light years away.There are probably more than 100 billion (1011) galaxies in the
observable universe.Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants
with one trillion (1012) stars, all orbiting the galaxy's center of mass. A 2010 study by astronomers estimated that
the observable universe contains 300 sextillion (3×1023) stars.
The observable matter is spread
homogeneously (uniformly) throughout
the universe, when averaged over
distances longer than 300 million
light-years.[26] 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-kelvins.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.[30] 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 most precise estimate of the universe's age is 13.72±0.12 billion years old, based on observations of the cosmic
microwave background radiation.Independent estimates (based on measurements such as radioactive dating)
agree at 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.[33] 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 also appears to have neither net momentum nor angular
momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and
the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite.
The universe appears to have a smooth
space-time continuum consisting of three
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 behave in a manner
that regularly follows a set of 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.
Fine tuning
It appears that many of the properties of the universe have special values in the sense that a universe where these
properties only differ slightly would not be able to support intelligent life.Not all scientists agree that this
fine-tuning exists. In particular, it is not known under what conditions intelligent life could form and what
form or shape that would take. A relevant observation in this discussion is that for an observer to exist to observe
fine-tuning, the universe must be able to support intelligent life. As such the conditional probability of observing a
universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle and is
particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of
properties exist (see below).
Historical models
Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the
then-available data and conceptions of the universe. Historically, cosmologies and cosmogonies were based on
narratives of gods acting in various ways. Theories of an impersonal universe governed by physical laws were first
proposed by the Greeks and Indians. Over the centuries, improvements in astronomical observations and theories of
motion and gravitation led to ever more accurate descriptions of the universe. The modern era of cosmology began
with Albert Einstein's 1915 general theory of relativity, which made it possible to quantitatively predict the origin,
evolution, and conclusion of the universe as a whole. Most modern, accepted theories of cosmology are based on
general relativity and, more specifically, the predicted Big Bang; however, still more careful measurements are
required to determine which theory is correct.
Creation
Many cultures have stories describing the origin of the world, which may be roughly grouped into common types. In
one type of story, the world is born from a world egg; such stories include the Finnish epic poem Kalevala, the
Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the creation idea is caused 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 god Atum
story, or the Genesis creation narrative. In another type of story, the world is created from the union of male and
female deities, as in the Maori story of Rangi and Papa. In other stories, the universe is created by crafting it from
pre-existing materials, such as the corpse of a dead god — as from Tiamat in the Babylonian epic Enuma Elish or
from the giant Ymir in Norse mythology – or from chaotic materials, as in Izanagi and Izanami in Japanese
mythology. In other stories, the universe emanates from fundamental principles, such as Brahman and Prakrti, or the
yin and yang of the Tao.
Philosophical models
Further information: Cosmology
From the 6th century BCE, the pre-Socratic Greek philosophers developed the earliest known philosophical models
of the universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand
the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice
to water to steam) and several philosophers proposed that all the apparently different materials of the world are
different forms of a single primordial material, or arche. The first to do so was Thales, who proposed this material is
Water. Thales' student, Anaximander, proposed that everything came from the limitless apeiron. Anaximenes
proposed 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). Heraclitus proposed fire (and
spoke of logos). Empedocles proposed the elements: earth, water, air and fire. His four element theory became very
popular. Like Pythagoras, Plato believed that all things were composed of number, with the Empedocles' elements
taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus—proposed that
the universe was composed of indivisible atoms moving through void (vacuum). Aristotle did not believe that was
feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover,
without resistance, it would do so indefinitely fast.
Although Heraclitus argued for eternal change, his quasi-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 τὸ ἐν (The One). Parmenides' theory seemed implausible to many Greeks, but his student
Zeno of Elea challenged them with several famous paradoxes. Aristotle responded to these paradoxes by developing
the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and
unchanging cycles of time, he believed the world was bounded by the celestial spheres, and thus magnitude was only
finitely multiplicative.
The Indian philosopher Kanada, founder of the Vaisheshika school, developed a theory 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 theory 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, Al-Ghazali (Algazel). Borrowing from Aristotle's Physics and Metaphysics,
they employed two logical arguments against an infinite past, the first being the "argument from the impossibility of
the existence of an actual infinite", which states:
"An actual infinite cannot exist."
"An infinite temporal regress of events is an actual infinite."
" An infinite temporal regress of events cannot exist."
The second argument, the "argument from the impossibility of completing an actual infinite by successive addition",
states:
"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
" The temporal series of past events cannot be an actual infinite."
Both arguments were adopted by Christian philosophers and theologians, and the second argument in particular
became more famous after it was adopted by Immanuel Kant in his thesis of the first antinomy concerning time.
Astronomical models
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. 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 that at the center of the universe was a "central fire" around which the Earth, Sun,
Moon and Planets revolved in uniform circular motion.The Greek astronomer 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' 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.
The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was
Seleucus of Seleucia, a Hellenistic astronomer who lived a century after 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 theory were probably related to the phenomenon of
tides.[50] 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 the heliocentric theory by determining the constants of a geometric model for the heliocentric theory
and by developing methods to compute planetary positions using this model, like what Nicolaus Copernicus later did
in the 16th century. During the Middle Ages, heliocentric models may have also been proposed by the Indian
astronomer, Aryabhata, and by the Persian astronomers, Albumasar and Al-Sijzi.
The Aristotelian model was accepted in the Western world for roughly
two millennia, until Copernicus revived Aristarchus' theory that the
astronomical data could be explained more plausibly if the earth
rotated on its axis and if the sun were placed at the center of the
universe.
“In the center rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can
illuminate everything at the same time? ”
—Nicolaus Copernicus, in Chapter 10, Book 1 of De Revolutionibus Orbium Coelestrum (1543)
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,
Christian scholar Nicholas of Cusa also proposed that the Earth rotates on its axis in his book, On Learned Ignorance
(1440). Aryabhata (476–550), Brahmagupta (598–668), Albumasar and Al-Sijzi, also proposed that the Earth
rotates on its axis. The first empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets,
was given by Tusi (1201–1274) and Ali Qushji (1403–1474).
This cosmology was accepted by Isaac Newton, Christiaan Huygens
and later scientists. Edmund Halley (1720)and Jean-Philippe de
Cheseaux (1744)[59] 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.
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.
Theoretical models
Of the four fundamental interactions, gravitation is dominant at
cosmological length scales; that is, the other three forces play a
negligible role in determining structures at the level of planetary
systems, galaxies and larger-scale structures. 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 theory of relativity
Given gravitation's predominance in shaping cosmological structures,
accurate predictions of the universe's past and future require an
accurate theory of gravitation. The best theory available is Albert
Einstein's general theory of relativity, which has passed all
experimental tests hitherto. However, since rigorous experiments have
not been carried out on cosmological length scales, general relativity
could conceivably be inaccurate. Nevertheless, its cosmological
predictions appear to be consistent with observations, so there is no
compelling reason to adopt another theory.
General relativity provides a set of ten nonlinear partial differential equations for the spacetime metric (Einstein's
field equations) that must be solved from the 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),[hat 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.
Special relativity and space-time
The universe has at least three spatial and
one temporal (time) dimension. It was long
thought that the spatial and temporal
dimensions were different in nature and
independent of one another. However,
according to the special theory of relativity,
spatial and temporal separations are
interconvertible (within limits) by changing
one's motion.
To understand this interconversion, it is
helpful to consider the analogous
interconversion of spatial separations along
the three spatial dimensions. Consider the
two endpoints of a rod of length L. The
length can be determined from the
differences in the three coordinates Δx, Δy
and Δz of the two endpoints in a given
reference frame
using the Pythagorean theorem. In a rotated
reference frame, the coordinate differences
differ, but they give the same length
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
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.
Solving Einstein's field equations
The distances between the spinning galaxies increase with time, but the distances between the stars within each
galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann
universe with zero cosmological constant Λ; such a universe oscillates between a Big Bang and a Big Crunch.
In non-Cartesian (non-square) or curved coordinate systems, the Pythagorean theorem holds only on infinitesimal
length scales and must be augmented with a more general metric tensor gμν, which can vary from place to place and
which describes the local geometry in the particular coordinate system. However, assuming the cosmological
principle that the universe is homogeneous and isotropic everywhere, every point in space is like every other point;
hence, the metric tensor must be the same everywhere. That leads to a single form for the metric tensor, called the
Friedmann–Lemaître–Robertson–Walker metric
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. 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 13.7 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.
Space has no boundary – that is empirically more certain than any external observation. However, that does not imply that space is
infinite...(translated, original German)
Bernhard Riemann (Habilitationsvortrag, 1854)
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, because it depends critically on the curvature index k and the
cosmological constant Λ. If the universe is sufficiently dense, k equals +1, meaning that its average curvature
throughout is positive and the universe will eventually recollapse in 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 speed of the universe is not decreasing as originally expected, but increasing; if this continues indefinitely,
the universe will eventually rip itself to shreds (the Big Rip). Experimentally, the universe has an overall density that
is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations
are needed to resolve the question.
Big Bang model
The prevailing Big Bang model accounts for many of the experimental observations described above, such as the
correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous,
isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space; as
the space itself expands, the wavelength of a photon traveling through space likewise increases, decreasing its
energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from
more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an
important problem in experimental physical cosmology.
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.
Multiverse theory
Some speculative theories have proposed that this 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 theories are distinct from
concepts such as alternate planes of consciousness and simulated
reality, although the idea of a larger universe is not new; for example,
Bishop Étienne Tempier of Paris ruled in 1277 that God could create as
many universes as he saw fit, a question that was being hotly debated
by the French theologians.
Max Tegmark developed a four part classification scheme for the
different types of multiverses that scientists have suggested in various
problem domains. An example of such a theory is the chaotic inflation model of the early universe. Another is the
many-worlds interpretation of quantum mechanics. Parallel worlds are generated in a manner similar to quantum
superposition and decoherence, with all states of the wave function being realized in separate worlds. Effectively, the
multiverse evolves as a universal wavefunction. If the big bang that created our multiverse created an ensemble of
multiverses, the wave function of the ensemble would be entangled in this sense.
The least controversial category of multiverse in Tegmark's scheme is Level I, which describes distant space-time
events "in our own universe". If space is infinite, or sufficiently large and uniform, identical instances of the history
of Earth's entire Hubble volume occur every so often, simply by chance. Tegmark calculated our nearest so-called
doppelgänger, is 1010115 meters away from us (a double exponential function larger than a googolplex). In
principle, it would be impossible to scientifically verify an identical Hubble volume. However, it does follow as a
fairly straightforward consequence from otherwise unrelated scientific observations and theories. Tegmark suggests
that statistical analysis exploiting the anthropic principle provides an opportunity to test multiverse theories in some
cases. Generally, science would consider a multiverse theory that posits neither a common point of causation, nor the
possibility of interaction between universes, to be an idle speculation.
Shape of the universe
The shape or geometry of the universe includes both local geometry in the observable universe and global geometry,
which we may or may not be able to measure. Shape can refer to curvature and topology. More formally, the subject
in practice investigates which 3-manifold corresponds to the spatial section in comoving coordinates of the
four-dimensional space-time of the universe. Cosmologists normally work with a given space-like slice of spacetime
called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the
backward light cone (points within the cosmic light horizon, given time to reach a given observer). If the observable
universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot
determine the global structure by observation: one is limited to a small patch.
Among the Friedmann–Lemaître–Robertson–Walker (FLRW) models, the presently most popular shape of the
Universe found to fit observational data according to cosmologists is the infinite flat model,[77] while other FLRW
models include the Poincaré dodecahedral space[78][79] and the Picard horn.[80] The data fit by these FLRW models
of space especially include the Wilkinson Microwave Anisotropy Probe (WMAP) maps of cosmic background
radiation. NASA released the first WMAP cosmic background radiation data in February 2003. In 2009 the Planck
observatory was launched to observe the microwave background at higher resolution than WMAP, possibly
providing more information on the shape of the Universe. The data should be released in late 2012.