The Guide to the Universe

Guide to the Universe – 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
Greeks, who proposed that the universe possesses infinite space and has
existed eternally, but contains a single set of concentric spheres of
finite size – corresponding to the fixed stars, the Sun and
various planets –
rotating about a spherical but unmoving Earth.
Over the centuries, more precise observations and improved theories of gravity
led to Copernicus’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 millions 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.

This high-resolution image of the Hubble
ultra deep fieldshows a diverse range of galaxies,
each consisting of billions of stars.
The equivalent area of sky that the picture occupies is shown in the
lower left corner. The smallest, reddest galaxies, about 100, are
some of the most distant galaxies to have been imaged by an optical
telescope, existing at the time shortly after the Big Bang.

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)
ofcosmic
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
theultimate
fate of the universe.

Current interpretations of astronomical
observations indicate that the age
of the universe is 13.75 ±0.17 billion years,^[4] and
that the diameter of the observable
universe is at least 93 billion light
years, or 8.80
× 10²⁶ metres.^[5] 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

See also: Cosmos, Nature, World
(philosophy), and Celestial
spheres

The word universe derives
from the Old
French word Univers,
which in turn derives from the Latin word universum.^[6] The
Latin word was used by Cicero and
later Latin authors in many of the same senses as the modern English word
is used.^[7] 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").^[7] Lucretius
used the word in the sense "everything rolled into one, everything combined into
one".

Artistic rendition (highly exaggerated) of a Foucault
pendulum showing that
the Earth is not stationary, but rotates.

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,
"something transported in a circle", originally used to describe a course of a
meal, the food being carried around the circle of dinner guests.^[8] This
Greek word refers to an
early Greek model of the universe, in which all matter was contained within
rotating spheres centered on the Earth; according to Aristotle,
the rotation of the
outermost sphere was responsible
for the motion and change of everything within. It was natural for the Greeks to
assume that the Earth was stationary and that the heavens rotated about the Earth,
because carefulastronomical and
physical measurements (such as the Foucault
pendulum) are required to prove otherwise.

The most common term for "universe" among the ancient Greek
philosophers from Pythagoras onwards
was τὸ
πᾶν (The All), defined as all
matter (τὸ ὅλον) and all space (τὸ
κενόν).^[9]^[10] Other
synonyms for the universe among the ancient Greek philosophers included κόσμος (meaning
the world,
thecosmos)
and φύσις (meaning Nature,
from which we derive the word physics).^[11] The
same synonyms are found in Latin authors (totum, mundus,natura)^[12] 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).^[13]

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. Time is not considered in Eriugena’s
definition; thus, his definition includes everything that exists, has existed
and will exist, as well as everything that does not exist, has never existed and
will never exist. This all-embracing definition was not adopted by most later
philosophers, but something not entirely dissimilar reappears in quantum
physics, perhaps most obviously in the path-integral
formulation of Feynman.^[14]According
to that formulation, the probability
amplitudes for the various
outcomes of an experiment given a perfectly defined initial state of the system
are determined by summing over all possible paths by which the system could
progress from the initial to final state. Naturally, an experiment can have only
one outcome; in other words, only one possible outcome is made real in this
universe, via the mysterious process of quantum
measurement, also known as the collapse
of the wavefunction (but see the many-worlds
hypothesis below in the Multiversesection).
In this well-defined mathematical sense, even that which does not exist (all
possible paths) can influence that which does finally exist (the experimental
measurement). As a specific example, every electron is
intrinsically identical to every other; therefore, probability amplitudes must
be computed allowing for the possibility that they exchange positions, something
known as exchange
symmetry. This conception of the universe embracing both the existent and
the non-existent loosely parallels the Buddhist doctrines
of shunyata andinterdependent
development of reality, and Gottfried
Leibniz‘s more modern concepts of contingency and
the identity
of indiscernibles.

Definition as
reality

Definition as connected space-time

Definition as observable reality

See also: Observable
universe and Observational
cosmology

According to a still-more-restrictive definition, the universe is everything
within our connected space-time that
could have a chance to interact with us and vice versa.^{[citation

needed]}According to the general
theory of relativity, some regions of space may
never interact with ours even in the lifetime of the universe, due to the finite speed
of light and the ongoingexpansion
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 may 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

Main articles: Observable
universe, Age
of the universe, Large-scale
structure of the universe, and Abundance
of the chemical elements

The universe is very large and possibly infinite in volume. The region visible
from Earth (the observable universe) is about 92 billion light
years across,^[16] 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.^[17] As
an example, our Milky
Way Galaxy is roughly 100,000
light years in diameter,^[18] and
our nearest sister galaxy, the Andromeda
Galaxy, is located roughly 2.5 million light years away.^[19] There
are probably more than 100 billion (10¹¹) galaxies in
the observable
universe.^[20] Typical
galaxies range from dwarfs with
as few as ten million^[21] (10⁷) stars up
to giants with one trillion^[22] (10¹²)
stars, all orbiting the galaxy’s center of mass. Thus, a very rough estimate
from these numbers would suggest there are around one sextillion (10²¹)
stars in the observable universe; though a 2003 study by Australian National
University astronomers resulted in a figure of 70 sextillion (7 x 10²²)^[23].

The universe is believed to be mostly composed of dark
energy and dark
matter, both of which are poorly understood at present. Less
than 5% of the universe is ordinary matter, a relatively small
perturbation.

The observable matter is spread uniformly (homogeneously) throughout the
universe, when averaged over distances longer than 300 million light-years.^[24] 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
structuressuch 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.^[25] The
universe is also bathed in a highly isotropic microwave radiation that
corresponds to a thermal
equilibrium blackbody
spectrum of roughly 2.725 kelvin.^[26] The
hypothesis that the large-scale universe is homogeneous and isotropic is known
as the cosmological
principle,^[27]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.^[28] The
properties of dark energy and dark matter are largely unknown. Dark mattergravitates as
ordinary matter, and thus works to slow the expansion
of the universe; by contrast, dark energy accelerates
its expansion.

The universe is old and
evolving. The most
precise estimate of the
universe’s age is 13.73±0.12 billion years old, based on observations of the cosmic
microwave background radiation.^[29]Independent
estimates (based on measurements such as radioactive
dating) agree, although they are less precise, ranging from 11–20 billion
years^[30] to
13–15 billion years.^[31] 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 longerwavelengths 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.^[32] The
universe seems to have much more matter than antimatter,
an asymmetry possibly related to the observations of CP
violation.^[33] 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 netmomentum 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.^[34]

The elementary
particles from which
the universe is constructed. Six leptons and
six quarks comprise
most of thematter;
for example, the protons and neutrons of atomic
nuclei are composed
of quarks, and the ubiquitous electron is
a lepton. These particles interact via the gauge
bosonsshown in the middle row, each corresponding to a
particular type of gauge
symmetry. The Higgs
boson (as yet
unobserved) is believed to confer mass on
the particles with which it is connected. The graviton,
a supposed gauge boson for gravity,
is not shown.

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
geometryis experimentally true with high accuracy throughout most of the
Universe.^[35] 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.^[36]

The universe appears to behave in a manner that regularly follows a set of physical
laws and physical
constants.^[37] According
to the prevailing Standard
Model of physics, all matter is
composed of three generations of leptons and quarks,
both of which arefermions.
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; andgravity,
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
momentumand energy;
in many cases, these conservation laws can be related to symmetries or mathematical
identities.

Fine tuning

Main article: fine-tuned
universe

It appears that many of 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.^[38]^[39] Not
all scientists agree that this fine-tuning exists.^[40]^[41] 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 existence of an observer to observe fine-tuning, requires
that the universe supports 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

See also: Cosmology and Timeline
of cosmology

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

Main articles: Creation
myth and Creator
deity

Sumerian account of
the creatrix goddess Nammu,
the precursor of theAssyrian goddess Tiamat;
perhaps the earliest surviving creation myth.

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 his or herself, as in the Tibetan
Buddhismconcept of Adi-Buddha,
the ancient
Greek story of Gaia (Mother
Earth), the Aztec goddess Coatlicue myth,
the ancient
Egyptian god Atumstory,
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 andIzanami 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
(wood, metal, etc.) are all different forms of a single material, the arche.
The first to do so was Thales,
who called this material Water.
Following him, Anaximenes called
it Air,
and posited that there must be attractive and repulsive forces that
cause the arche to condense or dissociate into different forms. Empedocles proposed
that multiple fundamental materials were necessary to explain the diversity of
the universe, and proposed that all four classical elements (Earth, Air, Fire
and Water) existed, albeit in different combinations and forms. This
four-element theory was adopted by many of the subsequent philosophers. Some
philosophers before Empedocles advocated less material things for the arche; Heraclitus argued
for a Logos, Pythagoras believed
that all things were composed of numbers,
whereas Thales’ student, Anaximander,
proposed that everything was composed of a chaotic substance known as apeiron,
roughly corresponding to the modern concept of a quantum
foam. Various modifications of the apeiron theory were proposed, most
notably that of Anaxagoras,
which proposed that the various matter in the world was spun off from a rapidly
rotating apeiron, set in motion by the principle of Nous (Mind).
Still other philosophers — most notably Leucippus and
Democritus — proposed that the universe was composed of indivisible atoms moving
through empty space, a vacuum; Aristotle opposed
this view ("Nature abhors a vacuum") on the grounds that resistance
to motion increases with density;
hence, empty space should offer no resistance to motion, leading to the
possibility of infinite speed.

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 resolved these paradoxes by developing the notion of an infinitely
divisible continuum, and applying it to space and time.

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.^[42] 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.^[43]

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. 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).
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:^[44]

"An
actual infinite cannot exist."

"An
infinite temporal regress of events is an actual infinite."

" $\therefore$ 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:^[44]

"An
actual infinite cannot be completed by successive addition."

"The temporal series of past events has been completed by successive
addition."

" $\therefore$ The
temporal series of past events cannot be an actual infinite."

Both arguments were adopted by later 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.^[44]

Astronomical models

Main article: History
of astronomy

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 this model, space and time are infinite and eternal,
the Earth is spherical and stationary, and all other matter is confined to
rotating concentric spheres. This model was refined by Callippus and Aristotle,
and brought into nearly perfect agreement with astronomical observations by Ptolemy.
The success of this model is largely due to the mathematical fact that any
function (such as the position of a planet) can be decomposed into a set of
circular functions (the Fourier
modes). However, not all Greek scientists accepted the geocentric model of
the universe. 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.^[45] The Greek
astronomer Aristarchus
of Samos was the first known
individual to propose aheliocentric model
of the universe. Though the original text has been lost, a reference in
Archimedes’ book The Sand Reckoner describes Aristarchus’ heliocentric theory. Archimedeswrote:
(translated into English)

You King Gelon are aware the ‘universe’ is the name given by most
astronomers to the sphere the center of which is the center of the Earth,
while its radius is equal to the straight line between the center of the Sun
and the center of the Earth. This is the common account as you have heard
from astronomers. But Aristarchus has brought out a book consisting of
certain hypotheses, wherein it appears, as a consequence of the assumptions
made, that the universe is many times greater than the ‘universe’ just
mentioned. His hypotheses are that the fixed stars and the Sun remain
unmoved, that the Earth revolves about the Sun on the circumference of a
circle, the Sun lying in the middle of the orbit, and that the sphere of
fixed stars, situated about the same center as the Sun, is so great that the
circle in which he supposes the Earth to revolve bears such a proportion to
the distance of the fixed stars as the center of the sphere bears to its
surface.

Aristarchus thus believed the stars to be very far away, and saw this as the
reason why there was no visible parallax, that is, an observed movement of the
stars relative to each other as the Earth moved around the Sun. The stars are in
fact much farther away than the distance that was generally assumed in ancient
times, which is why stellar parallax is only detectable with telescopes. The
geocentric model, consistent with planetary parallax, was assumed to be an
explanation for the unobservability of the parallel phenomenon, stellar
parallax. The rejection of the heliocentric view was apparently quite strong, as
the following passage from Plutarch suggests (On the Apparent Face in the Orb of
the Moon):

Cleanthes [a contemporary of
Aristarchus and head of the Stoics] thought it was the duty of the Greeks to
indict Aristarchus of Samos on the charge of impiety for putting in motion
the Hearth of the universe [i.e. the earth], . . . supposing the heaven to
remain at rest and the earth to revolve in an oblique circle, while it
rotates, at the same time, about its own axis. [1]

The only other astronomer from antiquity known by name who supported
Aristarchus’ heliocentric model was Seleucus
of Seleucia, a Hellenized Babylonian astronomer
who lived a century after Aristarchus.^[46]^[47]^[48] 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.^[49] 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.^[50] 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.^[51] During
the Middle
Ages, heliocentric models may have also been proposed by the Indian
astronomer, Aryabhata,^[52] and
by the Persian
astronomers, Albumasar^[53] andAl-Sijzi.^[54]

Model of the Copernican universe
byThomas
Digges in 1576, with
the amendment that the stars are no longer confined to a sphere, but
spread uniformly throughout the space surrounding the planets.

The Aristotelian model was accepted in the Western
world for roughly two millennia,
until Copernicus revived
Aristarchus’ 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?	”
—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).^[55] Aryabhata
(476–550), Brahmagupta (598–668), Albumasar andAl-Sijzi,
also proposed that the Earth rotates on its axis.^{[citation

needed]} The first empirical
evidence for the Earth’s rotation
on its axis, using the phenomenon of comets,
was given by Tusi (1201–1274)
and Ali
Kuşçu (1403–1474).^{[citation

needed]} Tusi, however,
continued to support the Aristotelian universe, thus Kuşçu was the first to
refute the Aristotelian notion of a stationary Earth on an empirical basis,
similar to how Copernicus later justified the Earth’s rotation. Al-Birjandi (d.
1528) further developed a theory of "circular inertia"
to explain the Earth’s rotation, similar to how Galileo
Galilei explained it.^[56]^[57]

Johannes Kepler published
theRudolphine
Tables containing
a star catalog and planetary tables using Tycho
Brahe‘s measurements.

Copernicus’ heliocentric
model allowed the stars to be
placed uniformly through the (infinite) space surrounding the planets, as first
proposed by Thomas
Digges in his Perfit
Description of the Caelestiall Orbes according to the most aunciente doctrine of
the Pythagoreans, latelye revived by Copernicus and by Geometricall
Demonstrations approved (1576).^[58] Giordano
Bruno accepted the idea that
space was infinite and filled with solar systems similar to our own; for the
publication of this view, he was burned
at the stake in the Campo
dei Fiori in Rome on 17 February
1600.^[58]

This cosmology was accepted provisionally by Isaac
Newton, Christiaan
Huygens and later scientists,^[58] although
it had several paradoxes that were resolved only with the development of general
relativity. The first of these was that it assumed that space and time were
infinite, and that the stars in the universe had been burning forever; however,
since stars are constantly radiating energy,
a finite star seems inconsistent with the radiation of infinite energy.
Secondly, Edmund Halley (1720)^[59] and Jean-Philippe
de Cheseaux (1744)^[60] 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.^[61] Third,
Newton himself showed that an infinite space uniformly filled with matter would
cause infinite forces and instabilities causing the matter to be crushed inwards
under its own gravity.^[58] This
instability was clarified in 1902 by the Jeans
instability criterion.^[62] One
solution to these latter two paradoxes is the Charlier
universe, in which the matter is arranged hierarchically (systems of
orbiting bodies that are themselves orbiting in a larger system, ad
infinitum) in a fractal way
such that the universe has a negligibly small overall density; such a
cosmological model had also been proposed earlier in 1761 by Johann
Heinrich Lambert.^[63] A
significant astronomical advance of the 18th century was the realization by Thomas
Wright, Immanuel
Kant and others that stars are
not distributed uniformly throughout space; rather, they are grouped into galaxies.^[64]

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.^[65] This
theory and its implications will be discussed in more detail in the following
section.

Theoretical models

High-precision test of general relativity by the Cassini space
probe (artist’s impression): radio signals
sent between the Earth and the probe (green wave) aredelayed by
the warping of space
and time(blue lines) due to the Sun‘s
mass.

Of the four fundamental
interactions, gravitation is
dominant at cosmological length scales; that is, the other three forces are
believed to play a negligible role in determining structures at the level of
planets, stars, galaxies and larger-scale structures. Since all matter and
energy gravitate, gravity’s effects are cumulative; by contrast, the effects of
positive and negative charges tend to cancel one another, making
electromagnetism relatively insignificant on cosmological length scales. The
remaining two interactions, the weak and strong
nuclear forces, decline very rapidly with distance; their effects are
confined mainly to sub-atomic length scales.

General
theory of relativity

Main articles: Introduction
to general relativity, General
relativity, and Einstein’s
field equations

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 of a set of ten nonlinear partial differential
equations for the spacetime
metric (Einstein’s
field equations) that must be solved from the distribution of mass-energy and momentum throughout
the universe. Since these are unknown in exact detail, cosmological models have
been based on the cosmological
principle, which states that the universe is homogeneous and isotropic. In
effect, this principle asserts that the gravitational effects of the various
galaxies making up the universe are equivalent to those of a fine dust
distributed uniformly throughout the universe with the same average density. The
assumption of a uniform dust makes it easy to solve Einstein’s field equations
and predict the past and future of the universe on cosmological time scales.

Einstein’s field equations include a cosmological
constant (Λ),^[65]^[66] that
corresponds to an energy density of empty space.^[67] 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,^[68] recent
astronomical observations of type
Ia supernovae have detected a
large amount of "dark
energy" that is accelerating the universe’s expansion.^[69] Preliminary
studies suggest that this dark energy corresponds to a positive Λ,
although alternative theories cannot be ruled out as yet.^[70] 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.^[71] Evidence
for such zero-point energy is observed in the Casimir
effect.

Special
relativity and space-time

Main articles: Introduction
to special relativity and Special
relativity

Only its length L is
intrinsic to the rod (shown in black); coordinate differences
between its endpoints (such as Δx, Δy or Δξ, Δη) depend on their
frame of reference (depicted in blue and red, respectively).

The universe has at least three spatial and
one temporal (time)
dimension. It was long thought that the spatial and temporal dimensions were
different in nature and independent of one another. However, according to the special
theory of relativity, spatial and temporal separations are interconvertible
(within limits) by changing one’s motion.

To understand this interconversion, it is helpful to consider the analogous
interconversion of spatial separations along the three spatial dimensions.
Consider the two endpoints of a rod of length L.
The length can be determined from the differences in the three coordinates Δx,
Δy and Δz of the two endpoints in a given reference frame

L 2 = Δ x 2 + Δ y 2 + Δ z 2

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

L 2 = Δξ 2 + Δη 2 + Δζ 2 .

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

$<br /> s^{2} = L_{1}^{2} - c^{2} \Delta t_{1}^{2} = L_{2}^{2} - c^{2} \Delta t_{2}^{2}$

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

See also: Big
Bang and Ultimate
fate of the universe

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.

Animation illustrating the metric
expansion of the universe

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 g_μν,
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

$<br /> ds^2 = -c^{2} dt^2 +<br /> R(t)^2 \left( \frac{dr^2}{1-k r^2} + r^2 d\theta^2 + r^2 \sin^2 \theta \, d\phi^2 \right)$

where (r, θ, φ) correspond to a spherical
coordinate system. This metric has
only two undetermined parameters: an overall length scale R that
can vary with time, and a curvature index k that
can be only 0, 1 or −1, corresponding to flat Euclidean
geometry, or spaces of positive or negative curvature.
In cosmology, solving for the history of the universe is done by calculating R as
a function of time, given k and
the value of the cosmological
constant Λ, which is a
(small) parameter in Einstein’s field equations. The equation describing how R varies
with time is known as the Friedmann
equation, after its inventor, Alexander
Friedmann.^[72]

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 thespeed
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.

Prevailing model of the origin and expansion of spacetime and
all that it contains.

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

Main articles: Big
Bang, Timeline
of the Big Bang, Nucleosynthesis,
and Lambda-CDM
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.

Chief nuclear reactions responsible for the relative
abundances of light atomic
nuclei observed
throughout the universe.

Other experimental observations can be explained by combining the overall
expansion of space with nuclearand 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
negativeelectrons,
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-energycontent
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.

Untestable proposals

Multiverse theory

Main articles: Multiverse, Many-worlds
hypothesis, Bubble
universe theory, and Parallel
universe (fiction)

Depiction of a multiverse of
seven"bubble"
universes, which are separatespacetime continua,
each having differentphysical
laws, physical
constants, and perhaps even different numbers ofdimensions or topologies.

Some speculative theories have proposed that this universe is but one of a set of
disconnected universes, collectively denoted as themultiverse,
altering the concept that the universe encompasses everything.^[15]^[73] By
definition, there is no possible way for anything in one universe to affect
another; if two "universes" could affect one another, they would be part of a
single universe. Thus, although some fictional characters travel between parallel
fictional "universes", this is, strictly speaking, an incorrect usage of the
term universe. The
disconnected universes are conceived as being physical, in the sense that each
should have its own space and time, its own matter and energy, and its own
physical laws — that also challenges the definition of parallelity as these
universes don’t exist synchronously (since they have their own time) or in a
geometrically parallel way (since there’s no interpretable relation between
spatial positions of the different universes). Such physically disconnected
universes should be distinguished from the metaphysical conception
of alternate
planes of consciousness, which are not thought to be physical places and are
connected through the flow of information. The concept of a multiverse of
disconnected universes is very old; 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.^[74]

There are two scientific senses in which multiple universes are discussed.
First, disconnected spacetime continua may exist; presumably, all forms of
matter and energy are confined to one universe and cannot "tunnel" between them.
An example of such a theory is the chaotic
inflation model of the early
universe.^[75] Second,
according to the many-worlds
hypothesis, a parallel universe is born with every quantum
measurement; the universe "forks" into parallel copies, each one
corresponding to a different outcome of the quantum measurement. However, both
senses of the term "multiverse" are speculative and may be considered unscientific;
no known experimental test in one universe could reveal the existence or
properties of another non-interacting universe.

Shape of the
universe

For more details on this topic, see 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-timeof
the universe. Analysis of data from WMAP implies
that the universe is spatially
flat with only a 2% margin of
error.^[76]

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.

In October 2001, NASA began collecting data with the Wilkinson
Microwave Anisotropy Probe (WMAP)
on cosmic background radiation. Like visible light from distant stars and
galaxies, cosmic background radiation allows scientists to peer into the past to
the time when the universe was in its infancy. Density fluctuations in this
radiation can also tell scientists much about the physical nature of space.^[77] NASA
released the first WMAP cosmic background radiation data in February 2003. In
2009 the Planck
observatory was launched which
will be able to analyze the microwave background at higher resolution, providing
more information on the shape of the early universe. The preliminary data will
be released in December 2010.

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