The Big Bang theory is the prevailing cosmological
model for the observable universe from the
earliest known periods through its subsequent
large-scale evolution. The model describes
how the universe expanded from a very high-density
and high-temperature state, and offers a comprehensive
explanation for a broad range of phenomena,
including the abundance of light elements,
the cosmic microwave background (CMB), large
scale structure and Hubble's law (the farther
away galaxies are, the faster they are moving
away from Earth). If the observed conditions
are extrapolated backwards in time using the
known laws of physics, the prediction is that
just before a period of very high density
there was a singularity which is typically
associated with the Big Bang. Physicists are
undecided whether this means the universe
began from a singularity, or that current
knowledge is insufficient to describe the
universe at that time. Detailed measurements
of the expansion rate of the universe place
the Big Bang at around 13.8 billion years
ago, which is thus considered the age of the
universe. After its initial expansion, the
universe cooled sufficiently to allow the
formation of subatomic particles, and later
simple atoms. Giant clouds of these primordial
elements (mostly hydrogen, with some helium
and lithium) later coalesced through gravity,
eventually forming early stars and galaxies,
the descendants of which are visible today.
Astronomers also observe the gravitational
effects of dark matter surrounding galaxies.
Though most of the mass in the universe seems
to be in the form of dark matter, Big Bang
theory and various observations seem to indicate
that it is not made out of conventional baryonic
matter (protons, neutrons, and electrons)
but it is unclear exactly what it is made
out of.
Since Georges Lemaître first noted in 1927
that an expanding universe could be traced
back in time to an originating single point,
scientists have built on his idea of cosmic
expansion. The scientific community was once
divided between supporters of two different
theories, the Big Bang and the Steady State
theory, but a wide range of empirical evidence
has strongly favored the Big Bang which is
now universally accepted. In 1929, from analysis
of galactic redshifts, Edwin Hubble concluded
that galaxies are drifting apart; this is
important observational evidence consistent
with the hypothesis of an expanding universe.
In 1964, the cosmic microwave background radiation
was discovered, which was crucial evidence
in favor of the Big Bang model, since that
theory predicted the existence of background
radiation throughout the universe before it
was discovered. More recently, measurements
of the redshifts of supernovae indicate that
the expansion of the universe is accelerating,
an observation attributed to dark energy's
existence. The known physical laws of nature
can be used to calculate the characteristics
of the universe in detail back in time to
an initial state of extreme density and temperature.
== Overview ==
The Belgian astronomer and Catholic priest
Georges Lemaître proposed on theoretical
grounds that the universe is expanding, which
was observationally confirmed soon afterwards
by Edwin Hubble. In 1927 in the Annales de
la Société Scientifique de Bruxelles (Annals
of the Scientific Society of Brussels) under
the title "Un Univers homogène de masse constante
et de rayon croissant rendant compte de la
vitesse radiale des nébuleuses extragalactiques"
("A homogeneous Universe of constant mass
and growing radius accounting for the radial
velocity of extragalactic nebulae"), he presented
his new idea that the universe is expanding
and provided the first observational estimation
of what is known as the Hubble constant. What
later will be known as the "Big Bang theory"
of the origin of the universe, he called his
"hypothesis of the primeval atom" or the "Cosmic
Egg".American astronomer Edwin Hubble observed
that the distances to faraway galaxies were
strongly correlated with their redshifts.
This was interpreted to mean that all distant
galaxies and clusters are receding away from
our vantage point with an apparent velocity
proportional to their distance: that is, the
farther they are, the faster they move away
from us, regardless of direction. Assuming
the Copernican principle (that the Earth is
not the center of the universe), the only
remaining interpretation is that all observable
regions of the universe are receding from
all others. Since we know that the distance
between galaxies increases today, it must
mean that in the past galaxies were closer
together. The continuous expansion of the
universe implies that the universe was denser
and hotter in the past.
Large particle accelerators can replicate
the conditions that prevailed after the early
moments of the universe, resulting in confirmation
and refinement of the details of the Big Bang
model. However, these accelerators can only
probe so far into high energy regimes. Consequently,
the state of the universe in the earliest
instants of the Big Bang expansion is still
poorly understood and an area of open investigation
and speculation.
The first subatomic particles to be formed
included protons, neutrons, and electrons.
Though simple atomic nuclei formed within
the first three minutes after the Big Bang,
thousands of years passed before the first
electrically neutral atoms formed. The majority
of atoms produced by the Big Bang were hydrogen,
along with helium and traces of lithium. Giant
clouds of these primordial elements later
coalesced through gravity to form stars and
galaxies, and the heavier elements were synthesized
either within stars or during supernovae.
The Big Bang theory offers a comprehensive
explanation for a broad range of observed
phenomena, including the abundance of light
elements, the CMB, large scale structure,
and Hubble's Law. The framework for the Big
Bang model relies on Albert Einstein's theory
of general relativity and on simplifying assumptions
such as homogeneity and isotropy of space.
The governing equations were formulated by
Alexander Friedmann, and similar solutions
were worked on by Willem de Sitter. Since
then, astrophysicists have incorporated observational
and theoretical additions into the Big Bang
model, and its parametrization as the Lambda-CDM
model serves as the framework for current
investigations of theoretical cosmology. The
Lambda-CDM model is the current "standard
model" of Big Bang cosmology, consensus is
that it is the simplest model that can account
for the various measurements and observations
relevant to cosmology.
== Timeline ==
=== 
Singularity ===
Extrapolation of the expansion of the universe
backwards in time using general relativity
yields an infinite density and temperature
at a finite time in the past. This singularity
indicates that general relativity is not an
adequate description of the laws of physics
in this regime. Models based on general relativity
alone can not extrapolate toward the singularity
beyond the end of the Planck epoch.
This primordial singularity is itself sometimes
called "the Big Bang", but the term can also
refer to a more generic early hot, dense phase
of the universe. In either case, "the Big
Bang" as an event is also colloquially referred
to as the "birth" of our universe since it
represents the point in history where the
universe can be verified to have entered into
a regime where the laws of physics as we understand
them (specifically general relativity and
the standard model of particle physics) work.
Based on measurements of the expansion using
Type Ia supernovae and measurements of temperature
fluctuations in the cosmic microwave background,
the time that has passed since that event
— otherwise known as the "age of the universe"
— is 13.799 ± 0.021 billion years. The
agreement of independent measurements of this
age supports the ΛCDM model that describes
in detail the characteristics of the universe.
Despite being extremely dense at this time—far
denser than is usually required to form a
black hole—the universe did not re-collapse
into a black hole. This may be explained by
considering that commonly-used calculations
and limits for gravitational collapse are
usually based upon objects of relatively constant
size, such as stars, and do not apply to rapidly
expanding space such as the Big Bang.
=== Inflation and baryogenesis ===
The earliest phases of the Big Bang are subject
to much speculation. In the most common models
the universe was filled homogeneously and
isotropically with a very high energy density
and huge temperatures and pressures and was
very rapidly expanding and cooling. Approximately
10−37 seconds into the expansion, a phase
transition caused a cosmic inflation, during
which the universe grew exponentially during
which time density fluctuations that occurred
because of the uncertainty principle were
amplified into the seeds that would later
form the large-scale structure of the universe.
After inflation stopped, reheating occurred
until the universe obtained the temperatures
required for the production of a quark–gluon
plasma as well as all other elementary particles.
Temperatures were so high that the random
motions of particles were at relativistic
speeds, and particle–antiparticle pairs
of all kinds were being continuously created
and destroyed in collisions. At some point,
an unknown reaction called baryogenesis violated
the conservation of baryon number, leading
to a very small excess of quarks and leptons
over antiquarks and antileptons—of the order
of one part in 30 million. This resulted in
the predominance of matter over antimatter
in the present universe.
=== Cooling ===
The universe continued to decrease in density
and fall in temperature, hence the typical
energy of each particle was decreasing. Symmetry
breaking phase transitions put the fundamental
forces of physics and the parameters of elementary
particles into their present form. After about
10−11 seconds, the picture becomes less
speculative, since particle energies drop
to values that can be attained in particle
accelerators. At about 10−6 seconds, quarks
and gluons combined to form baryons such as
protons and neutrons. The small excess of
quarks over antiquarks led to a small excess
of baryons over antibaryons. The temperature
was now no longer high enough to create new
proton–antiproton pairs (similarly for neutrons–antineutrons),
so a mass annihilation immediately followed,
leaving just one in 1010 of the original protons
and neutrons, and none of their antiparticles.
A similar process happened at about 1 second
for electrons and positrons. After these annihilations,
the remaining protons, neutrons and electrons
were no longer moving relativistically and
the energy density of the universe was dominated
by photons (with a minor contribution from
neutrinos).
A few minutes into the expansion, when the
temperature was about a billion (one thousand
million) kelvin and the density was about
that of air, neutrons combined with protons
to form the universe's deuterium and helium
nuclei in a process called Big Bang nucleosynthesis.
Most protons remained uncombined as hydrogen
nuclei.As the universe cooled, the rest mass
energy density of matter came to gravitationally
dominate that of the photon radiation. After
about 379,000 years, the electrons and nuclei
combined into atoms (mostly hydrogen); hence
the radiation decoupled from matter and continued
through space largely unimpeded. This relic
radiation is known as the cosmic microwave
background radiation. The chemistry of life
may have begun shortly after the Big Bang,
13.8 billion years ago, during a habitable
epoch when the universe was only 10–17 million
years old.
=== Structure formation ===
Over a long period of time, the slightly denser
regions of the nearly uniformly distributed
matter gravitationally attracted nearby matter
and thus grew even denser, forming gas clouds,
stars, galaxies, and the other astronomical
structures observable today. The details of
this process depend on the amount and type
of matter in the universe. The four possible
types of matter are known as cold dark matter,
warm dark matter, hot dark matter, and baryonic
matter. The best measurements available, from
Wilkinson Microwave Anisotropy Probe (WMAP),
show that the data is well-fit by a Lambda-CDM
model in which dark matter is assumed to be
cold (warm dark matter is ruled out by early
reionization), and is estimated to make up
about 23% of the matter/energy of the universe,
while baryonic matter makes up about 4.6%.
In an "extended model" which includes hot
dark matter in the form of neutrinos, then
if 
the "physical baryon density"
Ω
b
h
2
{\displaystyle \Omega _{\text{b}}h^{2}}
is estimated at about 0.023 (this is different
from the 'baryon density'
Ω
b
{\displaystyle \Omega _{\text{b}}}
expressed as a fraction of the total matter/energy
density, which as noted above is about 0.046),
and the corresponding cold dark matter density
Ω
c
h
2
{\displaystyle \Omega _{\text{c}}h^{2}}
is about 0.11, the corresponding neutrino
density
Ω
v
h
2
{\displaystyle \Omega _{\text{v}}h^{2}}
is estimated to be less than 0.0062.
=== Cosmic acceleration ===
Independent lines of evidence from Type Ia
supernovae and the CMB imply that the universe
today is dominated by a mysterious form of
energy known as dark energy, which apparently
permeates all of space. The observations suggest
73% of the total energy density of today's
universe is in this form. When the universe
was very young, it was likely infused with
dark energy, but with less space and everything
closer together, gravity predominated, and
it was slowly braking the expansion. But eventually,
after numerous billion years of expansion,
the growing abundance of dark energy caused
the expansion of the universe to slowly begin
to accelerate.Dark energy in its simplest
formulation takes the form of the cosmological
constant term in Einstein's field equations
of general relativity, but its composition
and mechanism are unknown and, more generally,
the details of its equation of state and relationship
with the Standard Model of particle physics
continue to be investigated both through observation
and theoretically.All of this cosmic evolution
after the inflationary epoch can be rigorously
described and modeled by the ΛCDM model of
cosmology, which uses the independent frameworks
of quantum mechanics and Einstein's General
Relativity. There is no well-supported model
describing the action prior to 10−15 seconds
or so. Apparently a new unified theory of
quantum gravitation is needed to break this
barrier. Understanding this earliest of eras
in the history of the universe is currently
one of the greatest unsolved problems in physics.
== Features of the model ==
The Big Bang theory depends on two major assumptions:
the universality of physical laws and the
cosmological principle. The cosmological principle
states that on large scales the universe is
homogeneous and isotropic.
These ideas were initially taken as postulates,
but today there are efforts to test each of
them. For example, the first assumption has
been tested by observations showing that largest
possible deviation of the fine structure constant
over much of the age of the universe is of
order 10−5. Also, general relativity has
passed stringent tests on the scale of the
Solar System and binary stars.If the large-scale
universe appears isotropic as viewed from
Earth, the cosmological principle can be derived
from the simpler Copernican principle, which
states that there is no preferred (or special)
observer or vantage point. To this end, the
cosmological principle has been confirmed
to a level of 10−5 via observations of the
CMB. The universe has been measured to be
homogeneous on the largest scales at the 10%
level.
=== Expansion of space ===
General relativity describes spacetime by
a metric, which determines the distances that
separate nearby points. The points, which
can be galaxies, stars, or other objects,
are themselves specified using a coordinate
chart or "grid" that is laid down over all
spacetime. The cosmological principle implies
that the metric should be homogeneous and
isotropic on large scales, which uniquely
singles out the Friedmann–Lemaître–Robertson–Walker
metric (FLRW metric).
This metric contains a scale factor, which
describes how the size of the universe changes
with time. This enables a convenient choice
of a coordinate system to be made, called
comoving coordinates. In this coordinate system,
the grid expands along with the universe,
and objects that are moving only because of
the expansion of the universe, remain at fixed
points on the grid. While their coordinate
distance (comoving distance) remains constant,
the physical distance between two such co-moving
points expands proportionally with the scale
factor of the universe.The Big Bang is not
an explosion of matter moving outward to fill
an empty universe. Instead, space itself expands
with time everywhere and increases the physical
distance between two comoving points. In other
words, the Big Bang is not an explosion in
space, but rather an expansion of space. Because
the FLRW metric assumes a uniform distribution
of mass and energy, it applies to our universe
only on large scales—local concentrations
of matter such as our galaxy are gravitationally
bound and as such do not experience the large-scale
expansion of space.
=== Horizons ===
An important feature of the Big Bang spacetime
is the presence of particle horizons. Since
the universe has a finite age, and light travels
at a finite speed, there may be events in
the past whose light has not had time to reach
us. This places a limit or a past horizon
on the most distant objects that can be observed.
Conversely, because space is expanding, and
more distant objects are receding ever more
quickly, light emitted by us today may never
"catch up" to very distant objects. This defines
a future horizon, which limits the events
in the future that we will be able to influence.
The presence of either type of horizon depends
on the details of the FLRW model that describes
our universe.Our understanding of the universe
back to very early times suggests that there
is a past horizon, though in practice our
view is also limited by the opacity of the
universe at early times. So our view cannot
extend further backward in time, though the
horizon recedes in space. If the expansion
of the universe continues to accelerate, there
is a future horizon as well.
== History ==
=== Etymology ===
English astronomer Fred Hoyle is credited
with coining the term "Big Bang" during a
1949 BBC radio broadcast, saying: "These theories
were based on the hypothesis that all the
matter in the universe was created in one
big bang at a particular time in the remote
past."It is popularly reported that Hoyle,
who favored an alternative "steady state"
cosmological model, intended this to be pejorative,
but Hoyle explicitly denied this and said
it was just a striking image meant to highlight
the difference between the two models.
=== Development ===
The Big Bang theory developed from observations
of the structure of the universe and from
theoretical considerations. In 1912 Vesto
Slipher measured the first Doppler shift of
a "spiral nebula" (spiral nebula is the obsolete
term for spiral galaxies), and soon discovered
that almost all such nebulae were receding
from Earth. He did not grasp the cosmological
implications of this fact, and indeed at the
time it was highly controversial whether or
not these nebulae were "island universes"
outside our Milky Way. Ten years later, Alexander
Friedmann, a Russian cosmologist and mathematician,
derived the Friedmann equations from Albert
Einstein's equations of general relativity,
showing that the universe might be expanding
in contrast to the static universe model advocated
by Einstein at that time. In 1924 Edwin Hubble's
measurement of the great distance to the nearest
spiral nebulae showed that these systems were
indeed other galaxies. Independently deriving
Friedmann's equations in 1927, Georges Lemaître,
a Belgian physicist, proposed that the inferred
recession of the nebulae was due to the expansion
of the universe.In 1931 Lemaître went further
and suggested that the evident expansion of
the universe, if projected back in time, meant
that the further in the past the smaller the
universe was, until at some finite time in
the past all the mass of the universe was
concentrated into a single point, a "primeval
atom" where and when the fabric of time and
space came into existence.Starting in 1924,
Hubble painstakingly developed a series of
distance indicators, the forerunner of the
cosmic distance ladder, using the 100-inch
(2.5 m) Hooker telescope at Mount Wilson Observatory.
This allowed him to estimate distances to
galaxies whose redshifts had already been
measured, mostly by Slipher. In 1929 Hubble
discovered a correlation between distance
and recession velocity—now known as Hubble's
law. Lemaître had already shown that this
was expected, given the cosmological principle.
In the 1920s and 1930s almost every major
cosmologist preferred an eternal steady state
universe, and several complained that the
beginning of time implied by the Big Bang
imported religious concepts into physics;
this objection was later repeated by supporters
of the steady state theory. This perception
was enhanced by the fact that the originator
of the Big Bang theory, Georges Lemaître,
was a Roman Catholic priest. Arthur Eddington
agreed with Aristotle that the universe did
not have a beginning in time, viz., that matter
is eternal. A beginning in time was "repugnant"
to him. Lemaître, however, thought thatIf
the world has begun with a single quantum,
the notions of space and time would altogether
fail to have any meaning at the beginning;
they would only begin to have a sensible meaning
when the original quantum had been divided
into a sufficient number of quanta. If this
suggestion is correct, the beginning of the
world happened a little before the beginning
of space and time.
During the 1930s other ideas were proposed
as non-standard cosmologies to explain Hubble's
observations, including the Milne model, the
oscillatory universe (originally suggested
by Friedmann, but advocated by Albert Einstein
and Richard Tolman) and Fritz Zwicky's tired
light hypothesis.After World War II, two distinct
possibilities emerged. One was Fred Hoyle's
steady state model, whereby new matter would
be created as the universe seemed to expand.
In this model the universe is roughly the
same at any point in time. The other was Lemaître's
Big Bang theory, advocated and developed by
George Gamow, who introduced big bang nucleosynthesis
(BBN) and whose associates, Ralph Alpher and
Robert Herman, predicted the CMB. Ironically,
it was Hoyle who coined the phrase that came
to be applied to Lemaître's theory, referring
to it as "this big bang idea" during a BBC
Radio broadcast in March 1949. For a while,
support was split between these two theories.
Eventually, the observational evidence, most
notably from radio source counts, began to
favor Big Bang over Steady State. The discovery
and confirmation of the CMB in 1964 secured
the Big Bang as the best theory of the origin
and evolution of the universe. Much of the
current work in cosmology includes understanding
how galaxies form in the context of the Big
Bang, understanding the physics of the universe
at earlier and earlier times, and reconciling
observations with the basic theory.
In 1968 and 1970 Roger Penrose, Stephen Hawking,
and George F. R. Ellis published papers where
they showed that mathematical singularities
were an inevitable initial condition of general
relativistic models of the Big Bang. Then,
from the 1970s to the 1990s, cosmologists
worked on characterizing the features of the
Big Bang universe and resolving outstanding
problems. In 1981, Alan Guth made a breakthrough
in theoretical work on resolving certain outstanding
theoretical problems in the Big Bang theory
with the introduction of an epoch of rapid
expansion in the early universe he called
"inflation". Meanwhile, during these decades,
two questions in observational cosmology that
generated much discussion and disagreement
were over the precise values of the Hubble
Constant and the matter-density of the universe
(before the discovery of dark energy, thought
to be the key predictor for the eventual fate
of the universe).In the mid-1990s, observations
of certain globular clusters appeared to indicate
that they were about 15 billion years old,
which conflicted with most then-current estimates
of the age of the universe (and indeed with
the age measured today). This issue was later
resolved when new computer simulations, which
included the effects of mass loss due to stellar
winds, indicated a much younger age for globular
clusters. While there still remain some questions
as to how accurately the ages of the clusters
are measured, globular clusters are of interest
to cosmology as some of the oldest objects
in the universe.
Significant progress in Big Bang cosmology
has been made since the late 1990s as a result
of advances in telescope technology as well
as the analysis of data from satellites such
as COBE, the Hubble Space Telescope and WMAP.
Cosmologists now have fairly precise and accurate
measurements of many of the parameters of
the Big Bang model, and have made the unexpected
discovery that the expansion of the universe
appears to be accelerating.
== Observational evidence ==
The earliest and most direct observational
evidence of the validity of the theory are
the expansion of the universe according to
Hubble's law (as indicated by the redshifts
of galaxies), discovery and measurement of
the cosmic microwave background and the relative
abundances of light elements produced by Big
Bang nucleosynthesis. More recent evidence
includes observations of galaxy formation
and evolution, and the distribution of large-scale
cosmic structures, These are sometimes called
the "four pillars" of the Big Bang theory.Precise
modern models of the Big Bang appeal to various
exotic physical phenomena that have not been
observed in terrestrial laboratory experiments
or incorporated into the Standard Model of
particle physics. Of these features, dark
matter is currently subjected to the most
active laboratory investigations. Remaining
issues include the cuspy halo problem and
the dwarf galaxy problem of cold dark matter.
Dark energy is also an area of intense interest
for scientists, but it is not clear whether
direct detection of dark energy will be possible.
Inflation and baryogenesis remain more speculative
features of current Big Bang models. Viable,
quantitative explanations for such phenomena
are still being sought. These are currently
unsolved problems in physics.
=== Hubble's law and the expansion of space
===
Observations of distant galaxies and quasars
show that these objects are redshifted—the
light emitted from them has been shifted to
longer wavelengths. This can be seen by taking
a frequency spectrum of an object and matching
the spectroscopic pattern of emission lines
or absorption lines corresponding to atoms
of the chemical elements interacting with
the light. These redshifts are uniformly isotropic,
distributed evenly among the observed objects
in all directions. If the redshift is interpreted
as a Doppler shift, the recessional velocity
of the object can be calculated. For some
galaxies, it is possible to estimate distances
via the cosmic distance ladder. When the recessional
velocities are plotted against these distances,
a linear relationship known as Hubble's law
is observed:
v
=
H
0
D
{\displaystyle v=H_{0}D}
where
v
{\displaystyle v}
is the recessional velocity of the galaxy
or other distant object,
D
{\displaystyle D}
is the comoving distance to the object, and
H
0
{\displaystyle H_{0}}
is Hubble's constant, measured to be 70.4+1.3−1.4
km/s/Mpc by the WMAP probe.Hubble's law has
two possible explanations. Either we are at
the center of an explosion of galaxies—which
is untenable given the Copernican principle—or
the universe is uniformly expanding everywhere.
This universal expansion was predicted from
general relativity by Alexander Friedmann
in 1922 and Georges Lemaître in 1927, well
before Hubble made his 1929 analysis and observations,
and it remains the cornerstone of the Big
Bang theory as developed by Friedmann, Lemaître,
Robertson, and Walker.
The theory requires the relation
v
=
H
D
{\displaystyle v=HD}
to hold at all times, where
D
{\displaystyle D}
is the comoving distance, v is the recessional
velocity, and
v
{\displaystyle v}
,
H
{\displaystyle H}
, and
D
{\displaystyle D}
vary as the universe expands (hence we write
H
0
{\displaystyle H_{0}}
to denote the present-day Hubble "constant").
For distances much smaller than the size of
the observable universe, the Hubble redshift
can be thought of as the Doppler shift corresponding
to the recession velocity
v
{\displaystyle v}
. However, the redshift is not a true Doppler
shift, but rather the result of the expansion
of the universe between the time the light
was emitted and the time that it was detected.That
space is undergoing metric expansion is shown
by direct observational evidence of the Cosmological
principle and the Copernican principle, which
together with Hubble's law have no other explanation.
Astronomical redshifts are extremely isotropic
and homogeneous, supporting the Cosmological
principle that the universe looks the same
in all directions, along with much other evidence.
If the redshifts were the result of an explosion
from a center distant from us, they would
not be so similar in different directions.
Measurements of the effects of the cosmic
microwave background radiation on the dynamics
of distant astrophysical systems in 2000 proved
the Copernican principle, that, on a cosmological
scale, the Earth is not in a central position.
Radiation from the Big Bang was demonstrably
warmer at earlier times throughout the universe.
Uniform cooling of the CMB over billions of
years is explainable only if the universe
is experiencing a metric expansion, and excludes
the possibility that we are near the unique
center of an explosion.
=== Cosmic microwave background radiation
===
In 1964 Arno Penzias and Robert Wilson serendipitously
discovered the cosmic background radiation,
an omnidirectional signal in the microwave
band. Their discovery provided substantial
confirmation of the big-bang predictions by
Alpher, Herman and Gamow around 1950. Through
the 1970s the radiation was found to be approximately
consistent with a black body spectrum in all
directions; this spectrum has been redshifted
by the expansion of the universe, and today
corresponds to approximately 2.725 K. This
tipped the balance of evidence in favor of
the Big Bang model, and Penzias and Wilson
were awarded a Nobel Prize in 1978.
The surface of last scattering corresponding
to emission of the CMB occurs shortly after
recombination, the epoch when neutral hydrogen
becomes stable. Prior to this, the universe
comprised a hot dense photon-baryon plasma
sea where photons were quickly scattered from
free charged particles. Peaking at around
372±14 kyr, the mean free path for a photon
becomes long enough to reach the present day
and the universe becomes transparent.
In 1989, NASA launched the Cosmic Background
Explorer satellite (COBE), which made two
major advances: in 1990, high-precision spectrum
measurements showed that the CMB frequency
spectrum is an almost perfect blackbody with
no deviations at a level of 1 part in 104,
and measured a residual temperature of 2.726
K (more recent measurements have revised this
figure down slightly to 2.7255 K); then in
1992, further COBE measurements discovered
tiny fluctuations (anisotropies) in the CMB
temperature across the sky, at a level of
about one part in 105. John C. Mather and
George Smoot were awarded the 2006 Nobel Prize
in Physics for their leadership in these results.
During the following decade, CMB anisotropies
were further investigated by a large number
of ground-based and balloon experiments. In
2000–2001 several experiments, most notably
BOOMERanG, found the shape of the universe
to be spatially almost flat by measuring the
typical angular size (the size on the sky)
of the anisotropies.In early 2003, the first
results of the Wilkinson Microwave Anisotropy
Probe (WMAP) were released, yielding what
were at the time the most accurate values
for some of the cosmological parameters. The
results disproved several specific cosmic
inflation models, but are consistent with
the inflation theory in general. The Planck
space probe was launched in May 2009. Other
ground and balloon based cosmic microwave
background experiments are ongoing.
=== Abundance of primordial elements ===
Using the Big Bang model it is possible to
calculate the concentration of helium-4, helium-3,
deuterium, and lithium-7 in the universe as
ratios to the amount of ordinary hydrogen.
The relative abundances depend on a single
parameter, the ratio of photons to baryons.
This value can be calculated independently
from the detailed structure of CMB fluctuations.
The ratios predicted (by mass, not by number)
are about 0.25 for
He
4
/
H
{\displaystyle {\ce {^4He/H}}}
, about 10−3 for
H
2
/
H
{\displaystyle {\ce {^2H/H}}}
, about 10−4 for
He
3
/
H
{\displaystyle {\ce {^3He/H}}}
and about 10−9 for
Li
7
/
H
{\displaystyle {\ce {^7Li/H}}}
.The measured abundances all agree at least
roughly with those predicted from a single
value of the baryon-to-photon ratio. The agreement
is excellent for deuterium, close but formally
discrepant for
He
4
{\displaystyle {\ce {^4He}}}
, and off by a factor of two for
Li
7
{\displaystyle {\ce {^7Li}}}
; in the latter two cases there are substantial
systematic uncertainties. Nonetheless, the
general consistency with abundances predicted
by Big Bang nucleosynthesis is strong evidence
for the Big Bang, as the theory is the only
known explanation for the relative abundances
of light elements, and it is virtually impossible
to "tune" the Big Bang to produce much more
or less than 20–30% helium. Indeed, there
is no obvious reason outside of the Big Bang
that, for example, the young universe (i.e.,
before star formation, as determined by studying
matter supposedly free of stellar nucleosynthesis
products) should have more helium than deuterium
or more deuterium than
He
3
{\displaystyle {\ce {^3He}}}
, and in constant ratios, too.
=== Galactic evolution and distribution ===
Detailed observations of the morphology and
distribution of galaxies and quasars are in
agreement with the current state of the Big
Bang theory. A combination of observations
and theory suggest that the first quasars
and galaxies formed about a billion years
after the Big Bang, and since then, larger
structures have been forming, such as galaxy
clusters and superclusters.Populations of
stars have been aging and evolving, so that
distant galaxies (which are observed as they
were in the early universe) appear very different
from nearby galaxies (observed in a more recent
state). Moreover, galaxies that formed relatively
recently, appear markedly different from galaxies
formed at similar distances but shortly after
the Big Bang. These observations are strong
arguments against the steady-state model.
Observations of star formation, galaxy and
quasar distributions and larger structures,
agree well with Big Bang simulations of the
formation of structure in the universe, and
are helping to complete details of the theory.
=== Primordial gas clouds ===
In 2011, astronomers found what they believe
to be pristine clouds of primordial gas by
analyzing absorption lines in the spectra
of distant quasars. Before this discovery,
all other astronomical objects have been observed
to contain heavy elements that are formed
in stars. These two clouds of gas contain
no elements heavier than hydrogen and deuterium.
Since the clouds of gas have no heavy elements,
they likely formed in the first few minutes
after the Big Bang, during Big Bang nucleosynthesis.
=== Other lines of evidence ===
The age of the universe as estimated from
the Hubble expansion and the CMB is now in
good agreement with other estimates using
the ages of the oldest stars, both as measured
by applying the theory of stellar evolution
to globular clusters and through radiometric
dating of individual Population II stars.The
prediction that the CMB temperature was higher
in the past has been experimentally supported
by observations of very low temperature absorption
lines in gas clouds at high redshift. This
prediction also implies that the amplitude
of the Sunyaev–Zel'dovich effect in clusters
of galaxies does not depend directly on redshift.
Observations have found this to be roughly
true, but this effect depends on cluster properties
that do change with cosmic time, making precise
measurements difficult.
=== Future observations ===
Future gravitational waves observatories might
be able to detect primordial gravitational
waves, relics of the early universe, up to
less than a second after the Big Bang.
== Problems and related issues in physics
==
As with any theory, a number of mysteries
and problems have arisen as a result of the
development of the Big Bang theory. Some of
these mysteries and problems have been resolved
while others are still outstanding. Proposed
solutions to some of the problems in the Big
Bang model have revealed new mysteries of
their own. For example, the horizon problem,
the magnetic monopole problem, and the flatness
problem are most commonly resolved with inflationary
theory, but the details of the inflationary
universe are still left unresolved and many,
including some founders of the theory, say
it has been disproven. What follows are a
list of the mysterious aspects of the Big
Bang theory still under intense investigation
by cosmologists 
and astrophysicists.
=== Baryon asymmetry ===
It is not yet understood why the universe
has more matter than antimatter. It is generally
assumed that when the universe was young and
very hot it was in statistical equilibrium
and contained equal numbers of baryons and
antibaryons. However, observations suggest
that the universe, including its most distant
parts, is made almost entirely of matter.
A process called baryogenesis was hypothesized
to account for the asymmetry. For baryogenesis
to occur, the Sakharov conditions must be
satisfied. These require that baryon number
is not conserved, that C-symmetry and CP-symmetry
are violated and that the universe depart
from thermodynamic equilibrium. All these
conditions occur in the Standard Model, but
the effects are not strong enough to explain
the present baryon asymmetry.
=== Dark energy ===
Measurements of the redshift–magnitude relation
for type Ia supernovae indicate that the expansion
of the universe has been accelerating since
the universe was about half its present age.
To explain this acceleration, general relativity
requires that much of the energy in the universe
consists of a component with large negative
pressure, dubbed "dark energy".Dark energy,
though speculative, solves numerous problems.
Measurements of the cosmic microwave background
indicate that the universe is very nearly
spatially flat, and therefore according to
general relativity the universe must have
almost exactly the critical density of mass/energy.
But the mass density of the universe can be
measured from its gravitational clustering,
and is found to have only about 30% of the
critical density. Since theory suggests that
dark energy does not cluster in the usual
way it is the best explanation for the "missing"
energy density. Dark energy also helps to
explain two geometrical measures of the overall
curvature of the universe, one using the frequency
of gravitational lenses, and the other using
the characteristic pattern of the large-scale
structure as a cosmic ruler.
Negative pressure is believed to be a property
of vacuum energy, but the exact nature and
existence of dark energy remains one of the
great mysteries of the Big Bang. Results from
the WMAP team in 2008 are in accordance with
a universe that consists of 73% dark energy,
23% dark matter, 4.6% regular matter and less
than 1% neutrinos. According to theory, the
energy density in matter decreases with the
expansion of the universe, but the dark energy
density remains constant (or nearly so) as
the universe expands. Therefore, matter made
up a larger fraction of the total energy of
the universe in the past than it does today,
but its fractional contribution will fall
in the far future as dark energy becomes even
more dominant.
The dark energy component of the universe
has been explained by theorists using a variety
of competing theories including Einstein's
cosmological constant but also extending to
more exotic forms of quintessence or other
modified gravity schemes. A cosmological constant
problem, sometimes called the "most embarrassing
problem in physics", results from the apparent
discrepancy between the measured energy density
of dark energy, and the one naively predicted
from Planck units.
=== Dark matter ===
During the 1970s and the 1980s, various observations
showed that there is not sufficient visible
matter in the universe to account for the
apparent strength of gravitational forces
within and between galaxies. This led to the
idea that up to 90% of the matter in the universe
is dark matter that does not emit light or
interact with normal baryonic matter. In addition,
the assumption that the universe is mostly
normal matter led to predictions that were
strongly inconsistent with observations. In
particular, the universe today is far more
lumpy and contains far less deuterium than
can be accounted for without dark matter.
While dark matter has always been controversial,
it is inferred by various observations: the
anisotropies in the CMB, galaxy cluster velocity
dispersions, large-scale structure distributions,
gravitational lensing studies, and X-ray measurements
of galaxy clusters.Indirect evidence for dark
matter comes from its gravitational influence
on other matter, as no dark matter particles
have been observed in laboratories. Many particle
physics candidates for dark matter have been
proposed, and several projects to detect them
directly are underway.Additionally, there
are outstanding problems associated with the
currently favored cold dark matter model which
include the dwarf galaxy problem and the cuspy
halo problem. Alternative theories have been
proposed that do not require a large amount
of undetected matter, but instead modify the
laws of gravity established by Newton and
Einstein; yet no alternative theory has been
as successful as the cold dark matter proposal
in explaining all extant observations.
=== Horizon problem ===
The horizon problem results from the premise
that information cannot travel faster than
light. In a universe of finite age this sets
a limit—the particle horizon—on the separation
of any two regions of space that are in causal
contact. The observed isotropy of the CMB
is problematic in this regard: if the universe
had been dominated by radiation or matter
at all times up to the epoch of last scattering,
the particle horizon at that time would correspond
to about 2 degrees on the sky. There would
then be no mechanism to cause wider regions
to have the same temperature.A resolution
to this apparent inconsistency is offered
by inflationary theory in which a homogeneous
and isotropic scalar energy field dominates
the universe at some very early period (before
baryogenesis). During inflation, the universe
undergoes exponential expansion, and the particle
horizon expands much more rapidly than previously
assumed, so that regions presently on opposite
sides of the observable universe are well
inside each other's particle horizon. The
observed isotropy of the CMB then follows
from the fact that this larger region was
in causal contact before the beginning of
inflation.Heisenberg's uncertainty principle
predicts that during the inflationary phase
there would be quantum thermal fluctuations,
which would be magnified to cosmic scale.
These fluctuations serve as the seeds of all
current structure in the universe. Inflation
predicts that the primordial fluctuations
are nearly scale invariant and Gaussian, which
has been accurately confirmed by measurements
of the CMB.If inflation occurred, exponential
expansion would push large regions of space
well beyond our observable horizon.A related
issue to the classic horizon problem arises
because in most standard cosmological inflation
models, inflation ceases well before electroweak
symmetry breaking occurs, so inflation should
not be able to prevent large-scale discontinuities
in the electroweak vacuum since distant parts
of the observable universe were causally separate
when the electroweak epoch ended.
=== Magnetic monopoles ===
The magnetic monopole objection was raised
in the late 1970s. Grand unified theories
predicted topological defects in space that
would manifest as magnetic monopoles. These
objects would be produced efficiently in the
hot early universe, resulting in a density
much higher than is consistent with observations,
given that no monopoles have been found. This
problem is also resolved by cosmic inflation,
which removes all point defects from the observable
universe, in the same way that it drives the
geometry to flatness.
=== Flatness problem ===
The flatness problem (also known as the oldness
problem) is an observational problem associated
with a Friedmann–Lemaître–Robertson–Walker
metric (FLRW). The universe may have positive,
negative, or zero spatial curvature depending
on its total energy density. Curvature is
negative if its density is less than the critical
density; positive if greater; and zero at
the critical density, in which case space
is said to be flat.
The problem is that any small departure from
the critical density grows with time, and
yet the universe today remains very close
to flat. Given that a natural timescale for
departure from flatness might be the Planck
time, 10−43 seconds, the fact that the universe
has reached neither a heat death nor a Big
Crunch after billions of years requires an
explanation. For instance, even at the relatively
late age of a few minutes (the time of nucleosynthesis),
the density of the universe must have been
within one part in 1014 of its critical value,
or it would not exist as it does today.
== Cause ==
Physics may conclude that time did not exist
before 'Big Bang', but 'started' with the
Big Bang and hence there might be no 'beginning',
'before' or potentially 'cause' and instead
always existed. Quantum fluctuations, or other
laws of physics that may have existed at the
start of the Big Bang could then create the
conditions for matter to occur.
== Ultimate fate of the universe ==
Before observations of dark energy, cosmologists
considered two scenarios for the future of
the universe. If the mass density of the universe
were greater than the critical density, then
the universe would reach a maximum size and
then begin to collapse. It would become denser
and hotter again, ending with a state similar
to that in which it started—a Big Crunch.Alternatively,
if the density in the universe were equal
to or below the critical density, the expansion
would slow down but never stop. Star formation
would cease with the consumption of interstellar
gas in each galaxy; stars would burn out,
leaving white dwarfs, neutron stars, and black
holes. Very gradually, collisions between
these would result in mass accumulating into
larger and larger black holes. The average
temperature of the universe would asymptotically
approach absolute zero—a Big Freeze. Moreover,
if the proton were unstable, then baryonic
matter would disappear, leaving only radiation
and black holes. Eventually, black holes would
evaporate by emitting Hawking radiation. The
entropy of the universe would increase to
the point where no organized form of energy
could be extracted from it, a scenario known
as heat death.Modern observations of accelerating
expansion imply that more and more of the
currently visible universe will pass beyond
our event horizon and out of contact with
us. The eventual result is not known. The
ΛCDM model of the universe contains dark
energy in the form of a cosmological constant.
This theory suggests that only gravitationally
bound systems, such as galaxies, will remain
together, and they too will be subject to
heat death as the universe expands and cools.
Other explanations of dark energy, called
phantom energy theories, suggest that ultimately
galaxy clusters, stars, planets, atoms, nuclei,
and matter itself will be torn apart by the
ever-increasing expansion in a so-called Big
Rip.
== Misconceptions ==
The following is a partial list of misconceptions
about the Big Bang model:
The Big Bang as the origin of the universe:
One of the common misconceptions about the
Big Bang model is the belief that it was the
origin of the universe. However, the Big Bang
model does not comment about how the universe
came into being. Current conception of the
Big Bang model assumes the existence of energy,
time, and space, and does not comment about
their origin or the cause of the dense and
high temperature initial state of the universe.The
Big Bang was "small": It is misleading to
visualize the Big Bang by comparing its size
to everyday objects. When the size of the
universe at Big Bang is described, it refers
to the size of the observable universe, and
not the entire universe.Hubble's law violates
the special theory of relativity: Hubble's
law predicts that galaxies that are beyond
Hubble Distance recede faster than the speed
of light. However, special relativity does
not apply beyond motion through space. Hubble's
law describes velocity that results from expansion
of space, rather than through space.Doppler
redshift vs cosmological red-shift: Astronomers
often refer to the cosmological red-shift
as a normal Doppler shift, which is a misconception.
Although similar, the cosmological red-shift
is not identical to the Doppler redshift.
The Doppler redshift is based on special relativity,
which does not consider the expansion of space.
On the contrary, the cosmological red-shift
is based on general relativity, in which the
expansion of space is considered. Although
they may appear identical for nearby galaxies,
it may cause confusion if the behavior of
distant galaxies is understood through the
Doppler redshift.
== Speculations ==
While the Big Bang model is well established
in cosmology, it is likely to be refined.
The Big Bang theory, built upon the equations
of classical general relativity, indicates
a singularity at the origin of cosmic time;
this infinite energy density is regarded as
impossible in physics. Still, it is known
that the equations are not applicable before
the time when the universe cooled down to
the Planck temperature, and this conclusion
depends on various assumptions, of which some
could never be experimentally verified. (Also
see Planck epoch.)
One proposed refinement to avoid this would-be
singularity is to develop a correct treatment
of quantum gravity.It is not known what could
have preceded the hot dense state of the early
universe or how and why it originated, though
speculation abounds in the field of cosmogony.
Some proposals, each of which entails untested
hypotheses, are:
Models including the Hartle–Hawking no-boundary
condition, in which the whole of space-time
is finite; the Big Bang does represent the
limit of time but without any singularity.
Big Bang lattice model, states that the universe
at the moment of the Big Bang consists of
an infinite lattice of fermions, which is
smeared over the fundamental domain so it
has rotational, translational and gauge symmetry.
The symmetry is the largest symmetry possible
and hence the lowest entropy of any state.
Brane cosmology models, in which inflation
is due to the movement of branes in string
theory; the pre-Big Bang model; the ekpyrotic
model, in which the Big Bang is the result
of a collision between branes; and the cyclic
model, a variant of the ekpyrotic model in
which collisions occur periodically. In the
latter model the Big Bang was preceded by
a Big Crunch and the universe cycles from
one process to the other.
Eternal inflation, in which universal inflation
ends locally here and there in a random fashion,
each end-point leading to a bubble universe,
expanding from its own big bang.Proposals
in the last two categories see the Big Bang
as an event in either a much larger and older
universe or in 
a multiverse.
== Religious and philosophical interpretations
==
As a description of the origin of the universe,
the Big Bang has significant bearing on religion
and philosophy. As a result, it has become
one of the liveliest areas in the discourse
between science and religion. Some believe
the Big Bang implies a creator, and some see
its mention in their holy books, while others
argue that Big Bang cosmology makes the notion
of 
a creator superfluous.
== See also ==
Big Bounce – A hypothetical cosmological
model for the origin of the known universe
Big Crunch – Theoretical scenario for the
ultimate fate of the universe
Cold Big Bang – A designation of an absolute
zero temperature at the beginning of the Universe
Cosmic Calendar
Eureka: A Prose Poem – A lengthy non-fiction
work by American author Edgar Allan Poe, Edgar
Allan Poe's Big Bang speculation
Shape of the universe – The local and global
geometry of the universe
== Notes
