The chronology of the universe describes the
history and future of the universe according
to Big Bang cosmology. The earliest stages
of the universe's existence are estimated
as taking place 13.8 billion years ago, with
an uncertainty of around 21 million years
at the 68% confidence level.
== Outline ==
For the purposes of this summary, it is convenient
to divide the chronology of the universe since
it originated, into five parts. It is generally
considered meaningless or unclear whether
time existed before this chronology:
1. The very early universe – the first picosecond
(10−12) of cosmic time. It includes the
Planck epoch, during which currently understood
laws of physics may not apply; the emergence
in stages of the four known fundamental interactions
or forces – first gravity, and later the
strong, weak and electromagnetic interactions;
and the expansion of space and supercooling
of the still immensely hot universe due to
cosmic inflation, which is believed to have
been triggered by the separation of the strong
and electroweak interaction.Tiny ripples in
the universe at this stage are believed to
be the basis of large-scale structures that
formed much later. Different stages of the
very early universe are understood to different
extents. The earlier parts are beyond the
grasp of practical experiments in particle
physics but can be explored through other
means.2. The early universe, lasting around
377,000 years. Initially, various kinds of
subatomic particles are formed in stages.
These particles include almost equal amounts
of matter and antimatter, so most of it quickly
annihilates, leaving a small excess of matter
in the universe.At about one second, neutrinos
decouple; these neutrinos form the cosmic
neutrino background. If primordial black holes
exist, they are also formed at about one second
of cosmic time. Composite subatomic particles
emerge – including protons and neutrons
– and from about 3 minutes, conditions are
suitable for nucleosynthesis: around 25% of
the protons and all the neutrons fuse into
heavier elements, mainly helium-4.By 20 minutes,
the universe is no longer hot enough for fusion,
but far too hot for neutral atoms to exist
or photons to travel far. It is therefore
an opaque plasma. At around 47,000 years,
as the universe cools, its behavior begins
to be dominated by matter rather than radiation.At
about 377,000 years, the universe finally
becomes cool enough for neutral atoms to form
("recombination"), and as a result it also
became transparent for the first time. The
newly formed atoms – mainly hydrogen and
helium with traces of lithium – quickly
reach their lowest energy state (ground state)
by releasing photons ("photon decoupling"),
and these photons can still be detected today
as the cosmic microwave background (CMB).
This is currently the oldest observation we
have of the universe.3. Dark Ages and large-scale
structure emergence, from 377,000 years until
about 1 billion years. After recombination
and decoupling, the universe was transparent
but the clouds of hydrogen only collapsed
very slowly to form stars and galaxies, so
there were no new sources of light. The only
photons (electromagnetic radiation, or "light")
in the universe were those released during
decoupling (visible today as the cosmic microwave
background) and 21 cm radio emissions occasionally
emitted by hydrogen atoms. The decoupled photons
would have filled the universe with a brilliant
pale orange glow at first, gradually redshifting
to non-visible wavelengths after about 3 million
years, leaving it without visible light. This
period is known as the Dark Ages.Between about
10 and 17 million years the universe's average
temperature was suitable for liquid water
(273 – 373K) and there has been speculation
whether rocky planets or indeed life could
have arisen briefly, since statistically a
tiny part of the universe could have had different
conditions from the rest, and gained warmth
from the universe as a whole.At some point
around 400 to 700 million years, the earliest
generations of stars and galaxies form, and
early large structures gradually emerge, drawn
to the foam-like dark matter filaments which
have already begun to draw together throughout
the universe. The earliest generations of
stars have not yet been observed astronomically.
They may have been huge and non-metallic with
very short lifetimes compared to most stars
we see today, so they commonly finish burning
their hydrogen fuel and explode as supernovae
after mere millions of years. Other theories
suggest that they may have included small
stars, some perhaps still burning today. In
either case, these early generations of supernovae
created most of the everyday elements we see
around us today, and seeded the universe with
them.Galaxy clusters and superclusters emerge
over time. At some point, high energy photons
from the earliest stars, dwarf galaxies and
perhaps quasars led to a period of reionization.
The universe gradually transitioned into the
universe we see around us today, and the Dark
Ages only fully came to an end at about 1
billion years.4. The universe as it appears
today. From 1 billion years, and for about
12.8 billions of years, the universe has looked
much as it does today. It will continue to
appear very similar for many billions of years
into the future. The thin disk of our galaxy
began to form at about 5 billion years (8.8
bn years ago), and the solar system formed
at about 9.2 billion years (4.6 bn years ago),
with the earliest traces of life on Earth
emerging by about 10.3 billion years (3.5
bn years ago).From about 9.8 billion years
of cosmic time, the slowing expansion of space
gradually begins to accelerate under the influence
of dark energy, which may be a scalar field
throughout our universe. The present-day universe
is understood quite well, but beyond about
100 billion years of cosmic time (about 86
billion years in the future), uncertainties
in current knowledge mean that we are less
sure which path our universe will take.5.
The far future. At some time the Stelliferous
Era will end as stars are no longer being
born, and the expansion of the universe will
mean that the observable universe becomes
limited to local galaxies. There are various
scenarios for the far future and ultimate
fate of the universe. More exact knowledge
of our current universe will allow these to
be better understood.
== A more detailed summary ==
Earliest stages of chronology shown below
(before neutrino decoupling) are an active
area of research and based on ideas which
are still speculative and subject to modification
as scientific knowledge improves.
"Time" column is based on extrapolation of
observed metric expansion of space back in
the past. For the earliest stages of chronology
this extrapolation may be invalid. To give
one example, eternal inflation theories propose
that inflation lasts forever throughout most
of the universe, making the notion of "N seconds
since Big Bang" ill-defined.
The radiation temperature refers to the cosmic
background radiation and is given by 2.725·(1+z),
where z is the redshift.
== Very early universe ==
=== Planck epoch ===
Times shorter than 10−43 seconds (Planck
time)
The Planck epoch is an era in traditional
(non-inflationary) Big Bang cosmology immediately
after the event which began our known universe.
During this epoch, the temperature and average
energies within the universe were so inconceivably
high compared to any temperature we can observe
today, that everyday subatomic particles could
not form, and even the four fundamental forces
that shape our universe—electromagnetism,
gravitation, weak nuclear interaction, and
strong nuclear interaction—were combined
and formed one fundamental force. Little is
understood about physics at this temperature;
different hypotheses propose different scenarios.
Traditional big bang cosmology predicts a
gravitational singularity before this time,
but this theory relies on the theory of general
relativity, which is thought to break down
for this epoch due to quantum effects.
In inflationary models of cosmology, times
before the end of inflation (roughly 10−32
second after the Big Bang) do not follow the
same timeline as in traditional big bang cosmology.
Models that aim to describe the universe and
physics during the Planck epoch are generally
speculative and fall under the umbrella of
"New Physics". Examples include the Hartle–Hawking
initial state, string landscape, string gas
cosmology, and the ekpyrotic universe.
=== Grand unification epoch ===
Between 10−43 seconds and 10−36 seconds
after the Big Bang
As the universe expanded and cooled, it crossed
transition temperatures at which forces separated
from each other. These phase transitions can
be visualised as similar to condensation and
freezing phase transitions of ordinary matter.
At certain temperatures/energies, water molecules
change their behaviour and structure, and
they will behave completely differently. Like
steam turning to water, the fields which define
our universe's fundamental forces and particles
also completely change their behaviors and
structures when the temperature/energy falls
below a certain point. This is not apparent
in everyday life, because it only happens
at much, much, higher temperatures than we
usually see in our present universe.
These phase transitions are believed to be
caused by a phenomenon of quantum fields called
"symmetry breaking".
In everyday terms, as the universe cools,
it becomes possible for the quantum fields
that create the forces and particles around
us, to settle at lower energy levels and with
higher levels of stability. In doing so, they
completely shift how they interact. Forces
and interactions arise due to these fields,
so the universe can behave very differently
above and below a phase transition. For example,
in a later epoch, a side effect of one phase
transition is that suddenly, many particles
that had no mass at all acquire a mass (they
begin to interact with the Higgs boson), and
a single force begins to manifest as two separate
forces.
The grand unification epoch began with a phase
transitions of this kind, when gravitation
separated from the universal combined gauge
force. This caused two forces to now exist:
gravity, and an electrostrong interaction.
There is no hard evidence yet, that such a
combined force existed, but many physicists
believe it did. The physics of this electrostrong
interaction would be described by a so-called
grand unified theory (GUT).
The grand unification epoch ended with a second
phase transition, as the electrostrong interaction
in turn separated, and began to manifest as
two separate interactions, called the strong
and electroweak interactions.
=== Electroweak epoch ===
Between 10−36 seconds (or the end of inflation)
and 10−32 seconds after the Big Bang
Depending on how epochs are defined, and the
model being followed, the electroweak epoch
may be considered to start before or after
the inflationary epoch. In some models it
is described as including the inflationary
epoch. In other models, the electroweak epoch
is said to begin after the inflationary epoch
ended, at roughly 10−32 seconds.
According to traditional big bang cosmology,
the electroweak epoch began 10−36 seconds
after the Big Bang, when the temperature of
the universe was low enough (1028 K) for the
Electronuclear Force to begin to manifest
as two separate interactions, called the strong
and the electroweak interactions. (The electroweak
interaction will also separate later, dividing
into the electromagnetic and weak interactions).
The exact point where electrostrong symmetry
was broken is not certain, because of the
very high energies of this event.
=== Inflationary epoch and the rapid expansion
of space ===
Before ca. 10−32 seconds after the Big Bang
At this point, the very early universe suddenly
and very rapidly expanded to at least 1078
times its previous volume (and possibly much
more). This is equivalent to a linear increase
of at least 1026 times in every spatial dimension
– equivalent to an object 1 nanometer (10−9
m, about half the width of a molecule of DNA)
in length, expanding to one approximately
10.6 light years (about 62 trillion miles)
long in a tiny fraction of a second. This
change is known as inflation.
Although light and objects within spacetime
cannot travel faster than the speed of light,
in this case it was the metric governing the
size and geometry of spacetime itself that
changed in scale. Changes to the metric are
not limited by the speed of light.
There is good evidence that inflation happened,
and it is widely accepted that it did take
place. But the exact reasons why it happened
are still being explored. So a range of models
exist that explain why and how it took place
- it is not yet clear which explanation is
correct.
In several of the more prominent models, it
is thought to have been triggered by the separation
of the strong and electroweak interactions
which ended the grand unification epoch. One
of the theoretical products of this phase
transition was a scalar field called the inflaton
field. As this field settled into its lowest
energy state throughout the universe, it generated
an enormous repulsive force that led to a
rapid expansion of space itself. Inflation
explains several observed properties of the
current universe that are otherwise difficult
to account for, including explaining how today's
universe has ended up so exceedingly homogeneous
(similar) on a very large scale, even though
it was highly disordered in its earliest stages.
It is not known exactly when the inflationary
epoch ended, but it is thought to have been
between 10−33 and 10−32 seconds after
the Big Bang. The rapid expansion of space
meant that elementary particles remaining
from the grand unification epoch were now
distributed very thinly across the universe.
However, the huge potential energy of the
inflation field was released at the end of
the inflationary epoch, as the inflaton field
decayed into other particles, known as "reheating".
This heating effect led to the universe being
repopulated with a dense, hot mixture of quarks,
anti-quarks and gluons. In other models, reheating
is often considered to mark the start of the
electroweak epoch, and some theories, such
as warm inflation, avoid a reheating phase
entirely.
In non-traditional versions of Big Bang theory
(known as "inflationary" models), inflation
ended at a temperature corresponding to roughly
10−32 second after the Big Bang, but this
does not imply that the inflationary era lasted
less than 10−32 second. To explain the observed
homogeneity of the universe, the duration
in these models must be longer than 10−32
second. Therefore, in inflationary cosmology,
the earliest meaningful time "after the Big
Bang" is the time of the end of inflation.
After inflation ended, the universe continued
to expand, but at a very slow rate. The slow
expansion began to speed up after several
billion years, believed to be due to dark
energy, and is still expanding today.
On March 17, 2014, astrophysicists of the
BICEP2 collaboration announced the detection
of inflationary gravitational waves in the
B-mode power spectrum which was interpreted
as clear experimental evidence for the theory
of inflation. However, on June 19, 2014, lowered
confidence in confirming the cosmic inflation
findings was reported and finally, on February
2, 2015, a joint analysis of data from BICEP2/Keck
and Planck satellite concluded that the statistical
"significance [of the data] is too low to
be interpreted as a detection of primordial
B-modes" and can be attributed mainly to polarized
dust in the Milky Way.
=== Electroweak symmetry breaking ===
10−12 seconds after the Big Bang
As the universe's temperature continued to
fall below a certain very high energy level,
a third symmetry breaking occurs. So far as
we currently know, it was the final symmetry
breaking event in the formation of our universe.
It is believed that below some energies unknown
yet, the Higgs field spontaneously acquires
a vacuum expectation value. When this happens,
it breaks electroweak gauge symmetry. This
has two related effects:
Via the Higgs mechanism, all elementary particles
interacting with the Higgs field become massive,
having been massless at higher energy levels.
As a side-effect, the weak force and electromagnetic
force, and their respective bosons (the W
and Z bosons and photon) now begin to manifest
differently in the present universe. Before
electroweak symmetry breaking these bosons
were all massless particles and interacted
over long distances, but at this point the
W and Z bosons abruptly become massive particles
only interacting over distances smaller than
the size of an atom, while the photon remains
massless and remains a long-distance interaction.After
electroweak symmetry breaking, the fundamental
interactions we know of – gravitation, electromagnetism,
the strong interaction and the weak interaction
– have all taken their present forms, and
fundamental particles have mass, but the temperature
of the universe is still too high to allow
the formation of many fundamental particles
we now see in the universe.
=== Supersymmetry breaking (speculative) ===
If supersymmetry is a property of our universe,
then it must be broken at an energy that is
no lower than 1 TeV, the electroweak scale.
The masses of particles and their superpartners
would then no longer be equal. This very high
energy could explain why no superpartners
of known particles have ever been observed.
== Early universe ==
After cosmic inflation ends, the universe
is filled with a hot quark–gluon plasma,
the remains of reheating. From this point
onwards the physics of the early universe
is much better understood, and the energies
involved in the Quark epoch are directly amenable
to experiment.
=== The quark epoch ===
Between 10−12 seconds and 10−6 seconds
after the Big Bang
The quark epoch began approximately 10−12
seconds after the Big Bang. This was the period
in the evolution of the early universe immediately
after electroweak symmetry breaking, when
the fundamental interactions of gravitation,
electromagnetism, the strong interaction and
the weak interaction had taken their present
forms, but the temperature of the universe
was still too high to allow quarks to bind
together to form hadrons.
During the quark epoch the universe was filled
with a dense, hot quark–gluon plasma, containing
quarks, leptons and their antiparticles. Collisions
between particles were too energetic to allow
quarks to combine into mesons or baryons.
The quark epoch ended when the universe was
about 10−6 seconds old, when the average
energy of particle interactions had fallen
below the binding energy of hadrons.
==== Baryogenesis ====
Perhaps by 10−11 secondsBaryons are subatomic
particles such as protons and neutrons, that
are composed of three quarks. It would be
expected that both baryons, and particles
known as antibaryons would have formed in
equal numbers. However, this does not seem
to be what happened – as far as we know,
the universe was left with far more baryons
than antibaryons. In fact, almost no antibaryons
are observed in nature. It is not clear how
this came about. Any explanation for this
phenomenon must allow the Sakharov conditions
related to baryogenesis to have been satisfied
at some time after the end of cosmological
inflation. Current particle physics suggests
asymmetries under which these conditions would
be met, but these asymmetries appear to be
too small to account for the observed baryon-antibaryon
asymmetry of the universe.
=== Hadron epoch ===
Between 10−6 second and 1 second after the
Big Bang
The quark–gluon plasma that composes the
universe cools until hadrons, including baryons
such as protons and neutrons, can form.
Initially, hadron/anti-hadron pairs could
form, so matter and anti-matter were in thermal
equilibrium. However, as the temperature of
the universe continued to fall, new hadron/anti-hadron
pairs were no longer produced, and most of
the newly formed hadrons and anti-hadrons
annihilated each other, giving rise to pairs
of high-energy photons. A comparatively small
residue of hadrons remained at about 1 second
of cosmic time, when this epoch ended.
Theory predicts that about 1 neutron remained
for every 7 protons. We believe this to be
correct because, at a later stage, all the
neutrons and some of the protons fused, leaving
hydrogen, a hydrogen isotope called deuterium,
helium and other elements, which we can measure.
A 1:7 ratio of hadrons at the end of this
epoch would indeed produce the observed element
ratios in the early as well as current universe.
=== Neutrino decoupling and cosmic neutrino
background ===
Around 1 second after the Big Bang
At approximately 1 second after the Big Bang
neutrinos decouple and begin traveling freely
through space. As neutrinos rarely interact
with matter, these neutrinos still exist today,
analogous to the much later cosmic microwave
background emitted during recombination, around
377,000 years after the Big Bang. The neutrinos
from this event have a very low energy, around
10−10 times smaller than is possible with
present-day direct detection. Even high energy
neutrinos are notoriously difficult to detect,
so this cosmic neutrino background (CNB) may
not be directly observed in detail for many
years, if at all.However, Big Bang cosmology
makes many predictions about the CNB, and
there is very strong indirect evidence that
the cosmic neutrino background exists, both
from Big Bang nucleosynthesis predictions
of the helium abundance, and from anisotropies
in the cosmic microwave background. One of
these predictions is that neutrinos will have
left a subtle imprint on the cosmic microwave
background (CMB). It is well known that the
CMB has irregularities. Some of the CMB fluctuations
were roughly regularly spaced, because of
the effect of baryonic acoustic oscillations.
In theory, the decoupled neutrinos should
have had a very slight effect on the phase
of the various CMB fluctuations.In 2015, it
was reported that such shifts had been detected
in the CMB. Moreover, the fluctuations corresponded
to neutrinos of almost exactly the temperature
predicted by Big Bang theory (1.96 +/-0.02K
compared to a prediction of 1.95K), and exactly
three types of neutrino, the same number of
neutrino flavours currently predicted by the
Standard Model.
=== Possible formation of primordial black
holes ===
May have occurred within about 1 second after
the Big BangPrimordial black holes are a hypothetical
type of black hole proposed in 1966, that
may have formed during the so-called radiation
dominated era, due to the high densities and
inhomogeneous conditions within the first
second of cosmic time. Random fluctuations
could lead to some regions becoming dense
enough to undergo gravitational collapse,
forming black holes. Current understandings
and theories place tight limits on the abundance
and mass of these objects.
Typically, primordial black hole formation
requires density contrasts (regional variations
in the Universe's density) of around
δ
ρ
/
ρ
∼
0.1
{\displaystyle \delta \rho /\rho \sim 0.1}
(10%), where
ρ
{\displaystyle \rho }
is the average density of the Universe. Several
mechanisms could produce dense regions meeting
this criterion during the early universe,
including reheating, cosmological phase transitions
and (in so-called "hybrid inflation models")
axion inflation. Since primordial black holes
didn't form from stellar gravitational collapse,
their masses can be far below stellar mass
(~2×1033 g). Stephen Hawking calculated
in 1971 that primordial black holes could
weigh as little as 10−5 g. But they can
have any size, so they could also be large,
and may have contributed to the formation
of galaxies.
=== Lepton epoch ===
Between 1 second and 10 seconds after the
Big Bang
The majority of hadrons and anti-hadrons annihilate
each other at the end of the hadron epoch,
leaving leptons (such as the electron, muons
and certain neutrinos) and anti-leptons, dominating
the mass of the universe.
The lepton epoch follows a similar path to
the earlier hadron epoch. Initially leptons
and anti-leptons are produced in pairs. About
10 seconds after the Big Bang the temperature
of the universe falls to the point at which
new lepton/anti-lepton pairs are no longer
created and most remaining leptons and anti-leptons
quickly annihilate each other, giving rise
to pairs of high energy photons, and leaving
a small residue of non-annihilated leptons.
=== Photon epoch ===
Between 10 seconds and 377,000 years after
the Big Bang
After most leptons and anti-leptons are annihilated
at the end of the lepton epoch, most of the
mass-energy in the universe is left in the
form of photons. (Much of the rest of its
mass-energy is in the form of neutrinos and
other relativistic particles). Therefore the
energy of the universe, and its overall behavior,
is dominated by its photons. These photons
continue to interact frequently with charged
protons, electrons and (eventually) nuclei.
They continue to do so for about the next
377,000 years.
=== Nucleosynthesis of light elements ===
Between 2 minutes and 20 minutes after the
Big Bang
Between about 2 and 20 minutes after the Big
Bang, the temperature and pressure of the
universe allow nuclear fusion to occur, giving
rise to nuclei of a few light elements beyond
hydrogen ("Big Bang nucleosynthesis"). About
25% of the protons, and all the neutrons fuse
to form deuterium, a hydrogen isotope, and
most of the deuterium quickly fuses to form
helium-4.
Atomic nuclei will easily unbind (break apart)
above a certain temperature, related to their
binding energy. From about 2 minutes, the
falling temperature means that deuterium no
longer unbinds, and is stable, and starting
from about 3 minutes, helium and other elements
formed by the fusion of deuterium also no
longer unbind and are stable.The short duration
and falling temperature means that only the
simplest and fastest fusion processes can
occur. Only tiny amounts of nuclei beyond
helium are formed, because nucleosynthesis
of heavier elements is difficult and requires
thousands of years even in stars. Small amounts
of tritium (another hydrogen isotope) and
beryllium-7 and -8 are formed, but these are
unstable and are quickly lost again. A small
amount of deuterium is left unfused because
of the very short duration.Therefore, the
only stable nuclides created by the end of
Big Bang nucleosynthesis are protium (single
proton/hydrogen nucleus), deuterium, helium-3,
helium-4, and lithium-7. By mass, the resulting
matter is about 75% hydrogen nuclei, 25% helium
nuclei, and perhaps 10−10 by mass of Lithium-7.
The next most common stable isotopes produced
are lithium-6, beryllium-9, boron-11, carbon,
nitrogen and oxygen ("CNO"), but these have
predicted abundances of between 5 and 30 parts
in 1015 by mass, making them essentially undetectable
and negligible.The amounts of each light element
in the early universe can be estimated from
old galaxies, and is strong evidence for the
Big Bang. For example, the Big Bang should
produce about 1 neutron for every 7 protons,
allowing for 25% of all nucleons to be fused
into helium-4 (2 protons and 2 neutrons out
of every 16 nucleons), and this is the amount
we find today, and far more than can be easily
explained by other processes. Similarly, deuterium
fuses extremely easily; any alternative explanation
must also explain how conditions existed for
deuterium to form, but also left some of that
deuterium unfused and not immediately fused
again into helium. Any alternative must also
explain the proportions of the various light
elements and their isotopes. A few isotopes,
such as lithium-7, were found to be present
in amounts that differed from theory, but
over time, these differences have been resolved
by better observations.
=== Matter domination ===
47,000 years after the Big Bang
Until now, the universe's large scale dynamics
and behavior have been determined mainly by
radiation – meaning, those constituents
that move relativistically (at or near the
speed of light), such as photons and neutrinos.
As the universe cools, from around 47,000
years (z=3600), the universe's large scale
behavior becomes dominated by matter instead.
This occurs because the energy density of
matter begins to exceed both the energy density
of radiation and the vacuum energy density.
Around or shortly after this time, the densities
of non-relativistic matter (atomic nuclei)
and relativistic radiation (photons) become
equal, the Jeans length, which determines
the smallest structures that can form (due
to competition between gravitational attraction
and pressure effects), begins to fall and
perturbations, instead of being wiped out
by free-streaming radiation, can begin to
grow in amplitude.
According to the Lambda-CDM model, by this
stage, the matter in the universe is around
84.5% cold dark matter and 15.5% "ordinary"
matter. (However the total matter in the universe
is only 31.7%, much smaller than the 68.3%
of dark energy). There is overwhelming evidence
that dark matter exists and dominates our
universe, but since the exact nature of dark
matter is still not understood, Big Bang theory
does not presently cover any stages in its
formation.
From this point on, and for several billion
years to come, the presence of dark matter
accelerates the formation of structure in
our universe. In the early universe, dark
matter gradually gathers in huge filaments
under the effects of gravity. This amplifies
the tiny inhomogeneities (irregularities)
in the density of the universe which was left
by cosmic inflation. Over time, slightly denser
regions become denser and slightly rarefied
(emptier) regions become more rarefied. Ordinary
matter eventually gathers together faster
than it would otherwise do, because of the
presence of these concentrations of dark matter.
=== Recombination, photon decoupling, and
the cosmic microwave background (CMB) ===
ca. 377,000 years after the Big Bang
About 377,000 years after the Big Bang, two
connected events occurred: recombination and
photon decoupling. Recombination describes
the ionized particles combining to form the
first neutral atoms, and decoupling refers
to the photons released ("decoupled") as the
newly formed atoms settle into more stable
energy states.
Just before recombination, the baryonic matter
in the universe was at a temperature where
it formed a hot ionized plasma. Most of the
photons in the universe interacted with electrons
and protons, and could not travel significant
distances without interacting with ionized
particles. As a result, the universe was opaque
or "foggy". Although there was light, it was
not possible to see, nor can we observe that
light through telescopes.
At around 377,000 years, the universe has
cooled to a point where free electrons can
combine with the hydrogen and helium nuclei
to form neutral atoms. This process is relatively
fast (and faster for the helium than for the
hydrogen), and is known as recombination.
The name is slightly inaccurate and is given
for historical reasons: in fact the electrons
and atomic nuclei were combining for the first
time.
Directly combining in a low energy state (ground
state) is less efficient, so these hydrogen
atoms generally form with the electrons still
in a high energy state, and once combined,
the electrons quickly release energy in the
form of one or more photons as they transition
to a low energy state. This release of photons
is known as photon decoupling. Some of these
decoupled photons are captured by other hydrogen
atoms, the remainder remain free. By the end
of recombination, most of the protons in the
universe have formed neutral atoms. This change
from charged to neutral particles means that
the mean free path photons can travel before
capture in effect becomes infinite, so any
decoupled photons that have not been captured
can travel freely over long distances (see
Thomson scattering). The universe has become
transparent to visible light, radio waves
and other electromagnetic radiation for the
first time in its history.
The photons released by these newly formed
hydrogen atoms initially had a temperature/energy
of around ~ 4000 K. This would have been visible
to the eye as a pale yellow/orange tinted,
or "soft", white color. Over billions of years
since decoupling, as the universe has expanded,
the photons have been red-shifted from visible
light to radio waves (microwave radiation
corresponding to a temperature of about 2.7
K). Red shifting describes the photons acquiring
longer wavelengths and lower frequencies as
the universe expanded over billions of years,
so that they gradually changed from visible
light to radio waves. These same photons can
still be detected as radio waves today. They
form the cosmic microwave background ("CMB"),
and they provide crucial evidence of the early
universe and how it developed.
Around the same time as recombination, existing
pressure waves within the electron-baryon
plasma – known as baryon acoustic oscillations
– became embedded in the distribution of
matter as it condensed, giving rise to a very
slight preference in distribution of large-scale
objects. Therefore, the cosmic microwave background
is a picture of the universe at the end of
this epoch including the tiny fluctuations
generated during inflation (see diagram),
and the spread of objects such as galaxies
in the universe is an indication of the scale
and size of the universe as it developed over
time.
== The Dark Ages and large-scale structure
emergence ==
ca. 380 thousand to about 1 billion years
after the Big Bang
=== 
Dark Ages ===
After recombination and decoupling, the universe
was transparent and had cooled enough to allow
light to travel long distances, but there
were no light-producing structures such as
stars and galaxies. Stars and galaxies are
formed when dense regions of gas form due
to the action of gravity, and this takes a
long time within a near-uniform density of
gas and on the scale required, so it is estimated
that stars did not exist for perhaps hundreds
of millions of years after recombination.
This period, known as the Dark Ages, began
around 377,000 years after the Big Bang. During
the Dark Ages, the temperature of the universe
cooled from some 4000 K down to about 60 K,
and only two sources of photons existed: the
photons released during recombination/decoupling
(as neutral hydrogen atoms formed), which
we can still detect today as the cosmic microwave
background (CMB), and photons occasionally
released by neutral hydrogen atoms, known
as the 21 cm spin line of neutral hydrogen.
The hydrogen spin line is in the microwave
range of frequencies, and within 3 million
years, the CMB photons had redshifted out
of visible light to infrared; from that time
until the first stars, there were no visible
light photons. Other than perhaps some rare
statistical anomalies, the universe was truly
dark.
The October 2010 discovery of UDFy-38135539,
the first observed galaxy to have existed
during the following reionization epoch, gives
us a window into these times. The galaxy earliest
in this period observed and thus also the
most distant galaxy ever observed is currently
on the record of Leiden University's Richard
J. Bouwens and Garth D. Illingsworth from
UC Observatories/Lick Observatory. They found
the galaxy UDFj-39546284 to be at a time some
480 million years after the Big Bang or about
halfway through the Cosmic Dark Ages at a
distance of about 13.2 billion light-years.
More recently, the UDFy-38135539, EGSY8p7
and GN-z11 galaxies were found to be around
380–550 million years after the Big Bang
and at a distance of around 13.4 billion light-years.
There is also currently an observational effort
underway to detect the faint 21 cm spin line
radiation, as it is in principle an even more
powerful tool than the cosmic microwave background
for studying the early universe.
Structures may have begun to emerge from around
150 million years, and stars and early galaxies
gradually emerged from around 400 to 700 million
years. As they emerged, the Dark Ages gradually
ended. Because this process was gradual, the
Dark Ages only fully ended around 1 billion
(1000 million) years, as the universe took
its present appearance.
==== Speculative "habitable epoch" ====
ca. 10–17 million years after the Big BangFor
about 6.6 million years, between about 10
to 17 million years after the Big Bang (redshift
137–100), the background temperature was
between 373 K and 273 K, a temperature compatible
with liquid water and common biological chemical
reactions. Loeb (2014) speculated that primitive
life might in principle have appeared during
this window, which he called "the Habitable
Epoch of the Early Universe". Loeb argues
that carbon-based life might have evolved
in a hypothetical pocket of the early universe
that was dense enough both to generate at
least one massive star that subsequently releases
carbon in a supernova, and that was also dense
enough to generate a planet. (Such dense pockets,
if they existed, would have been extremely
rare.) Life would also have required a heat
differential, rather than just uniform background
radiation; this could be provided by naturally-occurring
geothermal energy. Such life would likely
have remained primitive; it is highly unlikely
that intelligent life would have had sufficient
time to evolve before the hypothetical oceans
freeze over at the end of the habitable epoch.
=== Earliest structures and stars emerge ===
Around 150 million to 1 billion years after
the Big Bang
The matter in the universe is around 84.5%
cold dark matter and 15.5% "ordinary" matter.
Since the start of the matter-dominated era,
the dark matter has gradually been gathering
in huge spread out (diffuse) filaments under
the effects of gravity. Ordinary matter eventually
gathers together faster than it would otherwise
do, because of the presence of these concentrations
of dark matter. It is also slightly more dense
at regular distances due to early baryon acoustic
oscillations (BAO) which became embedded into
the distribution of matter when photons decoupled.
Unlike dark matter, ordinary matter can lose
energy by many routes, which means that as
it collapses, it can lose the energy which
would otherwise hold it apart, and collapse
more quickly, and into denser forms. Ordinary
matter gathers where dark matter is denser,
and in those places it collapses into clouds
of mainly hydrogen gas. The first stars and
galaxies form from these clouds. Where numerous
galaxies have formed, galaxy clusters and
superclusters will eventually arise. Large
voids with few stars will develop between
them, marking where dark matter became less
common.
Structure formation in the big bang model
proceeds hierarchically, due to gravitational
collapse, with smaller structures forming
before larger ones. The earliest structures
to form are the first stars (known as population
III stars), dwarf galaxies, and quasars (which
are thought to be bright, early active galaxies
containing a supermassive black hole surrounded
by a inward-spiralling accretion disk of gas).
Before this epoch, the evolution of the universe
could be understood through linear cosmological
perturbation theory: that is, all structures
could be understood as small deviations from
a perfect homogeneous universe. This is computationally
relatively easy to study. At this point non-linear
structures begin to form, and the computational
problem becomes much more difficult, involving,
for example, N-body simulations with billions
of particles. The Bolshoi Cosmological Simulation
is a high precision simulation of this era.
These Population III stars are also responsible
for turning the few light elements that were
formed in the Big Bang (hydrogen, helium and
small amounts of lithium) into many heavier
elements. They can be huge as well as perhaps
small – and non-metallic (no elements except
hydrogen and helium). The larger stars have
very short lifetimes compared to most Main
Sequence stars we see today, so they commonly
finish burning their hydrogen fuel and explode
as supernovae after mere millions of years,
seeding the universe with heavier elements
over repeated generations. They mark the start
of the Stelliferous (starry) era.
As yet, no Population III stars have been
found, so our understanding of them is based
on computational models of their formation
and evolution. Fortunately, observations of
the Cosmic Microwave Background radiation
can be used to date when star formation began
in earnest. Analysis of such observations
made by the European Space Agency's Planck
telescope in 2016 concluded that the first
generation of stars formed 700 million years
after the Big Bang.Quasars provides some additional
evidence of early structure formation. Their
light shows evidence of elements such as carbon,
magnesium, iron and oxygen. This is evidence
that by the time quasars formed, a massive
phase of star formation had already taken
place, including sufficient generations of
population III stars to give rise to these
elements.
=== Reionization ===
As the first stars, dwarf galaxies and quasars
gradually form, the intense radiation they
emit reionizes much of the surrounding universe;
splitting the neutral hydrogen atoms back
into a plasma of free electrons and protons
for the first time since recombination and
decoupling.
Reionization is evidenced from observations
of quasars. Quasars are a form of active galaxy,
and the most luminous objects observed in
the universe. Electrons in neutral hydrogen
have a specific patterns of absorbing photons,
related to electron energy levels and called
the Lyman series. Ionized hydrogen does not
have electron energy levels of this kind.
Therefore, light travelling through ionized
hydrogen and neutral hydrogen shows different
absorption lines. In addition, the light will
have travelled for billions of years to reach
us, so any absorption by neutral hydrogen
will have been redshifted by varied amounts,
rather than by one specific amount, indicating
when it happened. These features make it possible
to study the state of ionization at many different
times in the past. They show that reionization
began as "bubbles" of ionized hydrogen which
became larger over time. They also show that
the absorption was due to the general state
of the universe (the intergalactic medium)
and not due to passing through galaxies or
other dense areas. Reionization might have
started as early as z=16 (250 million years
of cosmic time) and was complete by around
z=9 or 10 (500 million years). The epoch of
reionization probably ended by around z=5
or 6 (1 billion years) as the era of Population
III stars and quasars – and their intense
radiation – came to an end, and the ionized
hydrogen gradually reverted to neutral atoms.These
observations have narrowed down the period
of time during which reionization took place,
but the source of the photons that caused
reionization is still not completely certain.
To ionize neutral hydrogen, an energy larger
than 13.6 eV is required, which corresponds
to ultraviolet photons with a wavelength of
91.2 nm or shorter, implying that the sources
must have produced significant amount of ultraviolet
and higher energy. Protons and electrons will
recombine if energy is not continuously provided
to keep them apart, which also sets limits
on how numerous the sources were and their
longevity. With these constraints, it is expected
that quasars and first generation stars and
galaxies were the main sources of energy.
The current leading candidates from most to
least significant are currently believed to
be population III stars (the earliest stars)
(possibly 70%), dwarf galaxies (very early
small high-energy galaxies) (possibly 30%),
and a contribution from quasars (a class of
active galactic nuclei).However, by this time,
matter had become far more spread out due
to the ongoing expansion of the universe.
Although the neutral hydrogen atoms were again
ionized, the plasma was much more thin and
diffuse, and photons were much less likely
to be scattered. Despite being reionized,
the universe remained largely transparent
during reionization. As the universe continued
to cool and expand, reionization gradually
ended.
=== Galaxies, clusters and superclusters ===
Matter continues to draw together under the
influence of gravity, to form galaxies. The
stars from this time period, known as Population
II stars, are formed early on in this process,
with more recent Population I stars formed
later. Gravitational attraction also gradually
pulls galaxies towards each other to form
groups, clusters and superclusters. The Hubble
Ultra Deep Field observatory has identified
a number of small galaxies merging to form
larger ones, at 800 million years of cosmic
time (13 billion years ago) (this age estimate
is now believed to be slightly overstated).Johannes
Schedler's project has identified a quasar
CFHQS 1641+3755 at 12.7 billion light-years
away, when the universe was just 7% of its
present age. On July 11, 2007, using the 10-metre
Keck II telescope on Mauna Kea, Richard Ellis
of the California Institute of Technology
at Pasadena and his team found six star forming
galaxies about 13.2 billion light years away
and therefore created when the universe was
only 500 million years old. Only about 10
of these extremely early objects are currently
known. More recent observations have shown
these ages to be shorter than previously indicated.
The most distant galaxy observed as of October
2016, GN-z11, has been reported to be 32 billion
light years away, a vast distance made possible
through space-time expansion (redshift z=11.1;
comoving distance of 32 billion light-years;
lookback time of 13.4 billion years).
== Current appearance of the Universe ==
The universe has appeared much the same as
it does now, for many billions of years. It
will continue to look similar for many more
billions of years into the future.
Based upon the emerging science of nucleocosmochronology,
the Galactic thin disk of the Milky Way is
estimated to have been formed 8.8 ± 1.7 billion
years ago.
=== Dark 
energy dominated era ===
From about 9.8 billion years after the Big
bangFrom about 9.8 billion years of cosmic
time, the universe's large-scale behavior
is believed to have gradually changed for
the third time in its history. Its behavior
had originally been dominated by radiation
(relativistic constituents such as photons
and neutrinos) for the first 47,000 years,
and since about 377,000 years of cosmic time,
its behavior had been dominated by matter.
During its matter-dominated era, the expansion
of the universe had begun to slow down, as
gravity reigned in the initial outward expansion.
But from about 9.8 billion years of cosmic
time, observations show that that the expansion
of the universe slowly stops decelerating,
and gradually begins to accelerate again,
instead.
While the precise cause is not known, the
observation is accepted as correct by the
cosmologist community. By far the most accepted
understanding is that this is due to an unknown
form of energy which has been given the name
"dark energy". "Dark" in this context means
that it is not directly observed, but can
currently only be studied by examining the
effect it has on the universe. Research is
ongoing to understand this dark energy. Dark
energy is now believed to be the single largest
component of the universe, as it constitutes
about 68.3% of the entire mass-energy of the
physical universe.
Dark energy is believed to act like a cosmological
constant - a scalar field that exists throughout
space. Unlike gravity, the effects of such
a field do not diminish (or only diminish
slowly) as the universe grows. While matter
and gravity have a greater effect initially,
their effect quickly diminishes as the universe
continues to expand. Objects in the universe,
which are initially seen to be moving apart
as the universe expands, continue to move
apart, but their outward motion gradually
slows down. This slowing effect becomes smaller
as the universe becomes more spread out. Eventually,
the outward and repulsive effect of dark energy
begins to dominate over the inward pull of
gravity. Instead of slowing down and perhaps
beginning to move inward under the influence
of gravity, from about 9.8 billion years of
cosmic time, the expansion of space starts
to slowly accelerate outward at a gradually
increasing rate.
== Far future and ultimate fate ==
The universe has existed for around 13.8 billion
years, and we believe that we understand it
well enough to predict its large-scale development
for many billions of years into the future
– perhaps as much as 100 billion years of
cosmic time (about 86 billion years from now).
Beyond that, we need to better understand
the universe to make any accurate predictions.
Therefore, the universe could follow a variety
of different paths beyond this time.
There are several competing scenarios for
the possible long-term evolution of the universe.
Which of them will happen, if any, depends
on the precise values of physical constants
such as the cosmological constant, the possibility
of proton decay, the energy of the vacuum
(meaning, the energy of "empty" space itself),
and the natural laws beyond the Standard Model.
If the expansion of the universe continues
and it stays in its present form, eventually
all but the nearest galaxies will be carried
away from us by the expansion of space at
such a velocity that our observable universe
will be limited to our own gravitationally
bound local galactic cluster. In the very
long term (after many trillions – thousands
of billions – of years, cosmic time), the
Stelliferous Era will end, as stars cease
to be born and even the longest-lived stars
gradually die. Beyond this, all objects in
the universe will cool and (with the possible
exception of protons) gradually decompose
back to their constituent particles and then
into subatomic particles and very low level
photons and other fundamental particles, by
a variety of possible processes. But this
will take a duration of time that is almost
inconceivable to most people, compared to
which the entire 13.8 billion years of the
universe would be a tiny instant in time.
Ultimately, in the extreme future, the following
scenarios have been proposed for the ultimate
fate of the universe.
In this kind of extreme timescale, extremely
rare quantum phenomena may also occur that
are extremely unlikely to be seen on a timescale
smaller than trillions of years. These may
also lead to unpredictable changes to the
state of the universe which would not be likely
to be significant on any smaller timescale.
For example, on a timescale of millions of
trillions of years, black holes might appear
to evaporate almost instantly, uncommon quantum
tunneling phenomena would appear to be common,
and quantum (or other) phenomena so unlikely
that they might occur just once in a trillion
years may occur many times.
== See also
