# Chronology of the universe

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The chronology of the universe describes the history and future of the universe according to Big Bang cosmology.
Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.

## Overview

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.

### Very early universe

The first picosecond (10$^{−12}$ seconds) of cosmic time includes the Planck epoch, during which currently established laws of physics may not have applied; the emergence in stages of the four known fundamental interactions or forces—first gravitation, and later the electromagnetic, weak and strong interactions; and the accelerated expansion of the universe due to cosmic inflation.
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 the extrapolation of known physical laws to extremely high temperatures.

### Early universe

This period lasted around 380,000 years. Initially, various kinds of subatomic particles were formed in stages. These particles included almost equal amounts of matter and antimatter, so most of it quickly annihilated, leaving a small excess of matter in the universe.
At about one second, neutrinos decoupled; these neutrinos formed the cosmic neutrino background (CνB). 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 two minutes, conditions are suitable for nucleosynthesis: around 25% of the protons and all the neutrons fuse into heavier elements, initially deuterium which itself quickly fuses into mainly helium-4.
By 20 minutes, the universe is no longer hot enough for nuclear fusion, but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma.
The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form He$^+$ . At around 47,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. Much later, hydrogen and helium hydride react to form molecular hydrogen (H$_2$), the fuel needed for the first stars. At about 370,000 years, neutral hydrogen atoms finish forming ("recombination"), greatly reducing the Thomson scattering of photons.$^{:22.4.1}$ No longer scattered by free electrons, the photons were ("decoupled") and propagated freely. This vast collection of photons from the earliest times of the universe can still be detected today as the cosmic microwave background (CMB).$^{:22.4.3}$ This is the oldest direct observation we currently have of the universe.

### Gravity builds cosmic structure

This period measures from 380,000 years until about 1 billion years. Even before recombination and decoupling, matter began to accumulate around clumps of dark matter.$^{:4.1}$ Clouds of hydrogen collapsed very slowly to form stars and galaxies, so there were few sources of light and the emission from these sources was immediately absorbed by hydrogen atoms. 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. This period is known as the cosmic Dark Ages.
At some point around 200 to 500 million years, the earliest generations of stars and galaxies form (exact timings are still being researched), 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 very massive (100–300 solar masses) 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 highly energetic pair-instability 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 every day 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 lead to a period of reionization that commences gradually between about 250–500 million years and finishes by about 1 billion years (exact timings still being researched). The Dark Ages only fully came to an end at about 1 billion years as the universe gradually transitioned into the universe we see around us today, but denser, hotter, more intense in star formation, and richer in smaller (particularly unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies.
The earliest galaxies that have been observed, around from 330 million years after the Big Bang, or 13.4 billion years ago (redshift of z=13.2), have few elements heavier than hydrogen (metal poor) and show spectroscopic evidence of being surrounded by neutral hydrogen as expected. Other analysis suggests these galaxies formed rapidly in an environment of intense radiation.

### Universe as it appears today

From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today and it will continue to appear very similar for many billions of years into the future. The thin disk of our galaxy began to form when the universe was about 5 billion years old or 9 ± 2 Gya. The Solar System formed at about 9.2 billion years (4.6 Gya),$^{:22.2.3}$ with the earliest evidence of life on Earth emerging by about 10 billion years (3.8 Gya).
The thinning of matter over time reduces the ability of the matter to gravitationally decelerate the expansion of the universe; in contrast, dark energy is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point about five or six billion years ago when the universe entered the modern "dark-energy-dominated era" where the universe's expansion is now accelerating rather than decelerating. The present-day universe is quite well understood, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), we are less sure which path the universe will take.

### Far future and ultimate fate

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 the present-day universe may allow these to be better understood.

### Tabular summary

    Note: The radiation temperature in the table below refers to the cosmic microwave background radiation and is given by 2.725 K·(1 + z), where z is the redshift.
| Epoch | Time | Redshift | Radiation temperature (Energy) | Description |
| Planck epoch | < 10$^{−43}$ s | | > 10$^{32}$ K ( > 10$^{19}$ GeV) | The Planck scale is the physical scale beyond which current physical theories may not apply and cannot be used to reliably predict any events. During the Planck epoch, cosmology and physics are assumed to have been dominated by the quantum effects of gravity. |
| Grand unification epoch | < 10$^{−36}$ s | | > 10$^{29}$ K ( > 10$^{16}$ GeV) | The three forces of the Standard Model are still unified (assuming that nature is described by a Grand Unified Theory, gravity not included). |
| Inflationary epoch Electroweak epoch | < 10$^{−32}$ s | | 10$^{28}$ K ~ 10$^{22}$ K (10$^{15}$ ~ 10$^9$ GeV) | Cosmic inflation expands space by a factor of the order of 10$^{26}$ over a time of the order of 10$^{−36}$ to 10$^{−32}$ seconds. The universe is supercooled from about 10$^{27}$ down to 10$^{22}$ Kelvins. The strong interaction becomes distinct from the electroweak interaction. |
| Electroweak epoch ends | 10$^{−12}$ s | | 10$^{15}$ K (150 GeV) | Before temperature falls below 150 GeV, the average energy of particle interactions is high enough that it is more succinct to describe them as an exchange of W$_1$, W$_2$, W$_3$, and B vector bosons (electroweak interactions) and H$^+$, H$^−$, H$^0$, H$^{0⁎}$ scalar bosons (Higgs interaction). In this picture, the vacuum expectation value of the Higgs field is zero (therefore, all fermions are massless), all electroweak bosons are massless (they had not yet subsumed a component of the Higgs field to become massive), and photons (γ) do not yet exist (they will exist after a phase transition as a linear combination of B and W$_3$ bosons, γ = B cos θ$_W$ + W$_3$ sin θ$_W$, where θ$_W$ is the Weinberg angle). These are the highest energies directly observable in the Large Hadron Collider. The sphere of space that will become the observable universe is approximately 300 light-seconds (~0.6 AU) in radius at this time. |
| Quark epoch | 10$^{−12}$ s ~ 10$^{−5}$ s | | 10$^{15}$ K ~ 10$^{12}$ K (150 GeV ~ 150 MeV) | The forces of the Standard Model have reorganized into the "low-temperature" form: Higgs and electroweak interactions rearranged into massive Higgs boson H$^0$, weak force carried by massive W$^+$, W$^–$, and Z$^0$ bosons, and electromagnetism carried by massless photons. Higgs field has a nonzero vacuum expectation value, making fermions massive. Energies are too high for quarks to coalesce into hadrons, instead forming a quark–gluon plasma. |
| Baryogenesis | 10$^{−5}$ s ~ 1 s | | 10$^{12}$ K ~ 10$^{10}$ K (150 MeV ~ 1 MeV) | Quarks are bound into hadrons. A slight matter-antimatter asymmetry from the earlier phases (baryon asymmetry) results in an elimination of anti-baryons. Until 0.1 s, muons and pions are in thermal equilibrium, and outnumber baryons by about 10:1. Close to the end of this epoch, only light-stable baryons (protons and neutrons) remain. Due to the sufficiently high density of leptons, protons and neutrons rapidly change into one another under the action of weak force. Due to the higher mass of neutron the neutron:proton ratio, which is initially 1:1, starts to decrease. |
| Neutrino decoupling | 1 s | | 10$^{10}$ K (1 MeV) | Neutrinos cease interacting with baryonic matter, and form cosmic neutrino background. Neutron:proton ratio freezes at approximately 1:6. The sphere of space that will become the observable universe is approximately 10 light-years in radius at this time. |
| Lepton epoch | 1 s ~ 10 s | | 10$^{10}$ K ~ 10$^9$ K (1 MeV ~ 100 keV) | Leptons and antileptons remain in thermal equilibrium—energy of photons is still high enough to produce electron-positron pairs. |
| Big Bang nucleosynthesis | 10 s ~ 10$^3$ s | | 10$^9$ K ~ 10$^7$ K (100 keV ~ 1 keV) | Protons and neutrons are bound into primordial atomic nuclei: hydrogen and helium-4. Trace amounts of deuterium, helium-3, and lithium-7 also form. At the end of this epoch, the spherical volume of space which will become the observable universe is about 300 light-years in radius, baryonic matter density is on the order of 4 grams per m$^3$ (about 0.3% of sea level air density)—however, most energy at this time is in electromagnetic radiation. |
| Photon epoch | 10 s ~ 370 ka | | 10$^9$ K ~ 4000 K (100 keV ~ 0.4 eV) | The universe consists of a plasma of nuclei, electrons, and photons; temperature is too low to create electron-positron pairs (or any other pairs of massive particles), but too high for the binding of electrons to nuclei. |
| Recombination | 18 ka ~ 370 ka | 6000 ~ 1100 | 4000 K (0.4 eV) | Electrons and atomic nuclei first become bound to form neutral atoms. Photons are no longer in thermal equilibrium with matter and the universe first becomes transparent. Recombination lasts for about 100 ka, during which the universe is becoming more and more transparent to photons. The photons of the cosmic microwave background radiation originate at this time. The spherical volume of space that will become the observable universe is 42 million light-years in radius at this time. The baryonic matter density at this time is about 500 million hydrogen and helium atoms per m$^3$, approximately a billion times higher than today. This density corresponds to pressure on the order of 10$^{−17}$ atm. |
| Dark Ages | 370 ka ~ 150 Ma? (Only fully ends by about 1 Ga) | 1100 ~ 20 | 4000 K ~ 60 K | The time between recombination and the formation of the first stars. During this time, the only source of photons was hydrogen emitting radio waves at hydrogen line. Freely propagating CMB photons quickly (within about 3 million years) red-shifted to infrared, and the universe was devoid of visible light. |
| Star and galaxy formation and evolution | Earliest galaxies: from about 300–400 Ma? (first stars: similar or earlier) Modern galaxies: 1 Ga ~ 10 Ga (Exact timings being researched) | From about 20 | From about 60 K | The earliest known galaxies existed by about 380 Ma. Galaxies coalesce into "proto-clusters" from about 1 Ga (redshift z = 6 ) and into galaxy clusters beginning at 3 Ga ( z = 2.1 ), and into superclusters from about 5 Ga ( z = 1.2 ). See: list of galaxy groups and clusters, list of superclusters. |
| Reionization | 200 Ma ~ 1 Ga (Exact timings being researched) | 20 ~ 6 | 60 K ~ 19 K | The most distant astronomical objects observable with telescopes date to this period; as of 2016, the most remote galaxy observed is GN-z11, at a redshift of 11.09. The earliest "modern" Population I stars are formed in this period. |
| Present time | 13.8 Ga | 0 | 2.7 K | Farthest observable photons at this moment are CMB photons. They arrive from a sphere with a radius of 46 billion light-years. The spherical volume inside it is commonly referred to as the observable universe. |
| Alternative subdivisions of the chronology (overlapping several of the above periods) |||||
| Radiation-dominated era | From inflation (~ 10$^{−32}$ sec) ~ 47 ka | > 3600 | > 10$^4$ K | During this time, the energy density of massless and near-massless relativistic components such as photons and neutrinos, which move at or close to the speed of light, dominate both matter density and dark energy. |
| Matter-dominated era | 47 ka ~ 9.8 Ga | 3600 ~ 0.4 | 10$^4$ K ~ 4 K | During this time, the energy density of matter dominates both radiation density and dark energy, resulting in a decelerated expansion of the universe. |
| Dark energy dominated era | > 9.8 Ga | < 0.4 | < 4 K | Matter density falls below dark energy density (vacuum energy), and expansion of space begins to accelerate. This time happens to correspond roughly to the time of the formation of the Solar System and the evolutionary history of life. |
| Stelliferous Era | 150 Ma ~ 100 Ta | 20 ~ −0.99 | 60 K ~ 0.03 K | The time between the first formation of Population III stars until the cessation of star formation, leaving all stars in the form of degenerate remnants. |
| Far future | > 100 Ta | < −0.99 | < 0.1 K | The Stelliferous Era will end as stars eventually die and fewer are born to replace them, leading to a darkening universe. Various theories suggest a number of subsequent possibilities. Assuming proton decay, the matter may eventually evaporate into a Dark Era (heat death). Alternatively, the universe may collapse in a Big Crunch. Other suggested ends include a false vacuum catastrophe or a Big Rip as possible ends to the universe. |

## Big Bang

The observed expansion of the universe lead to a physical model call the Big Bang. The concept includes an early state of extreme temperature and density; a high precision version of the Big Bang model using conventional physics, known as Lambda-CDM, agrees with a wide array of astrophysical observations. Extrapolating backwards in time to higher temperatures and densities leads to conditions where physical modeling has not resulted in quantitative predictions matching observations or into territory where the existing laws of physics are known to fail.$^{:305}$

### Singularity

Approaching infinite temperature, a scale factor of zero, or time at zero is known to be outside of our physical models. Speculating about an initial gravitational singularity is not sensible: the conditions are outside of the range of the theory.$^{:305}$

### Planck epoch

    Times shorter than 10$^{−43}$ seconds (Planck time)
Since the standard model of cosmology predicts expansion of the universe from a very hot time in the distant past, it can be followed back to smaller and smaller scales. However, it cannot be followed back to zero space. Below distance known as a Planck length, the basis for the equations breaks down. The energy of particles in this time is so large that quantum effects take over from Einstein equations for gravity. The Planck time,10$^{−43}$ seconds, is therefore the beginning time for the Big Bang model of cosmology.$^{:274}$

### Grand unification epoch

    Between 10$^{−43}$ seconds and 10$^{−36}$ seconds after the Big Bang
After the Planck era, the universe could, in principle, be modeled by extensions of the Standard model of particle physics, for example, those called grand unified theories. Many such theories have proposed but none been successful producing quantitative agreement with the results of modern astrophysical observations. Nevertheless, the time between 10$^{−43}$ and 10$^{−36}$ seconds has been called the grand unification epoch.
Before the GUT epoch, the temperature of the universe exceeded 10$^{15}$ GeV. As the universe expanded and cooled, it may have crossed a cosmological phase transition which may have resulted in the large ratio of matter to antimatter we observe today. This phase transition is a thermodynamic effect similar to condensation of a gas or freezing of a liquid. While the transition in the GUT epoch is speculative, electroweak and quark-hadron transitions which happen later are supported by theoretical models with some successful predictions.$^{:305}$

### 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.

## Inflation

    Before c. 10$^{−32}$ seconds after the Big Bang
At this point of the very early universe, the universe is thought to have expanded by a factor of at least 10$^{78}$ in volume. This is equivalent to a linear increase of at least 10$^{26}$ times in every spatial dimension—equivalent to an object 1 nanometre (10$^{−9}$ m, about half the width of a molecule of DNA) in length, expanding to one approximately 10.6 light-years (100 trillion kilometres) long in a tiny fraction of a second. This phase of the cosmic expansion history is known as inflation.
The mechanism that drove inflation remains unknown, although many models have been put forward. 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 the universe.
Inflation explains several observed properties of the current universe that are otherwise difficult to account for, including explaining how today's universe has galaxies and clusters of galaxies when it started out spatially uniform (homogeneous).$^{:324}$ Tiny quantum fluctuations in the universe, amplified by inflation, are believed to be the basis of large-scale structures that formed much later.
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 inflaton 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.
After inflation ended, the universe continued to expand, but at a decelerating rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in the universe's large-scale behavior. It is still expanding (and accelerating), today.

## 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 accessible in particle physics experiments and other detectors.

### Electroweak epoch

    Starting anywhere between 10$^{−22}$ and 10$^{−15}$ seconds after the Big Bang, until 10$^{−12}$ seconds after the Big Bang
Sometime after inflation, the created particles went through thermalization, where mutual interactions lead to thermal equilibrium. Before the electroweak symmetry breaking, at a temperature of around 10$^{15}$ K, approximately 10$^{−15}$ seconds after the Big Bang, the electromagnetic and weak interaction have not yet separated, and the gauge bosons and fermions have not yet gained mass through the Higgs mechanism. This epoch ended with electroweak symmetry breaking, potentially through a phase transition. In some extensions of the Standard Model of particle physics, baryogenesis also happened at this stage, creating an imbalance between matter and anti-matter (though in extensions to this model, this may have happened earlier). Little is known about the details of these processes.

#### Electroweak symmetry breaking

    10$^{−12}$ seconds after the Big Bang
As the universe's temperature continued to fall below 159.5±1.5 GeV, electroweak symmetry breaking happened. So far as we know, it was the penultimate symmetry breaking event in the formation of the universe, the final one being chiral symmetry breaking in the quark sector. 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 nuclear 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, electromagnetic, weak and strong interactions—have all taken their present forms, and fundamental particles have their expected masses, but the temperature of the universe is still too high to allow the stable formation of many particles we now see in the universe, so there are no protons or neutrons, and therefore no atoms, atomic nuclei, or molecules. (More exactly, any composite particles that form by chance, almost immediately break up again due to the extreme energies.)

### Quark epoch

    Between 10$^{−12}$ seconds and 10$^{−5}$ 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$^{−5}$ seconds old, when the average energy of particle interactions had fallen below the mass of the lightest hadron, the pion.

#### Baryogenesis

    Perhaps by 10$^{−11}$ seconds
Baryons 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.
Theory predicts that about 1 neutron remained for every 6 protons, with the ratio falling to 1:7 over time due to neutron decay. This is believed to be correct because, at a later stage, the neutrons and some of the protons fused, leaving hydrogen, a hydrogen isotope called deuterium, helium and other elements, which can be measured. A 1:7 ratio of hadrons would indeed produce the observed element ratios in the early and current universe.

### Neutrino decoupling and cosmic neutrino background (CνB)

    Around 1 second after the Big Bang
At approximately 1 second after the Big Bang neutrinos decouple and begin travelling 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 370,000 years after the Big Bang. The neutrinos from this event have a very low energy, around 10$^{−10}$ times the amount of those observable with present-day direct detection. Even high-energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all.
However, Big Bang cosmology makes many predictions about the CνB, and there is very strong indirect evidence that the CνB exists, both from Big Bang nucleosynthesis predictions of the helium abundance, and from anisotropies in the cosmic microwave background (CMB). One of these predictions is that neutrinos will have left a subtle imprint on the 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±0.02 K compared to a prediction of 1.95 K), and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.

### Possible formation of primordial black holes

    May have occurred within about 1 second after the Big Bang
Primordial black holes are a hypothetical type of black hole proposed in 1966, that may have formed during the so-called radiation-dominated era, if the universe was sufficiently inhomogeneous. Random fluctuations could lead to some regions becoming dense enough to undergo gravitational collapse, forming black holes.

### Lepton epoch

    Between 1 second and 10 seconds after the Big Bang
The majority of hadrons and anti-hadrons annihilate each other leaving leptons (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe. Initially leptons and antileptons are produced in pairs. About 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton–antilepton pairs are no longer created and most remaining leptons and antileptons quickly annihilated 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 370,000 years after the Big Bang
After most leptons and antileptons 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 particles, i.e., electrons, protons and (eventually) nuclei. They continue to do so for about the next 370,000 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 nuclear fusion reactions convert a 1/7 mixture of neutrons and protons in to a mix of protons, deuterium (a proton fused with a neutron), $^3$He, $^4$He, with trace amounts of $^7$Li and $^7$Be. These reactions end when the temperature falls below the 0.07MeV needed for nuclear fusion. The final mixture depends upon the reaction rates, the temperature, and the density of the components. The reaction rates can be measured in nuclear physics laboratories while the temperature and densities can be calculated from models of the expansion of the universe.$^{:313}$
About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and almost all of the deuterium quickly fuses to form helium-4. Helium-4 has much higher binding energy than nuclei with 5 to 8 nucleons so only trace amounts of those nuclei are created. Heavier nuclei produced in stars do not appear because they require the combination of three Helium-4 nuclei and the density of Helium-4 is too low for many three way collisions to occur before the expansion cools the universe below the fusion temperature.$^{:65}$ Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and are quickly decay. A small amount of deuterium is left unfused.$^{:70}$
The amounts of each light element in the early universe can be estimated from old galaxies, and is strong evidence for the Big Bang.$^{:313}$ 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 explained by production in stars.$^{:305}$ 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.

### 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 (redshift 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 47,000 years, 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. There is overwhelming evidence that dark matter exists and dominates the universe, but since the exact nature of dark matter is still not understood, the 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 the universe. In the early universe, dark matter gradually gathers in huge filaments under the effects of gravity, collapsing faster than ordinary (baryonic) matter because its collapse is not slowed by radiation pressure. 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.
The properties of dark matter that allow it to collapse quickly without radiation pressure, also mean that it cannot lose energy by radiation either. Losing energy is necessary for particles to collapse into dense structures beyond a certain point. Therefore, dark matter collapses into huge but diffuse filaments and haloes, and not into stars or planets. Ordinary matter, which can lose energy by radiation, forms dense objects and also gas clouds when it collapses.

### Recombination, photon decoupling, and the cosmic microwave background (CMB)

About 370,000 years after the Big Bang, two connected events occurred: the ending of 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.
Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium nuclei to form He$^+$ atoms. Neutral helium nuclei then start to form at around 100,000 years, with neutral hydrogen formation peaking around 260,000 years. This process 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.
At around 100,000 years, the universe had cooled enough for helium hydride, the first molecule, to form. In April 2019, this molecule was first announced to have been observed in interstellar space, in NGC 7027, a planetary nebula within this galaxy. (Much later, atomic hydrogen reacted with helium hydride to create molecular hydrogen, the fuel required for star formation.)
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 background of this box approximates the original 4000 K color of the photons released during decoupling, before they became redshifted to form the cosmic microwave background. The entire universe would have appeared as a brilliantly glowing fog of a color similar to this and a temperature of 4000 K, at the time.
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, 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 9-year WMAP image), 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.

## Gravity builds cosmic structure

    370 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 370,000 years after the Big Bang. During the Dark Ages, the temperature of the universe cooled from some 4000 K to about 60 K (3727 °C to about −213 °C), 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 first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang. These stars were the first source of visible light in the universe after recombination. Structures may have begun to emerge from around 150 million years, and early galaxies emerged from around 180 to 700 million years. As they emerged, the Dark Ages gradually ended. Because this process was gradual, the Dark Ages only ended fully at around 1 billion years, as the universe took on its present appearance.

#### Oldest observations of stars and galaxies

At present, the oldest observations of stars and galaxies are from shortly after the start of reionization, with galaxies such as GN-z11 (Hubble Space Telescope, 2016) at about z≈11.1 (about 400 million years cosmic time). Hubble's successor, the James Webb Space Telescope, launched December 2021, is designed to detect objects up to 100 times fainter than Hubble, and much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time). This is believed to be earlier than the first galaxies, and around the era of the first stars.
There is also 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.

### 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, 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.
The exact timings of the first stars, galaxies, supermassive black holes, and quasars, and the start and end timings and progression of the period known as reionization, are still being actively researched, with new findings published periodically. As of 2019: the earliest confirmed galaxies (for example GN-z11) date from around 380–400 million years, suggesting surprisingly fast gas cloud condensation and stellar birth rates; and observations of the Lyman-alpha forest, and of other changes to the light from ancient objects, allow the timing for reionization and its eventual end to be narrowed down. But these are all still areas of active research.
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 an inward-spiraling 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 Era.
As yet, no Population III stars have been found, so the 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 Planck microwave space telescope in 2016 concluded that the first generation of stars may have formed from around 300 million years after the Big Bang.
Quasars provide 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 specific patterns of absorbing ultraviolet 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. Ionized hydrogen in the intergalactic medium (particularly electrons) can scatter light through Thomson scattering as it did before recombination, but the expansion of the universe and clumping of gas into galaxies resulted in a concentration too low to make the universe fully opaque by the time of reionization. Because of the immense distance travelled by light (billions of light years) to reach Earth from structures existing during reionization, any absorption by neutral hydrogen is redshifted by various amounts, rather than by one specific amount, indicating when the absorption of then-ultraviolet light happened. These features make it possible to study the state of ionization at many different times in the past.
Reionization began as "bubbles" of ionized hydrogen which became larger over time until the entire intergalactic medium was ionized, when the absorption lines by neutral hydrogen become rare. 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 to happen as early as z = 16 (250 million years of cosmic time) and was mostly complete by around z = 9 or 10 (500 million years), with the remaining neutral hydrogen becoming fully ionized z = 5 or 6 (1 billion years), when Gunn-Peterson troughs that show the presence of large amounts of neutral hydrogen disappear. The intergalactic medium remains predominantly ionized to the present day, the exception being some remaining neutral hydrogen clouds, which cause Lyman-alpha forests to appear in spectra.
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 due how sparse the intergalactic medium was. Reionization gradually ended as the intergalactic medium became virtually completely ionized, although some regions of neutral hydrogen do exist, creating Lyman-alpha forests.
In August 2023, images of black holes and related matter in the very early universe by the James Webb Space Telescope were reported and discussed.

### 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. Hubble Ultra Deep Field observations 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).

## Present and future

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. The galactic disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago but only the age of the Sun, 4.567 billion years, is known precisely.

### Dark energy-dominated era

    From about 9.8 billion years after the Big Bang
From 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 370,000 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 reined in the initial outward expansion. But from about 9.8 billion years of cosmic time, observations show 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 its existence can be deduced by examining the gravitational 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

There are several competing scenarios for the 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 the observable universe will be limited to our own gravitationally bound local galaxy 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.
Ultimately, in the extreme future, the following scenarios have been proposed for the ultimate fate of the universe:
| Scenario | Scenario | Description |
| Heat Death | As expansion continues, the universe becomes larger, colder, and more dilute; in time, all structures eventually decompose to subatomic particles and photons. | In the case of indefinitely continuing cosmic expansion, the energy density in the universe will decrease until, after an estimated time of 10$^{1000}$ years, it reaches thermodynamic equilibrium and no more structure will be possible. This will happen only after an extremely long time because first, some (less than 0.1%) matter will collapse into black holes, which will then evaporate extremely slowly via Hawking radiation. The universe in this scenario will cease to be able to support life much earlier than this, after some 10$^{14}$ years or so, when star formation ceases.$^{,§IID}$ In some Grand Unified Theories, proton decay after at least 10$^{34}$ years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons.$^{,§IV,§VF}$ In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium.$^{,§VIB,VID}$ The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson (Lord Kelvin), who extrapolated the classical theory of heat and irreversibility (as embodied in the first two laws of thermodynamics) to the universe as a whole. |
| Big Rip | Expansion of space accelerates and at some point becomes so extreme that even subatomic particles and the fabric of spacetime are pulled apart and unable to exist. | For any value of the dark energy content of the universe where the negative pressure ratio is less than −1, the expansion rate of the universe will continue to increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, and ultimately the Solar System will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Even atomic nuclei will be torn apart. Finally, forces and interactions even on the Planck scale—the smallest size for which the notion of "space" currently has a meaning—will no longer be able to occur as the fabric of spacetime itself is pulled apart and the universe as we know it will end in an unusual kind of singularity. |
| Big Crunch | Expansion eventually slows and halts, then reverses as all matter accelerates towards its common centre. Currently considered to be likely incorrect. | In the opposite of the "Big Rip" scenario, the expansion of the universe would at some point be reversed and the universe would contract towards a hot, dense state. This is a required element of oscillatory universe scenarios, such as the cyclic model, although a Big Crunch does not necessarily imply an oscillatory universe. Current observations suggest that this model of the universe is unlikely to be correct, and the expansion will continue or even accelerate. |
| Vacuum instability | Collapse of the quantum fields that underpin all forces, particles and structures, to a different form. | Cosmology traditionally has assumed a stable or at least metastable universe, but the possibility of a false vacuum in quantum field theory implies that the universe at any point in spacetime might spontaneously collapse into a lower-energy state (see Bubble nucleation), a more stable or "true vacuum", which would then expand outward from that point with the speed of light. |
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 tunnelling 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.