History of the universe

See also: Timeline of the universe
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

### Chronology in five stages

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 itself 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 2 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 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 100,000 years, helium hydride is the first molecule. (Much later, hydrogen and helium hydride react to form molecular hydrogen, the fuel needed for the first stars)
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 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 huge (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 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 leads to a period of reionization that commences gradually between about 250-500 million years, is complete by about 700-900 million years, and diminishes by about 1 billion years (exact timings still being researched). 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 our 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.

### Tabular summary

Further information: Timeline of cosmological epochs, Timeline of natural history, Geologic time scale, Timeline of the evolutionary history of life, Timeline of the far future
Further information: Graphical timeline of the universe, Graphical timeline of the Big Bang, Graphical timeline from Big Bang to Heat Death, Graphical timeline of the Stelliferous Era
*Note: The radiation temperature in the table below, refers to the cosmic background radiation and is given by 2.725·(1+*z*), where* z *is the redshift.*

## The Big Bang

Main articles: Big Bang, Origin of the Universe, Problem of why there is anything at all
The standard model of cosmology is based on a model of space-time called the Friedmann–Lemaître–Robertson–Walker ("FLRW") metric. A metric provides a measure of distance between objects, and the FLRW metric is the exact solution of Einstein's field equations if some key properties of space such as homogeneity and isotropy are assumed to be true. The FLRW metric very closely matches overwhelming other evidence, showing that the universe has expanded since the Big Bang.

If we assume that the FLRW metric equations are valid all the way back to the beginning of our universe, then we can follow them back in time, to a point where the equations suggest all distances between objects in the universe were zero or infinitesimally small. (This does **not** necessarily mean the universe was physically small at the Big Bang, although that is indeed one of the possibilities.) Going forward, this provides a model of our universe which matches all current physical observations extremely closely. This initial period of the Universe's chronology is called the "Big Bang". The standard model of cosmology does not attempt to explain *why* the universe began to exist; it explains only *how* the universe physically developed once that moment happened.

We interpret the singularity from the FLRW metric as meaning that current theories are inadequate to describe what actually happened at the start of the Big Bang itself. It is widely believed that a correct theory of quantum gravity may allow a more correct description of that event, but no such theory has yet been developed. After that moment, all distances throughout the universe began to increase from (perhaps) zero because the FLRW metric itself changed over time, affecting distances between all non-bound objects everywhere. For this reason we say that the Big Bang "happened everywhere".

## Early universe

### Neutrino decoupling and cosmic neutrino background

*Around 1 second after the Big Bang*
Main articles: Neutrino decoupling, Cosmic neutrino background
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 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*, 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*
Main articles: Lepton epoch
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*
Main articles: Photon epoch
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 particles, i.e., electrons, protons 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*
Main articles: Big Bang nucleosynthesis
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*
Main articles: Matter-dominated era, Structure formation
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 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. (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, 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 our 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.

### First molecules

*100,000 years after the Big Bang*
At around 100,000 years, the universe has cooled enough for helium hydride, the first molecule, to form. In April 2019, this molecule was first announced to have been discovered in interstellar space. (Much later, atomic hydrogen reacts with helium hydride to create molecular hydrogen, the fuel required for star formation.)

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

Main articles: Recombination (cosmology), decoupling (cosmology)
*c. 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

 *380 thousand to about 1 billion years after the Big Bang*
See also: Hydrogen line, List of the most distant astronomical objects
### Dark Ages

See also: 21 centimeter radiation
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 exact timings of the first stars (known as "first light"), 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 date from around 380 - 400 million years (for example GN-z11), suggesting surprisingly fast gas cloud condensation and stellar birth rates, and observations of the Lyman-alpha forest and other changes to the light from ancient objects allows the timing for reionization, and its eventual end, to be narrowed down. But these are all still areas of active research.

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. Subsequently, Leiden University's Richard J. Bouwens and Garth D. Illingsworth from UC Observatories/Lick Observatory found the galaxy UDFj-39546284 to be even older, at a time some 480 million years after the Big Bang or about halfway through the Cosmic Dark Ages 13.2 billion years ago. In December 2012 the first candidate galaxies dating to before reionization were discovered, when UDFy-38135539, EGSY8p7 and GN-z11 galaxies were found to be around 380–550 million years after the Big Bang, 13.4 billion years ago and at a distance of around 32 billion light-years (9.8 billion parsecs).

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 early galaxies gradually emerged from around 380 to 700 million years. (We do not have separate observations of very early individual stars; the earliest observed stars are discovered as participants in very early galaxies). 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"

*c. 10–17 million years after the Big Bang*
For 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*
See also: Stellar formation, Dwarf galaxy,  Baryon acoustic oscillations, Observable universe#Large-scale structure, Structure formation, Large-scale structure of the cosmos, Stelliferous era

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 an 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 may have formed from around 300 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

See also: Reionization, Dwarf galaxy, Quasar
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 to happen as early as *z*=16 (250 million years of cosmic time) and was complete by around *z*=9 or 10 (500 million years)before gradually diminishing and probably coming to an end 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

See also: Galaxy formation and evolution

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

Main articles: dark energy, Scale factor (cosmology)
*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 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 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

Main articles: Ultimate fate of the universe, Timeline of the far future
Further information: Future of an expanding universe, Heat death of the universe
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.

Ultimately, in the extreme future, the following scenarios have been proposed for the ultimate fate of the universe.

 **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.
 **Big Crunch** | Expansion eventually slows and halts, then reverses as all matter accelerates towards its common centre. Not now considered likely.
 **Vacuum instability** | Collapse of the quantum fields that underpin all forces, particles and structures, to a different form.
The effect would be that the quantum fields that underpin all forces, particles and structures, would undergo a transition to a more stable form. New forces and particles would replace the present ones we know of, with the side effect that all current particles, forces and structures would be destroyed and subsequently (if able) reform into different particles, forces and structures.

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.