do not read
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.[1]
Nature timelineThis box: viewtalkedit−13 —–−12 —–−11 —–−10 —–−9 —–−8 —–−7 —–−6 —–−5 —–−4 —–−3 —–−2 —–−1 —–0 —Dark AgesReionizationMatter-dominated
eraAccelerated expansionWater on EarthSingle-celled lifePhotosynthesisMulticellular
lifeVertebrates←Earliest Universe←Earliest stars←Earliest galaxy←Earliest quasar / black hole←Omega Centauri←Andromeda Galaxy←Milky Way spirals←NGC 188 star cluster←Alpha Centauri←Earth / Solar System←Earliest known life←Earliest oxygen←Atmospheric oxygen←Sexual reproduction←Earliest fungi←Earliest animals / plants←Cambrian explosion←Earliest mammals←Earliest apes / humansL
i
f
e(billion years ago)Overview[edit]Diagram of evolution of the (observable part) of the universe from the Big Bang (left), the CMB-reference afterglow, to the present
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.[citation needed]
Very early universe[edit]
The first picosecond (10−12 seconds) of cosmic time includes the Planck epoch,[2] 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.[3] 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[edit]
This period lasted around 380,000 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 (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 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 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,[4] 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 (H2) the fuel needed for the first stars. At about 370,000 years,[5]: 22.4.1 [6][7][8] neutral hydrogen atoms finish forming ("recombination") greatly reducing the Thomson scattering of photons. 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).[5]: 22.4.3 This is the oldest direct observation we currently have of the universe.
Gravity builds cosmic structure[edit]The age of the universe by redshift z=5 to 20. For early objects, this relationship is calculated using the cosmological parameters for mass Ωm and dark energy ΩΛ, in addition to redshift and the Hubble parameter H0.[9]
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.[10]: 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.[citation needed]
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.[11] 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.
The lookback time of extragalactic observations by their redshift up to z=20.[9]
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.[12][13] Other analysis suggests these galaxies formed rapidly in an environment of intense radiation.[14]
Universe as it appears today[edit]
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.[15] The Solar System formed at about 9.2 billion years (4.6 Gya),[5]: 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 gravity to decelerate the expansion of the universe; in contrast, dark energy (believed to be a constant scalar field throughout the visible universe) 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.[16][17]
Far future and ultimate fate[edit]
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.
Duration: 50 seconds.0:50Hubble Space Telescope—Ultra Deep Field galaxies to Legacy Field zoom out (video 00:50; 2 May 2019)Tabular summary[edit]Further information: Timeline of the early universe, Timeline of natural history, Geologic time scale, Timeline of the evolutionary history of life, and Timeline of the far futureNote: 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.EpochTimeRedshiftRadiation
temperature
(Energy)
[verification needed]DescriptionPlanck epoch< 10−43 s> 1032 K
( > 1019 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> 1029 K
( > 1016 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 s1028 K ~ 1022 K
(1015 ~ 109 GeV)Cosmic inflation expands space by a factor of the order of 1026 over a time of the order of 10−36 to 10−32 seconds. The universe is supercooled from about 1027 down to 1022 Kelvins.[18] The strong interaction becomes distinct from the electroweak interaction.Electroweak epoch ends10−12 s1015 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 W1, W2, W3, and B vector bosons (electroweak interactions) and H+, H−, H0, H0⁎ 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 W3 bosons, γ = B cos θW + W3 sin θW, where θW 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 epoch10−12 s ~ 10−5 s1015 K ~ 1012 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 H0, weak force carried by massive W+, W–, and Z0 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.Hadron epoch10−5 s ~ 1 s1012 K ~ 1010 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 decoupling1 s1010 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 epoch1 s ~ 10 s1010 K ~ 109 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 nucleosynthesis10 s ~ 103 s109 K ~ 107 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 m3 (about 0.3% of sea level air density)—however, most energy at this time is in electromagnetic radiation.Photon epoch10 s ~ 370 ka109 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.Recombination18 ka ~ 370 ka6000 ~ 11004000 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 m3, approximately a billion times higher than today. This density corresponds to pressure on the order of 10−17 atm.Dark Ages370 ka ~ 150 Ma?
(Only fully ends by about 1 Ga)1100 ~ 204000 K ~ 60 KThe 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 evolutionEarliest galaxies: from about 300–400 Ma?
(first stars: similar or earlier)
Modern galaxies: 1 Ga ~ 10 Ga
(Exact timings being researched)From about 20From about 60 KThe 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.Reionization200 Ma ~ 1 Ga
(Exact timings being researched)20 ~ 660 K ~ 19 KThe 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 time13.8 Ga02.7 KFarthest 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 eraFrom inflation (~ 10−32 sec) ~ 47 ka> 3600> 104 KDuring 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 era47 ka ~ 9.8 Ga[4]3600 ~ 0.4104 K ~ 4 KDuring 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[16]< 0.4< 4 KMatter 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 Era150 Ma ~ 100 Ta[19]20 ~ −0.9960 K ~ 0.03 KThe 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[19]< −0.99< 0.1 KThe 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[edit]Main articles: Big Bang, Origin of the universe, and "Why is there anything at all?"
The concordance model of cosmology, called the Lambda-CDM model, is based on a model of spacetime that starts with the assumption that the density of mass is homogeneous and isotropic. These assumptions lead to the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, a measure of distance between objects. With this metric Einstein field equations reduce to a simpler form called Friedmann equations which can be solved by treating spacetime as perfect fluid characterized by only pressure and density. The Lambda-CDM model closely matches high precision data across many kinds of astrophysical measurements, leading to the widespread acceptance of the Big Bang model.[5]
Very early universe[edit]Planck epoch[edit]Times shorter than 10−43 seconds (Planck time)See also: Planck units § In cosmology
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.[20]: 274
Grand unification epoch[edit]Between 10−43 seconds and 10−36 seconds after the Big Bang[21]Main article: Grand unification epoch
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. Neverthe less, the time between 10−43 and 10−36 seconds haas been called the grand unification epoch.[22][23]
As the universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These cosmological phase transitions can be visualized as similar to condensation and freezing phase transitions of ordinary matter. At certain temperatures/energies, water molecules change their behavior and structure, and they will behave completely differently. Like steam turning to water, the fields which define the universe's fundamental forces and particles also completely change their behaviors and structures when the temperature/energy falls below a certain point. This is not apparent in everyday life, because it only happens at far higher temperatures than usually seen in the present-day universe.
These phase transitions in the universe's fundamental forces are believed to be caused by a phenomenon of quantum fields called "symmetry breaking".
In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability. In doing so, they completely shift how they interact. Forces and interactions arise due to these fields, so the universe can behave very differently above and below a phase transition. For example, in a later epoch, a side effect of one phase transition is that suddenly, many particles that had no mass at all acquire a mass (they begin to interact differently with the Higgs field), and a single force begins to manifest as two separate forces.
Assuming that nature is described by a so-called Grand Unified Theory (GUT), the grand unification epoch began with a phase transition of this kind, when gravitation separated from the universal combined gauge force. This caused two forces to now exist: gravity, and an electrostrong interaction. There is no hard evidence yet that such a combined force existed, but many physicists believe it did. The physics of this electrostrong interaction would be described by a Grand Unified Theory.
The grand unification epoch ended with a second phase transition, as the electrostrong interaction in turn separated, and began to manifest as two separate interactions, called the strong and the electroweak interactions.
Electroweak epoch[edit]Between 10−36 seconds (or the end of inflation) and 10−32 seconds after the Big Bang[21]Main article: Electroweak epoch
Depending on how epochs are defined, and the model being followed, the electroweak epoch may be considered to start before or after the inflationary epoch. In some models, it is described as including the inflationary epoch. In other models, the electroweak epoch is said to begin after the inflationary epoch ended, at roughly 10−32 seconds.
According to traditional Big Bang cosmology, the electroweak epoch began 10−36 seconds after the Big Bang, when the temperature of the universe was low enough (1028 K) for the electronuclear force to begin to manifest as two separate interactions, the strong and the electroweak interactions. (The electroweak interaction will also separate later, dividing into the electromagnetic and weak interactions.) The exact point where electrostrong symmetry was broken is not certain, owing to speculative and as yet incomplete theoretical knowledge.
Inflationary epoch and the rapid expansion of space[edit]Before c. 10−32 seconds after the Big BangMain articles: Inflation (cosmology) and Expansion of the universe
At this point of the very early universe, the universe is thought to have expanded by a factor of at least 1078 in volume. This is equivalent to a linear increase of at least 1026 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.[24] 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 ended up so exceedingly homogeneous (spatially uniform) on a very large scale, even though it was highly disordered in its earliest stages.
It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10−33 and 10−32 seconds after the Big Bang. The rapid expansion of space meant that elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe. However, the huge potential energy of the 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.
In non-traditional versions of Big Bang theory (known as "inflationary" models), inflation ended at a temperature corresponding to roughly 10−32 seconds after the Big Bang, but this does not imply that the inflationary era lasted less than 10−32 seconds. To explain the observed homogeneity of the universe, the duration in these models must be longer than 10−32 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation.
After inflation ended, the universe continued to expand, but at a 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.
On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-modes power spectrum which was interpreted as clear experimental evidence for the theory of inflation.[25][26][27][28][29] However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported [28][30][31] and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the European Space Agency's Planck microwave space telescope concluded that the statistical "significance [of the data] is too low to be interpreted as a detection of primordial B-modes" and can be attributed mainly to polarized dust in the Milky Way.[32][33][34]
Supersymmetry breaking (speculative)[edit]Main article: Supersymmetry breaking
If supersymmetry is a property of the universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak scale. The masses of particles and their superpartners would then no longer be equal. This very high energy could explain why no superpartners of known particles have ever been observed.
Early universe [edit]
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 and early thermalization[edit]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 1015 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.
Thermalization[edit]See also: Big Bang § Thermalization
The number density of each particle species was, by a similar analysis to Stefan–Boltzmann law:
n=2σT3/ckB≈1053 m−3,
which is roughly just (kBT/ℏc)3. Since the interaction was strong, the cross-section σ was approximately the particle wavelength squared, which is roughly n−2/3. The rate of collisions per particle species can thus be calculated from the mean free path, giving approximately:
σ⋅n⋅c≈n1/3⋅c≈1026s−1.
For comparison, since the cosmological constant was negligible at this stage, the Hubble parameter was:
H≈8πGρ/3≈8πG3c2xnkBT≈ 3⋅1010 s−1,
where x ~ 102 was the number of available particle species.[notes 1]
Thus H is orders of magnitude lower than the rate of collisions per particle species. This means there was plenty of time for thermalization at this stage.
At this epoch, the collision rate is proportional to the third root of the number density, and thus to a−1, where a is the scale parameter. The Hubble parameter, however, is proportional to a−2. Going back in time and higher in energy, and assuming no new physics at these energies, a careful estimate gives that thermalization was first possible when the temperature was:[35]
Tthermalization≈2.5⋅1014 GeV≈1027 K,
approximately 10−22 seconds after the Big Bang.
Electroweak symmetry breaking[edit]10−12 seconds after the Big BangMain article: Electroweak symmetry breaking
As the universe's temperature continued to fall below 159.5±1.5 GeV, electroweak symmetry breaking happened.[36] 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[edit]Between 10−12 seconds and 10−5 seconds after the Big BangMain article: Quark epoch
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.[37][38][better source needed]
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.[37]
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.[37]
Baryogenesis[edit]Perhaps by 10−11 seconds[citation needed]Main article: BaryogenesisFurther information: Leptogenesis (physics)
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.
Hadron epoch[edit]Between 10−5 second and 1 second after the Big BangMain article: Hadron epoch
The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. Initially, hadron/anti-hadron pairs could form, so matter and antimatter were in thermal equilibrium. However, as the temperature of the universe continued to fall, new hadron/anti-hadron pairs were no longer produced, and most of the newly formed hadrons and anti-hadrons annihilated each other, giving rise to pairs of high-energy photons. A comparatively small residue of hadrons remained at about 1 second of cosmic time, when this epoch ended.
Theory predicts that about 1 neutron remained for every 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.[39]
Neutrino decoupling and cosmic neutrino background (CνB)[edit]Around 1 second after the Big BangMain articles: Neutrino decoupling and Cosmic neutrino background
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.[40] 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.[40]
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.[40]
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.[40]
Possible formation of primordial black holes[edit]May have occurred within about 1 second after the Big BangMain article: Primordial black hole
Primordial black holes are a hypothetical type of black hole proposed in 1966,[41] 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. Any primordial black holes would have to have less mass than an asteroid to avoid detection.[42]: 23
Lepton epoch[edit]Between 1 second and 10 seconds after the Big BangMain article: 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 antileptons, dominating the mass of the universe.
The lepton epoch follows a similar path to the earlier hadron epoch. 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.[43][44][45]
Photon epoch[edit]Between 10 seconds and 370,000 years after the Big BangMain article: Photon epoch
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.[45] (Much of the rest of its mass–energy is in the form of neutrinos and other relativistic particles.[citation needed]) 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[edit]Between 2 minutes and 20 minutes after the Big Bang[46]Main article: Big Bang nucleosynthesis
Between about 2 and 20 minutes after the Big Bang, the temperature and pressure of the universe allowed 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[39] 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.[47]
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.[39] Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and are quickly lost again.[39] A small amount of deuterium is left unfused because of the very short duration.[39]
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.[48] 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.[49][50]
The amounts of each light element in the early universe can be estimated from old galaxies, and is strong evidence for the Big Bang.[39] 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.[39] 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.[39] 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.[39]
Matter domination[edit]47,000 years after the Big BangMain articles: Matter-dominated era and 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.[51] As the universe cools, from around 47,000 years (redshift z = 3600),[4] 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.[52] 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)[edit]Main articles: Recombination (cosmology) and decoupling (cosmology)
9-year WMAP image of the cosmic microwave background radiation (2012).[53][54] The radiation is isotropic to roughly one part in 100,000.[55]
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.[56] This process is known as recombination.[57] 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.[58] In April 2019, this molecule was first announced to have been observed in interstellar space, in NGC 7027, a planetary nebula within this galaxy.[58] (Much later, atomic hydrogen reacted with helium hydride to create molecular hydrogen, the fuel required for star formation.[58])
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.[59] 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.[60]
Gravity builds cosmic structure [edit]370 thousand to about 1 billion years after the Big Bang[61]See also: Hydrogen line and List of the most distant astronomical objectsDark Ages [edit]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.