Chronology of the universe#The very early universe

{{Further|Timeline of the universe|Timeline of natural history|Geologic time scale|Timeline of the evolutionary history of life|Timeline of the far future}}

: Note: The radiation temperature in the table below refers to the cosmic microwave background radiation and is given by 2.725 K·(1 + {{mvar|z}}), where {{mvar|z}} is the redshift.

{{Main|Big Bang|Cosmogony|l2=Origin of the universe|Why there is anything at all|l3="Why is there anything at all?"}}

The observed expansion of the universe led to a physical model called 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.{{rp|305}}

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.{{rp|305}}

: 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.{{Cite book |last=Peacock |first=J. A. |url=https://www.cambridge.org/core/product/identifier/9780511804533/type/book |title=Cosmological Physics |date=1998-12-28 |publisher=Cambridge University Press |isbn=978-0-521-41072-4 |edition=1 |doi=10.1017/cbo9780511804533}}{{rp|274}}

: Between 10⁻⁴³ seconds and 10⁻³⁶ seconds after the Big Bang{{harvnb|Ryden|2003|p=196}}

{{Main|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. Nevertheless, the time between 10⁻⁴³ and 10⁻³⁶ seconds has been called the grand unification epoch.{{Cite book |last=Silk |first=Joseph |author-link=Joseph Silk |url=https://www.google.com/books/edition/On_the_Shores_of_the_Unknown/JowbGgYiSDYC?hl=en&gbpv=1&dq=%22Grand+unification+epoch%22+-wikipedia&pg=PA89&printsec=frontcover |title=On the Shores of the Unknown: A Short History of the Universe |date=2005-01-03 |publisher=Cambridge University Press |isbn=978-0-521-83627-2 |pages=87 |language=en}}{{Cite book |last=Baggott |first=Jim |author-link=Jim Baggott |url=https://www.google.com/books/edition/Origins/f19gDwAAQBAJ?hl=en&gbpv=1&dq=%22Grand+unification+epoch%22+-wikipedia&pg=PA31&printsec=frontcover |title=Origins: The Scientific Story of Creation |date=2018-06-07 |publisher=Oxford University Press |isbn=978-0-19-256196-1 |pages=30 |language=en}}

Before the GUT epoch, the temperature of the universe exceeded 10¹⁵ 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.{{rp|305}}

: Between 10⁻³⁶ seconds (or the end of inflation) and 10⁻³² seconds after the Big Bang

{{Main|Electroweak epoch}}

Depending on how epochs are defined, and the model being followed, the electroweak epoch may be considered to have started 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 have begun after the inflationary epoch ended, at roughly 10⁻³² seconds.

: Before c. 10⁻³² seconds after the Big Bang

{{Main|Inflation (cosmology)|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 {{convert|10.6|ly|e12km|abbr=off}} 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.{{Cite journal |last=Lidsey |first=James E. |date=1997 |title=Reconstructing the inflaton potential—an overview |url=https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.69.373 |journal=Reviews of Modern Physics |volume=69 |issue=2 |pages=373–410 |doi=10.1103/RevModPhys.69.373|arxiv=astro-ph/9508078 }} 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).{{rp|324}}

Tiny quantum fluctuations in the universe, amplified by inflation, are believed to be the basis of large-scale structures that formed much later.{{sfn|Durrer|2008|p=2}}

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.

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.

: 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.

: 10⁻¹² seconds after the Big Bang

{{Main|Higgs mechanism|l1=Electroweak symmetry breaking}}

As the universe's temperature continued to fall below 159.5±1.5 GeV, electroweak symmetry breaking happened.{{cite journal |last1=D'Onofrio |first1=Michela |last2=Rummukainen |first2=Kari |date=15 January 2016 |title=Standard model cross-over on the lattice |journal=Physical Review D |volume=93 |number=2 |page=025003 |arxiv=1508.07161 |doi=10.1103/PhysRevD.93.025003 |bibcode=2016PhRvD..93b5003D |s2cid=119261776 }} 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:

1. Via the Higgs mechanism, all elementary particles interacting with the Higgs field become massive, having been massless at higher energy levels.
2. 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.)

: Between 10⁻¹² seconds and 10⁻⁵ seconds after the Big Bang

{{Main|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.{{harvnb|Petter|2013|p=[https://books.google.com/books?id=Ne69AQAAQBAJ&pg=PA68 68]}}{{harvnb|Morison|2015|p=[https://books.google.com/books?id=GZx7BAAAQBAJ&pg=PA298 298]}}{{better source needed|reason=The original text for section 4.1 is from a 24 Feb. 2007 edit by Gandalf61 citing no source. The two references provided by Gandalf61 for the main 'Quark epoch' article are more likely the proper sources for the text. I do not have complete access to Allday (2002) or to the undated Britt's(?) 'Physics 175: Stars and Galaxies' (PDF) (Wayback Machine from 6 Feb. 2012 could not find the archived PDF). I provided referneces to books by Petter (2013) and Morison (2015), but I have a feeling that these may be rather tenuous sources. This pattern is repeated elsewhere in this article.|date=January 2020}}

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.

: Perhaps by 10⁻¹¹ seconds{{citation needed|date=April 2018}}

{{Main|Baryogenesis}}

{{Further|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.

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.

: Around 1 second after the Big Bang

{{Main|Neutrino decoupling|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.{{cite web |url=https://www.forbes.com/sites/startswithabang/2016/09/09/cosmic-neutrinos-detected-confirming-the-big-bangs-last-great-prediction |url-status=live |title=Cosmic Neutrinos Detected, Confirming The Big Bang's Last Great Prediction |last=Siegel |first=Ethan |author-link=Ethan Siegel |date=9 September 2016 |department=Science |website=Forbes |location=Jersey City, NJ |format=Blog |issn=0015-6914 |archive-url=https://web.archive.org/web/20160910124614/http://www.forbes.com/sites/startswithabang/2016/09/09/cosmic-neutrinos-detected-confirming-the-big-bangs-last-great-prediction/#6209dfdd63e1 |archive-date=10 September 2016 |access-date=7 January 2020}}

• Coverage of original paper: {{cite journal |last1=Follin |first1=Brent |last2=Knox |first2=Lloyd |last3=Millea |first3=Marius |last4=Pan |first4=Zhen |display-authors=3 |title=First Detection of the Acoustic Oscillation Phase Shift Expected from the Cosmic Neutrino Background |journal=Physical Review Letters |date=26 August 2015 |volume=115 |issue=9 |pages=091301 |arxiv=1503.07863 |bibcode=2015PhRvL.115i1301F |doi=10.1103/PhysRevLett.115.091301 |pmid=26371637 |s2cid=24763212 }} 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 ({{val|1.96|0.02|u=K}} compared to a prediction of {{val|1.95|u=K}}), and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.

: May have occurred within about 1 second after the Big Bang

{{Main|Primordial black hole}}

Primordial black holes are a hypothetical type of black hole proposed in 1966,{{cite journal |last1=Zel'dovitch |first1=Yakov B. |author1-link=Yakov Zeldovich |last2=Novikov |first2=Igor D. |author2-link=Igor Dmitriyevich Novikov |date=January–February 1967 |title=The Hypothesis of Cores Retarded During Expansion and the Hot Cosmological Model |journal=Soviet Astronomy |volume=10 |issue=4 |pages=602–603 |bibcode=1967SvA....10..602Z}}

• Translated from: {{cite journal |last1=Zel'dovitch |first1=Yakov B. |author1-link=Yakov Zeldovich |last2=Novikov |first2=Igor D. |author2-link=Igor Dmitriyevich Novikov |date=July–August 1966 |title=The Hypothesis of Cores Retarded During Expansion and the Hot Cosmological Model |journal=Astronomicheskii Zhurnal |volume=43 |issue=4 |pages=758–760 |bibcode=1966AZh....43..758Z}} 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.

: Between 1 second and 10 seconds after the Big Bang

{{Main|Lepton epoch}}

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.{{cite web |url=https://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf |url-status=live |title=Thermal history of the universe and early growth of density fluctuations |last=Kauffmann |first=Guinevere |author-link=Guinevere Kauffmann |publisher=Max Planck Institute for Astrophysics |location=Garching |type=Lecture |archive-url=https://web.archive.org/web/20190811021409/https://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf |archive-date=11 August 2019 |access-date=7 January 2020}}{{cite web |url=https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/fr_1/fr_1_part3.html |url-status=live |title=First Few Minutes |last=Chaisson |first=Eric J. |author-link=Eric Chaisson |year=2013 |website=Cosmic Evolution |publisher=Harvard–Smithsonian Center for Astrophysics |location=Cambridge, MA |archive-url=https://web.archive.org/web/20190702121327/https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/fr_1/fr_1_part3.html |archive-date=2 July 2019 |access-date=7 January 2020}}{{cite web |url=https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html |url-status=live |title=Timeline of the Big Bang |website=The Physics of the Universe |archive-url=https://web.archive.org/web/20190722222738/https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html |archive-date=22 July 2019 |access-date=7 January 2020}}

: Between 10 seconds and 370,000 years after the Big Bang

{{Main|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. (Much of the rest of its mass–energy is in the form of neutrinos and other relativistic particles.{{citation needed|date=September 2018}}) 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.

: Between 2 minutes and 20 minutes after the Big Bang{{cite web |url=http://www.astro.ucla.edu/~wright/BBNS.html |url-status=live |title=Big Bang Nucleosynthesis |last=Wright |first=Edward L. |author-link=Edward L. Wright |date=26 September 2012 |website=Ned Wright's Cosmology Tutorial |publisher=Division of Astronomy & Astrophysics, University of California, Los Angeles |location=Los Angeles |archive-url=https://web.archive.org/web/20190905050415/http://www.astro.ucla.edu/~wright/BBNS.html |archive-date=5 September 2019 |access-date=21 September 2018}}

{{Main|Big Bang nucleosynthesis}}

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), ³He, ⁴He, with trace amounts of ⁷Li and ⁷Be. 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.{{Cite journal |last=Schramm |first=David N. |last2=Turner |first2=Michael S. |date=1998-01-01 |title=Big-bang nucleosynthesis enters the precision era |url=https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.70.303 |journal=Reviews of Modern Physics |volume=70 |issue=1 |pages=303–318 |doi=10.1103/RevModPhys.70.303|arxiv=astro-ph/9706069 }}{{rp|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.{{Cite book |last=Dodelson |first=Scott |title=Modern cosmology |date=2003 |publisher=Academic Press |isbn=978-0-12-219141-1 |location=San Diego, Calif}}{{rp|65}}

Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and are quickly decay.{{cite journal |last=Karki |first=Ravi |date=May 2010 |title=The Foreground of Big Bang Nucleosynthesis |url=https://www.nepjol.info/index.php/HP/article/download/5186/4314 |url-status=live |format=PDF |journal=The Himalayan Physics |volume=1 |issue=1 |pages=79–82 |doi=10.3126/hj.v1i0.5186 |archive-url=https://web.archive.org/web/20180921114731/https://www.nepjol.info/index.php/HP/article/download/5186/4314 |archive-date=21 September 2018 |access-date=21 September 2018|doi-access=free }} A small amount of deuterium is left unfused.{{rp|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.{{rp|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.{{rp|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.{{cite journal|author1=Elcio Abdalla|author2=Guillermo Franco Abellán|author3=Amin Aboubrahim|display-authors=2|title=Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies|journal=Journal of High Energy Astrophysics |arxiv=2203.06142v1|date=11 Mar 2022|volume=34 |page=49 |doi=10.1016/j.jheap.2022.04.002 |bibcode=2022JHEAp..34...49A |s2cid=247411131 }}

: 47,000 years after the Big Bang

{{Main|Scale factor (cosmology)#Matter-dominated era|l1=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.{{harvnb|Ryden|2006}} 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.{{sfn|Zeilik|Gregory|1998|p=497}} 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.

{{Main|Recombination (cosmology)|decoupling (cosmology)}}

{{anchor|9-year WMAP image}}File:WMAP 2012.png image of the cosmic microwave background radiation (2012).{{cite web |url=https://www.space.com/19027-universe-baby-picture-wmap.html |url-status=live |title=New 'Baby Picture' of Universe Unveiled |last=Gannon |first=Megan |date=21 December 2012 |website=Space.com |location=New York |publisher=Future plc |archive-url=https://web.archive.org/web/20191029114309/https://www.space.com/19027-universe-baby-picture-wmap.html |archive-date=29 October 2019 |access-date=10 January 2020}}{{cite journal |last1=Bennett |first1=Charles L. |author1-link=Charles L. Bennett |last2=Larson |first2=Davin |last3=Weiland |first3=Janet L. |date=October 2013 |title=Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results |arxiv=1212.5225 |display-authors=etal |doi=10.1088/0067-0049/208/2/20 |volume=208 |issue=2 |page=Article 20 |journal=The Astrophysical Journal Supplement Series |bibcode=2013ApJS..208...20B |s2cid=119271232 }} The radiation is isotropic to roughly one part in 100,000.{{harvnb|Wright|2004|p=291}}]]

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 {{chem|He|+}} atoms. Neutral helium nuclei then start to form at around 100,000 years, with neutral hydrogen formation peaking around 260,000 years.{{cite journal |last1=Sunyaev |first1=R. A. |last2=Chluba |first2=J. |title=Signals From the Epoch of Cosmological Recombination |journal=Astronomical Notes |date=August 2009 |volume=330 |issue=7 |pages=657–674 |doi=10.1002/asna.200911237 |arxiv=0908.0435 |doi-access=free }} This process is known as recombination.{{sfn|Mukhanov|2005|p=120}} 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.{{cite web |url=https://www.space.com/astronomers-detect-universe-first-molecule-space.html |url-status=live |last=Mathewson |first=Samantha |date=18 April 2019 |title=Astronomers Finally Spot Universe's First Molecule in Distant Nebula |website=Space.com |location=New York |publisher=Future plc |archive-url=https://web.archive.org/web/20191117101703/https://www.space.com/astronomers-detect-universe-first-molecule-space.html |archive-date=17 November 2019 |access-date=10 January 2020}} 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 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.{{cite web |url=https://www.mediacollege.com/lighting/colour/colour-temperature.html |title=Color Temperature Chart |website=MediaCollege.com |location=Te Awamutu |publisher=Wavelength Media |access-date=21 September 2018}} 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.{{cite news |last=Amos |first=Jonathan |date=13 November 2012 |title=Quasars illustrate dark energy's roller coaster ride |url=https://www.bbc.com/news/science-environment-20303592 |url-status=live |department=Science & Environment |work=BBC News |location=London |publisher=BBC |archive-url=https://web.archive.org/web/20191221020427/https://www.bbc.com/news/science-environment-20303592 |archive-date=21 December 2019 |access-date=11 January 2020}}

: 370 thousand to about 1 billion years after the Big Bang{{cite magazine |last=Loeb |first=Abraham |author-link=Avi Loeb |date=November 2006 |title=The Dark Ages of the Universe |url=https://www.cfa.harvard.edu/~loeb/sciam.pdf |url-status=live |magazine=Scientific American |doi=10.1038/scientificamerican1106-46 |volume=295 |issue=5 |pages=46–53 |archive-url=https://web.archive.org/web/20190326231454/https://www.cfa.harvard.edu/~loeb/sciam.pdf |archive-date=26 March 2019 |access-date=11 January 2020 }}

{{See also|Hydrogen line|List of the most distant astronomical objects}}

{{See also|Hydrogen line|l1=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 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,{{citation needed|date=October 2018}} 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.

{{anchor|First light}}The first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang.{{cite web |url=http://www.astro.caltech.edu/~rse/firstlight/ |url-status=live |title=Searching for First Light in the Early Universe |last=Ellis |first=Richard |author-link=Richard Ellis (astronomer) |website=Richard Ellis's Homepage |location=Pasadena, CA |publisher=Astronomy Department, California Institute of Technology |archive-url=https://web.archive.org/web/20011212014043/http://www.astro.caltech.edu/~rse/firstlight/ |archive-date=12 December 2001 |access-date=21 January 2007}} 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.{{Citation needed|date=January 2023}} 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.{{Citation needed|date=January 2023}}

File:NASA-WMAP-first-stars.jpg

{{main|Hubble Space Telescope|James Webb Space Telescope|List of the most distant astronomical objects}}

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).{{Cite web |url=http://news.yale.edu/2016/03/03/shattering-cosmic-distance-record-once-again |title=Shattering the cosmic distance record, once again |publisher=Yale University |first=Jim |last=Shelton |date=March 3, 2016 |access-date=March 4, 2016}}{{cite web |url=http://www.spacetelescope.org/news/heic1604/ |title=Hubble breaks cosmic distance record |website=SpaceTelescope.org |id=heic1604 |date=March 3, 2016 |access-date=March 3, 2016}}{{cite journal |title=A Remarkably Luminous Galaxy at z=11.1 Measured with Hubble Space Telescope Grism Spectroscopy |journal=The Astrophysical Journal |first1=P. A. |last1=Oesch |first2=G. |last2=Brammer |first3=P. |last3=van Dokkum |display-authors=etal |volume=819 |issue=2 |at=129 |date=March 2016 |arxiv=1603.00461 |bibcode=2016ApJ...819..129O |doi=10.3847/0004-637X/819/2/129|s2cid=119262750 |doi-access=free }}{{Cite news|url=https://www.sciencealert.com/even-when-hubble-looks-as-far-back-in-time-as-possible-it-still-can-t-find-the-first-stars|title=Hubble Has Looked Back in Time as Far as It Can And Still Can't Find The First Stars|first=Nancy |last=Atkinson |work=Universe Today |via=ScienceAlert}} 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).{{Cite web|url=https://briankoberlein.com/blog/deeper-sky|title=A Deeper Sky | by Brian Koberlein|website=briankoberlein.com}}{{Cite web|url=https://jwst.nasa.gov/content/forScientists/faqScientists.html|title=FAQ for Scientists Webb Telescope/NASA|website=jwst.nasa.gov}} 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.

: Around 150 million to 1 billion years after the Big Bang

{{See also|Star formation|l1=Stellar formation|Dwarf galaxy|Baryon acoustic oscillations|Observable universe#Large-scale structure|l4=Large-scale structure|Structure formation|Future of an expanding universe#Stelliferous Era|l6=Stelliferous Era}}

File:Hubble ultra deep field.jpgs often showcase galaxies from an ancient era that tell us what the early Stelliferous Era was like.]]

File:Hubble - infant galaxy.jpg

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.{{cite web |url=https://sci.esa.int/web/planck/-/58193-first-stars-formed-even-later-than-previously-thought |url-status=bot: unknown |title=First stars formed even later than we thought |date=31 August 2016 |website=ESA Science & Technology |location=Paris |publisher=European Space Agency |archive-url=https://web.archive.org/web/20200129094032/https://sci.esa.int/web/planck/-/58193-first-stars-formed-even-later-than-previously-thought |archive-date=29 January 2020 |access-date=12 January 2020 }}

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.

{{See also|Reionization|Dwarf galaxy|Quasar}}

File:Conditions During the Era of Reionization (Illustration) (2023-122).jpg

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.{{cite journal |first=Mark |last=Dijkstra |date=22 October 2014 |title=Lyα Emitting Galaxies as a Probe of Reionization |journal=Publications of the Astronomical Society of Australia |volume=31 |page=e040 |arxiv=1406.7292 |doi=10.1017/pasa.2014.33 |bibcode=2014PASA...31...40D |s2cid=119237814 }} 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.{{cite journal |last1=Madau |first1=Piero |last2=Haardt |first2=Francesco |last3=Rees |first3=Martin J. |author3-link=Martin Rees |date=1 April 1999 |title=Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Source |journal=The Astrophysical Journal |volume=514 |issue=2 |pages=648–659 |arxiv=astro-ph/9809058 |bibcode=1999ApJ...514..648M |doi=10.1086/306975 |s2cid=17932350 }} With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy.{{cite journal |last1=Barkana |first1=Rennan |last2=Loeb |first2=Abraham |author-link2=Avi Loeb |date=July 2001 |title=In the Beginning: The First Sources of Light and the Reionization of the Universe |journal=Physics Reports |volume=349 |issue=2 |pages=125–238 |arxiv=astro-ph/0010468 |bibcode=2001PhR...349..125B |doi=10.1016/S0370-1573(01)00019-9 |s2cid=119094218 }} The current leading candidates from most to least significant are currently believed to be Population III stars (the earliest stars; possibly 70%),{{cite journal |last1=Gnedin |first1=Nickolay Y. |last2=Ostriker |first2=Jeremiah P. |date=10 September 1997 |title=Reionization of the Universe and the Early Production of Metals |journal=The Astrophysical Journal |volume=486 |issue=2 |pages=581–598 |arxiv=astro-ph/9612127 |bibcode=1997ApJ...486..581G |doi=10.1086/304548 |s2cid=5758398 }}{{cite arXiv |last1=Lu |first1=Limin |last2=Sargent |first2=Wallace L. W. |author2-link=Wallace L. W. Sargent |last3=Barlow |first3=Thomas A. |last4=Rauch |first4=Michael |display-authors=3 |date=13 February 1998 |title=The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium |eprint=astro-ph/9802189}} dwarf galaxies (very early small high-energy galaxies; possibly 30%),{{cite journal |last1=Bouwens |first1=Rychard J. |author1-link=Rychard Bouwens |last2=Illingworth |first2=Garth D. |last3=Oesch |first3=Pascal A. |display-authors=etal |title=Lower-luminosity Galaxies Could Reionize the Universe: Very Steep Faint-end Slopes to the UV Luminosity Functions at z ≥ 5–8 from the HUDF09 WFC3/IR Observations |date=10 June 2012 |journal=The Astrophysical Journal Letters |volume=752 |issue=1 |page=Article L5 |arxiv=1105.2038 |bibcode=2012ApJ...752L...5B |doi=10.1088/2041-8205/752/1/L5 |s2cid=118856513 }} and a contribution from quasars (a class of active galactic nuclei).{{cite journal |last1=Shapiro |first1=Paul R.|author1-link=Paul R. Shapiro|last2=Giroux |first2=Mark L. |date=15 October 1987 |title=Cosmological H II Regions and the Photoionization of the Intergalactic Medium |journal=The Astrophysical Journal |volume=321 |pages=L107–L112 |bibcode=1987ApJ...321L.107S |doi=10.1086/185015 |doi-access=free }}{{cite journal |last1=Xiaohu |first1=Fan |author1-link=Xiaohui Fan |last2=Narayanan |first2=Vijay K. |last3=Lupton |first3=Robert H. |display-authors=etal |date=December 2001 |title=A Survey of z > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z ~ 6 |journal=The Astrophysical Journal |volume=122 |issue=6 |pages=2833–2849 |arxiv=astro-ph/0108063 |bibcode=2001AJ....122.2833F |doi=10.1086/324111 |s2cid=119339804 }}

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.{{cite news |last=Wood |first=Charlie |title=JWST Spots Giant Black Holes All Over the Early Universe – Giant black holes were supposed to be bit players in the early cosmic story. But recent James Webb Space Telescope observations are finding an unexpected abundance of the beasts. |url=https://www.quantamagazine.org/jwst-spots-giant-black-holes-all-over-the-early-universe-20230814/ |date=14 August 2023 |work=Quanta Magazine |url-status=live |archiveurl=https://archive.today/20230815000505/https://www.quantamagazine.org/jwst-spots-giant-black-holes-all-over-the-early-universe-20230814/ |archivedate=15 August 2023 |accessdate=5 November 2023 }}

{{See also|Galaxy formation and evolution}}

File:Large-scale structure of light distribution in the universe.jpg |agency=UCL Media Relations |date=27 August 2014 |archive-url=https://web.archive.org/web/20161005231610/http://www.ucl.ac.uk/mathematical-physical-sciences/news-events/maps-news-publication/maps1423 |archive-date=5 October 2016 |access-date=14 January 2020}}]]

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).{{cite web |url=https://apod.nasa.gov/apod/ap040309.html |title=The Hubble Ultra Deep Field |editor1-last=Nemiroff |editor1-first=Robert J. |editor1-link=Robert J. Nemiroff |editor2-last=Bonnell |editor2-first=Jerry |date=9 March 2004 |website=Astronomy Picture of the Day |publisher=NASA; Michigan Technological University |location=Washington, D.C.; Houghton, MI |archive-url=https://web.archive.org/web/20191007075825/https://apod.nasa.gov/apod/ap040309.html |archive-date=7 October 2019 |access-date=22 September 2018}} (This age estimate is now believed to be slightly overstated).{{cite news |last=Landau |first=Elizabeth |author-link=Elizabeth Landau |date=25 October 2013 |orig-year=Originally published 23 October 2013 |title=Scientists confirm most distant galaxy ever |url=https://www.cnn.com/2013/10/23/tech/innovation/most-distant-galaxy/index.html |url-status=live |work=CNN |location=New York |publisher=Warner Media, LLC |archive-url=https://web.archive.org/web/20131024035400/https://www.cnn.com/2013/10/23/tech/innovation/most-distant-galaxy/index.html |archive-date=24 October 2013 |access-date=21 September 2018}}

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.{{Cite journal |last=Soderblom |first=David R. |date=2010-08-01 |title=The Ages of Stars |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-081309-130806 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=48 |issue=1 |pages=581–629 |doi=10.1146/annurev-astro-081309-130806 |issn=0066-4146|arxiv=1003.6074 }}

: From about 9.8 billion years after the Big Bang

{{Main|Dark energy|Scale factor (cosmology)}}

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".{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=20 February 2017 |title=Cosmos Controversy: The Universe Is Expanding, but How Fast? |url=https://www.nytimes.com/2017/02/20/science/hubble-constant-universe-expanding-speed.html |url-status=live |url-access=registration |department=Out There |newspaper=The New York Times |issn=0362-4331 |archive-url=https://web.archive.org/web/20191112051927/https://www.nytimes.com/2017/02/20/science/hubble-constant-universe-expanding-speed.html |archive-date=12 November 2019 |access-date=21 February 2017}} "A version of this article appears in print on Feb. 21, 2017, Section D, Page 1 of the New York edition with the headline: A Runaway Universe."{{cite journal |last1=Peebles |first1=P. J. E. |author-link1=Jim Peebles |last2=Ratra |first2=Bharat |author-link2=Bharat Ratra |date=22 April 2003 |title=The cosmological constant and dark energy |journal=Reviews of Modern Physics |arxiv=astro-ph/0207347 |volume=75 |issue=2 |pages=559–606 |doi=10.1103/RevModPhys.75.559 |bibcode=2003RvMP...75..559P |s2cid=118961123 }} "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.

{{Main|Ultimate fate of the universe|Timeline of the far future}}

{{Further|Future of an expanding universe|Heat death of the universe}}File:Red dwarf lifetime.png star plotted against its mass relative to the Sun{{harvnb|Adams|Laughlin|Graves|2004}}]]

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.{{cn|date=January 2025}}

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.{{cn|date=January 2025}}

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

In this kind of extreme timescale, extremely rare quantum phenomena may also occur that are extremely unlikely to be seen on a timescale smaller than trillions of years. These may also lead to unpredictable changes to the state of the universe which would not be likely to be significant on any smaller timescale. For example, on a timescale of millions of trillions of years, black holes might appear to evaporate almost instantly, uncommon quantum 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.{{citation needed|date=January 2020}}

{{div col|colwidth=30em}}

• {{annotated link|Age of the universe}}
• {{annotated link|Cosmic Calendar}} (age of the universe scaled to a single year)
• {{annotated link|Cyclic model}}
• {{annotated link|Scale factor (cosmology)#Dark-energy-dominated era|Dark-energy-dominated era}}
• {{annotated link|Dyson's eternal intelligence}}
• {{annotated link|Entropy (arrow of time)}}
• {{annotated link|Illustris project}}
• {{annotated link|Scale factor (cosmology)#Matter-dominated era|Matter-dominated era}}
• {{annotated link|Scale factor (cosmology)#Radiation-dominated era|Radiation-dominated era}}
• {{annotated link|Timeline of the early universe}}
• {{annotated link|Timeline of the far future}}
• {{annotated link|Ultimate fate of the universe}}

{{div col end}}

{{reflist|group=notes}}

{{reflist|30em}}

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• {{cite conference |url=http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf |url-status=live |title=Red Dwarfs and the End of the Main Sequence |last1=Adams |first1=Fred C. |author1-link=Fred Adams |last2=Laughlin |first2=Gregory |author2-link=Gregory P. Laughlin |last3=Graves |first3=Genevieve J. M. |date=December 2004 |conference=First Astrophysics meeting of the Observatorio Astronómico Nacional held in Ensenada, Baja California, Mexico, December 8–12, 2003 |editor1-last=García-Segura |editor1-first=G. |editor2-last=Tenorio-Tagle |editor2-first=G. |editor3-last=Franco |editor3-first=J. |editor4-last=Yorke |editor4-first=H. W. |display-editors=3 |volume=22 |book-title=Gravitational Collapse: From Massive Stars to Planets |pages=46–49 |series=Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias |location=Mexico City |publisher=Instituto de Astronomía, Universidad Nacional Autónoma de México |isbn=978-9-703-21160-9 |oclc=58527824 |issn=1405-2059 |archive-url=https://web.archive.org/web/20190711072446/http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf |archive-date=11 July 2019 |access-date=15 January 2020 }}
• {{cite book|last=Durrer |first=Ruth |author-link=Ruth Durrer |title=The Cosmic Microwave Background |year=2008 |publisher=Cambridge University Press |isbn=978-0-521-84704-9}}
• {{cite journal |last1=Hinshaw |first1=G. |last2=Weiland |first2=J. L. |last3=Hill |first3=R. S. |last4=Odegard |first4=N. |last5=Larson |first5=D. |last6=Bennett |first6=C. L. |last7=Dunkley |first7=J. |last8=Gold |first8=B. |last9=Greason |first9=M. R. |last10=Jarosik |first10=N. |last11=Komatsu |first11=E. |last12=Nolta |first12=M. R. |last13=Page |first13=L. |last14=Spergel |first14=D. N. |last15=Wollack |first15=E. |last16=Halpern |first16=M. |last17=Kogut |first17=A. |last18=Limon |first18=M. |last19=Meyer |first19=S. S. |last20=Tucker |first20=G. S. |last21=Wright |first21=E. L. |title=Five-year Wilkinson microwave anisotropy probe observations: data processing, sky maps, and basic results |journal=The Astrophysical Journal Supplement Series |date=1 February 2009 |url=https://iopscience.iop.org/article/10.1088/0067-0049/180/2/225/pdf |volume=180 |issue=2 |pages=225–245 |doi=10.1088/0067-0049/180/2/225|arxiv=0803.0732 |bibcode=2009ApJS..180..225H |hdl=2152/43109 |s2cid=3629998 }}
• {{cite book |editor1-last=Gibbons |editor1-first=Gary W. |editor1-link=Gary Gibbons |editor2-last=Hawking |editor2-first=Stephen W. |editor2-link=Stephen Hawking |editor3-last= Siklos |editor3-first=Stephen T.C. |year=1983 |title=The Very Early Universe: Proceedings of the Nuffield Workshop, Cambridge, 21 June to 9 July, 1982 |location=Cambridge, UK; New York |publisher=Cambridge University Press |isbn=978-0-521-25349-9 |lccn=83007330 |oclc=9488764 }}
• {{cite book |last=Morison |first=Ian |author-link=Ian Morison |year=2015 |title=A Journey Through the Universe: Gresham Lectures on Astronomy |isbn=978-1-107-07346-3 |location=Cambridge, UK |publisher=Cambridge University Press |lccn=2014016830 |oclc=910903969 }}
• {{cite book |last=Mukhanov |first=Viatcheslav |author-link=Viatcheslav Mukhanov |year=2005 |title=Physical Foundations of Cosmology |location=Cambridge, UK |publisher=Cambridge University Press |isbn=978-0-511-79055-3 |lccn=2006295735 |oclc=859642394 }}
• {{cite book |last=Petter |first=Patrick |year=2013 |title=Basic Knowledge of Astrophysic: A New Way |location=Berlin |publisher=epubli GmbH |isbn=978-3-8442-7203-1 |oclc=863893991 }}
• {{cite book |last=Ryden |first=Barbara Sue |author-link1=Barbara Ryden |year=2003 |title=Introduction to Cosmology |location=San Francisco |publisher=Addison-Wesley |isbn=978-0-8053-8912-8 |lccn=2002013176 |oclc=1087978842 }}
• {{cite book |last=Ryden |first=Barbara Sue |date=13 January 2006 |title=Introduction to Cosmology }}
• {{cite book |last=Ryden |first=Barbara Sue |year=2017 |title=Introduction to Cosmology |edition=2nd |location=New York |publisher=Cambridge University Press |isbn=978-1-107-15483-4 |lccn=2016040124 |oclc=1123190939 }}
• {{cite journal |author=Navas, S. |collaboration=Particle Data Group |year=2024|title=Review of Particle Physics |journal=Physical Review D |volume=110 |issue=3 |pages=1–708 }}
• {{cite book |last=Wright |first=Edward L. |author-link=Edward L. Wright |year=2004 |chapter=Theoretical Overview of Cosmic Microwave Background Anisotropy |editor-last=Freedman |editor-first=Wendy L. |editor-link=Wendy Freedman |title=Measuring and Modeling the Universe |series=Carnegie Observatories Astrophysics Series |volume=2 |pages=291 |location=Cambridge, UK |publisher=Cambridge University Press |arxiv=astro-ph/0305591 |bibcode=2004mmu..symp..291W |isbn=978-0-521-75576-4 |lccn=2005277053 |oclc=937330165 }}
• {{cite book |last1=Zeilik |first1=Michael |last2=Gregory |first2=Stephen A. |year=1998 |title=Introductory Astronomy & Astrophysics |series=Saunders Golden Sunburst Series |edition=4th |location=Fresno, CA |publisher=Thomson Learning |isbn=978-0-03-006228-5 |lccn=97069268 |oclc=813279385 }}

{{refend}}

• {{cite AV media |people=Carroll, Sean M. |date=14 January 2011 |title=Cosmology and the arrow of time: Sean Carroll at TEDxCaltech |medium=Video |url=https://www.youtube.com/watch?v=WMaTyg8wR4Y |url-status=live |location=New York; Vancouver, British Columbia |publisher=TED Conferences LLC |archive-url=https://web.archive.org/web/20191220093224/https://www.youtube.com/watch?v=WMaTyg8wR4Y |archive-date=20 December 2019 |access-date=20 January 2020 }}
• {{cite web |url=https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/splash.html |url-status=live |last=Chaisson |first=Eric J. |author-link=Eric Chaisson |title=Cosmic Evolution: From Big Bang to Humankind |year=2013 |publisher=Harvard–Smithsonian Center for Astrophysics |location=Cambridge, MA |archive-url=https://web.archive.org/web/20190827032825/https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/splash.html |archive-date=27 August 2019 |access-date=19 January 2020}}
• {{cite web |url=https://www.pbs.org/deepspace/timeline/ |url-status=live |title=History of the Universe Timeline |year=2000 |website=Mysteries of Deep Space |location=Arlington, VA |publisher=PBS Online |archive-url=https://web.archive.org/web/20190701171715/https://www.pbs.org/deepspace/timeline/ |archive-date=1 July 2019 |access-date=24 March 2005}}
• {{cite web |url=https://hubblesite.org/ |url-status=dead |title=HubbleSite |location=Baltimore, MD |publisher=Space Telescope Science Institute's Office of Public Outreach |archive-url=https://web.archive.org/web/20200118030438/https://hubblesite.org/ |archive-date=18 January 2020 |access-date=24 March 2005 }}
• {{cite AV media |people=Krauss, Lawrence M. (Speaker); Cornwell, R. Elisabeth (Producer) |date=21 October 2009 |title='A Universe From Nothing' by Lawrence Krauss, AAI 2009 |medium=Video |url=https://www.youtube.com/watch?v=7ImvlS8PLIo |url-status=live |access-date=3 February 2020 |location=Washington, D.C. |publisher=Richard Dawkins Foundation for Reason and Science |archive-url=https://web.archive.org/web/20191221045755/https://www.youtube.com/watch?v=7ImvlS8PLIo |archive-date=21 December 2019}}
• {{cite AV media |people=Lucas, Tom (Director, Writer); Grupper, Jonathan (Director, Writer) |date=18 May 2007 |url=https://exploringtime.org/?page=segments |title=Exploring Time |medium=Television documentary miniseries |location=Silver Spring, MD |publisher=Twin Cities Public Television, Red Hill Studios, and NHK for The Science Channel |access-date=19 January 2020}}
• {{cite web |url=https://onceuponauniverse.com/ |url-status=live |title=Once Upon a Universe |date=26 March 2013 |location=Swindon, UK |publisher=Science and Technology Facilities Council |archive-url=https://web.archive.org/web/20190509150726/https://onceuponauniverse.com/ |archive-date=9 May 2019 |access-date=20 January 2020}}
• {{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=17 March 2006 |title=Astronomers Find the Earliest Signs Yet of a Violent Baby Universe |url=https://www.nytimes.com/2006/03/17/science/space/astronomers-find-the-earliest-signs-yet-of-a-violent-baby.html |url-access=registration |newspaper=The New York Times |location=New York |issn=0362-4331 |access-date=19 January 2020 }}
• {{cite AV media |people=Plait, Phil |date=14 January 2016 |title=Deep Time: Crash Course Astronomy #45 |medium=Video |url=https://www.youtube.com/watch?v=jDF-N3A60DE |url-status=live |publisher=PBS Digital Studios |archive-url=https://web.archive.org/web/20160115202426/https://www.youtube.com/watch?v=jDF-N3A60DE |archive-date=15 January 2016 |access-date=2 October 2016 }}
• {{cite web |url=https://www.fnal.gov/pub/presspass/vismedia/gallery/graphics.html |url-status=dead |title=Press Pass - Photo Gallery - Graphics and Illustrations |date=1 January 2004 |website=Fermilab |location=Batavia, IL |archive-url=https://web.archive.org/web/20051227154314/https://www.fnal.gov/pub/presspass/vismedia/gallery/graphics.html |archive-date=27 December 2005 |access-date=19 January 2020}} (See: "Energy time line from the Big Bang to the present" (1984) and "History of the Universe Poster" (1989).)
• {{cite web |url=http://members.bellatlantic.net/~vze3fs8i/hist/hist.html |url-status=dead |title=The History of the Universe in 200 Words or Less |last=Schulman |first=Eric |author-link=Eric Schulman |year=1997 |archive-url=https://web.archive.org/web/20051124060752/http://members.bellatlantic.net/~vze3fs8i/hist/hist.html |archive-date=24 November 2005 |access-date=24 March 2005 }}
• {{cite web |url=https://universeadventure.org/ |url-status=live |title=The Universe Adventure |year=2007 |location=Berkeley, CA |publisher=Lawrence Berkeley National Laboratory |archive-url=https://web.archive.org/web/20190622181127/https://universeadventure.org/ |archive-date=22 June 2019 |access-date=21 January 2020}}
• {{cite web |url=http://www.astro.ucla.edu/~wright/cosmology_faq.html |url-status=live |title=Frequently Asked Questions in Cosmology |last=Wright |first=Edward L. |author-link=Edward L. Wright|date=24 May 2013 |location=Los Angeles |publisher=Division of Astronomy & Astrophysics, University of California, Los Angeles |archive-url=https://web.archive.org/web/20191210001221/http://www.astro.ucla.edu/~wright/cosmology_faq.html |archive-date=10 December 2019 |access-date=19 January 2020}}

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