The chronology of the universe describes the history and future of the universe according to the current understanding of physical cosmology. In this model, the earliest stage that is supported by observational evidence is known as inflation, which occurred 13.8 billion years ago. During this epoch, space underwent a period of extremely rapid expansion in a tiny fraction of a second. Once inflation ended, the energy that was responsible for it was converted into particles and radiation, heating up the universe into a very hot and dense state, initiating the Big Bang.
At some early stage, baryogenesis produced a small excess of matter over antimatter. Most matter and antimatter particles annihilated each other in pairs, leaving behind a small excess of matter and a large amount of radiation. As the universe cooled further, many other heavy particles annihilated each other or decayed, eventually leaving behind a plasma that was dominated by protons, neutrons, electrons, photons, and neutrinos.
After the first second, the plasma became dilute enough so that neutrinos ceased interacting efficiently with the other particles and instead started free streaming through the universe, producing the primordial neutrino background. Five seconds later, pair production created electrons and positrons annihilated each other, transferring their energy into the rest of the plasma.
Within the first three minutes, the temperature became low enough to enable stable nuclei to start forming in a process known as Big Bang nucleosynthesis, resulting in hydrogen, helium, and a small amount of lithium. In most models of cosmology, dark matter is usually assumed to have been produced by this epoch, although its production mechanism is unknown. Around 380,000 years later, electrons were captured by the nucleons, forming stable atoms. With this, the universe became transparent to photons, producing the cosmic microwave background.
The next phase of the universe involved the gradual gravitational collapse of the atomic gas. As the gas was compressed and heated, it eventually ignited nuclear fusion, creating the first stars. Gravitational attraction over the subsequent millions of years formed galaxies and the larger structures visible today. After a further nine billion years, the universe transitioned from a matter-dominated universe into a dark energy–dominated universe, leading to the accelerated cosmic expansion that we see today.
Many modifications to the standard timeline have been proposed, introducing novel phases of expansion, particles, or other cosmological mechanisms. These often focus on modifications to the chronology before Big Bang nucleosynthesis, since there is a lack of direct observational evidence to determine exactly what happened. Viable alternatives to inflation that still result in the observed large-scale structure have also been proposed.
Background Expansion
The current accepted model of the history of the universe is based on the concept of inflationary cosmology and the Big Bang. During inflation, the universe expanded at an exponential rate. Once this ended, cosmic expansion subsequently decelerated to much slower rates. The energy driving inflation was converted to a hot and dense plasma, initiating the hot Big Bang, during which the universe cooled as it expanded. Different particles interact during each major stage in the expansion; as the universe expands the density falls and some particle interactions cease to be important. The character of the universe changes. Moreover, the rate of the expansion itself depends upon the nature of the existing particles, creating an interplay between cosmology and particle physics.
Time
In cosmology, time and space are connected: space expands as time increases. Time at each point in space (for example a galaxy) can be uniquely defined in terms of an imaginary clock at that point. These clocks move with the point in space as the universe expands; they are synchronized to a single point in the distant past.
Light from distant galaxies is emitted in the past then travels at the speed of light: knowledge about a distant galaxy is limited to one point in time called the lookback time. During the journey from a distant point, the universe continues to expand, stretching the wavelength of the light along the way, an effect called cosmological redshift. The redshift can be measured by comparing incoming light to known spectroscopic lines and the resulting value can be related to the comoving distance to the emitter. Consequently, experimental knowledge about the chronology of the universe is derived by observing distant light.
Overview
The chronology of the universe can be divided into four parts: Inflation, the first era supported by experimental evidence, a period of exponential expansion that ends with the conversion of energy into particles during reheating, Hot Big Bang, the universe cools and expands from a hot dense state, forming baryons, nuclei, and the Cosmic Microwave Background, Gravity builds cosmic structure, reduced density allows matter to dominate over radiation for control of expansion and gravitational attraction builds stars, galaxies, and clusters of galaxies. Cosmic acceleration, continued expansion allows dark energy to overcome gravitational force, inhibiting larger structures. With these large subsections are many events and transitions. Older models divided the chronology differently, using different terminology or emphasis.
Tabular summary Modern cosmological chronologies begin with inflation, the earliest time period supported by solid observational evidence. Anything earlier is considered non-standard cosmology, the subject of a great deal of as-yet-unconfirmed research. Article subsection Cosmic time Redshift Temperature Description Inflation unknown not applicable Cosmic inflation expands space by a factor of the order of 10 over a time of the order of 10 to 10 seconds. Reheating unknown unknown Converts the energy in the inflation field into a thermal bath of Standard Model particles, initiating the Hot Big Bang. Many mechanisms have been proposed. Baryogenesis unknown unknown Matter and antimatter are created with one extra particle of matter for every 1010 pairs. The pairs annihilate producing photons and leaving the matter particles. Many mechanisms have been proposed but no observations select one. Electroweak phase transition 10 K(100 GeV/kB) The electroweak interaction becomes manifest as two distinct forces, the electromagnetic interaction and weak interaction. Matter particles have mass. The sphere of space that will become the observable universe is approximately 300 light-seconds (~0.6 AU) in radius at this time. Quantum chromodynamics phase transition The quark–gluon plasma of matter particles coalesce into hadrons: mostly protons, neutrons, and pions. Neutrino decoupling 1 s (1 MeV/kB) Neutrinos cease interacting with baryonic matter, and form cosmic neutrino background. The sphere of space that will become the observable universe is approximately 10 light-years in radius at this time. Electron–positron annihilation As the temperature falls, photons no longer have sufficient energy to produce electron/positron pairs. Electrons and positrons annihilate, leaving photons. Big Bang nucleosynthesis Protons and neutrons are bound into primordial atomic nuclei: hydrogen and helium-4. Trace amounts of deuterium, helium-3, and lithium-7 also form. At the end of this epoch, the spherical volume of space which will become the observable universe is about 300 light-years in radius, baryonic matter density is on the order of 4 grams per m (about 0.3% of sea level air density)—however, most energy at this time is in electromagnetic radiation. Recombination 290 ka – 370 ka 1090 – 1270 4000 K(0.4 eV/kB) 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 cubic metre, approximately a billion times higher than today. This density corresponds to pressure on the order of 10 atm. Dark Ages (Only fully ends by about 1 Ga) The time between recombination and the formation of the first stars. During this time, the only source of photons was hydrogen emitting radio waves at hydrogen line. Freely propagating CMB photons quickly (within about 3 million years) red-shifted to infrared, and the universe was devoid of visible light. Star and galaxy formation and evolution Earliest galaxies: from about 300–400 Ma?(first stars: similar or earlier)Modern galaxies: (exact timings being researched) From about 20 From about 60 K The earliest known galaxies existed by about 280 Ma. Galaxies coalesce into "proto-clusters" from about 1 Ga (redshift ) and into galaxy clusters beginning at 3 Ga , and into superclusters from about 5 Ga (). See: list of galaxy groups and clusters, list of superclusters. Reionization (exact timings being researched) 20 – 6 The most distant astronomical objects observable with telescopes date to this period; , the most remote galaxy observed is MoM-z14, at a redshift of 14.44. The earliest "modern" Population I stars are formed in this period. Present time 13.8 Ga 0 2.7 K Farthest observable photons at this moment are CMB photons. They arrive from a sphere with a radius of 46 billion light-years. The spherical volume inside it is commonly referred to as the observable universe. Alternative subdivisions of the chronology (overlapping several of the above periods) Radiation-dominated era From inflation (~ > 3600 > 10 K During this time, the energy density of massless and near-massless relativistic components such as photons and neutrinos, which move at or close to the speed of light, dominate both matter density and dark energy. Matter-dominated era During this time, the energy density of matter dominates both radiation density and dark energy, resulting in a decelerated expansion of the universe. Dark energy–dominated era > 9.8 Ga < 0.4 < 4 K Matter density falls below dark energy density (vacuum energy), and expansion of space begins to accelerate. This time happens to correspond roughly to the time of the formation of the Solar System and the evolutionary history of life. Stelliferous Era The time between the first formation of Population III stars until the cessation of star formation, leaving all stars in the form of degenerate remnants. Far future > 100 Ta < −0.99 < 0.1 K The stelliferous era will end as stars eventually die and fewer are born to replace them, leading to a darkening universe. Various theories suggest a number of subsequent possibilities. Assuming proton decay, the matter may eventually evaporate into a Dark Era (heat death). Alternatively, the universe may collapse in a Big Crunch. Other suggested ends include a false vacuum catastrophe or a Big Rip as possible ends to the universe.
Inflationary cosmology
Inflation
At this point of the very early universe, the universe is thought to have expanded by at least a factor of in time on the order of . All of the mass-energy in all of the galaxies currently visible started in a sphere with a radius around , then grew to a sphere with a radius around 0.9m by the end of inflation. This phase of the cosmic expansion history is known as inflation or sometimes as the inflationary epoch.