Chronology of the universe
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.
Background
Expansion
The current accepted model of the history of the universe is based on the concept of the Big Bang: the universe started hot and dense then expanded and cooled.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 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 distance 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 five parts:- Inflation, the first era supported by experimental evidence, a period of exponential expansion that ends with the conversion of energy into particles,
- Quark soup, the initial particles cool and coalesce, dark matter forms,
- Big bang nucleosynthesis, combining nucleons create the cores of the first atoms,
- Gravity builds cosmic structure, reduced density allows matter to dominate over radiation for control of expansion, photons decouple to form the cosmic background radiation, and gravitational attraction builds stars, galaxies, and clusters of galaxies.
- Cosmic acceleration, continued expansion allows dark energy to overcome gravitational force, inhibiting larger structures.
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.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 0.09m by the end of inflation. This phase of the cosmic expansion history is known as inflation or sometimes as the inflationary epoch.Inflation explains how today's universe has concentrations of matter, like galaxies and clusters of galaxies, rather than having matter spatially uniform through the universe. Tiny quantum fluctuations in the universe, amplified by inflation, are believed to be the basis of large-scale structures that formed much later.
The mechanism that drove inflation remains unknown, although many models have been put forward. In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest-energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the universe.
The rapid expansion meant that any potential particles remaining from the time before inflation were now distributed very thinly across the universe.
Reheating
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 any elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe to the point where there is no physical temperature that can be associated with them. However, the large 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 Standard Model particles.After inflation ended, the universe continued to expand. A region the size of a melon at that time has since grown to be our entire observable universe.
Hot Big Bang
The physical model for the chronology of the universe with strong observational and theoretical support is called the hot Big Bang model. The concept includes an early state of extreme temperature and density followed by expansion of the universe continuing to this day. A high-precision version of the Big Bang model using conventional physics, known as Lambda-CDM, agrees with a wide array of astrophysical observations. The concept is not extrapolated back to zero time. Within the standard model of cosmology the initial state is set by a process called inflation. The relative timeline for the earliest phenomena is unclear. Speculation on processes occurring before inflation involves physics considered [|outside of standard cosmology].Electroweak phase transition
As the universe's temperature continued to fall below, electroweak symmetry breaking happened. So far as we know, it was the penultimate symmetry breaking event in the formation of the universe, the final one being chiral symmetry breaking in the quark sector. This has two related effects:- Via the Higgs mechanism, all elementary particles interacting with the Higgs field became massive, having been massless at higher energy levels.
- As a side-effect, the weak nuclear force and electromagnetic force, and their respective bosons began 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 became massive particles only interacting over distances smaller than the size of an atom, while the photon remained massless and remained a long-distance interaction.
Quantum chromodynamics phase transition
After cosmic inflation ended, the universe was 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.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. The quark epoch ended when the universe was about 10−5 seconds old; two non-equilibrium events must have occurred next, formation of baryons and of dark matter.
Neutrino decoupling and cosmic neutrino background (CνB)
At approximately 1 second after the Big Bang neutrinos decouple and begin travelling freely through space. As neutrinos rarely interact with matter, these neutrinos still exist today, analogous to the much later cosmic microwave background emitted during recombination, around 370,000 years after the Big Bang. The neutrinos from this event have a very low energy, around 10−10 times the amount of those observable with present-day direct detection. Even high-energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background 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. 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, and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.
Cosmological models of this early time remain unsettled.
The Standard Model of particle physics is only tested up to temperatures of order 1017K in particle colliders, such as the Large Hadron Collider. Moreover, new physical phenomena not yet covered by the Standard Model could have been important before the time of neutrino decoupling, when the temperature of the universe was about 1010K.