Chronology of the universe

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 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, 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, the energy driving inflation was converted to a hot and dense plasma, initiating the hot Big Bang, during which the universe 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 (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.

Inflationary cosmology

Inflation

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