Chronology of the universe
The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.
Outline
Chronology in five stages
For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts. It is generally considered meaningless or unclear whether time existed before this chronology:;The very early universe
The first picosecond of cosmic time. It includes the Planck epoch, during which currently understood laws of physics may not apply; the emergence in stages of the four known fundamental interactions or forces—first gravitation, and later the electromagnetic, weak and strong interactions; and the expansion of space itself and supercooling of the still immensely hot universe due to cosmic inflation, which is believed to have been triggered by the separation of the strong and electroweak interaction.
Tiny ripples in the universe at this stage are believed to be the basis of large-scale structures that formed much later. Different stages of the very early universe are understood to different extents. The earlier parts are beyond the grasp of practical experiments in particle physics but can be explored through other means.
;The early universe
Lasting around 370,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter, so most of it quickly annihilates, leaving a small excess of matter in the universe.
At about one second, neutrinos decouple; these neutrinos form the cosmic neutrino background. If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge—including protons and neutrons—and from about 2 minutes, conditions are suitable for nucleosynthesis: around 25% of the protons and all the neutrons fuse into heavier elements, initially deuterium which itself quickly fuses into mainly helium-4.
By 20 minutes, the universe is no longer hot enough for nuclear fusion, but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma. At around 47,000 years, as the universe cools, its behaviour begins to be dominated by matter rather than radiation. At about 100,000 years, helium hydride is the first molecule.
At about 370,000 years, the universe finally becomes cool enough for neutral atoms to form, and as a result it also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium—quickly reach their lowest energy state by releasing photons, and these photons can still be detected today as the cosmic microwave background. This is currently the oldest observation we have of the universe.
;The Dark Ages and large-scale structure emergence
From 370,000 years until about 1 billion years. After recombination and decoupling, the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies, so there were no new sources of light. The only photons in the universe were those released during decoupling and 21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually redshifting to non-visible wavelengths after about 3 million years, leaving it without visible light. This period is known as the cosmic Dark Ages.
Between about 10 and 17 million years the universe's average temperature was suitable for liquid water and there has been speculation whether rocky planets or indeed life could have arisen briefly, since statistically a tiny part of the universe could have had different conditions from the rest as a result of a very unlikely statistical fluctuation, and gained warmth from the universe as a whole.
At some point around 200 to 500 million years, the earliest generations of stars and galaxies form, and early large structures gradually emerge, drawn to the foam-like dark matter filaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been huge and non-metallic, with very short lifetimes compared to most stars we see today, so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instability supernovae after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of the everyday elements we see around us today, and seeded the universe with them.
Galaxy clusters and superclusters emerge over time. At some point, high energy photons from the earliest stars, dwarf galaxies and perhaps quasars leads to a period of reionization that commences gradually between about 250-500 million years, is complete by about 700-900 million years, and diminishes by about 1 billion years. The universe gradually transitioned into the universe we see around us today, and the Dark Ages only fully came to an end at about 1 billion years.
;The universe as it appears today
From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today. It will continue to appear very similar for many billions of years into the future. The thin disk of our galaxy began to form at about 5 billion years, and the Solar System formed at about 9.2 billion years, with the earliest traces of life on Earth emerging by about 10.3 billion years.
From about 9.8 billion years of cosmic time, the slowing expansion of space gradually begins to accelerate under the influence of dark energy, which may be a scalar field throughout our universe. The present-day universe is understood quite well, but beyond about 100 billion years of cosmic time, uncertainties in current knowledge mean that we are less sure which path our universe will take.
;The far future and ultimate fate
At some time the Stelliferous Era will end as stars are no longer being born, and the expansion of the universe will mean that the observable universe becomes limited to local galaxies. There are various scenarios for the far future and ultimate fate of the universe. More exact knowledge of our current universe will allow these to be better understood.
—Ultra Deep Field galaxies to Legacy Field zoom out