Chronology of the Universe

The Chronology of the Universe is a detailed history of the universe as described by the Big Bang cosmology. Research conducted in 2015 estimated the formation of the universe to have occurred 13.8 billion years ago, with an uncertainty of 21 million years at a 68% confidence level.

Five Stages of Chronology

  • Very Early Universe (0-1 picosecond): This period includes the Planck epoch, where current laws of physics may not have applied; the emergence in stages of the four known forces (first gravity, then electromagnetic, weak, and strong); and the accelerated expansion of the universe due to cosmic inflation.
  • Early Universe (1 picosecond - 380,000 years): Subatomic particles emerge in stages, with almost equal amounts of matter and antimatter annihilating quickly, leaving a small matter excess. Neutrinos decouple at one second. If primordial black holes exist, they also form at one second. Composite subatomic particles emerge, including protons and neutrons, and around two minutes, conditions are suitable for nucleosynthesis, fusing 25% of protons and all neutrons into heavier elements, initially deuterium. By 20 minutes, the universe is no longer hot enough for nuclear fusion but too hot for neutral atoms or photons to travel far, making it an opaque plasma. The recombination epoch begins at around 18,000 years as electrons combine with helium nuclei to form He+. By 47,000 years, matter begins to dominate over radiation. At 100,000 years, hydrogen and helium hydride react to form molecular hydrogen (H2), the fuel for the first stars. Neutral hydrogen atoms finish forming ("recombination"); photons released ("photon decoupling") form the cosmic microwave background (CMB), the oldest direct observation of the universe.
  • Dark Ages and Large-Scale Structure Emergence (380,000 years - 1 billion years): Hydrogen clouds condense very slowly into stars and galaxies, so there were few light sources, and emissions were immediately absorbed by hydrogen atoms. The only photons in the universe were the CMB released during decoupling and 21 cm radio emissions from hydrogen atoms. Early generations of stars may have formed, very massive and non-metallic with short lifetimes, exploding as supernovae after millions of years. Other theories suggest they may have included small stars. These supernovae created most of the elements around us today and seeded the universe with them. Galaxy clusters and superclusters emerge. Early star emissions lead to reionization. The Dark Ages fully end at one billion years as the universe transitions to its current state, but denser, hotter, and more intense with star formation, and richer in smaller (especially unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies.
  • Universe as It Appears Today (1 billion years - today): The universe has looked much as it does today for 12.8 billion years and will continue to do so for many billions of years into the future. Our galaxy's thin disk began to form at about 5 billion years (8.8 Gya), and the Solar System formed at 9.2 billion years (4.6 Gya), with the earliest evidence of life on Earth emerging by 10 billion years (3.8 Gya). The expansion of the universe passed an inflection point about five or six billion years ago when the universe entered the modern "dark-energy-dominated era" where the universe's expansion is now accelerating rather than decelerating.
  • Far Future and Ultimate Fate (Billions of Years from Now): 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 long-term evolution of the universe. More exact knowledge of the present-day universe may allow these to be better understood.