List of time periods


The categorization of the past into discrete, quantified named blocks of time is called periodization. This is a list of such named time periods as defined in various fields of study. Major categorization systems include cosmological, geological, anthropological and historical.

Human time periods

These can be divided broadly into prehistorical and historical periods.
In archaeology and anthropology, prehistory is subdivided around the three-age system, this list includes the use of the three-age system as well as a number of various designation used in reference to sub-ages within the traditional three.
The dates for each age can vary by region. On the geologic time scale, the Holocene epoch starts at the end of the last glacial period of the current ice age and continues to the present. The beginning of Mesolithic is usually considered to correspond to the beginning of the Holocene epoch.

General periods

Only for Late Modern Contemporary history.
The geologic time scale covers the extent of the existence of Earth, from about 4600 million years ago to the present day. It is marked by Global Boundary Stratotype Sections and Points. Geologic time units are eons, eras, periods, epochs, and ages; and the corresponding chronostratigraphic units, which measure "rock-time", are eonothems, erathems, systems, series, and stages.
The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. The Cenozoic is sometimes divided into the Quaternary and Tertiary periods, although the latter is no longer used officially.

Cosmological time periods

13.8 billion years ago: The Big Bang

Time PeriodDurationDescription
Planck EpochFrom the start to 10−43 seconds after the Big BangVery little concrete information is known about this epoch. Different theories propose different views on this particular time.
Grand Unification EpochBetween 10−43 to 10−36 seconds after the Big BangThe result of the universe expanding and cooling down during the Planck epoch. All fundamental forces except gravity are unified.
Electroweak EpochBetween 10−36 seconds to 10−12 seconds after the Big BangThe universe cools down to 1028 kelvin. The fundamental forces are split into the strong force and the electroweak force.
Inflationary EpochBetween 10−36 seconds to 10−32 seconds after the Big BangThe shape of the universe flattens due to cosmic inflation.
Quark EpochBetween 10−12 seconds to 10−6 seconds after the Big BangCosmic inflation has ended. Quarks are present in the universe at this point. The electroweak force is divided again into the weak force and electromagnetic force.
Hadron EpochBetween 10−6 seconds to 1 second after the Big BangThe universe has cooled enough for quarks to form hadrons, protons, neutrons.
Lepton EpochBetween 1 second to 10 seconds after the Big BangMost hadrons and anti-hadrons annihilate each other, leaving behind leptons and anti-leptons.
Photon EpochBetween 10 seconds to 370,000 years after the Big BangMost leptons and anti-leptons annihilate each other. The universe is dominated by photons.
NucleosynthesisBetween 3 minutes to 20 minutes after the Big BangThe temperature of the universe has cooled down enough to allow atomic nuclei to form via nuclear fusion.
RecombinationAbout 377,000 years after the Big BangHydrogen and helium atoms form.
ReionizationBetween 150 million and 1 billion years after the Big BangThe first stars and quasars form due to gravitational collapse.

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