30 articles
21-cm cosmology
21-cm cosmology is the branch of observational cosmology that uses the 21-centimeter line of neutral hydrogen, redshifted from the early universe, to map the cosmic dark ages, the Epoch of Reionization, and later structure.
Age of the universe
The age of the universe is the time elapsed since the Big Bang, estimated at 13.8 billion years from the cosmic microwave background and confirmed by the oldest stars.
Baryogenesis
Baryogenesis is the hypothetical physical process in the early universe that generated a small excess of baryons over antibaryons, producing all the matter in today's cosmos.
Baryon asymmetry
Baryon asymmetry is the excess of matter over antimatter in the universe, quantified by the baryon-to-photon ratio and generated by baryogenesis in the early universe.
Big Bang
The Big Bang, or Big Bang model, is the prevailing cosmological theory in which the universe expanded and cooled from an extremely hot, dense state 13.8 billion years ago.
Big Bang nucleosynthesis
Big Bang nucleosynthesis, also called primordial nucleosynthesis, is the production of the lightest nuclei, chiefly helium-4, deuterium, helium-3, and lithium-7, during the universe's first minutes of cosmic time.
Big Crunch
The Big Crunch is a hypothetical end of the universe in which cosmic expansion stops and reverses, and all matter collapses back into a hot, dense final state.
Chronology of the universe
The chronology of the universe, also called the timeline of the universe, divides cosmic history into epochs from the Big Bang 13.8 billion years ago to today.
Cosmogony
Cosmogony is the study or account of the origin of the universe, in mythic, theological, philosophical, or scientific form, distinct from cosmology, which studies structure and evolution.
Cosmological lithium problem
The cosmological lithium problem, also called the primordial lithium problem, is the discrepancy by which old stars hold three to four times less lithium-7 than big bang nucleosynthesis predicts.
Cosmos
The cosmos (Κόσμος) is the universe as a whole: all space, time, matter, energy, stars, and galaxies, about 13.8 billion years old, studied through cosmology.
Critical density (cosmology)
Critical density is the average mass-energy density a universe expanding at a given rate needs for no spatial curvature; today it is about 10⁻²⁶ kg/m³.
Density parameter
The density parameter, written Ω (Omega), is the ratio of the actual mean density of the universe or a component to the critical density, and the components sum to 1.
Heat death of the universe
The heat death of the universe, also called the Big Freeze, is the hypothetical final state of maximum entropy where no usable energy remains, proposed by Lord Kelvin in 1852.
History of the Big Bang theory
The history of the Big Bang theory traces the idea of a hot, dense origin of the universe, from Lemaître's 1927 primeval-atom hypothesis to modern satellite observations.
Initial singularity
The initial singularity, also called the Big-Bang singularity, is the hypothetical state of infinite density and temperature to which general relativity extrapolates the universe backward in time.
Matter-dominated era
The matter-dominated era is the period of cosmic history, beginning about 50,000 years after the Big Bang, when matter's density exceeded radiation's and drove cosmic expansion.
Matter–radiation equality
Matter–radiation equality is the epoch in the early universe, about 51,000 years after the Big Bang at redshift roughly 3400, when matter first outweighed radiation, allowing density fluctuations to grow.
Nucleosynthesis
Nucleosynthesis, also called nucleogenesis, is the set of nuclear processes that create atomic nuclei, from big bang hydrogen and helium to elements forged in stars.
Outer space
Outer space is the near-vacuum region beyond the atmospheres of celestial bodies, extending from the conventional boundary above Earth through interplanetary, interstellar, and intergalactic space.
Particle horizon
The particle horizon, also called the comoving horizon, is the maximum distance from which light could have reached us in the age of the universe.
Planck epoch
The Planck epoch is the earliest period of the Universe, lasting from the Big Bang to one Planck time, about 5.39 × 10⁻⁴⁴ seconds, when gravity was as strong as the other forces.
Primordial black hole
A primordial black hole (PBH) is a hypothetical black hole formed within the first second after the Big Bang from collapsing dense regions, not from dying stars.
Primordial deuterium abundance
The primordial deuterium abundance is the deuterium-to-hydrogen ratio made by Big Bang nucleosynthesis in the universe's first minutes and preserved in gas that has never passed through stars.
Primordial magnetic fields
Primordial magnetic fields are magnetic fields generated in the early universe, during inflation or cosmological phase transitions. None has been directly detected; they are constrained through the cosmic microwave background, nucleosynthesis, and gamma-ray observations.
Radiation-dominated era
The radiation-dominated era is the period of cosmic history, from the Big Bang until roughly 50,000 years after it, when radiation's energy density exceeded matter's and controlled the universe's expansion, ending at matter-radiation equality.
Recombination (cosmology)
In cosmology, recombination is the epoch about 380,000 years after the Big Bang when the universe cooled enough for neutral hydrogen to form, making the cosmos transparent to light.
Reionization
Reionization is the process in cosmic history, between roughly 200 million and one billion years after the Big Bang, when ultraviolet light from the first stars and galaxies ionized the universe's neutral hydrogen.
Scale factor (cosmology)
The cosmic scale factor, a(t), is the quantity in cosmology that describes how physical distances between comoving objects grow as the universe expands with cosmic time.
Universe
The universe, also known as the physical universe, is all of space, time, matter, and energy, roughly 13.8 billion years old and expanding at an accelerating rate.