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Growth index (cosmology)
The growth index is a cosmological parameter, written γ, that summarizes how fast cosmic structures grow; it is about 0.55 in general relativity and differs in modified gravity.
H.E.S.S. Galactic Plane Survey
The H.E.S.S. Galactic Plane Survey (HGPS) is a survey of the Milky Way in very high-energy gamma rays, observed with telescopes in Namibia from 2004 to 2013 and published in 2018.
H0LiCOW
H0LiCOW (H0 Lenses in COSMOGRAIL's Wellspring) is an international collaboration measuring the Hubble constant from lensed quasars, finding a value in tension with Planck CMB predictions.
Habitable zone
The habitable zone, also called the Goldilocks zone, is the region around a star where a rocky planet with a suitable atmosphere could hold liquid surface water.
Halo bias
Halo bias is the cosmological relation between the spatial distribution of dark matter haloes and the dark matter density field, quantifying how haloes cluster more strongly than matter itself.
Halo mass function
The halo mass function (HMF) is the comoving number density of dark matter halos per unit mass, a central prediction of structure formation models used to constrain cosmological parameters.
Halo occupation distribution
The halo occupation distribution (HOD) is a statistical model in cosmology giving the probability that a dark matter halo of a given mass contains a given number of galaxies.
Harvard-Smithsonian Center for Astrophysics
The Harvard-Smithsonian Center for Astrophysics, also known as the CfA, is a joint Harvard and Smithsonian astronomy research institution in Cambridge, Massachusetts, founded in 1973.
Haute-Provence Observatory
The Haute-Provence Observatory (Observatoire de Haute-Provence, OHP) is a French astronomy and atmospheric science site near Saint-Michel-l'Observatoire, where 51 Pegasi b, the first exoplanet around a Sun-like star, was discovered in 1995.
HAYSTAC
HAYSTAC (Haloscope at Yale Sensitive To Axion CDM) is a tunable microwave cavity experiment at Yale University searching for axion dark matter, and the first to use a squeezed-state receiver.
HDF-S
HDF-S, or Hubble Deep Field South, is a ten-day Hubble Space Telescope observation in 1998 of a small sky patch in the constellation Tucana, revealing thousands of distant galaxies.
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.
Heliocentrism
Heliocentrism, also known as the Copernican heliocentric system, is the astronomical model placing the Sun rather than Earth at the center, advanced by Aristarchus and published by Nicolaus Copernicus in 1543.
Hemispherical power asymmetry
The hemispherical power asymmetry is a cosmic microwave background anomaly where temperature fluctuations are larger on one hemisphere, first seen in WMAP data in 2004.
Henize 2-10
He 2-10 (Henize 2-10) is a blue compact dwarf starburst galaxy in Pyxis, about 30 million light-years away, whose massive black hole appears to trigger star formation.
HI intensity mapping
HI intensity mapping is a radio astronomy technique that maps the large-scale distribution of matter by measuring integrated redshifted 21-cm emission from neutral hydrogen, without resolving individual galaxies.
Hierarchical structure formation
Hierarchical structure formation is the cosmological model in which small structures such as dwarf galaxies and dark matter halos form first, later merging into larger galaxies and clusters.
Higgs inflation
Higgs inflation is a cosmological inflation model in which the Standard Model Higgs field itself drives the early Universe's accelerated expansion, proposed by Bezrukov and Shaposhnikov in 2008.
High-redshift quasar
A high-redshift quasar is a quasar, an active nucleus powered by a supermassive black hole, seen at redshift z above 6, within the Universe's first billion years.
High-Z Supernova Search Team
The High-Z Supernova Search Team, led by Brian Schmidt, is an international collaboration whose 1998 observations of Type Ia supernovae revealed that the universe's expansion is accelerating.
Hinode
Hinode (ひので, Japanese for "Sunrise"), known before launch as Solar-B, is a Japanese solar-observing satellite launched by JAXA in 2006 to study the Sun's magnetic field.
History of astronomy
The history of astronomy records how humans observed and explained the sky, from Babylonian eclipse prediction by 700 BCE to Hipparchus, Tycho Brahe, and space telescopes.
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.
Hobby-Eberly Telescope
The Hobby-Eberly Telescope (HET) is a 10-meter fixed-elevation optical telescope at McDonald Observatory in west Texas, built for $13.9 million in 1997 and run by UT Austin, Penn State, and German universities.
Hobby-Eberly Telescope Dark Energy Experiment
The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is a spectroscopic survey on the 10-meter Hobby-Eberly Telescope in Texas measuring dark energy in the early universe.
Holographic dark energy
Holographic dark energy (HDE) is a class of cosmological models in which dark energy's density is set by the holographic principle, proposed by Miao Li in 2004.
Horizon problem
The horizon problem is a puzzle in big-bang cosmology: causally disconnected regions of the cosmic microwave background are observed at the same temperature, named by Alan Guth in 1981.
Hot dark matter
Hot dark matter is dark matter made of light, fast-moving particles, the standard example being neutrinos; its free streaming erases small density fluctuations, favoring large structures forming first.
Hot Jupiter
A hot Jupiter is a gas-giant exoplanet like Jupiter orbiting very close to its star, with atmospheric temperatures in the thousands of degrees; 51 Pegasi b was the first found.
Hubble bubble hypothesis
The Hubble bubble hypothesis proposes that the Milky Way sits inside a large underdense region whose outflow makes the locally measured expansion rate of the universe appear higher.