Cosmology and observation

Hubble constant

The Hubble constant (H₀) is the present-day expansion rate of the universe, discovered by Edwin Hubble in 1929 and measured in kilometers per second per megaparsec.

Hubble Deep Field

The Hubble Deep Field is a small patch of sky in Ursa Major that the Hubble Space Telescope imaged for ten days in 1995, revealing about 3,000 distant galaxies.

Hubble diagram

A Hubble diagram is a plot of distance or brightness against redshift for astronomical objects, used to trace the expansion history of the universe and the dark energy driving it.

Hubble flow

The Hubble flow is the uniform expansion of space that carries galaxies away from one another, so a galaxy's recessional velocity grows in proportion to its distance.

Hubble Frontier Fields

The Hubble Frontier Fields was a Hubble and Spitzer program begun in 2013 that used six galaxy clusters as natural telescopes to magnify the most distant galaxies.

Hubble radius

The Hubble radius, the boundary of the Hubble sphere, is the distance at which galaxies recede at light speed in an expanding universe, about 10 billion light-years today.

The Hubble tension is the significant disagreement between direct local measurements of the Hubble constant, near 73, and lower early-universe values near 67, a serious challenge to the standard ΛCDM model.

Hubble Ultra-Deep Field

The Hubble Ultra-Deep Field is a 2004 Hubble Space Telescope image of a small patch of Fornax showing at least 10,000 galaxies, many extremely distant.

Hubble volume

A Hubble volume is the region of the universe within the Hubble radius, the distance at which recession from the observer reaches the speed of light.

Hubble's law

Hubble's law, also called the Hubble–Lemaître law, is the cosmological observation that a galaxy's recessional velocity is proportional to its distance, discovered by Edwin Hubble in 1929.

Hybrid inflation

Hybrid inflation is a class of cosmological inflation models introduced by Andrei Linde in 1994, in which a second, waterfall field ends inflation abruptly at a critical point.

Hydrogen Epoch of Reionization Array

The Hydrogen Epoch of Reionization Array, or HERA, is a radio telescope of 14 m dishes in the South African Karoo measuring neutral hydrogen from cosmic dawn.

Hydrostatic mass bias

Hydrostatic mass bias is the systematic underestimate of galaxy-cluster masses inferred from X-ray observations assuming hydrostatic equilibrium, an assumption violated by non-thermal pressure from turbulent gas motions.

Hyper Suprime-Cam

Hyper Suprime-Cam (HSC) is an 870-megapixel optical imaging camera at the prime focus of the 8.2 m Subaru Telescope on Maunakea, Hawaii, used for wide-field surveys.

Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layer optical imaging survey on the Subaru Telescope in Hawaii, begun in 2014 to study dark matter and dark energy.

IC 10

IC 10 is a small irregular dwarf galaxy in the constellation Cassiopeia and a member of the Local Group, the closest-known starburst galaxy, discovered by Lewis Swift in 1887.

Illustris

Illustris is a suite of large cosmological hydrodynamical simulations of galaxy formation, run with the moving-mesh code Arepo and completed in late 2013, producing about 40,000 well-resolved galaxies in a box 106.5 megaparsecs on a side.

Indian astronomy

Indian astronomy, also called Jyotisha, is the tradition of observing and mathematically modeling the Sun, Moon, planets, and stars that developed on the Indian subcontinent from Vedic times onward.

Infrared Astronomical Satellite

The Infrared Astronomical Satellite (IRAS) was the first space telescope to survey the entire sky in infrared light, a 1983 joint project of the United States, the Netherlands, and the United Kingdom.

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.

Integral field spectroscopy

Integral field spectroscopy (IFS) is an astronomical technique that records a spectrum for every position in a two-dimensional field at once, producing a data cube used to map galaxies.

Intensity interferometry

Intensity interferometry is an optical astronomy technique that measures stellar angular diameters by correlating light-intensity fluctuations at separate telescopes, introduced by Hanbury Brown and Twiss in 1956.

Intercept method

The intercept method, also called the Marcq Saint Hilaire method, is a celestial navigation technique that turns one altitude observation into a line of position.

Interferometric imaging

Interferometric imaging reconstructs an image of a distant or large object by coherently combining signals from separated antennas or apertures, a technique central to radio astronomy and used in InSAR.

International Ultraviolet Explorer

The International Ultraviolet Explorer (IUE) was a joint NASA, ESA, and UK space telescope launched on 26 January 1978 to perform ultraviolet spectroscopy of celestial objects.

Interplanetary scintillation

Interplanetary scintillation is a radio astronomy technique that measures rapid intensity fluctuations of compact radio sources caused by solar wind density irregularities, used for space weather remote sensing.

Intracluster light

Intracluster light (ICL) is faint, diffuse light from stars bound to a galaxy cluster's halo rather than to any individual galaxy, first proposed by Fritz Zwicky in 1937.

Isocurvature perturbations

Isocurvature perturbations, also called entropy perturbations, are primordial fluctuations in the relative amounts of dark matter, baryons, photons, or neutrinos that produce no net curvature.

J2000.0 epoch

J2000.0 is the standard astronomical reference epoch, the mean equator and equinox of January 1, 2000, Julian date 2451545.0, to which star catalogs and spacecraft coordinates are referred.