42,772 articles
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.
Horizontal branch
The horizontal branch is the Hertzsprung–Russell diagram region of core-helium-burning stars, appearing as a nearly horizontal sequence in metal-poor globular clusters and as a red clump in metal-richer populations.
Horizontal-to-vertical spectral ratio
The horizontal-to-vertical spectral ratio (HVSR) is a single-station seismological method that estimates a site's fundamental resonance frequency from ambient vibration or earthquake recordings using one three-component seismometer.
Horn clause
A Horn clause, also known as a Horn sentence, is a clause containing at most one positive literal, named after logician Alfred Horn, central to logic programming and Prolog.
Horn River Basin
The Horn River Basin is an unconventional shale gas play in northeastern British Columbia, Canada, home to the first modern Canadian shale gas production in 2006.
Hornblende
Hornblende is a root-name group of dark green to black calcic amphibole minerals, documented by Abraham Gottlieb Werner in 1789, found in rocks such as granite, diorite, gabbro, and schist.
Horndeski gravity
Horndeski gravity is the most general four-dimensional scalar-tensor theory of gravity with equations of motion no higher than second order, so it propagates no Ostrogradsky ghost.
Horner–Wadsworth–Emmons reaction
The Horner–Wadsworth–Emmons reaction, also known as the Wittig–Horner reaction or HWE reaction, converts aldehydes and ketones into alkenes using phosphonate carbanions; developed in 1961 by Wadsworth and Emmons, it yields mainly E-alkenes and water-soluble phosphate byproducts.
Horner's method
Horner's method, also called Horner's rule or Horner's scheme, is an algorithm for evaluating a polynomial by rewriting it in nested form, using only n multiplications and n additions.
Horng-Tzer Yau
Horng-Tzer Yau (姚鸿泽), born in Taiwan in 1959, is a Taiwanese-born mathematician and Merton Professor at Harvard known for proving universality of random matrix spectra with Erdős and Schlein.
Horse (zodiac)
The Horse is the seventh animal of the twelve-year East Asian zodiac, paired with the branch 午 and assigned to people born in Horse years like 2026.
Horse latitudes
The horse latitudes, also called the subtropical ridge, are subtropical high-pressure belts over the oceans near 30°–35° latitude, known for calm winds, hot dry weather, and nearby deserts like the Sahara.
Horsehead Nebula
The Horsehead Nebula (Barnard 33) is a dark nebula in Orion, about 1,300 light-years away, silhouetted against the bright nebula IC 434 and first recorded in 1888.
Horsepower
Horsepower (hp) is a unit of power, originally defined by James Watt in the 1780s to compare steam engines with draft horses; one mechanical horsepower equals about 745.7 watts.
Horseshoe prior
The horseshoe prior is a Bayesian shrinkage prior for sparse regression, introduced by Carvalho, Polson, and Scott in 2009, that shrinks noise hard while leaving strong signals unshrunk.
Horseshoe vortex
The horseshoe vortex is a closed loop of vortex filaments that models the lift and wake of a finite wing, introduced by Ludwig Prandtl in lifting-line theory.
Horst L. Störmer
Horst L. Störmer, born 1949, is a German-born American physicist who shared the 1998 Nobel Prize in Physics for discovering the fractional quantum Hall effect.
Horst Sachs
Horst Sachs (1927–2016) was a German mathematician and professor at the Technical University of Ilmenau, known for the 1964 Sachs subgraph theorem and the monograph Spectra of Graphs.
Horst Weller
Horst Weller (born 1954) is a German physical chemist, professor at Universität Hamburg since 1994, known for synthesizing colloidal nanoparticles and arranging them into ordered superlattices.
Horvitz–Thompson estimator
The Horvitz–Thompson estimator is a design-based estimator of a population total, introduced in 1952, that weights each sampled unit by the reciprocal of its inclusion probability.
Hoshin V. Gupta
Hoshin V. Gupta, also published as Hoshin Gupta, is a hydrologist and Regents Professor at the University of Arizona, known for the SCE-UA optimization method and the Kling-Gupta Efficiency.
Host–guest polymer networks
A host–guest polymer network is a supramolecular polymer network in which covalent polymer chains are connected by reversible host–guest complexes, most often a cyclodextrin host, giving self-healing, stimuli-responsive gels used in drug delivery.
Hosting and organization of the International Physics Olympiad
Hosting of the International Physics Olympiad rotates to a different country almost every year since 1967, with each host paying costs through a national organizing committee.
Hot air oven
A hot air oven, or dry heat sterilizer, is an electrically heated chamber that sterilizes glassware, metal instruments, powders, and oils by oxidation, requiring cycles of 60 to 150 minutes.
Hot bottom burning
Hot bottom burning (HBB) is proton-capture nucleosynthesis at the base of the convective envelope of massive asymptotic giant branch stars, which modifies surface abundances of carbon, nitrogen, and lithium.
Hot cell
A hot cell is a heavily shielded enclosure in which highly radioactive materials are handled remotely, protecting operators from radiation and contamination through manipulators and viewing windows.
Hot corrosion
Hot corrosion is accelerated corrosion of hot-section alloys, especially in gas turbine engines, caused by molten salt deposits such as sodium sulfate that damage protective surface oxides.
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 desert climate
The hot desert climate, Köppen code BWh, is an arid climate with mean annual temperature at or above 18 °C, covering roughly 16% of global land area.
Hot disk method
The hot disk method, formally the transient plane source (TPS) method, measures a material's thermal conductivity, diffusivity, and volumetric heat capacity in one measurement using a heated disk sensor.