640 articles
Rare Earth hypothesis
The Rare Earth hypothesis, or Rare Earth theory, argues that complex extraterrestrial life is likely rare even if microbial life is common across the universe.
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.
RedMaPPer
RedMaPPer (red-sequence Matched-filter Probabilistic Percolation) is a photometric galaxy-cluster finding algorithm introduced by Eli Rykoff and Eduardo Rozo in 2014, whose catalogs of red-sequence clusters are widely used for cluster cosmology.
Redshift drift
Redshift drift is the tiny, systematic change over time in a distant object's cosmological redshift caused by the universe's expansion, proposed by Allan Sandage in 1962.
Redshift estimation
Redshift estimation is the astronomical method of measuring how much an object's light is shifted to longer wavelengths, done spectroscopically or photometrically, with spectroscopy far more accurate.
Redshift survey
A redshift survey is an observational astronomy method that measures redshifts of a large, well-defined galaxy sample and uses them as distance proxies to map three-dimensional cosmic structure.
Redshift-space distortions
Redshift-space distortions (RSD) are anisotropic distortions of the observed galaxy distribution arising from peculiar velocities: the Kaiser effect on large scales and Fingers of God in clusters.
Regiomontanus
Regiomontanus, born Johann Müller of Königsberg, was a German astronomer and mathematician who completed the Epitome of the Almagest and founded the first printing house dedicated to scientific books.
Reheating (cosmology)
Reheating is the epoch at the end of cosmic inflation in which the energy of the inflaton field converts into particles and radiation, producing the hot Big Bang plasma.
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.
Reverberation mapping
Reverberation mapping is an astronomical technique measuring the time delay between an active galactic nucleus's continuum variations and its emission lines, giving black hole masses without needing distance.
Right ascension
Right ascension (RA) is the angular distance measured eastward along the celestial equator from the vernal equinox, forming the equatorial coordinate system with declination like longitude.
Rossiter–McLaughlin effect
The Rossiter–McLaughlin effect is a distortion of a rotating star's spectral lines as a planet or star transits, revealing spin-orbit alignment; predicted by Holt in 1893 and demonstrated in 1924.
RX J1347.5-1145
RX J1347.5-1145 (RXJ1347) is a massive galaxy cluster in the constellation Virgo at redshift z = 0.451, the brightest X-ray cluster in the ROSAT All-Sky Survey.
S5 0014+81
S5 0014+81 is a distant blazar quasar in Cepheus, one of the most luminous known, with a black hole of about 40 billion solar masses.
S5 0716+714
S5 0716+714 is a BL Lacertae object, a blazar whose jet pointed nearly at Earth produces a highly variable, featureless continuum. It was discovered in 1979.
S8 tension
The S8 tension is a cosmological discrepancy: the cosmic microwave background implies a higher amplitude of matter clustering than weak gravitational lensing and galaxy surveys measure at low redshift.
Sachs–Wolfe effect
The Sachs–Wolfe effect is the influence of gravitational potentials on cosmic microwave background temperature anisotropy, predicted in 1967; its integrated form, strongest on the largest scales, probes dark energy.
Scalar field dark matter
Scalar field dark matter, also called fuzzy or wave dark matter, is a model in which dark matter is an ultralight spinless bosonic field rather than cold particles.
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.
Segmented mirror
A segmented mirror is a telescope primary mirror built from smaller hexagonal segments aligned to act as one surface, used in the Keck telescopes and JWST.
Self-interacting dark matter
Self-interacting dark matter (SIDM) is a class of dark matter models in which particles scatter elastically off one another, proposed by Spergel and Steinhardt in 2000.
SH0ES
SH0ES (Supernovae H₀ for the Equation of State) is a research collaboration led by Adam Riess that measures the universe's expansion rate, the Hubble constant, with Hubble Space Telescope data.
Shape of the universe
The shape of the universe, also called cosmic topology, is the global geometry of space, covering curvature and whether space is finite; observations show it is flat.
Shooting Star
A shooting star is the popular name for a meteor, the streak of light produced when a meteoroid entering Earth's atmosphere at high speed burns up.
SIMBA (simulation)
SIMBA is a suite of cosmological hydrodynamic simulations of galaxy formation, introduced in 2019 as the successor to Mufasa and run with the Gizmo code.
Simons Observatory
The Simons Observatory is a cosmic microwave background observatory at 5,190 meters in Chile's Atacama Desert, combining one large and three small telescopes to test cosmic inflation.
Sky
The sky is the expanse of air and scattered sunlight, or stars at night, seen above Earth's horizon; its colors come from sunlight interacting with the atmosphere.
SkyServer
SkyServer is the web portal to the Sloan Digital Sky Survey's catalog data, offering public access to images, spectra, and SQL queries for astronomers and science education.
Slitless spectroscopy
Slitless spectroscopy is an astronomical technique in which a single exposure records spectra for every object in a field of view, with no entrance slit, the object itself acting as the slit.