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Ultimate fate of the universe
The ultimate fate of the universe, also called the death of the universe, ranges from eternal cold expansion to Big Crunch collapse or Big Rip, decided by dark energy.
Ultralight dark matter
Ultralight dark matter, also called fuzzy or scalar field dark matter, is a class of models where dark matter is bosons so light they behave as coherent waves.
UM 287
UM 287 is a distant quasar around which a giant gas nebula was discovered in 2014, seen as a possible direct image of a cosmic web filament.
Unidentified flying object
An unidentified flying object (UFO), also known as unidentified aerial phenomena (UAP), is any object or light in the sky that resists identification after investigation, often associated with alien spacecraft.
United States Naval Observatory
The United States Naval Observatory (USNO) is the U.S. Navy's scientific institution for timekeeping and astronomy in Washington, D.C., founded in 1830 and the official source of time for the Department of Defense.
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.
Varāhamihira (वराहमिहिर)
Varāhamihira (वराहमिहिर) was an Indian astronomer and polymath who worked at Ujjain in the 6th century and wrote the Pañcasiddhāntikā and the Bṛhat Saṃhitā.
Vatican Observatory
The Vatican Observatory (Specola Vaticana) is the Holy See's astronomical research institute, re-founded by Pope Leo XIII in 1891 and working today from Castel Gandolfo and Arizona.
Velocity dispersion
Velocity dispersion is the standard deviation of the velocities of galaxies in a cluster or stars in a galaxy, used with the virial theorem to estimate mass.
Very Large Array
The Very Large Array, officially the Karl G. Jansky Very Large Array since 2012, is a New Mexico radio observatory of 27 movable dishes operating as an interferometer.
Very Large Telescope
The Very Large Telescope is an array of four 8.2-meter optical telescopes operated by the European Southern Observatory on Cerro Paranal in Chile's Atacama Desert.
Very Long Baseline Array
The Very Long Baseline Array (VLBA) is a system of ten radio telescopes from Hawaii to the U.S. Virgin Islands working as one interferometric instrument.
Very-long-baseline interferometry
Very-long-baseline interferometry (VLBI) is a radio astronomy technique in which telescopes thousands of kilometers apart observe the same source simultaneously, achieving the highest angular resolution of any method.
VIPERS
VIPERS, the VIMOS Public Extragalactic Redshift Survey, mapped about 90,000 galaxies with ESO's Very Large Telescope to measure cosmic structure at half the Universe's age.
Virial radius
The virial radius is the conventional outer boundary of a dark matter halo, defined so the mean density inside is a fixed multiple of a reference density.
Visibility function
The visibility function is the probability density that a cosmic microwave background photon underwent its last scattering at a given time, peaking sharply at recombination.
VISTA Surveys
The VISTA Surveys are six public near-infrared imaging surveys of the southern sky made with the 4.1-meter VISTA telescope at ESO's Paranal Observatory in Chile, starting in 2010.
Void (astronomy)
A cosmic void, or supervoid, is a large underdense region of the universe, typically 5 to 100 megaparsecs across, bounded by the filaments of the cosmic web.
Void galaxy
A void galaxy is a galaxy inside a cosmic void, an underdense region of the Universe. They are blue, low-mass, gas-rich galaxies that evolve slowly.
W. M. Keck Observatory
The W. M. Keck Observatory is an astronomical observatory atop Maunakea in Hawaii that operates the twin 10-meter Keck I and Keck II telescopes, which began science operations in 1993.
Warm dark matter
Warm dark matter is a class of dark matter models whose particles move fast enough in the early universe to erase density fluctuations below a characteristic scale.
Warm inflation
Warm inflation is a class of cosmological inflation models in which the inflaton dissipates energy into a thermal bath, proposed by Arjun Berera in 1995.
Warm-hot intergalactic medium
The warm-hot intergalactic medium (WHIM) is diffuse, highly ionized gas at 10^5 to 10^7 K in cosmic-web filaments, proposed to hold half the universe's missing ordinary matter.
Wavefront reconstruction
Wavefront reconstruction is the computational step of an adaptive optics system that estimates a light wavefront's phase distortions from wavefront-sensor measurements, converting them into a phase map or deformable-mirror commands.
Weighing the Giants
Weighing the Giants (WtG) is a collaborative astronomy project that measured weak-gravitational-lensing masses for 51 X-ray luminous galaxy clusters, calibrating masses for cluster cosmology experiments.
WiggleZ Dark Energy Survey
The WiggleZ Dark Energy Survey was a spectroscopic galaxy survey at the Anglo-Australian Telescope in Australia from 2006 to 2011, measuring baryon acoustic oscillations to test dark energy models.
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe (WMAP) was a NASA observatory that mapped the cosmic microwave background from 2001 to 2010, measuring the universe's age and composition.
William Herschel Telescope
The William Herschel Telescope is a 4.2-meter optical and near-infrared telescope on La Palma in the Canary Islands, opened in 1987 and carrying the WEAVE spectrograph since 2023.
X-ray transient
An X-ray transient is an astronomical source whose X-ray emission appears, brightens, and fades over limited periods, often recurring, covering binary outbursts, stellar flares, and tidal disruption events.
XMM-Newton
XMM-Newton is a European Space Agency X-ray space observatory launched in 1999, the largest scientific satellite built in Europe, studying X-ray sources across the universe.