Very Large Telescope (VLT): Organized by the European Southern Observatory, all four telescopes in this array were operational beginning in 2006. The VLT consists of four 8 meter telescopes that can either work independently or in combined mode. Combined, the telescopes provide the light collecting power of a 16 meter telescope.

Virgo cluster: An irregular cluster of about 2,500 galaxies.

w: Dark energy equation-of-state parameter that indicates how dark energy reacts to varying temperatures and densities.

warm dark matter: A theoretical type of dark matter particle with higher temperature and velocity than those of cold dark matter.

weak lensing: Gravitational lensing in which the lens is not strong enough to produce multiple images, but merely stretches the image of the background object.

Wide-field Infrared Survey Explorer (WISE): An infrared space observatory designed to be an all-sky survey in wavelengths from 3.5 to 23 microns that will be up to 1,000 times more sensitive than the Infrared Astronomical Satellite. Launch is scheduled for November 2009.

Wilkinson Microwave Anisotropy Probe (WMAP): Launched by NASA in 2001, the WMAP maps temperature fluctuations in the cosmic microwave background with very high precision; see cosmic microwave background (CMB) radiation.

X-ray Evolving Universe Spectroscopy (XEUS): A follow-up to the European Space Agency’s Cornerstone X-Ray Spectroscopy Mission. It is a permanent spaceborne x-ray observatory whose goals are to probe dark matter and dark energy, to study massive blackholes at z ~ 10, to observe the structure of the galaxies near black holes, and to study matter under extreme conditions and the structure of highly collapsed stars.

X-ray Multi-Mirror Mission-Newton (XMM-Newton): Launched by the European Space Agency in 1999, the XMM-Newton carries three advanced x-ray telescopes. XMM-Newton has studied x-rays from accretion onto black holes, properties of exploding stars, the nature of exotic matter, and has observed gamma-ray bursts.

z: See redshift.

zenith: The point on the celestial sphere directly above the observer, that is, opposite to the direction of gravity.

zodiacal light: Sunlight scattered by interplanetary dust in our solar system.

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