Microlensing:

If a small, dark body is directly in the line of sight to a brightbackground star, the brightness of the background star may appearto increase because of bending of the light rays by the dark body.

Neutrinos:

Very light (possibly massless) particles that are emitted in theprocess of radioactive decay. There are three species, associatedwith electrons, muons, and tau-leptons. They interact with ordinarymatter through the weak force.

Nucleosynthesis:

The process by which the elements are built up from protons and neutrons.

Open universe:

A universe expanding faster than the retarding pull of gravity. Ithas less than critical density, Ω < 1, and expands forever.

Planck time:

The period of time immediately after the creation of the universe(10⁻⁴³ s) during which quantum gravity is the principal phenomenon governingthe evolution of the universe.

q₀:

The deceleration rate for the expansion of the universe, which isrelated to the curvature of space.

Quarks:

The elementary constituents of hadrons or baryons (such as protonsand neutrons).

Quasar:

An object with a large redshift and an inferred luminosity oftenhundreds of times that of a normal galaxy.

Redshift:

The shifting of light toward the red end of the spectrum that occurswhen the observed light source is receding from the observer.

SIRTF:

Space Infrared Telescope Facility. A proposed orbiting infrared telescope.

Standard candle:

A celestial object whose intrinsic brightness is known or can beestimated by some physical principle and whose observed brightnessis therefore useful as a tool to measure distance.

Supersymmetry:

A space-time symmetry that would imply the existence of partnersto all elementary particles, with quantum spins of one-half a unithigher or lower. Often used in constructing theories that unify gravitywith the three other forces.

Thermal spectrum:

The characteristic distribution of radiation as a function of frequencythat is emitted by a body at a well-defined temperature, also calleda black-body spectrum.

Tully-Fisher relation:

A method to determine galactic distances. Big, luminous galaxiesrotate faster than small, faint ones. The connection between thetwo is given by the Tully-Fisher relation.

Unification:

The concept that two or more forces that seem distinct in today's universe could, at higher energies (or temperature), merge to becomeone force.

Universe:

All of space and time taken together.

VLA:

The Very Large Array. An array of 27 radio telescopes in New Mexico,capable of adjustable spacing along a Y-shaped track, up to a radiusof 27 km.

VLBA:

Very Long Baseline Array. A newly completed radio interferometeroperated by the National Radio Astronomy Observatory. Capable ofproducing images with angular resolution of one thousandth of a secondof arc.

Weak interactions:

The interactions of elementary particles that are responsible forradioactive decay.

WIMP:

Weakly Interacting Massive Particle. A candidate for dark matter.