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.


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.


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.


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


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


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


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


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.


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.


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


All of space and time taken together.


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.


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.


Weakly Interacting Massive Particle. A candidate for dark matter.

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