L: The cosmological constant, which measures the energy density of a vacuum.
W: The ratio of the average total density of matter to the critical density required to close the universe and eventually stop its expansion. can be broken down into its components according to the type of matter involved.
WB: The ratio of the mean density of baryons to the critical density required to close the universe.
Anisotropy: Variation with direction.
Axion: A hypothetical elementary particle whose existence might explain certain particle physics experiments. A candidate for cold dark matter.
Baryon: A massive, strongly interacting elementary particle, such as a proton or a neutron.
Baryonic matter: Ordinary matter as we know it consists largely of baryons, as opposed to hypothetical matter that might theoretically exist. Both kinds have mass, and either kind can be dark matter.
Big Bang theory: The theory that the universe began with all matter and energy concentrated to very high density and temperature some 15 billion years ago. The present universe expanded from that epoch and is still expanding. In the hot Big Bang theory, the ratio of photons to atoms is large, say ~109, as astronomers now observe.
Black hole: An object that has become so dense that, through the effects of general relativity, its contents are no longer accessible to the outside universe. No light from the surface can escape to the outside, hence the term "black."
Brown dwarf: A hypothetical star not sufficiently massive to ignite hydrogen burning, with mass less than 10 percent of the mass of the Sun.
CCD: Charge-coupled device. An electronic image detector used in modern video cameras and astronomical instruments.
CERN: The European Laboratory for Particle Physics.
Closed universe: A universe expanding slowly enough to be braked by gravity. In this case, W = 1. If W > 1 (above critical density), the universe will eventually recollapse. If = W < 1 (at critical density), the universe will expand forever, but ever more slowly.
Cluster: An assemblage of many galaxies.
CMBR: Cosmic microwave background radiation.
COBE: The Cosmic Background Explorer satellite.
Cosmic remnant: A product of a primordial physical process. The cosmic microwave background radiation is a cosmic remnant.
Cosmic velocity flow: Alterations in the regular movement of celestial objects away from each other caused by the gravitational attraction of other nearby objects. These flows give an indication of total mass, both luminous and nonluminous.
Cosmology: The study of the contents, structure, and evolution of the universe from the beginning of time to the infinite future.
CP: Charge-parity conservation. The symmetry of properties under a reflection in space and reversal of charge.
Critical density: The density of matter that would just halt the expansion of the universe. The dividing line between a collapsing and an ever-expanding universe.
Dark matter: Matter that does not emit enough light or other radiation to be observed directly. Most of the matter in the universe is believed to be of this type. Cold dark matter had a low velocity compared to the speed of light during the epoch of recombination. An example would be elementary particles with mass about equal to that of a proton or higher. Hot dark matter had a high velocity (near the speed of light) during the epoch of recombination. An example would be light elementary particles.
Epoch: A period characterized by the dominance of a particular physical process, such as the formation of the light elements from protons and neutrons.
Epoch of photon decoupling: See epoch of recombination.
Epoch of recombination: The time when electrons and nuclei were combining to form atoms and the universe was 1,600 times smaller than its present size. Also called the epoch of photon decoupling and the epoch of atom formation.
eV: An electron-volt, a measure of energy equal to that gained by an electron passing through a potential difference of 1 volt. Also a unit of particle mass. Electrons have a mass of about 0.511 MeV (million electron-volts); protons have a mass of about 938 GeV (billion electron-volts).
Flat universe: A universe where space is euclidean (zero curvature). If L = 0, a flat universe has W = 1. If L is non-zero, then L + W = 1.
Forces of nature: The four basic forces of physics: gravity, electromagnetism, and the weak and strong interactions.
Freeze-out: The disequilibrium by which relics are formed in the universe.
Galaxy: A large assemblage of stars. Our own galaxy, the Milky Way, contains 1011 stars.
Grand unification era: The era when the universe cooled sufficiently for gravity to be described by Einstein's general relativity theory, but where the temperature was still sufficiently high that the other remaining three forces of nature remained unified.
Grand unified theories: Theories that combine the strong, electromagnetic, and weak interactions into one unified theory.
Gravitational instability: The process whereby a small lump in an expanding universe can grow under gravity, pulling in surrounding matter and ultimately collapsing to form an object like a galaxy or cluster of galaxies.
Gravitational lens: A celestial object that distorts the image of another object behind it by virtue of the fact that its gravity affects the propagation of the light from the background object.
H0: The Hubble constant. A measure of the expansion rate and age of the universe.
Hadron: A strongly interacting particle such as a proton or neutron.
Halo: The matter surrounding a galaxy.
Horizon: Edge of the portion of the universe visible to us. Light signals from beyond this point have not had time to reach Earth yet.
HST: Hubble Space Telescope.
Hubble's law: The principle that any two distant celestial objects (e.g., galaxies) move away from each other at a speed that is proportional to the distance between them, due to the homogenous expansion of space.
Inflation: A rapid expansion appearing as an early phase in some cosmological models, which solves several problems of cosmology.
Infrared: Light of wavelength longer than the reddest part of the visible spectrum.
Intergalactic medium: The material between galaxies.
Ionized: Under terrestrial conditions, most matter has an equal amount of positive and negative charge, so that its net charge is zero. At high temperatures, the charges separate in a process called ionization.
IRAS: The NASA, British, and Dutch Infrared Astronomy Satellite, which was flown in 1983.
Keck telescopes: The two new, state-of-the-art ground-based 10-meter optical telescopes located on Mauna Kea, Hawaii.
LEP: The Large Electron-Positron Collider. A particle accelerator at CERN.
Leptons: A class of elementary particles including electrons, muons, and tauons.
Microlensing: If a small, dark body is directly in the line of sight to a bright background star, the brightness of the background star may appear to increase because of bending of the light rays by the dark body.
Neutrinos: Very light (possibly massless) particles that are emitted in the process of radioactive decay. There are three species, associated with electrons, muons, and tau-leptons. They interact with ordinary matter 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. It has less than critical density, W < 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 governing the evolution of the universe.
q0: The deceleration rate for the expansion of the universe, which is related to the curvature of space.
Quarks: The elementary constituents of hadrons or baryons (such as protons and neutrons).
Quasar: An object with a large redshift and an inferred luminosity often hundreds of times that of a normal galaxy.
Redshift: The shifting of light toward the red end of the spectrum that occurs when 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 be estimated by some physical principle and whose observed brightness is therefore useful as a tool to measure distance.
Supersymmetry: A space-time symmetry that would imply the existence of partners to all elementary particles, with quantum spins of one-half a unit higher or lower. Often used in constructing theories that unify gravity with the three other forces.
Thermal spectrum: The characteristic distribution of radiation as a function of frequency that is emitted by a body at a well-defined temperature, also called a black-body spectrum.
Tully-Fisher relation: A method to determine galactic distances. Big, luminous galaxies rotate faster than small, faint ones. The connection between the two 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 become one 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 radius of 27 km.
VLBA: Very Long Baseline Array. A newly completed radio interferometer operated by the National Radio Astronomy Observatory. Capable of producing images with angular resolution of one thousandth of a second of arc.
Weak interactions: The interactions of elementary particles that are responsible for radioactive decay.
WIMP: Weakly Interacting Massive Particle. A candidate for dark matter.