Big bang: The singularity at the beginning of the universe.
Big crunch: The singularity at the end of the universe.
Black hole: A region of space-time from which nothing, not even light, can escape, because gravity is so strong.
Casimir effect: The attractive pressure between two flat, parallel metal plates placed very near to each other in a vacuum. The pressure is due to a reduction in the usual number of virtual particles in the space between the plates.
Chandrasekhar limit: The maximum possible mass of a stable cold star, above which it must collapse into a black hole.
Conservation of energy: The law of science that states that energy (or its equivalent in mass) can neither be created nor destroyed.
Coordinates: Numbers that specify the position of a point in space and time.
Cosmological constant: A mathematical device used by Einstein to give space-time an inbuilt tendency to expand.
Cosmology: The study of the universe as a whole.
Dark matter: Matter in galaxies, clusters, and possibly between clusters, that can not be observed directly but can be detected by its gravitational effect. As much as 90 percent of the mass of the universe may be in the form of dark matter.
Duality: A correspondence between apparently different theories that lead to the same physical results.
Einstein-Rosen bridge: A thin tube of space-time linking two black holes. Also see Wormhole.
Electric charge: A property of a particle by which it may repel (or attract) other particles that have a charge of similar (or opposite) sign.
Electromagnetic force: The force that arises between particles with electric charge; the second strongest of the four fundamental forces.
Electron: A particle with negative electric charge that orbits the nucleus of an atom.
Electroweak unification energy: The energy (around 100 GeV) above which the distinction between the electromagnetic force and the weak force disappears.
Elementary particle: A particle that, it is believed, cannot be subdivided.
Event: A point in space-time, specified by its time and place.
Event horizon: The boundary of a black hole.
Exclusion principle: The idea that two identical spin-1/2 particles cannot have (within the limits set by the uncertainty principle) both the same position and the same velocity.
Field: Something that exists throughout space and time, as opposed to a particle that exists at only one point at a time.
Frequency: For a wave, the number of complete cycles per second.
Gamma rays: Electromagnetic rays of very short wavelength, produced in radio-active decay or by collisions of elementary particles.
General relativity: Einstein’s theory based on the idea that the laws of science should be the same for all observers, no matter how they are moving. It explains the force of gravity in terms of the curvature of a four-dimensional space-time.
Geodesic: The shortest (or longest) path between two points.
Grand unification energy: The energy above which, it is believed, the electro-magnetic force, weak force, and strong force become indistinguishable from each other.
Grand unified theory (GUT): A theory which unifies the electromagnetic, strong, and weak forces.
Imaginary time: Time measured using imaginary numbers.
Light cone: A surface in space-time that marks out the possible directions for light rays passing through a given event.
Light-second (light-year): The distance traveled by light in one second (year).
Magnetic field: The field responsible for magnetic forces, now incorporated along with the electric field, into the electromagnetic field.
Mass: The quantity of matter in a body; its inertia, or resistance to acceleration.
Microwave background radiation: The radiation from the glowing of the hot early universe, now so greatly red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters). Also see COBE, on page 145.
Naked singularity: A space-time singularity not surrounded by a black hole.
Neutrino: An extremely light (possibly massless) particle that is affected only by the weak force and gravity.
Neutron: An uncharged particle, very similar to the proton, which accounts for roughly half the particles in an atomic nucleus.
Neutron star: A cold star, supported by the exclusion principle repulsion between neutrons.
No boundary condition: The idea that the universe is finite but has no boundary (in imaginary time).
Nuclear fusion: The process by which two nuclei collide and coalesce to form a single, heavier nucleus.