15.6 Key Terms
Average density (of the universe): the mass of matter (including the equivalent mass of energy) that would be contained in each unit of volume (say, 1 cubic centimetre) if all the stars, galaxies, and other objects were taken apart, atom by atom, and if all those particles, along with the light and other energy, were distributed throughout all of space with absolute uniformity. 15.3
Big Bang: the theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy). 15.2
Closed universe: a model in which the universe expands from a Big Bang, stops, and then contracts to a big crunch. 15.3
Cosmic microwave background (CMB): microwave radiation coming from all directions that is the redshifted afterglow of the Big Bang. 15.4
Cosmological constant: the term in the equations of general relativity that represents a repulsive force in the universe. 15.2
Cosmology: the study of the organization and evolution of the universe. 15.2
Critical density: in cosmology, the density that is just sufficient to bring the expansion of the universe to a stop after infinite time. 15.3
Flat universe: a model of the universe that has a critical density and in which the geometry of the universe is flat, like a sheet of paper. 15.4
Hubble constant: a constant of proportionality in the law relating the velocities of remote galaxies to their distances. 15.1
Hubble-Lemaître Law: a rule that the radial velocities of remove galaxies are proportional to their distances from us. 15.1
Open universe: a model in which the density of the universe is not high enough to bring the expansion of the universe to a halt. 15.3
Photon decoupling time: when radiation began to stream freely through the universe without interacting with matter. 15.4
Standard bulb (standard candle): some astronomical object with known luminosity that produces an enormous amount of energy and can be observed at distances of a billion light-years or more. 15.2
Weakly interacting massive particles (WIMPs): weakly interacting massive particles are one of the candidates for the composition of dark matter. 15.5
