Chapter 26 Cosmology
Big Bang
event that cosmologists consider the beginning of the universe, in which all matter and radiation in the entire universe came into being
dark energy
generic name given to the unknown cosmic force field thought to be responsible for the observed acceleration of the Hubble expansion
closed universe
geometry that the universe as a whole would have if the density of matter is above the critical value; this is finite in extent and has no edge, like the surface of a sphere; it has enough mass to stop the present expansion and will eventually collapse
open universe
geometry that the universe would have if the density of matter were less than the critical value; in this there is is not enough matter to halt the expansion of the universe; this is infinite in extent
primeval fireball
hot, dense state of the universe at very early times, just after the Big Bang
isotropy
looking the same in every direction; often applied to the universe as part of the cosmological principle
cosmological constant
quantity originally introduced by Einstein into general relativity to make his equations describe a static universe; now one of several candidates for the repulsive "dark energy" force responsible for the observed cosmic acceleration
cosmic microwave background
the almost perfectly isotropic radio signal that is the electromagnetic remnant of the Big Bang
critical density
the cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever
cosmology
the study of the structure and evolution of the entire universe
cosmological principle
two assumptions that make up the basis of cosmology, namely that the universe is homogeneous and isotropic on sufficiently large scales
critical universe
universe in which the density of matter is exactly equal to the critical density; the universe is infinite in extent and has zero curvature; the expansion will continue forever, but will approach and expansion speed of zero
Olbers's paradox
a thought experiment suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the Sun
homogeneity
assumed property of the universe such that the number of galaxies in an imaginary large cube of the universe is the same no matter where in the universe the cube is placed; more generally, "the same everywhere"