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"