Chapter 17 Cosmology
Olber's paradox
A thought expirement suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the sun.
hemogeneous
An 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.
isotropic
Assumed property of the universe such that the universe looks the same in every direction.
Grand Unified Theories
Class of theories describing the behavior of the single force that results from unification of the strong, weak, and electromagnetic forces in the early universe.
radiation dominated
Early epoch in the universe, when the equivalent density of radiation in the cosmos exceeded the density of matter.
decoupling
Event in the early universe when atoms first formed, after which photons could propagate freely through space.
big bang
Event that cosmolgists 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 world would have if the density of matter is above the critical value. A closed universe is finite in extent and has no edge, like the surface of a sphere. It has enough mass to stop the present explosion and what will eventually collapse.
open universe
Geometry that the universe would have if the density of matter were less than the critical value. In an open universe there is not enough matter to halt the expansion of the universe. An open universe is infinite in extent.
flatness problem
One of two conceptual problems with the standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density.
horizon problem
One of two conceptual problems with the standard Big Biang model, which is that some regions of the universe that have very similar properties are too far apart to have exchanged information within the age of the universe.
vacuum energy
Property of empty space when it is excited above its normal zero- energy state. Astronomers think that the temporary appearance of vacuum energy was responsible for one or more periods of inflation during the early universe.
cosmological constant
Quantity originally introduced by Einstien into general relativity to make his equations describe a static universe. Now one of the several candidates for the repulsive "dark energy" force responsible for the observed cosmic acceleration.
epoch of inflation
Short period of unchecked cosmic expansion early in the history of the universe. During inflation, the universe swelled in size by a factor of about 10^50.
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.
critical universe
The cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever.
primordial nuclearsynthesis
The production of elements having heavier than hydrogen by nuclear fusion in the high temperatures and densities that existed in the early universe.
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.