Chapter 17 Flashcards
(22 cards)
Homogeneous
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 of the universe such that the universe looks the same in every direction.
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.
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.
Big Bang
Event that cosmologists consider the beginning of the universe, in which all matter and radiation in the entire universe came into being.
Critical density
The cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever.
Closed universe
Geometry that the universe as a whole 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 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 an open universe there is not enough matter to halt the expansion of the universe. An open universe is infinite in extent.
Critical universe
Universe in which the density of matter is exactly to the critical density. The universe is infinite in extent and has zero curvature. The expansion will continue forever but will approach an expansion speed of zero.
Dark energy
Generic name given to the unknown cosmic force field thought to be responsible for the observed acceleration of the Hubble expansion.
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.
Radiation-dominated
Early Epoch in the universe, when the equivalent density of radiation in the cosmos exceeded the density of matter.
Primordial nucleosynthesis
The production of elements heavier than hydrogen by nuclear fusion in the high temperatures and densities that existed in the early universe.
Decoupling
Event in the early universe when atoms first formed, after which photons could propagate freely through space.
Horizon problem
One of the conceptual problems with the standard Big Bang 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.
Flatness problem
One of the 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.
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.
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.
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.
Notes
5% normal matter, 27% dark matter, and 68% dark energy.
