Ch 16 Galaxies and Matter

galaxy cluster themselves tend to clump together.

Large relatively empty region of the universe around which super clusters and "walls" of galaxies are organized

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

Nuclei containing the same number of protons but different numbers of neutrons. Most elements can exist in several isotopic forms. A common example of an isotope is deuterium, which differs from normal hydrogen by the presence of an extra neutron in the nucleus

the study of the structure and evolution of the entire universe

two assumptions that make up the basis of cosmology, namely that the universe is homogeneous and isotopic on sufficiently large scales

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

geometry that the universe would have if the density of matter were less than the critical value. In an open universe is infinite in extent

corresponding to the dividing line between a universe the re-collapses and one that expands forever

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

density of matter is exactly equal to the critical density. the universe is infinite in extend and has zero curvature. the expansion will continue forever bu will approach an expansion speed of zero