14.6 Key Terms

Cold dark matter: slow-moving massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation, and that make up most of the mass of galaxies and galaxy clusters. 14.4

Cosmological principle: the assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous. 14.2

Dark energy: an energy that is causing the expansion of the universe to accelerate; the source of this energy is not yet understood. 14.4

Homogeneous: having a consistent and even distribution of matter that is the same everywhere. 14.2

Hot dark matter: massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation, and that make up most of the mass of galaxies and galaxy clusters; hot dark matter is faster-moving material than cold dark matter. 14.4

Isotropic: the same in all directions. 14.2

Optical fibers: thin tubes of flexible glass that can transmit light from a source to the CCD that then records the spectrum. 14.3

Rotation curves: plots that show velocity versus distance from the galaxy centre for the observed rotation of spiral galaxies. 14.4

Supercluster: a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies. 14.3

Void: a region between clusters and superclusters of galaxies that appears relatively empty of galaxies. 14.3