- Radiation observed from all directions in the universe and regarded by astronomers as the fossil remnant of the ‘big bang’ at the birth of the cosmos. The radiation is very faint and corresponds to a temperature of only about 3° above absolute zero, because of the expansion of the universe which has ‘stretched’ the radiation and reduced its apparent temperature. The background radiation is one of the most powerful pieces of evidence for our present view of the universe. The COBE satellite and terrestrial observations have shown that it is highly homogeneous, but contains enough fluctuation to be consistent with the formation of structure in the universe itself, an elegant confirmation of our main ideas about cosmology.
- Radiation originating from sources such as cosmic rays, or arising from the small proportion of naturally occurring radioactive isotopes present in the atoms in any given environment.