cosmology

Branch of astronomy that deals with the structure and evolution of the universe as an ordered whole. Cosmologists construct ‘model universes’ mathematically and compare their large-scale properties with those of the observed universe.

Modern cosmology began in the 1920s with the discovery that the universe is expanding, which suggested that it began in an explosion, the Big Bang. An alternative – now discarded – view, the steady-state theory, claimed that the universe has no origin, but is expanding because new matter is being continually created.

There are a number of differences in the conclusions that can be drawn from the steady-state and Big Bang theories. For example, the number of galaxies per unit volume should not change with distance if the steady-state theory is correct, but should increase with distance if an evolutionary theory is correct for in looking over a distance we are also looking back in time with the universe gradually getting more compact. Counts of faint radio sources indicate an increase in the number per unit volume with distance; they support an evolutionary model. Again the mixture of old and new galaxies should be the same at both small and large distances according to the steady-state theory but the proportion of young objects should increase with distance for the evolutionary theories. Quasars are believed to be young objects, and they are found only with large red shifts (meaning far back in time), so their very existence seems to disprove the steady-state theory. Another piece of evidence for the Big Bang theory is the cosmic background radiation, which was first observed in 1965 and is believed to be the radiation predicted as a necessary consequence of the Big Bang. The present temperature of the background radiation can be used to calculate the temperature of the universe in the early stages of its expansion, and thus predict what the initial ratio of hydrogen to helium should have been, a prediction that turns out to be quite consistent with observation.

A great deal of effort has gone into establishing how the rate of expansion of the universe is changing with time, since this will determine the future fate of the cosmos. In the 1990s counts of supernovae appeared to establish that the expansion is accelerating, not decelerating as previously assumed. This conclusion is supported by other lines of evidence. Cosmologists speculate that the expansion is being accelerated by ‘dark energy’, filling all space and acting like negative pressure acting on matter.