Cosmological Constant:
A term added by Albert Einstein as a modification to his original theory of general relativity, in order to balance the attractive force of gravity and achieve a static or stationary universe. It represents the possibility that there is a density and pressure associated with apparently empty space, and that the overall mass-energy of the universe is actually much greater than currently estimated. Once dismissed as just a mathematical “fix”, it has been revived in recent years with the discovery of the apparent acceleration of the expansion of the universe.
Cosmological Principle:
The starting point for the General Theory of Relativity and theBig Bang theory is that, that averaged over large distances, one part of the universe looks approximately like any other part, and that, viewed on sufficiently large distance scales, there are no preferred directions or preferred places in the universe. Stated in more technical terms, on large spatial scales, the universe is homogeneous and isotropic.
Critical Mass (Critical Density):
As applied to the universe as a whole, critical mass refers to the total required mass of matter in the universe which will allow the effects of gravity to overcome its continued outward expansion. If the universe contains more than the critical mass of matter, its gravity will eventually reverse the expansion, causing the universe to collapse back to what has become known as the Big Crunch. If, however, it contains insufficientmatter, it will go on expanding forever. In the same way, critical density is that overall density of thematter in the universe which will just allow continued expansion.
In other contexts, critical mass is also used to refer to the amount of fissile material needed to sustainnuclear fission.
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