A type of universe in which the mean density of matter is precisely matched to the critical density. Such a model will not actually collapse, but will expand for ever with a continually decreasing expansion rate. This model lies on the dividing line between a closed Friedmann universe (which collapses) and an open Friedmann universe (which does not). This model has the mathematical virtue of simplicity, in that it is spatially flat (see Curvature of Spacetime). It is named after A. Einstein and W. de Sitter.
Subjects: Astronomy and Astrophysics.