Short answers, pulled from the story.
The Einstein de Sitter universe is a flat matter-only Friedmann Lemaître Robertson Walker metric proposed in 1932. It assumes both spatial curvature and the cosmological constant vanish completely to describe an expanding cosmos without extra parameters.
Albert Einstein and Willem de Sitter met in 1932 to discuss the nature of space and later that same year proposed the Einstein de Sitter universe. This proposal followed Edwin Hubble's discovery of a linear relation between galaxy redshifts and distance which forced Einstein to set his cosmological constant to zero within the Friedmann equations.
The current age of the Einstein de Sitter universe is exactly two-thirds of the Hubble time. The size of the universe evolves with time according to a simple power law derived from the equation where H0 squared equals k times rho divided by three.
Scientists lacked empirical evidence for either spatial curvature or a cosmological constant during that era so the model dominated cosmological thought due to its extreme simplicity. It served as a theoretical case where the universe sat precisely on the edge between eternal expansion and future collapse while researchers used it as a baseline for understanding cosmic history without complex variables.
Observations from the 1990s created increasingly serious problems for the standard model including measurements of the Hubble constant and galaxy clustering studies. The situation changed dramatically following the discovery of accelerating universe expansion in 1998 and subsequent observations of the cosmic microwave background occurred between 2000 and 2003 which showed dark energy makes up around 70 percent of present energy density.