The Friedmann-equation Λ=0 models have cosmological constant (AKA Lambda, Λ) set equal to zero.
Cosmological models of this kind were favored from circa 1965 (with the discovery of the cosmic microwave background (CMB): e.g., No-541,560--563) to circa 1995--1998 (e.g., Scott 2018).
The Lemaitre universe (1931) is a positive curvature Friedmann equation model that starts with point origin which we would now call a Big Bang. Georges Lemaitre (1894--1966) original idea was a primeval atom theory in 1931. After the point origin, the Lemaitre universe (1931) has 3 phases: 1) a matter dominated phase Ω ∼ 1/a**3, 2) a quasi Einstein universe phase, 3) A de Sitter universe phase.