Image 1 Caption: The evolution of the cosmic scale factor a(t) in 3 qualitatively distinct versions of the Friedmann equation Λ=0 models.
Features:
Many of these many people probably believe the FE models can only be extended to our pocket universe which is embedded in the multiverse.
The Einstein-de Sitter universe is actually the simplest of all FE models: it has cosmological constant Λ=0 and density parameter Ω=1 (i.e., it has zero curvature of space, and so has Euclidean geometry (AKA flat space geometry)).
The formula for the cosmic scale factor a(t) is just
a(t) = [(3/2)H_0*t]**(2/3) = [(3/2)(t/t_H)]**(2/3) = (t/t_0)**(2/3) ,where H_0 is Hubble constant, t_H is the Hubble time, and t_0=(2/3)t_H=(2/3)(1/H_0) is the age of the universe.
Albert Einstein (1879--1955) and Willem de Sitter (1872--1934) in their 1932 article, in fact, did want to present the simplest possible FE model that could account for the observations of the current observable universe (see O'Raifeartaigh et al. 2015, p. 1). They NOT present any theory about the POINT ORIGIN at t=0 which we now call Big Bang singularity. The reason for their simplest model approach and lack of POINT ORIGIN discussion is that they did NOT want indulge in highly speculative theorizing at least in their 1932 article which was rather brief (see O'Raifeartaigh et al. 2015, p. 1, 6, 23).
However, by 1995 it was beginning to be seen to be inadequate. At that time Ω_matter (i.e., the density parameter due to matter only: baryonic and dark) was being found to be ∼ 0.3$ and NOT 1. This is still the case (see Planck 2018, p. 15). But inflation cosmology (proposed 1979) (which had become highly favored as it still is) required Ω≅1. This suggested there was mass-energy that was NOT matter. So the idea of dark energy was introduced which in respect to the Friedmann equation (but NOT other respects) is equivalent to the cosmological constant Λ. Then in 1998 acceleration of the universe was discovered which could be accounted for by an nonzero cosmological constant Λ. Very soon the Λ-CDM model became standard model of cosmology (SMC) which as aforesaid, it still is circa 2022. After circa 1998, the Einstein-de Sitter universe became a subject for textbooks and history of cosmology.
UNDER CONSTRUCTION BELOW