A Schwarzschild radial coordinate R is presented for the Friedmann dust-filled cosmology models. It is shown that a worldline of constant Schwarzschild radial coordinate in the dust-filled universe is instantaneously null at Rn=2GM/c2, where M is the Schwarzschild mass inside the sphere R=Rn. It is also shown that Mp=3τc3/4G, where Mp is the proper mass inside R=Rn and τ is the age of the universe. The Rn=2GM/c2 result in Friedmann dust-filled cosmology is made physically significant by abandoning the cosmological principle and adjoining segments of Friedmann dust to segments of Schwarzschild vacuum. In the resulting cosmology model, the observable universe may lie inside a black or white hole.
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W. M. Stuckey (1994) studied this question.