  The recent Cosmic Microwave Background (CMB) experiments have shown that the
average density of the universe is close to the critical one and the universe
is asymptotically flat (Euclidean). Taking into account that the universe
remains flat and the total density of the universe $\Omega_{0}$ is conserved
equal to a unit during the cosmological expansion, the Schwarzschild radius of
the observable universe has been determined equal to the Hubble distance
$R_{s}=2GM/c^{2}=R\sim c/H$, where M is the mass of the observable universe, R
is the Hubble distance and H is the Hubble constant. Besides, it has been shown
that the speed of the light c appears the parabolic velocity for the observable
universe $c=\sqrt{2GM/R}=v_{p}$ and the recessional velocity $v_{r}=Hr$ of an
arbitrary galaxy at a distance r > 100 Mps from the observer, is equal to the
parabolic velocity for the sphere, having radius r and a centre, coinciding
with the observer. The requirement for conservation of $\Omega_{0}=1$ during
the expansion enables to derive the Hoyle-Carvalho formula for the mass of the
observable universe $M=c^{3}/(2GH)$ by a new approach.
  Key words: flat universe; critical density of the universe; Schwarzschild
radius; mass of the universe; parabolic velocity
