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 This theory concerns the question of the stability of the expansion of a homogenous gas under the influence of gravity
[40], p. 149. If we regard the universe, idealized, as a cloud with homogenously distributed matter, then there essentially act two forces in it: Pressure forces preventing a densification of matter, and gravitation forces favoring it. For a local disturbance, the equilibrium condition between both these forces leads to the relation [40], p. 152:
          (26)
 Since    is the adiabatic speed of sound cs in a gas, from eq. (26) results:
         (27)
 Applied to a gas volume with the size of the total universe (r = R), we receive for the plane universe which, as known, is featured by the perfect balance between eternal expansion and a contraction starting sometimes, the relation:
                (28)
With M = 4/3 R^3.pr  , there finally results:
                       (29)

or, resp.
                    (30)
Thus, there is:
         (31)
We receive, by the way, the same result when we regard the universe as a homogenous continuum. For the signal speed
(wave propagation speed c_w ) in this continuum results the relation (see Annex III):
                   (32)
 

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