deceleration of universal expansion --

    depends on mass density -- is there enough mass to

    gravitational bind the largest velocities

Energy : just bound if     1/2 m v2 = G M2 / R

classically, : Vesc = (2GM/R)1/2

                     Vesc / R = (8p G density / 3 )1/2

                      H0 => density > 9 x 10-27 kg/m3

equivalent to ~ 5 H atoms per m3

density of matter (luminous and dark) ~ 3 x 10-27 kg/m3

only 30% of critical density needed to close univ.