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.