Can collapsed stars close the Universe

Can black holes and other collapsed objects contribute enough mass to close the Universe (i.e., to bring the density of the Universe up to the critical density, rho/subc/). If the black holes are the collapsed remnants of an early generation of massive stars, current observations of the nightsky brightness may be used to place an upper limit on the mass contributed by such objects. We show that the density contributed by collapsed objects must be less than 0.01 rho/subc/ unless the stars formed before an epoch of 109 yr (z>5). Even more stringent limits may be set for many of the models considered