ar X iv : 0 81 1 . 18 33 v 1 [ as tr o - ph ] 1 2 N ov 2 00 8 Spherical Collapse in Modified Newtonian Dynamics ( MOND )

Modeling the structure formation in the universe, we extend the spherical collapse model in the context of MOND starting with the linear Newtonian structure formation followed by the MONDian evolution. In MOND the formation of structures speed up without a need for dark matter. Starting with the top-hat over-dense distribution of the matter, the structures virialize with a power–law profile of the distribution of matter. We show that the virialization process takes place gradually from the center of the structure to the outer layers. In this scenario the smaller structures enter to the MONDian regime earlier and evolve faster, hence they are older than larger structures. We also show that the virialization of the structures occur in the MONDian regime, in which the smaller structures have stronger gravitational acceleration than the larger ones. This feature of the dynamical behavior of the structures is in agreement with this fact that the smaller structures as the globular clusters or galactic bulges have been formed earlier and need less dark matter in CDM scenario. Subject headings: gravitation–galaxies: formation–cosmology: theory–dark matter– large scale structure of universe Department of Theoretical Physics and Astrophysics, University of Tabriz, P.O.Box 51664 Tabriz, Iran Department of Physics, Sharif University of Technology, P.O.Box 11365–9161, Tehran, Iran 3 Institute for Advanced Studies in Basic Sciences, P. O. Box 45195-1159, Zanjan, Iran Institute for Studies in Theoretical Physics and Mathematics, P.O.Box 19395–5531, Tehran, Iran