Cosmology with modified Newtonian dynamics (MOND)
Abstract
It is well known that the application of Newtonian dynamics to an expanding spherical region leads to the correct relativistic expression (the Friedmann equation) for the evolution of the cosmic scalefactor. Here, the cosmological implications of Milgrom's modified Newtonian dynamics (MOND) are considered by means of a similar procedure. Earlier work by Felten demonstrated that in a region dominated by modified dynamics the expansion cannot be uniform (separations cannot be expressed in terms of a scalefactor) and that any such region will eventually recollapse regardless of the initial expansion velocity and mean density. Here I show that, because of the acceleration threshold for the MOND phenomenology, a region dominated by MOND will have a finite size which, in the earlier Universe (z>3), is smaller than the horizon scale. Therefore, uniform expansion and homogeneity on the horizon scale are consistent with MONDdominated nonuniform expansion and the development of inhomogeneities on smaller scales. In the radiationdominated era, the amplitude of MONDinduced inhomogeneities is much smaller than that implied by observations of the cosmic background radiation, and the thermal and dynamical history of the Universe is identical to that of the standard big bang model. In particular, the standard results for primordial nucleosynthesis are retained. When matter first dominates the energy density of the Universe, the cosmology diverges from that of the standard model. Objects of galaxy mass are the first virialized objects to form (by z=10), and larger structure develops rapidly. At present, the Universe would be inhomogeneous out to a substantial fraction of the Hubble radius.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 June 1998
 DOI:
 10.1046/j.13658711.1998.01459.x
 arXiv:
 arXiv:astroph/9710335
 Bibcode:
 1998MNRAS.296.1009S
 Keywords:

 GRAVITATION;
 GALAXIES: FORMATION;
 COSMOLOGY: THEORY;
 LARGESCALE STRUCTURE OF UNIVERSE;
 Astrophysics
 EPrint:
 20 pages, LaTex (aaspp4.sty), 4 figures, submitted MNRAS