Dark matter: Astronomical aspects

Highlights of Astronomy, Vol. 19 International Astronomical Union, 2009 0. Engvold, ed. DARK MAT T E R : Astronomical Asp ects Virginia Trimble Department of Physics and Astronomy, University of California, Irvine CA 92697 USA, and Astronomy Depa1iment, University of Maryland, College Park MD 20742 USA Abstract . The chief evidence for appreciable dark matter in the universe comes from the monotonic increase in mass to light ratios measured for various astronomical systems as one looks on larger and larger length scales. Though the evidence comes from photons, most of the dark matter is non-photonic, and , for that matter, non-ba.ryonic. There remain several questions about the nature and behavior of dark matter to which conventional astronomical observations are (probably) relevant. The most germain to this JD is whether astronomers have seen decay or annihilation products from dark matter particles, to which the current answer seems to be no. We look at a few of the others. Single stars and clusters have mass-to-light ratios of order unity, in solar units, while the largest scale structures and global values reach about 300, cor- responding to a matter density of about 30% of closure density. If you think of this as a graph with a nearly straight line sloping from lower left to upper right, then the standard points on it pertain to the solar neighborhood (and inner bright parts of other galaxies), the outskirts of galaxies (rotation curves, globular cluster velocities, etc), binary galaxies and small groups, rich clusters, superclusters, and the universe as a whole. Curiously, by giving a little thought to the assorted distances scales in use at various times, you could have drawn this line before World War II, using the solar neighborhood numbers of Kapteyn (1922) and Jeans (1922), the Babcock (1939) rotation curve for M31, the bi- nary galaxies of Holmberg (1937) , and the work on the Coma and Virgo clusters by, respectively, Zwicky (1933) and Smith (1936). Holmberg drew attention to this qualitatively, by saying he thought it reasonable that his typical mass for a galaxy should come somewhere between those found by Hubble (for visible parts) and those found by Zwicky and Smith if you divide the mass of the whole clusters by the number of galaxies you see. Most astronomers working in extragalactic astronomy would endorse the previous paragraph. There remain, however, three sorts of alternatives. The first is that gravity might not behave as described by the equations of Newton and Einstein. Two examples are the MOdified Newtonian Dynamics (MOND) of Milgrom (2002) and the conformal gravity of Mannheim (2001). The second goes further, with non-standard contributions to the velocities we observe as well as to the forces exerted by matter. The quasi-steady state theory (Narlikar et al. 2003) is the best-known example in this class. Third is something that none of us has thought of yet, for which there are necessarily no references except to the speaker who was originally scheduled to give this talk. In the absence of direct laboratory detection of dark matter particles, most of the evidence we have for their properties comes from astronomical observa-