WEIGHING THE UNIVERSE Neta A. Bahcall Princeton University Why Weigh Universe? How much matter in Universe? Is there Dark-Matter? Where is it located? Is there Non-baryonic (exotic) dark-matter? What is it? [Baryon limit is ~4-5% of critical-density.] Most fundamental cosmological parameter
Cosmology; Evolution of Universe; Age of Universe; Galaxy Formation; Gravity Mass Density of Universe How much? How distributed?
Mass-to-Light Function Baryon Fraction Cluster Abundance and Evolution Other Large-Scale Structure Obs. All yield m m ~ 0.25 Mass ~ Light (on large scales) Mass-Density (Units) Critical mass-density (= density needed to
halt the Universe expansion): 2 2 critical critical = 3Hoo /8G -29g/cm3 3 ~ 6 p/m3 3 ~10-29 = /
mm = mm/crit crit mm = 1 is the critical density Flat Universe bb(baryons)(observed)
M/L M/L Function Function Flattens Flattens on on Large Large Scales Scales M M~ ~L
L (reaching end of Dark-Matter) Dark Matter located mostly in large galactic halos 100s Kpc) Group/Clusters: made up of Sp+E mix (+their DM halos); no significant additional DM Cluster M/L increases slightly with M (mergers?) Rich clusters M/LBB is Anti-biased (M/LBB>mean) Asymptotic Cluster M/Lii(22Mpc) is same for ALL
Groups and Clusters, 362+-54h ! Mass-Density Mass-Density of of Univers: Univers: m 0.2 ++- 0.03 0.03 m = 0.2
III. Cluster Abundance and Evolution Powerful method to determine m and 88 m and 88 = Amplitude of mass fluctuations (initial seeds) 0.6 ncl 0.6 ~ 0.35 (z~0) 88 m 0.35
cl (z~0) m nclcl (hi (hi z) z) Breaks degeneracy degeneracy m and 88=0.9+-0.1 m=0.2+-0.05 and
88 (galaxies)(obs) (galaxies)(obs) ~ ~ 0.9 0.9 If (m)~ ~ 0.9 If Mass
Mass ~ ~ Light Light (on (on large large scale) scale) 88(m) 0.9 Cluster Mass-Function (SDSS)
(SDSS) (Bahcall, (Bahcall, Dong, Dong, et et al al 02) 02) Best-fit MF: mm=0.2 and and 88=0.9
Fit: m=0.2 8=0.9 mm - 88 constraints from MF: mm = 0.2 and and 88 = 0.9 m=0.2, 8=0.9 m - 8 constraints from
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