The Dark Side of the Universe Neta A. Bahcall Princeton University
What is the Universe Made of? 10/23/2007 What is the Universe Made of? Luminous matter: Stars, Galaxies, Gas Dark stuff: Dark Matter: How know? How much? Where? What is it? Dark Energy: How know? How much?
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Weighing Galaxies Motion (v) of Stars and Gas: v2 = GM(<r)/r 10/23/2007 Weighing Galaxies Motion (v) of Stars and Gas: v2 = GM(<r)/r [ GM(<r)m/r2 ~ mv2/r ] Gravitational Lensing: M(<r) Data show velocity and mass in large halos around luminous galaxies Implies: Dark Matter in Galactic Halos!
Flat Rotation Curves 10/23/2007 M(<r) ~ v2r ~ r v r
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Mass Density of Universe 10/23/2007 How much? How distributed? Mass-to-Light Function Baryon Fraction Cluster Abundance and Evolution Other Large-Scale Structure Obs. All yield m ~ 0.25 Mass ~ Light (on large scales)
Mass-Density (Units) b (baryons) (observed) ~ 0.05 10/23/2007 Mass-Density (Units) Critical mass-density (= density needed to halt the Universe expansion): critical = 3Ho2/8G ~10-29g/cm3 ~ 6 p/m3 m = m/crit m = 1 is the critical density ‘Flat’ Universe b (baryons) (observed) ~ 0.05 [Mpc = 106pc; 1pc ~ 3 ly; Mo=2E33g]
Mass-to-Light Method m = m/critical 10/23/2007 Mass-to-Light Method <M/L>cl Luniv(Lo/Vol) = m(Mo/Vol) Weigh cluster mass, Mcl (<R~1Mpc) <M/L>cl = 300h m = m/critical m ~ 0.2 +-0.1
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Mass-to-Light Function (Bahcall, Lubin & Dorman ‘95; Bahcall and Fan ‘98) 10/23/2007
M/L (R) SDSS (Bahcall & Kulier 2010)
M/L Function: Conclusions 10/23/2007 M/L Function: Conclusions M/L Function Flattens on Large Scales: M ~ L (on large scales) reaching the end of growth of Dark-Matter Total Mass-Density of Universe: m = 0.2 +- 0.05
Dark Matter How know? Gravity: Motion, Gravitational Lensing 10/23/2007 How know? Gravity: Motion, Gravitational Lensing Where is it located? Large Galaxy Halos, Clusters How much? ~20% of Critical Density [atoms/luminous: only ~5%] What is it? New particles ???
Universe Expansion: Hubble Law 10/23/2007 Universe Expansion: Hubble Law
Hubble Expansion of Universe 10/23/2007 d
10/23/2007 Cosmic Acceleration
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‘Dark Energy’ Dark-Energy ‘Stretch’ > Mass-Density ‘pull’ 10/23/2007 ‘Dark Energy’ Universe acceleration Dark-Energy ‘Stretch’ > Mass-Density ‘pull’ (at present time) Mass-Density: m ~ 0.25 Acceleration: m - ~ - 0.5 ~ 0.75 What is it? ????
What is the Universe Made of? The Dark Side of the Universe 10/23/2007 What is the Universe Made of? The Dark Side of the Universe Luminous (‘Normal’) matter: 5% Dark Stuff: 95% Dark Matter: 20% Dark Energy: 75%
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Universe’s Baby Pictures (WMAP) 10/23/2007
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10/23/2007 The Universe
The Content of the Universe 10/23/2007 The Content of the Universe Luminous matter (5%): Stars, Galaxies, Gas Dark Matter (20%): New particles (?) Dark Energy (75%): Cosmological Constant?
The Cosmic Triangle [only 5% atoms/normal/luminous] 10/23/2007 The Cosmic Triangle Mass density of Universe: 25% of Critical [only 5% atoms/normal/luminous] Universe will expand forever Dark Energy in Universe: 75% Universe expansion accelerates Space Curvature: 0 Universe ‘Flat’
Fate of Universe Sparser and Sparser Darker and Darker 10/23/2007 Universe Will Expand Forever becoming: Larger and Larger Sparser and Sparser Darker and Darker Colder and Colder
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Sloan Digital Sky Survey 10/23/2007
Baryons in Clusters [Stars and Gas] 10/23/2007 Baryons in Clusters [Stars and Gas] Ωb/Ωm(cl) Mb/Mtot(cl) = 0.13 (gas) + 0.03 (stars) = 0.16 (h=0.7) Ωb(BBN; CMB) = 0.042 (h=0.7) Ωm = Ωb/(Ωb/Ωm) = 0.26+-0.04 0.24 +- 0.04 corrected for gas outflow
m from Baryon-Fraction 10/23/2007 m from Baryon-Fraction b/m = 0.18 +- 0.02 h=0.7 (Clusters; CMB) b = 0.042 +- 0.004 (BBN; CMB) m = 0.24 +- 0.04
Cluster Abundance and Evolution 10/23/2007 Cluster Abundance and Evolution Powerful method to determine m and 8 8 = Amplitude of mass fluctuations (initial ‘seeds’) ncl (z~0) 8 m0.6 ~ 0.35 ncl (hi z) Breaks degeneracy m=0.2+-0.05 and 8=0.9+-0.1
Cluster Mass-Function (SDSS) (Bahcall, Dong, et al ‘02) Best-fit MF: m=0.2 and 8=0.9 10/23/2007 Fit: m=0.2 8=0.9
Cluster Abundance Evolution 8 (Bahcall & Bode) 10/23/2007 Cluster Abundance Evolution 8 (Bahcall & Bode) 8
Weighing the Universe Universe Expands Forever! 10/23/2007 Weighing the Universe M/L Function m= 0.2 +- 0.05 Baryon Fraction 0.24 +- 0.04 Cluster Abundance 0.2 +- 0.05 and Evolution [8 = 0.9 +- 0.1] Supernovae Ia + Flat 0.25 +- 0.05 CMB + LSS + h + Flat 0.24 +- 0.04 m = 0.2 - 0.25 4-5% Baryons + ~20% Dark Matter Mass ~ Light Universe Expands Forever!
Accelerating Universe: Supernovae (Kirshner et al) 10/23/2007