The Dark Side of the Universe Neta A

The Dark Side of the Universe Neta A. Bahcall Princeton University 1

What is the Universe Made of? F Luminous matter: Stars, Galaxies, Gas F Dark stuff: Dark Matter: How know? Where? Dark Energy: How know? Where? How much? What is it? 2

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Weighing Galaxies F Motion (v) of Stars and Gas: v 2 = GM(<r)/r F Gravitational Lensing: M(<r) F Data show velocity and mass in large halos around luminous galaxies Implies: Dark Matter in Galactic Halos! 5

Rotation Curve 6

NGC 3198 7

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Sloan Digital Sky Survey 20

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Mass-Density ‘Critical Density’ of Universe: (mass-density needed to halt the Universal expansion): critical = 10 -29 g/cm 3 ~ 6 protons/m 3 Observed Mass Density of Universe: m = m/ critical ~ 0. 25 Atoms/Luminous matter: ~0. 05 22

Dark Matter F How know? Gravity: Motion, Gravitational Lensing F Where is it located? Large Galaxy Halos, Clusters F How much? ~20% of Critical Density [atoms/luminous: only ~5%] F What is it? ? ? ? 23

Universe Expansion: Hubble Law 24

Hubble Expansion of Universe d 25

Accelerating Universe: Supernovae (Kirshner et al) 26

Cosmic Acceleration 27

‘Dark Energy’ Universe acceleration Dark-Energy ‘Stretch’ > Mass-Density ‘pull’ (at present time) F Mass-Density: m ~ 0. 25 FAcceleration: m - ~ - 0. 5 ~ 0. 75 What is it? ? ? 28

The Dark Side of the Universe: What is the Universe Made of? Luminous (‘Normal’) matter: 5% Dark Stuff: 95% Dark Matter: 20% Dark Energy: 75% 29

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Universe’s Baby Pictures (WMAP) 31

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The Universe 33

The Cosmic Triangle Mass density of Universe: 25% of Critical [only 4 -5% atoms/normal/luminous] Universe will expand forever Dark Energy in Universe: 75% Universe expansion accelerates Space Curvature: 0 Universe ‘Flat’ 34

Fate of Universe Will Expand Forever becoming: Larger and Larger Sparser and Sparser Darker and Darker Colder and Colder But not for many-many billions years! 35

The End… “Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire, I hold with those who favor fire. ” Robert Frost “This is the way the world ends, with a whimper, not with a bang. ” T. S. Eliot 36

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Cosmic Acceleration: Supernovae 41

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