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Physics 133: Extragalactic Astronomy and Cosmology

Physics 133: Extragalactic Astronomy and Cosmology. Lecture 10; February 12 2014. Previously:. How do we measure cosmological parameters? The Hubble constant Measuring v (peculiar velocities) Measuring D (the cosmic distance ladder) Cosmography Luminosity distance and standard candles

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Physics 133: Extragalactic Astronomy and Cosmology

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  1. Physics 133: Extragalactic Astronomy and Cosmology Lecture 10; February 12 2014

  2. Previously: • How do we measure cosmological parameters? • The Hubble constant • Measuring v (peculiar velocities) • Measuring D (the cosmic distance ladder) • Cosmography • Luminosity distance and standard candles • Cosmic time • Ages of the oldest stars • Testing the expansion • Tolman’s test

  3. Outline: • Luminous and Dark Matters: • Luminous and baryonic matter • Dark matter in galaxies

  4. Luminous matter in galaxies • How much in stars? • Luminosity density • M/L • Ω*=0.004 • How much gas? • In galaxies a similar amount

  5. Intracluster medium X-ray (plasma at T~108K) Brehmsstrahlung Emission Mstars~15% Mgas

  6. Intergalactic medium • Not hot enough to be seen in X-ray but abundant • Can be seen in absorption in front of a luminous background source, e.g. quasar or gamma ray burst

  7. Inventory of baryons How do we know it’s 4% and not more?

  8. Circular Orbits [blackboard] Example: The Milky Way [blackboard] Galaxy rotation curves Stellar mass to light ratio ~ 5; total mass to light ratio up to150!

  9. Implications • There is dark matter, making up 40 times the stellar component: Ω=0.004*40 = 0.16>0.04=Ωb • Non baryonic dark matter • Gravity is wrong at small accelerations (modified newtonian dynamics) [blackboard]

  10. Dark matter in clusters. Virial Theorem • First evidence for dark matter was actually discovered in clusters • In the 1930s Zwicky used the virial theorem to derive the mass of the Coma cluster, finding it much higher than that of stars (he didn’t know about ICM) • Virial Theorem [Blackboard]

  11. Dark matter in clusters. Hydrostatic Equilibrium • A second method to determine cluster mass treats the X-ray emitting plasma as a fluid in hydrostatic equilibrium • Pressure gradients vs gravity • [Blackboard]

  12. Dark matter in clusters. Implications for cosmology • If clusters are fair samples of the Universe • m(baryons)/m(total)= Ωb/Ωm • Baryon density is known from primordial nucleosynthesis 0.04 -> Ωm~0.2-0.3

  13. The End See you on Wednesday! (Monday is Presidents Day)

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