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Matter Content of the Universe

Matter Content of the Universe. David Spergel March 2006 Valencia, Spain. Overview. What is the composition of the universe?. 4.5% Baryons 22.4% Dark Matter 73% Dark Energy <0.1% Neutrinos, Radiation. Baryon Density. Nucleosynthesis [D]/[H] CMB Power Spectrum Lyman a forest

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Matter Content of the Universe

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  1. Matter Content of the Universe David Spergel March 2006 Valencia, Spain

  2. Overview • What is the composition of the universe? 4.5% Baryons 22.4% Dark Matter 73% Dark Energy <0.1% Neutrinos, Radiation

  3. Baryon Density • Nucleosynthesis [D]/[H] • CMB Power Spectrum • Lyman a forest • Local Universe • Where are the missing baryons? • Fukugita, Hogan & Peebles, ApJ, 503, 518

  4. Baryon Abundance from Nucleosynthesis • Deuterium abundance is sensitive to the ratio of baryons/photons • Wbh2 = 0.022 \pm 0.02

  5. CMB & Baryon Abundance • Baryon abundance determines sound speed of baryon/photon fluid • Peak positions and peak heights are sensitive to baryon abundance.

  6. Lyman a Forest • Gas along the line of sight to a distant quasar absorbs light at the Lyman transition. Gas at different redshifts produces lines at different positions. The line strength is a measure of the gas column density at each position

  7. Local Universe: Galaxies 99

  8. Using the CMB to Find the Missing Baryons • Kinetic Sunyaev Zel’dovich Effect Sensitive to electron density. Since we can use the local density field to predict velocity, this is a tool to find the missing baryons CMB is not just a tool for studying the high redshift universe but also the growth of structure

  9. Atacama Cosmology Telescope • Operations start November • Full science operations in November 2007 • Studies both high redshift and low redshift universe

  10. Evidence for Dark Matter • Galaxy Motions in Clusters • Dynamical Measurements • Gravitational Lensing • CMB Observations

  11. A2142: A Rich Cluster • Images are 7’ on each side (moon is 30’) • X-ray temperature (5-10 keV) Contains 100’s of galaxies and enough gas for thousands more

  12. Weighing a Cluster • Galaxy velocity dispersion • Hydrostatic Equlibrium of Gas • Gravitational Lensing

  13. Weighing Galaxies: Rotation Curves • Galaxies contain more than just stars and gas. Roughly 10% of their mass is in non-visible matter

  14. Strong Gravitational Lensing

  15. Gravitational Lensing:A2218

  16. CMB Observations Measure Dark Matter Density at z = 1000 CMB observations imply the existence of significant amount of NON-BARYONIC dark matter Observations provide an accurate determination of dark matter density (More accurate soon!)

  17. Data will improve very soon…..

  18. Dwarf Spheroidals and 3 Myths about Dark Matter • Myth #1: MOND can fit all galactic observations • Myth #2: Observations imply the existence of light neutrinos • Myth #3: Locally dark matter is associated with tidal streams

  19. What is a dwarf spheroidal? • Dwarf spheroidal is a small galaxy composed of roughly a million old stars, dark matter and trace amounts of gas • Our Galaxy has ~10 satellite dwarf galaxies • Dwarf spheroidals are close enough that we can observe the motions of individual stars

  20. Dwarf Galaxies and MOND • MOND is an alternative to dark matter based on modifications on Newton’s equations • MOND fails dramatically in explaining dwarf spheroidals

  21. Dwarf Galaxies and the Nature of Dark Matter • Recent analysis claim that the dwarf galaxy observations imply a characteristic mass for the dark matter

  22. Dwarf Spheroidals have a minimum velocity disperson • Over a wide range of luminosity, dwarf spheroidals all have about the same velocity dispersion: 9 km/s Recent claim interpret this as a signature of dark matter properties mpv2 = 104 K = binding energy of Hydrogen

  23. Sagittarius • Dwarf spheroidal currently being torn apart by our Galaxy • Tidal stream goes through galactic neighborhood

  24. Sagittarius Dwarf

  25. Tidal Debris Model Law, Johnston & Majewski 2005

  26. Does the tidal debris affect the dark matter searches? • Freese et al: could be up to 20% of local dark matter density. Argue for important local contribution • PROBLEM: • Assumes copious amounts of dark matter • However, the width of the tidal stream depends on the dark matter density. • Can place upper limit on local density < 1% of the local dark matter density

  27. Conclusions We have not yet identified most of the mass in the local universe - dark baryons (hot gas?) - dark matter (SUSY particles??) Astronomical observations constrain dark matter properties: - CMB observations require non-baryonic dark matter - Be wary of strong claims based on dwarf spheroidals!

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