Matter Content of the Universe

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Matter Content of the Universe

• David Spergel
• March 2006
• Valencia, Spain

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Overview

• What is the composition of the universe?

4.5 Baryons 22.4 Dark Matter 73 Dark
Energy lt0.1 Neutrinos, Radiation
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Baryon Density

• Nucleosynthesis D/H
• CMB Power Spectrum
• Lyman a forest
• Local Universe
• Where are the missing baryons?
• Fukugita, Hogan Peebles, ApJ, 503, 518

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Baryon Abundance from Nucleosynthesis

• Deuterium abundance is sensitive to the ratio of
  baryons/photons
• Wbh2 0.022 \pm 0.02

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CMB Baryon Abundance

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

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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

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Local Universe Galaxies
Stars in disks 0.002
Stars in spheroids 0.005
Neutral atomic gas 0.0006
Molecular gas 0.0006
Plasma in clusters 0.003
Plasma in groups 0.007
Missing Warm Plasma 0.027
TOTAL 0.045
99
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Using the CMB to Find the Missing Baryons

• Kinetic Sunyaev Zeldovich 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

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Atacama Cosmology Telescope

• Operations start November
• Full science operations in November 2007
• Studies both high redshift and low redshift
  universe

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Evidence for Dark Matter

• Galaxy Motions in Clusters
• Dynamical Measurements
• Gravitational Lensing
• CMB Observations

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A2142 A Rich Cluster

• Images are 7 on each side (moon is 30)
• X-ray temperature
• (5-10 keV)
• Contains 100s of galaxies and enough gas for
• thousands more

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Weighing a Cluster

• Galaxy velocity dispersion
• Hydrostatic Equlibrium of Gas
• Gravitational Lensing

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Weighing Galaxies Rotation Curves

• Galaxies contain more than just stars and gas.
  Roughly 10 of their mass is in non-visible matter

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Strong Gravitational Lensing
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Gravitational LensingA2218
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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!)
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Data will improve very soon..
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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

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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

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Dwarf Galaxies and MOND

• MOND is an alternative to dark matter based on
  modifications on Newtons equations
• MOND fails dramatically in explaining dwarf
  spheroidals

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Dwarf Galaxies and the Nature of Dark Matter

• Recent analysis claim that the dwarf galaxy
  observations imply a characteristic mass for the
  dark matter

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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
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Sagittarius

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

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Sagittarius Dwarf
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Tidal Debris Model
Law, Johnston Majewski 2005
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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
• lt 1 of the local dark matter density

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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!