Chapter 16 Notes: Olivia Ward

Dark Matter, Dark Energy, and the Fate of the Universe
16.1 Unseen Influences in the Cosmos
What do we mean by dark matter and dark energy?

• Unseen Influences
  
  • Dark matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence.
  • Dark energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate.
• Contents of the Universe
  • Normal matter: ~ 4.6%
    
    • Normal matter inside stars: ~ 0.7%
    • Normal matter outside stars: ~ 3.9%
  • Dark matter: ~ 23%
  • Dark energy ~ 72%

16.2 Evidence for Dark Matter
What is the evidence for dark matter in galaxies?

• We measure the mass of the solar system using the orbits of planets.
  
  • Orbital Peroid
  • Average Distance
  • For Circles:
    
    • Orbital Velocity
    • Orbital Radius
  • Rotation Curve
    
    • A plot of orbital speed vs. orbital radius
    • Solar System's rotation curve declines because Sun has most of the mass.
    • The rotation curve of the Milky Way stays flat with distance.
    • Mass must be more spread out than in the Solar System.
      
      • The mass of the Milky Way is spread out over a larger region than the stars.
      • Most of the Milky Way's mass seems to be dark matter.
        • Mass within Sun's orbit: 1.0 X 10^11 MSun
        • Total mass: ~ 10^12 MSun
• The visible portion of a galaxy lies deep in the heat of a large halo of dark matter.
• We can measure orbital velocities in other spiral galaxies using the Doppler shift of the 21-cm line of atomic H.
  
  • Spiral galaxies all tend to have orbital velocities that remain constant at large radii.
• The broadening of spectral lines in elliptical galaxies tell us how fast the stars are orbiting.
  
  • These galaxies also have dark matter.

What is the evidence for dark matter in clusters of galaxies?

• We can measure the velocities of galaxies in a cluster from their Doppler shifts.
  
  • The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in the stars.
• Clusters contain large amounts of x-ray emitting hot gas.
  
  • The temperature of hot gas (particle motion) tells us about cluster mass:
    
    • 85% dark matter
    • 13% hot gas
    • 2% stars
• Gravitational lensing, the blending of light rays by gravity, can also tell us a cluster's mass.
  
  • A gravitational lens distorts our view of things behind it.
• All three methods of measuring cluster mass indicate similar amounts of dark matter.

Does dark matter really exist?

• Our Options
  
  • Dark matter really exists and we are observing the effects of its gravitational attraction.
  • Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.

What might dark matter be made of?

• How dark is it? Not as bright as a star.
• Two Basic Options
  
  • Ordinary Matter (MACHOs)
    
    • Massive Compact Halo Objects: dead or failed stars in halos of galaxies.
  • Exotic Particles (WIMPs)
    
    • Weakly Interacting Massive Particles: mysterious neutrino-like particles (the best bet).
• Compact starline objects occasionally make other stars appear bright through lensing.
• Why WIMPS?
  
  • There's not enough ordinary matter.
  • WIMPs could be left over from the Big Bang.
  • Models involving WIMPs explain how galaxy formation works.

16.3 Structure Formation
What is the role of dark matter in galaxy formation?

• Gravity of dark matter is what caused protogalactic clouds to contract early in time.
• WIMPs can't contract to the center because they don't radiate away from their orbital energy.
• Dark matter is still pulling things together.
  
  • After correcting Hubble's law, we can see that galaxies are flowing toward the densest region of space.

What are the largest structures in the universe?

• Maps of galaxy positions reveal extremely large structures: superclusters and voids.
• Models show that the galaxy of dark matter pulls mass into denser regions: the universe gros lumpier with time.
• Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs.
  

16.4 The Universe's Fate

Will the universe continue expanding forever?

• Does the universe have enough kinetic energy to escape from its own gravitational pull? Yes.
• Fate of the universe depends on the amount of dark matter.
  
  • Expansion appears to be speeding up.
  • Estimated age depends on both dark matter and dark energy. There is not enough dark matter to stop the acceleration.

Is the expansion of the universe accelerating?

•  The brightness of distance white dwarf supernovae tells us about how much the universe has expanded since they exploded.
• An accelerating universe is the best fit to supernova data.