Jessica Horn: Chapter 16 Notes

Chapter 16 Dark Matter, Dark Energy, and the Fate of the Universe
16.1 Unseen Influences in the Cosmos

• 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
• Normal Matter: Approximately 4.6%
• Normal matter inside stars: Approximately 0.7%
• Normal matter outside stars: Approximately 3.9%
• Dark matter: Approximately 23%
• Dark energy: Approximately 72%

16.2 Evidence for Dark Matter

• We measure the mass of the solar system using the orbits of planets
• orbital period
• average distance
• or for circles...
• orbital velocity
• orbital radius
• Rotation curve: A plot of orbital speed vs. orbital radius
• Solar system's rotation curve declines because the Sun has almost all the mass.
• Rotation curve of merry-go-round rises with radius
• The rotation curve of the Milky Way stays flat with distance
• Mass must be more spread out than in the solar system
• Most of the Milky Way's mass seems to be dark matter
• The visible portion of a galaxy lies deep in the heart of a large halo of dark matter
• We 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, indicating large amounts of dark matter
• The broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting
• These galaxies also have dark matter
• 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 approximately 50 times larger than the mass in stars
• Clusters contain large amounts of x ray emitting hot gas
• The temperature of hat gas tells us cluster mass:
• 85% dark matter, 13% hot gas, 2% stars
• Gravitational lensing: the bending of light rays by gravity, can also tell us a cluster's mass

Our Options

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

*Most astronomers prefer #1

• Ordinary Matter (MACHOs)
• Massive Compact Halo Objects: dead or failed stars in halos of galaxies
• Exotic Particles (WIMPs)
• Weakly Interacting Massive Particles: mysterious neurtino-like particles

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

• 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 their orbital energy
• Dark matter is still pulling things together
• Maps of galaxy positions reveal extremely large structures: superclusters and voids
• Structures in galaxy maps look very similar to the ones found in which dark matter is WIMPs

16.4 The Fate of the Universe

• Fate of the univers depends on the amount of dark matter
• Amount of matter is approximately 25% of the critical density, suggesting fate is external expansion
• Estimated age depends on both dark matter and dark energy
• The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded
• An accelerating univers is the best fit to supernova data