Chapter 22 Dark Matter, Dark Energy, and the Fate of the Universe

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Chapter 22Dark Matter, Dark Energy, and the
Fate of the Universe
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What do we mean by dark matter and dark energy?
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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.

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Contents of Universe

• Normal Matter 4.4
• Normal Matter inside stars 0.6
• Normal Matter outside stars 3.8
• Dark Matter 25
• Dark Energy 71

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What is the evidence for dark matter in galaxies?
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• We measure the mass of the solar system using the
  orbits of planets
• Orb. Period
• Avg. Distance
• Or for circles
• Orb. Velocity
• Orbital Radius

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Rotation curve A plot of orbital velocity versus
orbital radius Solar systems rotation curve
declines because Sun has almost all the mass
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Who has the largest orbital velocity? A, B, or C?
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Who has the largest orbital velocity? A, B, or C?
Answer C
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Rotation curve of merry-go-round rises with radius
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Rotation curve of Milky Way stays flat with
distance Mass must be more spread out than in
solar system
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Mass in Milky Way is spread out over a larger
region than the stars Most of the Milky Ways
mass seems to be dark matter!
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Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
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The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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Thought Question

• What would you conclude about a galaxy whose
  rotational velocity rises steadily with distance
  beyond the visible part of its disk?
• A. Its mass is concentrated at the center
• B. It rotates like the solar system
• C. Its especially rich in dark matter
• D. Its just like the Milky Way

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Thought Question

• What would you conclude about a galaxy whose
  rotational velocity rises steadily with distance
  beyond the visible part of its disk?
• A. Its mass is concentrated at the center
• B. It rotates like the solar system
• C. Its especially rich in dark matter
• D. Its just like the Milky Way

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What is the evidence for dark matter in clusters
of galaxies?
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We can measure the velocities of galaxies in a
cluster from their Doppler shifts
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The mass we find from galaxy motions in a cluster
is about 50 times larger than the mass in stars!
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Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
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Thought Question

• What kind of measurement does not tell us the
  mass of a cluster of galaxies?
• A. Measure velocities of cluster galaxies
• B. Measure total mass of clusters stars
• C. Measure temperature of its hot gas
• D. Measure distorted images of
• background galaxies

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Thought Question

• What kind of measurement does not tell us the
  mass of a cluster of galaxies?
• A. Measure velocities of cluster galaxies
• B. Measure total mass of clusters stars
• C. Measure temperature of its hot gas
• D. Measure distorted images of background
  galaxies

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Does dark matter really exist?
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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.

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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.
• Because gravity is so well tested, most
  astronomers prefer option 1.

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What might dark matter be made of?
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How dark is it?
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How dark is it?

• not as bright as a star.

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Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

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Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

The Best Bet
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MACHOs occasionally make other stars appear
brighter through lensing
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MACHOs occasionally make other stars appear
brighter through lensing but not enough
lensing events to explain all the dark matter.
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Why Believe in WIMPs?

• Theres not enough ordinary matter.
• WIMPs could be left over from Big Bang.
• Models involving WIMPs explain how galaxy
  formation works.

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What is the role of dark matter in galaxy
formation?
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Gravity of dark matter is what caused
protogalactic clouds to contract early in time.
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Dark matter is still pulling things
together. After correcting for Hubbles Law, we
can see that galaxies are flowing toward the
densest regions of space.
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What are the largest structures in the universe?
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Maps of galaxy positions reveal extremely large
structures superclusters and voids
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Time in billions of years
2.2
5.9
8.6
13.7
0.5
35
70
93
140
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Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time.
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Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time.
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Structures in galaxy maps look very similar to
the ones found in models in which dark matter is
WIMPs.
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Brightness of distant white-dwarf supernovae
tells us how much universe has expanded since
they exploded