More on Dark Matter

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More on Dark Matter

• Dr Bryce

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Just a reminder

• Clear Sky patrol
• Exam 3
• Homework deadlines
• http//news.bbc.co.uk/1/hi/sci/tech/6561391.stm

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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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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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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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Recap

• What is the evidence for dark matter in galaxies?
• Rotation curves of galaxies are flat, indicating
  that most of their matter lies outside their
  visible regions
• What is the evidence for dark matter in clusters
  of galaxies?
• Masses measured from galaxy motions, temperature
  of hot gas, and gravitational lensing all
  indicate that the vast majority of matter in
  clusters is dark

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• Does dark matter really exist?
• Either dark matter exists or our understanding of
  our gravity must be revised
• What might dark matter be made of?
• There does not seem to be enough normal
  (baryonic) matter to account for all the dark
  matter, so most astronomers suspect that dark
  matter is made of (non-baryonic) particles that
  have not yet been discovered

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?
Gravity of dark matter caused protogalactic
clouds to contract early in time
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WIMPs cant contract to center because they dont
radiate away their orbital energy
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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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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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Structures in galaxy maps look very similar to
the ones found in models in which dark matter is
WIMPs
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Doppler shifts can be measured with

• Visible light
• Radio waves
• Microwaves
• All of the above
• 1 and 3

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Doppler shifts can be measured with

• Visible light (how we measure stars)
• Radio waves (we measure the rotation of gas in
  spiral arms of galaxies)
• Microwaves (police give speeding tickets
  microwave is the same as radar)
• All of the above
• 1 and 3

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How have astronomers interpreted the unexpectedly
fast rotation of galaxies?

• The measurements must be wrong
• Galaxies may be breaking up
• There must be a lot of dark matter whose
  gravity can be felt but which isnt seen
• These galaxies must have had collisions

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What do the (fast) speeds of stars in elliptical
galaxies tell us?

• These galaxies may be breaking up
• There must be a lot of dark matter whose
  gravity can be felt but which isnt seen
• These galaxies must have had collisions
• None of the above

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What is the effect of dark matter on the
formation of the structure of galaxies and
clusters of galaxies?

• Nonethe matter is dark
• Small
• Key, since it has gravity and is abundant
• We dont know, because we dont know the
  distribution of dark matter
• Probably none. Since the matter is dark we cant
  be sure it is there