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More on Dark Matter

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More on Dark Matter. Dr Bryce. Just a reminder. Clear Sky patrol. Exam 3. Homework deadlines. http://news.bbc.co.uk/1/hi/sci/tech/6561391.stm. Our Options ... – PowerPoint PPT presentation

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Title: More on Dark Matter


1
More on Dark Matter
  • Dr Bryce

2
Just a reminder
  • Clear Sky patrol
  • Exam 3
  • Homework deadlines
  • http//news.bbc.co.uk/1/hi/sci/tech/6561391.stm

3
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

4
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

5
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

6
MACHOs occasionally make other stars appear
brighter through lensing …but not enough lensing
events to explain all the dark matter
7
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

8
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

9
  • 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

10
?
Gravity of dark matter caused protogalactic
clouds to contract early in time
11
WIMPs cant contract to center because they dont
radiate away their orbital energy
12
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
13
Maps of galaxy positions reveal extremely large
structures superclusters and voids
14
Time in billions of years
2.2
5.9
8.6
13.7
0.5
35
70
93
140
13
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
15
Structures in galaxy maps look very similar to
the ones found in models in which dark matter is
WIMPs
16
Doppler shifts can be measured with
  • Visible light
  • Radio waves
  • Microwaves
  • All of the above
  • 1 and 3

17
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

18
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

19
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

20
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
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