Galaxies

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Galaxies

• II

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Topics

• Galaxies and Dark Matter
• The Expanding Universe

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Island Universes

• 1924 Edwin Hubble
• Measured the distances to several galaxies using
  the Luminosity-Period relationship of Cepheid
  variables, discovered by Henrietta Leavitt (1912)

Hubble at the Mount Wilson Observatory
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Galaxy Types
Elliptical
Spiral
Irregular
Peculiar
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Galactic Clusters

• Clusters
• On very large scales galaxies are found to form
  clusters.
• The Milky Way, in particular, inhabits a cluster
  called the Local Group which contains about 30
  galaxies of which the Milky Way, M31 and M33 are
  the three largest.  

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Local Group Contains about 30 galaxies
http//en.wikipedia.org/wiki/Local_Group
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Galactic Clusters II

• SuperClusters
• On even greater scales the galaxy clusters
  themselves form clusters called superclusters
  that can be tens to hundreds of millions of light
  years across.
• These superclusters resemble huge ribbons or
  sheets in space.
• Between them lie great voids containing
  relatively few galaxies.

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Virgo Supercluster
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Slices of the Universe
1 billion light years
You are here
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Dark Matter
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Rotation Curve of Milky Way

• Rotation Curve
• This is a plot of the orbital speed of stars,
  about the galactic center, versus distance from
  the center.
• The speed is determined by the amount of matter
  enclosed within the stars orbit.

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Rotation Curve of Milky Way II

• How Much Mass?
• Newtons version of Keplers 3rd Law is
• If m1 gtgt m2

R
P Period of orbit v Speed of star
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Rotation Curve of Milky Way III

• How Much Mass?
• Mass enclosed in radius R
• v 200 km / sec
• R 26,000 light years
• m1 1011 Solar masses

R
P Period of orbit v Speed of star
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Rotation Curve Puzzle

• Puzzle
• Keplers Law predicts that the rotation curve
  should decrease with increasing distance from the
  center of gravity, like 1/vR, as is true in our
  solar system.
• But this law fails for stars orbiting the
  galactic center.

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Rotation Curve Puzzle Possible Solutions

• Possible Solution A New Law of Gravity
• Newtons law of gravity fails on very large
  scales.
• Possible Solution Dark Matter
• Newtons law holds even on large scales.
• But a non-luminous form of matter must exist.
• If so, the dark matter could be new forms of
  matter or dark stars, or a mixture of both.

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Dark Matter

• Sub-Stellar Objects (Failed Stars)
• Would-be stars that did not draw together enough
  matter to trigger thermonuclear reactions in
  their core.
• These are objects with masses less than about
  0.01 solar masses. They shine so feebly that they
  cannot be seen from Earth.
• Sub-stellar objects in our halo are referred to
  as Massive Compact Halo Objects (MACHOs). 
• Black Holes
• One intriguing possibility is that the dark
  matter may be in the form of black holes. 

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Dark Matter II

• Weakly Interacting Massive Particles (WIMPS)
• These particles could be everywhere.
• If the WIMP hypothesis is correct, these
  particles are traversing your body right now!
• But because they interact so weakly with matter
  it is unlikely that any will actually interact
  with the particles in your body!
• Experiments are being conducted worldwide to test
  the WIMP hypothesis.

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Dark Stars Gravitational Lenses
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Dark Stars Gravitational Lenses
Dark stars have been found but not enough to
explain the missing mass.
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The Expanding Universe

• 1929 Red Shift
• Drawing on his own observations and those of
  others, Edwin Hubble discovered that the red
  shift,
• z (lo - le) / le
• of the light from distant galaxies increases
  with distance d.
• le emitted wavelength
• lo observed wavelength

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The Expanding Universe II

• 1929 Hubbles Law
• Hubble assumed that the red shift is caused by
  the motion of galaxies away from us. In this case
  
• z v / c
• His observations are summarized in
• Hubbles Law
• v H0 d
• where H0 is called Hubbles constant.

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The Expanding Universe III

• Hubbles Law
• v H0 d
• The Hubble Time
• d v t
• d H0 d t
• t 1/H0
• For H0 70 km/s / Mpc
• t 14 billion years.

1 Mpc (Mega-parsec) 3.26 x 106 light years (ly)
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The Universal Scale Factor
a(t) is the scale factor of the Universe
t is cosmic time
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How Far Is Far ?
t0 t1 is the look-back time
d0 d(t0) is the proper distance between the two
galaxies now d1 d(t1) is the proper distance
between the two galaxies then