Announcements Friday April 21

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AnnouncementsFriday April 21

• Exam 4
• Wednesday, April 26
• 25 questions, 3 equations
• Hubble law, redshift, age from Hubble constant
• Covers Chapters 19 (Milky Way Galaxy), 20
  (Galaxies), 21 (Galaxy Evolution), 22 (Cosmology
  I)
• Practice exam will be posted by Sunday evening
• Course grading questions
• Lowest exam, homework dropped (but not PRS quiz)
• For lab students, final grade 75 lecture, 25
  lab

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Course grading

• Total 900 pts
• Exams 3x100 300 pts
• Final exam 150 pts
• Homework 12x20240
• PRS Quizes 6x25150 pts
• Observing test 60 pts
• EC
• Lab 300pts (1,200 pts total)

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Approximate Course Curve (No lab)
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Cosmology continued
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The farther we look into space, the farther back
in time we are seeing
Note that there is a farthest distance (particle
horizon) corresponding to VHubble c. Regions
farther away cannot be seen!
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Most distant object a galaxy ever seen (formed
500 Million yr after big bang 13.4 Billion yr
ago!
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Cosmic Background Radiation (CBR)

• In 1965 radio astronomers Penzias and Wilson
  accidentally discovered The microwave radiation
  that fills all space is evidence of a hot Big
  Bang.
• They were trying to measure the radio noise of
  the galactic halo, but found (an annoying) weak
  excess isotropic noise
• The signal was consistent with a radiation from
  a thermal source at 3 K.
• A paper had just been written (but not yet
  published) predicting exactly this radio noise
  from the big bang.
• Penzias Wilson won the Nobel Prize for this
  discovery in 1978

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The CBR Spectrum and Maps

• Since 1965 many observations have confirmed the
  CBR and measured its spectrum
• The satellites COBE (1990) and WMAP (2003) have
  made detailed maps of the CBR.
• These observation show
• Thermal spectrum, T 2.73K
• It is isotropic
• There are very small irregularities in the
  brightness across the sky

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The spectrum of the Cosmic Background Radiation
reveals a thermal spectrum with temperature T
2.73K (line is model, dots are measurements)
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The microwave sky map shows large-scale Doppler
shifts .
After removing effect of MW Galaxy motion
This implies that the galaxy is moving at a speed
V 400 km/s in the direction of the
constellation Aquarius
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Tiny temperature variations in the Cosmic
Microwave Background (CMB) are observed to be
about 3 x 10-4 K.
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Big Bang Decoupling Era (300,000 yr) and CBR
radiation

• Radiation dominated the early universe (first
  3105 yr) as universe cooled
• An observer would have seen blazing light,
  little matter
• At decoupling era (300,000 yr), temperature
  was 3,000?K, universe became transparent
• After decoupling matter dominates
• Decoupling occurred at a redshift z 1,000
• At present epoch (z0), photons from decoupling
  era are redshifted by factor of 1,000x
  (wavelength ix 1000x longer)
• For themal radiation, this means observed
  temperature 3,000?K ? 3? K

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Decoupling Era Matter and radiation no longer
interact if temperature is cooler than 3,000 K
Cool (T lt 3,000? K)
Hot (Tgtgt 3,000? K)
At an age 300,000 years, the universe was
finally cool enough from its initial primordial
fireball that electrons and protons could combine
to form atoms (era of recombination).
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(No Transcript)
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Galaxies first formed about 500-1000 million yrs
after Big Bang (computer simulation)
Movie. Click to play.
brown color represents neutral Hydrogen
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PRS Question

• Q. When was the decoupling era after t0?
• 1 sec
• 23 minutes
• 300,000 yr
• 1 milliom yrs
• 10 billion yrs

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Curvature of Space-time (Universe)

• General Relativity local space-time is curved
  (positively) by presence of masses
• What about OVERALL curvature of Universe (not
  near any masses)?
• The geometry of the universe depends on the
  combined average mass density of all forms of
  matter and energy. The three possibilities are
• ZERO CURVATURE Two parallel beams of light
  never intersect the universe is flat.
• POSITIVE CURVATURE Two initially parallel beams
  of light gradually converge the universe is
  spherical and is closed.
• NEGATIVE CURVATURE Two initially parallel beams
  of light gradually diverge the universe is
  hyperbolic and is open.
• In principle, we can measure (e.g. using long
  laser beams in a triangle, but not practical)

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Curvature of space-time
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Consequences of Curvature

• Open (negative curvature)
• Space is infinite
• Total density is less than critical density (O lt
  1)
• Hubble expansion continues forever
• A laser beam never returns
• Flat (zero curvature)
• Space is infinite
• Total density is equal to critical density (O
  1)
• Hubble expansion slowly stops (V ? 0 as t ?
  infinity)
• A laser beam never returns
• Closed (positive curvature)
• Space is bounded (finite)
• Total density is greater than critical density (O
  gt 1)
• Hubble expansion continues forever
• A laser beam returns (closed path)