Midterm

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Midterm

• average 61
• How will I curve the grades?
• Roughly 1/4-1/3 in this class will get A
• 1/3-1/2 will get B
• 1/4-1/6 will get C or worse
• Should I worry?
• If you are above 70 (and similar in homework)
• Keep on the good work, you can get A with a
  reasonable final
• If you are above 50
• You can still get A if doing better in final
• If you are below 45 and did poorly in homework
• You might not be able to get B if you dont work
  harder
• Extra credits? You can earn extra credit with an
  exceptional project report

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Lecture 16. Quest for the Edge of the Universe--
searching the most distant galaxies

• Oct 18, 2007

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Lookback Time

• Since the speed of light is finite
• it takes time for light to travel vast distances
• light reaching us from a galaxy 4 x 108 l.y. away
  took 4 x 108 years to arrive
• we refer to this as lookback time
• the farther out into the Universe we look, the
  farther back in time we see

• At vast distances, long lookback times
• it makes more sense to use lookback time instead
  of distance
• consider the spacetime diagram at right
• What is the distance to this galaxy?
• distance when the photons were emitted
• distance when the photons are received
• But there is no ambiguity that it took 4 x 108
  yrs for the photons to get here.

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Size of the Observable Universe

• As pointed out earlier, the Universe has no edge
• but it does have a horizon
• The cosmological horizon is the place where the
  lookback time equals the age of the Universe.
• we can not see beyond this horizon, because we
  would be looking at a time before the Universe
  even existed!
• our observable Universe grows 1 light year in
  size every year
• Current horizon is at 13.7 billion light years!
• Goal looking for the most distant, oldest, and
  highest redshift objects in the Universe that are
  close to our cosmic horizon!

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Redshift z

• Redshift measures of how fast an object is moving
  away from us ? they are redder, or their
  spectral lines are at longer (redder) wavelength
  (Doppler vcz)
• http//skyserver.sdss.org/en/proj/advanced/hubble/
  doppler.swf
• Expansion of the universe ? more distant objects
  move faster away from us, or at higher
  redshift, the universe was (1z) times smaller
  at that time
• Light from the most distant objects took the
  longest time to reach us ? highest redshift
  objects are the youngest, or earliest objects
  in the universe
• Searching for the first objects in the universe ?
  searching for the most distant, highest-redshift
  galaxies and quasars
• z7
• 13 billion light years away
• the universe was about 700 million years old 5
  of its current age.
• The universe was 8 times smaller than it is now

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Quasar Spectra at Different Redshifts
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How can redshift be larger than one?

• If Doppler equation tells us that v c z, then
  zgt1 would mean that vgtc! But nothing can travel
  faster than light!
• Thats right! The equation vc z is only an
  approximation that works when the velocity is
  very low the complete equation is
• So z1 when v0.6c, and when v0.9c, z3.4

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An offer from Vera Rubin
During the conference After the Dark Ages when
Galaxies were young In Oct 1998, near Washington
DC.
Martian meteorite Will be awarded the discoverer
of the first redshift higher than seven object
still unclaimed
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A brief cosmic history

• Big Bang the universe filled
• with hot gas

• Cosmic Dark Age no light
• no star, no galaxy

• First light the first galaxies
• and quasars in the universe

• Cosmic Renaissance universe lit up by young
  galaxies and quasars

• reionization completed,
• the universe is transpartent and
• the dark ages ended

? today
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A brief cosmic history

• Big Bang the universe filled
• with hot gas

• Cosmic Dark Age no light
• no star, no quasar

• First light the first galaxies
• and quasars in the universe

• Cosmic Renaissance universe lit up by young
  galaxies and quasars

• reionization completed,
• the universe is transpartent and
• the dark ages ended

? today
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The end of dark ages Movie
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Life as a Hydrogen atom at the end of cosmic dark
ages
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From Avi Loeb
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Finding the Most Distant Objects

• Discovery highest redshift
• Furthest objects (13 billion light years at z7)
• The oldest objects (13 billion years old)
• Probing the evolution of the Universe
• The Universe was only 700 million years, 5 its
  current age at z7
• The Universe was (1z) 8 times smaller then

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Difficulties in finding high-z galaxies and
quasars

• Faint the most distant galaxies are about
  1,000,000,000 times fainter than Andromeda
• Rare there were must fewer galaxies and quasars
  in the beginning by definition
• Red all spectral features shifted to the red
  part of the spectrum, due to the high redshift
• sky is very bright
• CCD is very insensitive
• And therefore, need the largest, most powerful
  telescopes and very smart technique!