5. (from activity 50, page 191) Who is most nearly correct, and why?

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5. (from activity 50, page 191)Who is most
nearly correct, and why?

1. Isabella, because the universe started out at one
   point
2. Isabella, because we are at the center of the
   observable universe
3. Diego, because the universe started out at one
   point
4. Diego, because the time we calculate corresponds
   to the time since the Big Bang

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What observed feature of the universe motivated
scientists to propose the Big Bang theory?

1. There is lots of debris in space, as would be
   expected from an explosion
2. Scientists thought the universe should have a
   definite beginning, not just be forever
3. The universe is expanding and the expansion
   traces back to a unique time in the past
4. None of the above

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17.2 Evidence for the Big Bang
Our Goals for Learning How do we observe the
radiation left over from the Big Bang? How do
the abundances of elements support the Big Bang?
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How do we observe the radiation left over from
the Big Bang?
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The cosmic microwave background the radiation
left over from the Big Bang was detected by
Penzias Wilson in 1965, using the radio
telescope shown here
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Background radiation from Big Bang has been
freely streaming across universe since atoms
formed at temperature 3,000 K visible/infrared
wavelengths
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Background has perfect thermal radiation spectrum
at temperature 2.73 K
Expansion of universe has redshifted thermal
radiation from that time to 1000 times longer
wavelength today microwaves
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WMAP (Wilkinson Microwave Anisotropy Probe) is
giving us detailed baby pictures of structure
in the universe
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How do the abundances of elements support the
Big Bang?
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Protons and neutrons combined to make
long-lasting helium nuclei when universe was 3
minutes old
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Big Bang theory prediction 75 H, 25 He (by
mass) Matches observations of nearly primordial
gases
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Abundances of other light elements agree with Big
Bang model having 4.4 normal matter more
evidence for dark matter!
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Which of the following abundance patterns is an
unrealistic chemical composition for a star?

1. 70 H, 28 He, 2 other
2. 95 H, 5 He, lt0.02 other
3. 75 H, 25 He, lt0.02 other

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What have we learned?

• How do we observe the radiation left over from
  the Big Bang?
• Telescopes that can detect microwaves allow us to
  observe the cosmic microwave backgroundradiation
  left over from the Big Bang. Its spectrum matches
  the characteristics expected of the radiation
  released at the end of the era of nuclei,
  spectacularly confirming a key prediction of the
  Big Bang theory.

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What have we learned?

• How do the abundances of elements support the
  Big Bang?
• The Big Bang theory predicts the ratio of protons
  to neutrons during the era of nucleosynthesis,
  and from this predicts that the chemical
  composition of the universe should be about 75
  hydrogen and 25 helium (by mass). This matches
  observations of the cosmic abundances, another
  spectacular confirmation of the Big Bang theory.

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Actual observed map of the sky at microwave
wavelengths redder means higher temperature,
bluer means lower
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If youre moving with respect to the cosmic
microwave background, then --- because of the
Doppler effect --- that radiation has

1. A shorter wavelength in the direction on the sky
   towards which youre moving
2. A longer wavelength in the direction on the sky
   towards which youre moving
3. A longer wavelength in the direction on the sky
   opposite to your motion
4. A shorter wavelength in the direction on the sky
   opposite to your motion
5. Both answers 1 and 3 are correct answers
6. Both answers 2 and 4 are correct answers

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The Doppler shift changes the peak wavelength ?
of the cosmic background radiation (CBR) across
the sky
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If you then convert the peak wavelength to a
temperature using Wiens law (?T constant),
youll get different temperatures at different
places in the sky.
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The Doppler shift changes the peak wavelength ?
of the cosmic background radiation (CBR) across
the sky

• If you then convert the peak wavelength to a
  temperature using Wiens law (?T constant),
  youll get different temperatures at different
  places in the sky.
• The amount by which the temperature changes
  across the sky is larger if youre moving faster
• So by measuring the temperature change, we can
  figure out how fast the Milky Way is moving

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The percentage difference in the temperature of
the cosmic background radiation across the sky is
the same as the percentage difference in peak
wavelength across the sky. If the temperature T
is 0.2 higher in your direction of motion, then

1. The peak wavelength of the cosmic background is
   0.2 longer in that direction
2. The peak wavelength of the cosmic background is
   0.2 shorter in that direction

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The percentage change in peak wavelength of the
cosmic background equals the speed of the Milky
Way as a percentage of the speed of light c
(c300,000 km/second). If the peak wavelength
changes by 0.2, the Milky Ways velocity is

1. 1,500,000 km/second
2. 60,000 km/second
3. 6,000 km/second
4. 600 km/second
5. 60 km/second
6. 6 km/second
7. 0.6 km/second

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Velocity is acceleration multiplied by time. If
the Milky Way is now travelling at 600 km/sec and
the universe is 15 billion years old, how fast is
the Milky Way speeding up (what is its
acceleration)? (Hint 1 km 1000 meters, 1
meter 100 cm)

1. Speeds up by 400 cm/sec every 1000 years
2. Speeds up by 40 cm/sec every 1000 years
3. Speeds up by 4 cm/sec every 1000 years
4. Speeds up by 0.4 cm/sec every 1000 years

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The Milky Way IS moving at 600 km/sec towards a
distant supercluster of galaxies in the
constellation of Lyra the Lyre (which contains
the bright star Vega).The Shapley Supercluster
is located 600 million light years away towards
Lyra. Knowing the distance d to that
supercluster, we can use our estimate of the
Milky Ways acceleration a to estimate the
superclusters mass M, since aGM/d2GMgalNgal/d2
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If a supercluster located d600 Mly
(million light years) away is responsible for
accelerating the Milky Way, what number N of
galaxies like the Milky Way are in it?
Use a0.01 N/d2 where a(4 cm/s)/1000 years and d
is in units of Mly.

1. 14
2. 240
3. 144,000
4. 900,000
5. 90,000,000
6. 144,000,000