lecture 37 cosmology [not on exam] · figure 27-3, freedman, geller, kaufmann 9th ed. universe, ......
TRANSCRIPT
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Structure of the Universe
• Does clustering of galaxies go on forever?
– Looked at very narrow regions of space to far
distances.
– On large scales the universe appears
homogeneous (smooth, same in all directions)
Figure 26.1, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Cosmological Principle
• The universe looks homogeneous on the
largest scales
• The universe is isotropic
• Both are assumptions about the universe
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Obler’s Paradox
• Assume
– cosmological principle
– universe is infinite
– universe is unchanging
in time.
• Results
– The entire sky should
be bright (BUT this is
not observed!!)
Figure 26.3, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Motions of Stars
• Transverse (Proper) Motion
– Motion of star across the sky.
– Measured by observing
change in position of star over
a long period of time.
– Only detectable for nearby
stars.
• Radial Motion (Radial Velocity)
– Motion along the line of sight
– Measured using Doppler shift of spectral lines.
– Can measure for very distant stars
Figure 17.4, Chaisson and McMillan,
6th ed. Astronomy Today, © 2008 Pearson Prentice Hall
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The Expanding Universe
• The universe is not unchanging over time, but
is expanding
– Primary evidence comes from Hubble’s Law
• Hubble’s Law
– The further away an
object is, the faster it is
moving away.
– predicts universe had a
beginning ~13 billion
years ago
• How does the expanding universe solve the
paradox?
– We can see only a finite distance (~13 billion
light years).
– For more distant objects the light has not had
time to reach us.
– Edge of universe is the cosmic light horizon
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The Big Bang
• Universe starts out almost infinitely small
• Goes through period of inflation, increases
size by factor of 1050
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Inflation Explains Why the
Universe Appears Flat
• The universe does not appear to be curved.
• Perhaps it is so large that we cannot observe
the curvature at this time.
Figure 27-3, Freedman, Geller, Kaufmann
9th ed. Universe,
© 2011 W. H. Freeman and Company
From what evidence do astronomers deduce
that the Universe is expanding?
A. They can see the disks of galaxies getting
smaller over time.
B. They see a redshift in the spectral lines of
distant galaxies.
C. They detect cosmic background X-ray
radiation.
D. They can see distant galaxies dissolve,
pulled apart by the expansion of space.
E. All of the above.
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From what evidence do astronomers deduce
that the Universe is expanding?
A. They can see the disks of galaxies getting
smaller over time.
B. They see a redshift in the spectral lines of
distant galaxies.
C. They detect cosmic background X-ray
radiation.
D. They can see distant galaxies dissolve,
pulled apart by the expansion of space.
E. All of the above.
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What is meant by inflation in the early
Universe? A. The force of gravity suddenly grew stronger in the
distant past.
B. Protons expanded to the size of stars, which was
how our Sun formed.
C. The Universe increased dramatically in size in an
extremely brief period of time.
D. The number of galaxies that we see at large
distances is much greater than the number we can
see near to us.
E. The diameter of distant galaxies is much greater
than the diameter of galaxies near to us.
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What is meant by inflation in the early
Universe? A. The force of gravity suddenly grew stronger in the
distant past.
B. Protons expanded to the size of stars, which was
how our Sun formed.
C. The Universe increased dramatically in size in
an extremely brief period of time.
D. The number of galaxies that we see at large
distances is much greater than the number we can
see near to us.
E. The diameter of distant galaxies is much greater
than the diameter of galaxies near to us.
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Evidence for the Big Bang
• Universe would be very hot (> 1 trillion K) right
after the Big Bang (photon dominated).
• Early universe would emit black body radiation.
• As universe expanded, wavelengths were stretched
• Radiation would appear much cooler = Cosmic
Microwave Background Radiation.
Figure 26.6, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Cosmic Microwave Background
• CMB has
now cooled
to about 3 K
Figure 17.7, Arny and Schneider,
5th ed. Explorations,
© 2008 The McGraw-Hill Companies
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Wilkinson Microwave Anisotropy
Probe (WMAP)
• Launched June
2001
• Measured
fluctuations in
Cosmic
Microwave
Background
Image by NASA/WMAP Science Team
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Cosmic Microwave Background
Image from the COBE data set
http://en.wikipedia.org/wiki/Cosmic_Background_Explorer
The background is nearly uniform at 2.725 K. If we subtract
out that signal, we can see the “dipole” pattern caused by the
Doppler shift due to Earth’s motion through the CMB.
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Temperature and Density Fluctuations
of the Early Universe
Image by NASA/WMAP Science Team
http://map.gsfc.nasa.gov/media/121238/index.html
See also: http://scienceblogs.com/startswithabang/2008/02/26/q-a-why-is-the-microwave-background-so-uniform/
If we subtract out the Doppler shift dipole pattern, we’re left
with only tiny fluctuations on the order of 0.00003 K.
What is meant by the cosmic microwave
background?
A. It is radiation from distant quasars.
B. It is radiation from hot gas in intergalactic
space.
C. It is radiation from the first stars formed
when the Universe was young.
D. It is radiation created during the early days
of the Universe.
E. It is the explanation of Olbers’ paradox.
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What is meant by the cosmic microwave
background?
A. It is radiation from distant quasars.
B. It is radiation from hot gas in intergalactic
space.
C. It is radiation from the first stars formed
when the Universe was young.
D. It is radiation created during the early
days of the Universe.
E. It is the explanation of Olbers’ paradox.
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Creating Matter
• Universe is initially dominated by photons
• Matter and anti-matter created via pair production.
– Most pairs annihilate each other.
– Inflation is so fast that some pairs are separated
– But more matter than anti matter survives – Why?
• By 300,000 yrs from Big Bang, universe becomes matter dominated.
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Galaxies Form
• Galaxies form from density
fluctuations in the early universe.
– Early universe would have been
very uniform.
– Minor fluctuations in density form
stars and galaxies.
– First stars form ~ 100-400 million
years after Big Bang
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Results from WMAP
• Universe is “flat” (parallel lines never meet)
• Universe is composed of:
– Baryonic matter (matter made of atoms)
– Cold dark matter (massive particles as yet
undiscovered)
– “Dark Energy”????
Image by NASA/WMAP Science Team
• Universe is 13.7
billion years old
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Is the Universe Accelerating? • Recent observations of supernova indicate
that the universe may be accelerating
instead of slowing down.
• Universe may expand forever.
Figure 17.10, Arny and Schneider,
5th ed. Explorations,
© 2008 The McGraw-Hill Companies
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How Will the Universe Evolve?
• Depends on density (gravitational pull) and
velocity of galaxies.
• Critical density = density of universe
needed to stop expansion
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• Open Universe (unbound, or low density)
– Density Critical Density
– Not enough gravity to stop expansion.
– Universe expands forever.
Figure 26.14,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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• Closed Universe (bound, or high density)
– Density > Critical Density
– Gravity is stronger than expansion.
– Universe will initially expand, stop, and eventually
fall back into itself.
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Predicting the Fate of the
Universe
• If we know the density of the universe, we
can predict the future of the universe
– Measure mass in large volume of space and
divide mass by volume
• Current Estimate: The Universe is open; it
will expand forever.
What would happen if the density of the Universe
were greater than the critical density?
A. Matter would never have formed.
B. Galaxies would not form.
C. Too many galaxies would form.
D. All the matter formed would be neutrons.
E. The Universe would eventually recollapse.
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