Download Cosmology

Lecture 29
Cosmology
December 14, 2015
1
2
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
3
Cosmological Principle
• The universe looks homogeneous on the
largest scales
• The universe is isotropic
• Both are assumptions about the universe
4
Implications
• The universe can have no edge
• The universe can have no center
5
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
6
Motions of Stars
• Radial Motion (Radial Velocity)
– Motion along the line of sight
– Measured using Doppler shift of spectral lines.
– Can measure for very distant 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.
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
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• 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 (from our perspective) is the
cosmic light horizon
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Why does the observable universe have an “edge”?
A. We can only see as far as light has traveled over the
100%
age of the universe.
B. The density of neutrinos at the “edge” becomes so
large that photons cannot penetrate this barrier.
C. Dust prevents us from seeing beyond a certain
distance.
D. There are so many galaxies in the universe that they
block our view.
0%
A.
B.
0%
C.
0%
D.
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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
12
From what evidence do astronomers deduce
that the Universe is expanding?
A. They can see the disks of galaxies getting
93%
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. 5%
2%
0%
0%
E. All of the above.
A.
B.
C.
D.
E.
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What is meant by inflation in the early
Universe?
A. The force of gravity suddenly grew stronger in the
95%
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.
5%
0%
0%
E. The diameter of distant galaxies is much greater0%
than the diameter of galaxies
near
to us.
A.
B.
C.
D.
E.
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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
15
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
16
Wilkinson Microwave Anisotropy
Probe (WMAP)
• Launched June
2001
• Measured
fluctuations in
Cosmic
Microwave
Background
Image by NASA/WMAP Science Team
17
Temperature and Density
Fluctuations of Early Universe
Image by NASA/WMAP Science Team
18
What is meant by the cosmic microwave
background?
A. It is radiation from distant quasars.
91%
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.
4%
4%
2%
0%
A.
B.
C.
D.
E.
19
History of Universe
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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.
21
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 to 400
million years after Big Bang
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Formation of Large-Scale Structure
Figure 18.14
Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015)
23
Small irregularities in the cosmic microwave
background are important because
A. they show us that the Big Bang explosion was very
nonuniform.
63%
B. they were the seeds of the quasars we can still see
today.
C. they are thought to have led to the creation of galaxy
38%
superclusters.
D. they are thought to contain most of the elusive
“missing matter.”
0%
A.
B.
0%
C.
D.
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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”????
• Universe is 13.7
billion years old
Figure 18.17
Bennett et al, The Essential Cosmic
Perspective, 7th ed. (Pearson, 2015)
25
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
26
How Will the Universe Evolve?
• Depends on density (gravitational pull) and
velocity of galaxies.
• Critical density = density of universe
needed to stop expansion
27
Future of Universe
Figure 18.15
Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015)
28
• 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
29
• 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.
30
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. It may also be
accelerating.
31
Evidence for acceleration
Figure 18.16
Bennett et al, The Essential Cosmic
Perspective, 7th ed. (Pearson, 2015)
32
What will happen if the universe is closed ?
A. The universe will expand past its maximum size, then
fragment into mini-universes.
98%
B. The universe will eventually fall back in on itself,
heading toward a “Big Crunch.”
C. The universe will expand forever.
D. The universe will reach a maximum size and remain
there, like a balloon being blown up.
2%
0%
A.
B.
C.
0%
D.