Miller's Astronomy 1 lecture notes on The Big Bang

1. The Birth of the Universe
LACC: §28.2, 28.4, 28.5
• Olber’s Paradox
• Hubble’s Law
• The Big Bang Theory
An attempt to answer the “big questions”: How did we
get here?
Thursday, May 20, 2010 1

2. Olber’s Paradox
http://www.williams.edu/astronomy/Course-Pages/330/images/
olbers_paradox.gif
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3. Olber’s Paradox
Why is the Sky Dark at Night?
If the universe were infinite and filled
with stars in a uniform distribution,
then every line of sight would
terminate on the surface of a star and
should be bright. To be sure, those
further away would be fainter, but
there would be more of them. Careful
analysis suggests that the sky
should be as bright as the
surface of an average star.
http://hyperphysics.phy-astr.gsu.edu/HBASE/Astro/olbers.html
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4. Olber’s Paradox
There are many possible explanations which have been considered.
Here are a few:
1.
There's too much dust to see the distant stars.
2.
The Universe has only a finite number of stars.
3.
The distribution of stars is not uniform. So, for example, there could
be an infinity of stars, but they hide behind one another so that only a
finite angular area is subtended by them.
4.
The Universe is expanding, so distant stars are red-shifted into
obscurity.
5.
The Universe is young. Distant light hasn't even reached us yet.
http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html
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5. The Expanding Universe
& Hubble’s Law
The further away a galaxy is
away from us, the greater
the red-shift of its spectrum.
The accepted explanation is
that the further away a
galaxy is, the faster it is
moving away from us. This
means galaxies are moving
away from each other, i.e.
the universe itself is
expanding.
3:52
http://www.youtube.com/watch?v=IwMFBqzpxDU
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6. The Expanding Universe
But if its space that is expanding why do we see redshifts?
The light itself is stretched as space expands.
The more distant an object is the more space has expanded while
it was traveling
http://www.pas.rochester.edu/~afrank/A105/LectureXVI/LectureXVI.html
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7. Smoking Gun of the Big Bang
The cosmic microwave background is a thermal relic of a hot, dense phase in the early universe. The
subsequent expansion of the universe shifts the radiation to colder temperatures but does not otherwise change
the spectrum: in the absence of later non-equilibrium interactions, the cosmic microwave background will follow a
blackbody spectrum.
http://www.pas.rochester.edu/~afrank/A105/LectureXVI/LectureXVI.html
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8. The Cosmic Microwave
Background [CMB]
This map shows a range of 0.0005 K from the coldest
(blue) to the hottest (red) parts of the sky. Note that there
is no part of the Earth at right that is not included in the oval,
and thus there is nothing "outside" the WMAP map.
http://www.pas.rochester.edu/~afrank/A105/LectureXVI/LectureXVI.html
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9. Evidence of the Big Bang:
Cosmic Microwave
Background
When this recombination event took place, the light
from the Big Bang peaked at about 1 micrometer in
the infrared. At that time the gas would have been
about 3,000 Kelvin and would have glowed orange-red
in the visible spectrum. However, the Universe has
expanded 1,000 times since, and the light within space
has been redshifted to longer and longer wavelengths.
Today the peak wavelength is close to 1 mm
(1 micrometer x 1,000 = 1 mm) and corresponds to a
gas temperature around 3 Kelvin (3, 000K ÷ 1, 000 =
3K).
http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php
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10. Olber’s Paradox
There are many possible explanations which have been considered.
Here are a few:
1.
There's too much dust to see the distant stars.
2.
The Universe has only a finite number of stars.
3.
The distribution of stars is not uniform. So, for example, there could
be an infinity of stars, but they hide behind one another so that only a
finite angular area is subtended by them.
4.
The Universe is expanding, so distant stars are red-shifted into
obscurity.
5.
The Universe is young. Distant light hasn't even reached us yet.
http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html
Thursday, May 20, 2010 10

11. Olber’s Paradox
1. The first explanation is just plain wrong. In a black body, the dust will heat up too. It does act
like a radiation shield, exponentially damping the distant starlight. But you can't put
enough dust into the universe to get rid of enough starlight without also
obscuring our own Sun. So this idea is bad.
2. The premise of the second explanation may technically be correct. But the number of
stars, finite as it might be, is still large enough to light up the entire sky, i.e.,
the total amount of luminous matter in the Universe is too large to allow this escape. The
number of stars is close enough to infinite for the purpose of lighting up the sky.
3. The third explanation might be partially correct. We just don't know. If the stars are
distributed fractally, then there could be large patches of empty space, and
the sky could appear dark except in small areas.
4. But the final two possibilities are surely each correct and partly responsible. There are
numerical arguments that suggest that the effect of the finite age of the Universe is the larger
effect. We live inside a spherical shell of "Observable Universe" which has radius equal to the
lifetime of the Universe. Objects more than about 13.7 [billion] years old (the latest figure)
are too far away for their light ever to reach us.
5. Historically, after Hubble discovered that the Universe was expanding, but before the Big Bang
was firmly established by the discovery of the cosmic background radiation, Olbers' paradox
was presented as proof of special relativity. You needed the red shift to get rid of the starlight.
This effect certainly contributes, but the finite age of the Universe is the most
important effect.
References: Ap. J. 367, 399 (1991). The author, Paul Wesson, is said to be on a personal crusade to end the confusion
surrounding Olbers' paradox.
Darkness at Night: A Riddle of the Universe, Edward Harrison, Harvard University Press, 1987
http://hyperphysics.phy-astr.gsu.edu/HBASE/Astro/olbers.html
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12. Cosmological Principles
Recall that there are two aspects of the [weak] cosmological principle:
• The universe is
homogeneous. This means
there is no preferred
observing position in the
universe.
• The universe is also
isotropic. This means you
see no difference in the
structure of the universe as
you look in different
directions.
http://www.astronomynotes.com/cosmolgy/s3.htm
The strong cosmological principle adds
• The universe looks the same at all times.
So, which describes our universe, the weak or the strong cosmological principle?
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13. The Birth of the Universe
LACC: §28.2, 28.4, 28.5
• Olber’s Paradox: Why is the night sky dark? The
universe is not of infinite age (and some objects are
so red-shifted we can’t see them).
• Hubble’s Law: Distance = H0 x Velocity implies the
universe is expanding and that at some point in the
past, the universe was a hot, dense, singularity.
• The Big Bang Theory: cosmological redshifts, cosmic
microwave background
An attempt to answer the “big questions”: How did we
get here?
Thursday, May 20, 2010 13

14. LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.
• Ch. 28, pp. 669: 5.
Due beginning of next class period.
Test covering chapters 24-28 next class period.
Thursday, May 20, 2010 14

15. The Fate of the Universe
LACC: §28.2, 28.4, 28.5
• The Big Crunch
• The Big Rip
• Heat Death
An attempt to answer the “big questions”:
What is going to happen to us?
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16. The Expanding Universe
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question28.html
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17. Big Bang Expansion
http://www.jwst.nasa.gov/firstlight.html
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18. Dark Energy
The diagram
[left] shows the
changes in the
rate of
expansion since
the universe's
birth 15 billion
years ago. The
more shallow
the curve, the
faster the rate
of expansion.
The curve changes noticeably about 7.5 billion years ago,
when objects in the universe began flying apart at a
faster rate. Astronomers theorize that the faster expansion rate
is due to a mysterious, dark force that is pulling galaxies apart.
Image courtesy of NASA/STScI/Ann Feild.
http://www.nasa.gov/missions/deepspace/f_dark-energy.html
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19. Dark Energy
Probing dark energy, the energy in empty space causing the expanding
universe to accelerate, calls for accurately measuring how that expansion
rate is increasing with time. Dark energy is thought to drive space apart.
Astronomers used NASA's Hubble Space Telescope to hunt for supernovae
(an energetic explosive event that occurs at the end of a star's lifetime),
using their brightness, astronomers could measure if the universe was
expanding faster or slower in the distant past.
In its search, Hubble discovered 42 new supernovae, including six that are
among the most distant ever found. The farthest supernovae show that
the universe was decelerating long ago, but then "changed gears" and
began to accelerate.
Cosmologists believe about 70 percent of the universe consists of
dark energy, 25 percent is dark matter, and only four percent normal
matter (the stuff that stars, planets and people are made of). Hubble
observations suggest the dark energy may be ... an energy percolating out
of the vacuum of the space between galaxies.
http://www.nasa.gov/missions/deepspace/f_dark-energy.html
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20. The Fate of the Universe:
The Big Crunch
Open Universe. In this
scenario, the universe will
expand forever
Flat Universe. It will
consume all of the energy
from the big bang and,
reaching equilibrium, coast to
a halt far into the future....it
will take, literally, forever for
the universe to reach the
equilibrium point.
Closed Universe. Its
expansion will slow down
until it reaches a maximum
size. Then it will [collapse]
back on itself.
http://science.howstuffworks.com/big-crunch3.htm
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21. The Fate of the Universe:
The Big Rip
The death of the universe
could rival its birth in explosive
drama if a puzzling form of
energy continues to accelerate
the expansion of space-time.
Since the 1920s astronomers
have thought the expansion
was slowing down, but recent
observations of distant stars
reveal that the stretching of space is actually speeding up. If it picks up
even more, the universe could be headed for a "big rip." An artist's
conception of this scenario—one of many possible fates—shows how,
some 20 billion years from now, unchecked expansion could tear
matter apart, from galaxies all the way down to atoms.
http://science.nationalgeographic.com/science/enlarge/universe-death.html
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22. The Fate of the Universe:
Heat Death
Basically, as the universe expands, it cools.
Eventually, everything is cold and dead (in 1
followed by a thousand 0’s years).
http://www.astroengine.com/?p=98
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23. The Fate of the Universe
LACC: §28.2, 28.4, 28.5
• The Big Crunch: if the universe exceeds some critical
density, gravity wins, the universe stops expanding
and collapses back on itself; a closed universe;
accelerated expansion implies this is not the case
• The Big Rip: dark energy goes crazy and starts
expanding everything--atoms eventually rip apart
• Heat Death: the universe expands forever; an open
universe; stars die out, black holes evaporate,
universe becomes cold, dead, and void of objects
An attempt to answer the “big questions”: What is going
to happen to us?
Thursday, May 20, 2010 23

24. LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.
• Ch. 28, pp. 669: 13.
Due beginning of next class period.
Test covering chapters 24-28 next class period.
Thursday, May 20, 2010 24

25. Review for the Test (5th of 5):
The Universe
[10 pts] Our Milky Way Galaxy [10 pts] Active Galaxies
• morphology: central bulge (barred, Sag A), disk • types: Seyfert Galaxies vs. Radio Galaxies, vs.
(spiral arms, Orion arm), halo (globular clusters) Quasars
• formation and evolution: collapse of protogalactic • feeding a supermassive black hole: accretion disk,
clouds, globular clusters, population I & II stars, normal part of early galaxy development and/or
collisions galaxy mergers
• observing our own galaxy: radio 21 cm (neutral • observing active galaxies: radio (radio lobes, radio
hydrogen), infrared (dust), visible (globular jets), visible (bright cores, parent galaxies), X-rays
clusters); distance ladder (stellar parallax, main- (hot jets)
sequence fitting, Cepheid variables)
[10 pts] The Universe
[10 pts] Normal Galaxies • dark matter: galactic rotation curves, missing
• galaxy types: Hubble Tuning Fork Diagram luminous matter in galaxy clusters (only ~10% of
(ellipticals, spirals, barred spirals, irregular, dwarf matter is visible!), dark matter candidates
galaxies, giant ellipticals); typical masses, sizes, (MACHOS, neutrinos, WIMPS etc.)
stellar populations, supermassive black holes, • Structure: walls, filaments, voids; Olber’s
mass-luminosity ratios, collisions, etc.; Paradox, cosmological principles (homogeneous,
• distance ladder (Cepheid variables, Tully-Fisher isotropic)
Relation, type 1 supernovae, brightest cluster • Evolution: The Big Bang (cosmological redshift,
galaxy, Hubble’s Law) cosmic microwave background, dark energy), The
• groupings of galaxies: clusters, superclusters; Big Crunch, Heat Death, The Big Rip
Local Group (LMC, SMC, Andromeda,
Sagittarius Dwarf Galaxy, Canis Major Dwarf [10 pts] Identify Objects from a picture
Galaxy, etc.); Local Supercluster (Local Cluster, • Milky Way galaxy: bulge, disc, spiral arms, halo
Virgo Cluster, etc.) • galaxy types: E0 elliptical, E7 elliptical, spiral,
barre spiral, irregular; galaxy collision; quasar
• Active Galactic Nuclei: supermassive black hole,
accretion disk, dust torus, X-ray jet(s), radio lobes
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