INST 2403
The Expanding Universe
INST 2403
The Expanding Universe
Dr. Uwe Trittmann
Utrittmann@Otterbein.edu
Office: Science 106
Phone: 823-1806
Secretary: Sandy Sallee (823-1316), Science 308
Office Hours: M 3-4pm, W 4-5pm or by appointment.
Course Materials
• Textbooks:
– Astronomy: A Beginner’s Guide to the Universe, 6th
Edition, by Chaisson and McMillan (Prentice Hall)
– Lecture Tutorials for Introductory Astronomy, 2nd ed., by
Edward E. Prather, Tim P. Slater, Jeff Adams, and Gina
Brissenden (Benjamin Cummings, 2007)
– Need to buy WebAssign access, too!
• Observatory schedule, lecture notes, study guides, the
syllabus, notices, online resources, …
Assignments and Grading
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Homework (20%)
Skylab (13%)
Rooftop Visit + Essay (2%)
Group discussions & Activities (8%)
Constellation Quiz (7%)
Three In-Class tests (each 10%)
Final exam (20%) [comprehensive!]
Homework
• We will use WebAssign, an online
homework system
• Procedure different than described in
syllabus
• You need/have already a WebAssign access
card
Skylab
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Equivalent of a term paper, but more experimental
5 different experiments, or one simulated experiment
You can work in groups of up to 4 on the Skylab
Start early - weather is always a factor
Ask questions!
• Due May 7, first draft February 24
(strict deadlines!)
A Glance at this Class
• The whole universe in 14 weeks!
Expect to work at least as hard
for this course as for your major
classes
• The focus is on concepts, not facts; on the
methods and tools of science:
– How do we know?
– How can we measure it?
– How can we predict it?
Science and
Quantitative
Reasoning
• Without quantitative reasoning there is no
progress in science (Galileo)
• However, (quantitative) reasoning is also
very useful in everyday life
– Interest rate, gas mileage, buying a used car,
that guy from Nigeria…
Making Sense of
Numbers
• Need to know how to read
graphs, functions
• Want to compare to relevant
scale  choose convenient units
• Underlying concepts have to be clear
– There is evidence that the disconnect between
scientists and the public starts at simple concepts,
like area, volume, ratios and graphs
– We’ll start there and we’ll move up to the stars!
Example: Relevant Scales
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The mass of the Earth is 6 x 1024 kg
This number can be used in equations easily
Nobody has a sense of how heavy that is
Need to compare to relevant scales
– Mass of Sun: 2 x 1030 kg  Sun’ mass = 300,000 Earth masses
– Mass of Jupiter: 2.4 x 1027 kg  300 Earth mass = 1 Jupiter mass
– Mass of Venus: 4.7 x 1024 kg  1 Venus mass =1 Earth mass
• Relevant Scale: 1 EM = 1 Earth mass
Example II
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The radius of the Earth is 6 x 106 m
Convert to miles (1mile = 1600m): 3800 miles
That is something we can fathom
But: use it to get insight!
Insight from Numbers
• We know ANY volume SCALES like the radius
to the third power
• Volume of Earth: 63 x (106) 3 m3 = 216 x1018 m3
= 1 EV
• Sun (radius 7 x108m): 1.5 million Earth Volumes
• But: only 300,000 times the Earth mass!
• Conclusion: the Sun is made from material that is
5 times lighter than the stuff the Earth is made of!
Getting a sense of how
things are working by
using (quantitative)
analysis
• Analyzing things meaning breaking down
complicated stuff we cannot understand into
smaller and smaller pieces, until we can
solve the little problems, then piece them
together again.
Asking questions to ask questions
• Scientists often come off as pretending to know
everything (Sheldon)
• In fact, they have a healthy self-confidence that
they can at least say something about everything
• Why? They simplify things enough to make a
rough estimate, then compare to reality
– Correct? Hah! I was right!
– Not correct? How interesting! We can explore more!
How many people live in
Australia?
• No idea.
• Say something:
– Can get size of this continent
– Know size & inhabitants of USA
– Proportionality tells us there should be XX
million Australians
• Conclusion: there are much fewer!
• Next Question: Why?
Chase the Demons Away (Sagan)
• The uninitiated tends to thinks things are
much more complicated than they are
• Tip of the iceberg
• There is nothing mysterious about the
natural world!
You can! …understand it, do it, calculate it,
ask relevant questions
The most important graph:
2
1/r
• Force of gravity (and EM) falls off like that
• Brightness of stars goes down like that
• Simple reason:
– things spread out over a sphere
– dilutes over the surface of the sphere
– surface scales like r2
Example: Two 100W light bulbs
• One APPEARS nine time brighter than the
other. How far away is it?
• We can’t compute how far away it is in
absolute terms, but it must be √9=3 times
farther than the other one.
• Which one is brighter?
Appearances
• Don’t be fooled by appearances!
– The sun and the moon appear to be the same
size in the sky (0.5 degrees)
– Alpha Centauri appears to be much dimmer
than the Sun
– Alpha Centauri and Vega appear to be equally
bright
Think!
• The moon and the sun COULD be at
different distances
• Alpha Centauri and Vega COULD be
different types of stars
Find out = Measure!
• How?
• That’s where the game of science starts!
The most important measurement
in Astronomy: Distance!
• The distances are astronomical!
• The distance scales are very different
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Solar system: light minutes
Stars: light years
Galaxies: 100,000 ly
Universe: billions of ly
• Need different “yardsticks”
Yardsticks and the Expanding
Universe
• Realizing (measuring) the distances to
objects means realizing how big the
universe is:
– We realized that the solar system is not the
universe
– We realized that our galaxy is not the universe
– We realized that the universe is not static