INST 2403

The Expanding Universe

INST 2403

The Expanding Universe

Dr. Uwe TrittmannUtrittmann@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

• 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

• 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

• 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

• 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: 1/r2

• 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– 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