Cosmology for High School Students

Lloyd Knox

Professor of Physics

University of California at Davis

What is Cosmology?

Not cosmetology!! Cosmetology is serious business that is actually helpful to people.

This is Cosmology:

From Webster’s: ‘a branch of astronomy that deals with the origin, structure, and space-time relationships of the universe’

California Science Standards: Earth Sciences - Grades 9 - 12 2. Earth-based and space-based astronomy reveal the structure, scale, and changes

in stars, galaxies, and the universe over time. As a basis for understanding this concept:

1. Students know the solar system is located in an outer edge of the disc-shaped Milky Way galaxy, which spans 100,000 light years.

2. Students know galaxies are made of billions of stars and comprise most of the visible mass of the universe.

3. Students know the evidence indicating that all elements with an atomic number greater than that of lithium have been formed by nuclear fusion in stars.

4. Students know that stars differ in their life cycles and that visual, radio, and X-ray telescopes may be used to collect data that reveal those differences.

5. * Students know accelerators boost subatomic particles to energy levels that simulate conditions in the stars and in the early history of the universe before stars formed.

6. * Students know the evidence indicating that the color, brightness, and evolution of a star are determined by a balance between gravitational collapse and nuclear fusion.

7. * Students know how the red-shift from distant galaxies and the cosmic background radiation provide evidence for the "big bang" model that suggests that the universe has been expanding for 10 to 20 billion years.

Why Teach Cosmology to High School Students?

• People* are naturally curious about the cosmos.• The curiosity provides an opportunity to teach them

about the process of science.• The scientific process is a good model for thinking.• We have learned a lot of fascinating things about the

Universe, and at the same time fundamental mysteries remain; what remains unknown can be as stimulating as what is known.

*Comment about H.S. students

Outline

• Our location in the Milky Way galaxy• A Universe of Galaxies• The Big Bang• The Discovery of the Expansion of the Universe

– How velocities are measured• Spectral Lines

• Doppler Effect

• Why Study Science?

After the break: we’ll engage in exercises I’ve prepared, and that you might do with your students.

The Sun is one of many stars in our galaxy. The distance from the Sun to the center of our galaxy is 28,000 light years.

Our galaxy would look something like this one. Why don’t I just show a picture of our own galaxy?

How many planets are known about around other stars? Currently 229! -- They’re called exoplanets.

A picture of our own galaxy taken from inside it.

Typical distance between galaxies: 2,000,000 light years

The Universe is a Very Big PlaceSDSS Collaboration 2002Each dot is

a galaxy

A Slice of the Universe

You are here=6 £ 108 l.y.

Distance to edge of observable Universe: 40 billion light years

a quasar about 15 billion light years away

What’s a quasar?

Size of entire Universe:

Size of entire Universe:

??

Our Place in the Universe

Credit: http://www.powersof10.com

107 light years = 3 Mpc

Outline• Our location in the Milky Way galaxy

• A Universe of Galaxies

• The Big Bang

• The Discovery of the Expansion of the Universe– How velocities are measured

• Spectral Lines

• Doppler Effect

• Why Study Science?

After the break: we’ll engage in exercises I’ve prepared, and that you might do with your students.

The Big Bang: An Expansion of Space Itself

• To understand the big bang, we have to change how we think about space.

• We tend to think of objects that move through space as the real actors, and space as merely the stage on which the action happens.

Space Is Dynamic• However, Einstein

taught us that space itself is an actor. It has properties that can change over time. In particular, space can be created.

• In fact, space is being created at a uniform rate in the Universe today.

Insert picture of Einstein

Before thinking more about the uniform expansion of space, let’s think about a fictitious uniform expansion of North America.

What are some consequences of a uniform expansion of space?

Sitting in our galaxy, what would we observe the other

galaxies doing?

Observable consequence of the expansion

Expanding Universe gets less dense with time, and was denser in the past.

= 72 km/sec/Mpc

What if space were being uniformly created in this room at some constant rate? What would we notice?

Outline• Our location in the Milky Way galaxy

• A Universe of Galaxies

• The Big Bang

• The Discovery of the Expansion of the Universe– How velocities are measured

• Spectral Lines

• Doppler Effect

• Why Study Science?

After the break: we’ll engage in exercises I’ve prepared, and that you might do with your students.

How did Hubble Measure Velocities and Distances?

We’ll address velocities.To do this we need to introduce

1) Spectral lines

2) The Doppler Effect

Along the way I’m going to make a general point about teaching hands-on science.

Spectral LinesLight from atoms split up by a prism into its components.

Atoms emit light at discrete wavelengths as they transition from a higher energy state to a lower energy one.

Wavelength is given by the axis at the top.

Gas Discharge Tube Time

Story of the ‘Extra Lines’

What do you do when the unexpected happens, the students ask you why,

and you don’t know the answer?

The Unknown

Threatening and/or stimulating?

Spectral Lines

Sodium Emission Line Spectrum Showing the Fainter non-Yellow lines also

Doppler Effect Java Applet

http://www.cbu.edu/~jvarrian/applets/doppler1/doppler.htm

Qualitative Exercise

• Object 1 is not moving with respect to the observer. Its spectrum is the ‘rest-frame’ spectrum.

• Which objects are moving away and which are moving towards the observer?

<== Shorter wavelength Longer wavelength ==>

1

2

3

4

5

Observable consequence of the expansion

Expanding Universe gets less dense with time, and was denser in the past.

= 72 km/sec/Mpc

What does this mean for how the density behaves over time?

Observable consequence of the expansion

Expanding Universe gets less dense with time, and was denser in the past.

= 72 km/sec/Mpc

Time of the big bang

Outline• Our location in the Milky Way galaxy

• A Universe of Galaxies

• The Big Bang

• The Discovery of the Expansion of the Universe– How velocities are measured

• Spectral Lines

• Doppler Effect

• Why Study Science?

After the break: we’ll engage in exercises I’ve prepared, and that you might do with your students.

Why Study Science?

• This is a great question, often asked by students (I believe) and deserves an answer. It is healthy to do something because you see how it will be useful in some way.

• There are a number of answers one could give. I believe the primary one, by far, is that science is a model for thinking and that studying science will make you a better thinker.

Why Study Science?As an example of how studying science can prepare one for

challenges outside of science, I have this quotation from one of our alumni:It was during my [engineering management] graduate courses that I realized that the analytical abilities, honed on physics problems at UCD, were exactly what I needed to attack the broad range of business case studies that were part of the graduate curriculum. Across Spectra-Physics, Graduate School and Microsoft, the one common element in all work is problem solving, the most

applicable background for problem solving is Physics. --Steven Guggenheimer

UCD Physics Class of 1987Vice President - Small Business and

SMS&P OperationsMicrosoft Corporation

(from http://www.physics.ucdavis.edu/testimonials.html)

“If you can solve physics problems, you can figure out how to sell toilets in Japan.” -- SG

Summary

• The solar system is located 28,000 light years from the center of our disk-shaped galaxy containing over 100 billion stars.

• The expansion of the Universe is an expansion of space itself, not simply stuff expanding out into pre-existing space.

• A direct observable consequence of the expansion is Hubble’s Law.

• To teach science in a worthwhile manner, the teacher has to be willing to deal with not knowing. Not knowing something is an opportunity for the teacher to model for the students how one goes about trying to find answers to questions.

A Homogeneous Universe?SDSS Collaboration 2002Each dot is

a galaxy

A Slice of the Universe

You are here=6 £ 108 l.y.

Homogeneity is a Question of ScaleSDSS Collaboration 2002

Conclusion: On large scales, the Universe is highly homogeneous

The Friedman-Robertson-Walker-Lemaitre Universe

• In the 1920’s a number of people worked out solutions to Einstein’s equations assuming a homogeneous Universe.

• Key result: a static Universe is impossible. In a homogeneous Universe, space must be uniformly increasing or uniformly decreasing.

The Friedman-Robertson-Walker-Lemaitre Universe

Aside: Einstein thought the Universe was static, and added a ‘cosmological constant’ to his equations in an attempt to reconcile his equations with a static Universe. He therefore missed the opportunity to predict the expansion.

Expansion FAQ

1. Is the Earth expanding?– No. Our description of uniform expansion is

only valid when describing very large length scales, not on small scales where our assumption of a homogeneous Universe is invalid.

2. What is the Universe expanding into?– It is not expanding into anything. Space is

being created uniformly everywhere.

Expansion FAQ continued

3. Are we at the center of the expansion?- No

4. Does space have an edge (a boundary)?– No. Space is either infinite (goes on forever),

or if you keep walking in one direction you come back to where you started.

Today

Not only is the Universe getting less dense with time, it’s also getting cooler.

The Universe is filled with thermal radiation with a temperature today of 2.73 degrees Kelvin (about -267 degrees Celsius, pretty chilly). This radiation is brightest in the microwave region of the spectrum and is called the cosmic microwave background.

As the Universe expands, the wavelengths stretch and the radiation cools. Conversely, at earlier times the radiation was hotter.