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 Place
Each dot is
a galaxy
A Slice of the
Universe
SDSS Collaboration 2002
=6 £ 108 l.y.
You are here
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
107 light years = 3 Mpc
Credit: http://www.powersof10.com
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
= 72 km/sec/Mpc
Expanding Universe
What if space were being uniformly
gets less dense with
time, and was denser in
created in this room at some constant
rate? What would we notice?the past.
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 Lines
Light 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
1
• 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?
2
3
4
5
<== Shorter wavelength Longer wavelength ==>
Observable consequence
of the expansion
= 72 km/sec/Mpc
Expanding Universe
gets
less dense with
What does this mean for how
the
density behaves over time? time, and was denser in
the past.
Observable consequence
of the expansion
= 72 km/sec/Mpc
Time of the big bang
Expanding Universe
gets less dense with
time, and was denser in
the past.
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 1987
Vice President - Small Business and
SMS&P Operations
Microsoft 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?
Each dot is
a galaxy
A Slice of the
Universe
SDSS Collaboration 2002
=6 £ 108 l.y.
You are here
Homogeneity is a Question of Scale
SDSS Collaboration 2002
Conclusion: On large
scales, the Universe is
highly homogeneous
The Friedman-RobertsonWalker-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-RobertsonWalker-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.
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.
Today