Day 8.14
The Very Big – The Cosmos
1) The spectral lines of the galaxies tell us what elements the stars contain and how they are moving
relative to us. Use the red shift of the lines to determine the velocity of each galaxy.
Ca-K Ca-H reference lines
Distance
Velocity
Galaxy
20
(x10 km)
(km/s)
3925
3950
3975
4000
4025
4050 Å
NGC
7.7
1357
NGC
13.6
3147
NGC
3.4
3368
NGC
22.1
5548
NGC
10.0
6764
NGC
19.7
6745
Velocity Scale (km/s)
2100
0
2000
4000
6000
speed ( km/s)
2) Plot the velocity of the galaxy on the y-axis and the distance to the galaxy on the x-axis.
600
0
500
0
400
0
300
0
200
0
100
0
5
10
15
20
25
20
distance (x10 km)
30
35
3) Almost of the galaxies are moving away from us and the farthest ones are moving fastest. Explain
why, using a model of paper clips (galaxies) linked by elastic bands (space).
4) Explain how to use this data to determine the age of the universe.
5) The theory that describes how the universe has expanded was named the ‘Big Bang’ by Fred Hoyle, a
British astronomer with a competing model. Explain why the name is a poor descriptor.
6) According to the Big Bang theory the early universe was denser, brighter, hotter and more uniform.
a) Explain why the universe used to be much denser using a balloon and a marker.
b) Explain why the universe used to be much brighter using a flashlight.
c) The universe used to be much hotter. The early universe was mostly made of light. Explain why
stretching light would make the universe cooler using a Slinky.
d) The universe used to be much more uniform. Right now, the universe contains galaxies of stars
and huge empty spaces in between the galaxies. The earlier, hotter universe would have been much
more uniform. Explain why using salad dressing that has separated into layers of oil, vinegar and spices.
7) According to the Big Bang theory the early universe was too hot for electrons and protons to stick
together as atoms. It was a soup containing electrons, protons and light moving rapidly, randomly and
colliding often. However, as the universe cooled, things changed. Once the temperature went below
3000 degrees K, the electrons and protons joined to form neutral hydrogen that does not interact with
the light. The light could now pass unimpeded and the universe became transparent. You are going to
act out what happens to the electrons, protons and light as the universe started to cool. You can use
props, but no words. How will you do this?
8) According to the Big Bang theory, this light should be hitting the Earth from all directions. The
original light has stretched 1000 times and should now have a peak wavelength of 1-mm which
corresponds to a temperature of 3 K or -270o C. This light is called the Cosmic Microwave
Background (CMB). Watch Minute Physics: Picture of the Big Bang
http://www.youtube.com/watch?v=_mZQ-5-KYHw Describe a part of the video helped you understand
something better or something new that you learnt.
9) The images above show temperature measurements of a small section of the CMB made by three
different satellites. In 1992, COBE measured the CMB temperature to be 2.73 K. In 2003 WMAP
measured it at 2.726 K and in 2013 Planck found that it was 2.72547 K. The variations in shade
represent differences in temperature of +/ 0.00002 degrees.
a) The Big Bang predicted that the CMB should be uniform temperature. Only the first image looks
uniform. Why do the later ones look different?
b) Your friend says that variation of the third image proves that the Big Bang theory is wrong.
Explain how the third image still shows a very uniform CMB. Using the ‘smoothness’ of the
Earth as an analogy.
c) The early universe was very uniform but the present universe is not. It is made of galaxies and
clusters of galaxies with large empty spaces in between them. Where did this structure come
from? The Big Bang theory predicts that there must have been some tiny variation in the early
universe to provide the seeds that grew to form the galaxies that we see now. Which of the
images supports this prediction? Explain.
10) The CMB images show how temperature and density varied. This variation resulted in galaxy
formation. Gravity pulls matter closer and light pressure pushes it apart. Galaxies can’t form unless
the original density is high enough.
a) Predict what will happen to the tiny variations in density as the universe evolves.
b) Observe a simulation at http://cosmicweb.uchicago.edu/filaments.html
c) Model this using magnetic marbles. Take magnetic marbles and see how many you can
place on a 20 cm by 20 cm piece of cardboard without them clumping together.
11) Astrophysicists cannot test their theories with experiments, so they make computer models and test
these. You will use a simplified version to determine the density of the early universe. Go to
http://www.learner.org/courses/physics/interactive/lab_interactives/cosmic.html
a) The picture on the bottom right shows what the distribution of galaxies and empty spaces of our
universe looks like. What does it look like the most?
A)
Swiss cheese
B) sand on a beach
C) a sponge D) cobwebs
b) The graph on the top right shows what the density of our universe is like.
The horizontal axis is density and the vertical axis shows how much
of the universe has that particular density. Label the graph on the right
where is shows that a small amount of universe is low density, a small
amount is high density and most of it is medium density.
c) Set the initial conditions on the graph (bottom left) for completely uniform
(slider at far left) and the maximum density (slider at very bottom). Compare
the universe that results to our present universe. Use both the image and the
graph to provide support for your answer.
d) What initial conditions match our universe?
12) The space between galaxies keeps getting bigger and bigger.
a) Explain why we should expect the expansion to slowdown.
b) The expansion is speeding up. How is this ‘explained’ by dark energy? Minute Physics 2011
Nobel Prize http://www.youtube.com/watch?v=v6o2bUPdxV0&list=PLED25F943F8D6081C