Topics Related to Cosmology
Astro C10 Fall 2015
GSI: Goni Halevi
Created on November 24, 2015
Note: This worksheet is adapted from the worksheets of previous GSI’s Alwin and Nat.
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Questions to Work Through
Please work in groups. Feel free to make use of your whiteboard. Ask each other, then me, if you get stuck.
Use your reader as a last resort. You will not be turning in this worksheet, but it is in your best interest to
complete it and understand the material.
1.1
Dark Energy
1.1.1
Why do we believe there is dark energy in the universe?
1.1.2
What would you tell a friend who thinks that dark matter and dark energy are the same thing?
(Practice explaining to each other).
1.2
Densities
1.2.1
If the universe were filled with only matter, how would the mass density decrease with the radius
(i.e. size) of the universe as it expands?
1.2.2
If the universe were filled with only photons, how would the energy density decrease with the radius
(i.e. size) of the universe as it expands? (Hint: the answer is different from 1.2.1. Take into account
the Doppler effect!)
1.2.3
If the universe were filled with only dark energy, how would the energy density decrease with the
radius (i.e. size) of the universe as it expands?
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1.2.4
2
Initially, the universe was dominated with photons (that period is called the radiation-dominated
era). How did that change into the present universe? i.e. where did all the radiation go?
Timeline of the Universe
2.1
Preparation Questions
These should help you fill out the chart with little memorization. You may need to reference your reader.
2.1.1
What are the 4 fundamental forces? Order them by strength.
2.1.2
The equation E = mc2 relates the energy of collisions to the particle mass. Order the particles you
know of by their mass.
2.1.3
How is Energy related to Temperature? How is Temperature related to the Age of the Universe?
Why?
2.1.4
Using your answers to 2.1.1 and 2.1.2 as a guide, think about how much energy it takes to do the
following things, and rank them:
– Remove quarks from a photon
– Energy released by an anti-proton/proton collision (opposite forms pairs)
– Energy released by an anti-electron/electron collison (opposite forms pairs)
– Remove electrons from an atom
– Escape from the Suns gravitational orbit (the opposite forms the solar system)
– Escape from the Galaxys gravitational field (the opposite is gas forming the galaxy)
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2.2
Key Terms
Use these terms to fill out the last column of the chart. Make sure that you can define each one vaguely, and
try to do this with as little memorization as possible.
Recombination
Last scattering surface
Electroweak
Galaxies formed
Solar system formed
CMB Radiation
Dark energy dominated
Matter dominated
Universe became transparent
Quarks combined to form Hadrons
Inflation
Quark-Gluon plasma
Last Leptons annihilate
GUT
Electrons bound to protons
Last Hadrons annihilate
Neutrons decay into protons
Nucleosynthesis
TOE
2.3
Alas, the Chart
Time
Temperature
< 10−43 s
1032 K
< 10−37 s
1029 K
< 10−12 s
1017 K
10−6 s
1013 K
1s
1010 K
100 s
109 K
380000 yr
3000 K
5 − 10 Gyr
15 K
14 Gyr
3K
What happened in the Universe?
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