Extragalactic Astronomy -
Fall 2012
Homework #4 - due September 17
Assume H0 = 72 (km/s)/Mpc.
1. Why is the night sky
dark?
2. How many cubes that are 100 Mpc on a side fit into a sphere with a
radius equal to the Hubble radius?
3. At what scale factor a did
radiation and matter have the same energy density? Give a purely
numerical value.
4. Integrate equation 4.34 to find the current age of the
universe. An answer accurate to 1% is sufficient.
5. How does the "temperature of the universe" vary as a function of
scale factor a?
6. Over what range of scale factors was the universe hot enough to make
electron-positron pairs?
7. What value of Omegab is predicted by Big Bang
nucleosynthesis?
8. Using python and matplotlib, make a plot of ionization fraction
versus redshift. Choose an appropriate range for redshift (i.e.
an range over which there is interesting variation in ionization
fraction).
9. What is the flatness problem?