ObservationalCosmology

Self-test problems

Use these questions as you proceed through the unit, to judge whether your coverage of the material and level of understanding are adequate. Answers are just a click away, via the      button, but you will greatly reduce the diagnostic value of the questions if you look at the solutions before making a serious attempt to answer the question yourself.

1. What is the redshift of a galaxy on our own "particle horizon"? How does the observed redshift of such a galaxy change with time, and why?  
2. If Omega were 0.4 at the present epoch, how close would it have been to unity at a redshift of 1000, assuming that a grows as t^2/3 (i.e. as a for a fairly flat matter-dominated universe)? 
3. The binding energy of the electron in a hydrogen atom is 13.6 eV.

(a) Calculate the temperature at which the mean blackbody photon energy has this value.
(b) At what redshift would the cosmic CMB radiation have this temperature?
(c) Why is the blackbody assumption a good one?
(d) Compare the redshift you calculated with that at which decoupling of matter and radiation actually occurs. What might account for the difference? 
4. What motivates the strange idea of cosmic inflation? 
5. In what way does the vacuum energy which drives inflation differ from that associated with Lambda? 
6. If the lifetime of free neutrons were 100 s, rather than ~1000 s, what would be the main effect on the abundances of the elements? 
7. Why are elements heavier than Li not synthesised in significant quantities in the Big Bang? 
8. Why should density fluctuations in the early Universe lead to temperature fluctuations in the CMB? Hint: look at section 3.1 in Bothun.
9. How would the Jeans Mass evolve with a after recombination, if the gas expands adiabatically with the Universe? 
10. What effect do you think the cosmological constant will have on the growth rate of density perturbations on scales of galaxies and clusters? 
11. Why is it reasonable to adopt the growth rate of density fluctuations for an Omega_matter=1 universe for much of the history of our Universe?
    Why does this fail at low redshift, and what observational consequences do you think this might have? 
12. What difference would you expect to observe between two systems of the same mass which virialised at z=2 and z=0 respectively? 
13. How would a typical spiral galaxy like the Milky Way (with a bulge and disc) be assembled according to current ideas about hierarchical galaxy formation? 
14. Why are theorists much more confident of their ability to simulate and  model the evolution of dark matter than of baryons, despite our ignorance of what dark matter actually consists of?