Sketch the Hubble sequence. Contrast Sa galaxies to Sc galaxies. Make a table or diagram which indicates the different length scales on which we see galaxies cluster. Consider the volume of the Universe occupied by these features. What difference would I see if I compared the spectra of AGNs to normal galaxies.
Black holes are the energy engines for AGNs.
Consider their sizes and their energy generation efficiencies. State Olbers's Paradox. Why or why not. Friedman found which could describe our Universe.