20.5  As the sensitivity of our instrumentation increases, we are able to look ever further into space and, therefore, ever further back in time.  When we reach the era of recombination, however, we run into a wall and can see no further back in time.  Explain why.

Up until the time of recombination, the universe was opaque, so we wouldn't be able to see through it!

20.7  Suppose you could view the early universe at a time when galaxies were first forming.  How would it be different from the universe we see today?

When galaxies were first forming, the universe would have been a far more active place than it is today.  Most of the stars we see today were forming, so there were probably a lot of supernovae (from huge stars that lived quickly), strong stellar winds from young, hot stars, and lots of hot gas.  We would see very few galaxies that look like today's galaxies.  Most would be small, irregular galaxies.

20.8  If we should eventually find life on Europa, what would this tell you about the probability of finding life on worlds around other stars?

It would make is very probable that we'd find life on other worlds - Earth clearly wouldn't be special!