(a) What was the temperature of
the cosmic background when the light we see from LPIF13 left that
quasar, at z=2.58?  Was it still a "microwave" background at that time?

(b) The human eye is sensitive to light with wavelengths ranging from about
400 nm to 700 nm.  At what redshift did the "color temperature" of the cosmic
background drop to a wavelength below the threshold where humans could see it? 

(c) At the time of matter-energy decoupling, electromagnetic radiation
was finally able to travel through the universe without being "hitched" to matter.
Thus, the universe is said to become "transparent" at this epoch in cosmic
history, which occurred at a redshift of 
approximately z = 1100.  hat was the background temperature of the
universe?  How does that compare to the temperature of the surfaces
of stars?  With this in mind, can you give a possible explanation of why 
decoupling might have occurred at z = 1100 and not much earlier or later?