Answer
See answers.
Work Step by Step
a. Astronomers expected that gravitational attraction would slow down the expansion of the universe, pulling stars and galaxies back together.
b. This is discussed in section 33-9, page 976.
Astronomers can measure the brightness and the redshift of light from Type Ia supernovae, whose intrinsic luminosity and spectra are well-known.The brightness as seen on Earth tells us how distant the supernova is, and the redshift tells us how fast it is moving.
See Hubble’s Law, equation 33-4, which states that the speed of a receding galaxy is proportional to its distance. An object twice as far ought to be moving twice as fast. If we find that very distant objects aren’t moving as fast as expected, this would suggest that in the distant past, the universe used to expand more slowly than it does now.
In short, by measuring a large variety of supernovae from many different eras, one can establish a history of the expansion rate of the universe and see whether it has changed.
