As the second essay in my series on inflationary cosmology, I would like to discuss the phenomenon of redshifts in the visible spectrum of astronomical bodies, which has allowed scientists to infer that such bodies are moving away from us.
This Wikipedia page offers an illustrative -- and colorful -- diagram of how "Absorption lines in the optical spectrum of a supercluster of distant galaxies" is redshifted away from the Sun. The same Wikipedia page notes that, among other mechanisms:
Another cause of redshift is the expansion of the universe, which explains the observation that the redshifts of distant galaxies, quasars, and intergalactic gas clouds increase in proportion to their distance from the earth. This mechanism is a key feature of the Big Bang model of physical cosmology.
Another webpage, this time from the Exploratorium, discusses the Doppler Effect and presents an analogy between the sound of a police car's siren and light waves (along with some nice graphics):
Like sound, light is a wave that can be described in terms of its frequency, the number of wave peaks that pass by each second. Just like a cosmic police car, a star zooming toward you has its light waves squeezed together. You see these light waves as having a higher frequency than normal. Since blue is at the high-frequency end of the visible spectrum, we say the light from an approaching star is shifted toward blue, or blueshifted.
Likewise, if a star is zooming away from you, any light it emits gets stretched. You see these stretched-out light waves as having a lower frequency. Since red is at the low-frequency end of the visible spectrum, we say that light from a receding star is shifted toward red, or redshifted.
A naive, layperson question that occurred to me is how we know the movement of galaxies and other objects away from us represents the expansion of the universe, rather than the movement of these objects into what was previously existing, empty space in the universe.
While not (apparently) addressing my exact question, Brian Greene's 2004 book, The Fabric of the Cosmos, explains the basis for why the universe must be expanding:
Just as the gravitational pull of the earth implies that a baseball popped high above the catcher must either be heading farther upward or must be heading downward but certainly cannot be staying put (except for the single moment when it reaches its highest point), [Alexander] Friedmann and [Georges] Lemaitre realized that the gravitational pull of the matter and radiation spread throughout the entire cosmos implies that the fabric of space must either be stretching or contracting, but that it could not be staying fixed in size (p. 230).