"Just as the pitch, or frequency, of a train whistle lowers as the train moves away from a listener, the frequency of light from a receding galaxy should appear lower to an observer than if the galaxy and observer were motionless relative to each other. This kind of change is called a "red shift" because visible light is shifted downward in frequency toward the long-wavelength, red end of the electromagnetic spectrum. The theory of an expanding universe was confirmed in 1929 by the astronomer Edwin Hubble of the United States, who showed from an analysis of the red shifts of distant galaxies that they are indeed receding from our galaxy and with velocity of recession that is proportional to distance. Hence, the universe is expanding."

Comments: The word "firmament" used in the above text does not refer to the so called "Theory of Firmament". It is a noun and according to the Webster's Dictionary, it means; the whole vault of the sky with its myriad stars etc. The word "expanse" comes from the Latin word, "expansum" and means; "to spread out".