In a terrestrial laboratory, the absorption line of singly-ionized calcium (Ca-II K) is measured at a wavelength of 3.934×10−7 m3.934 \times 10^{-7} \text{ m}3.934×10−7 m. When astronomers observe a distant galaxy cluster, they record the wavelength of this same line as 4.228×10−7 m4.228 \times 10^{-7} \text{ m}4.228×10−7 m.
Explain how this observation supports the theory that the Universe is expanding.