What astronomical phenomenon describes the stretching of light to longer wavelengths when galaxies move away from us?

• A. Blue Shift
• B. Red Shift
• C. Cosmic Microwave Background
• D. Parallax

Answer: (B)

Light from galaxies that are moving away from us is stretched as it travels, so its wavelengths become longer and the spectrum shifts toward the red end—this is red shift. This effect comes from the Doppler shift and, on cosmic scales, the expansion of the universe. The amount of red shift increases with the galaxy’s recessional speed, making more distant galaxies look redder. If a galaxy were moving toward us, the wavelengths would shorten and the spectrum would shift toward the blue—blue shift. The cosmic microwave background isn’t about light being stretched by motion in this way, and parallax measures distance from apparent position changes rather than spectral shifts.