Redshift
Redshift is the shift of light waves toward longer wavelengths, moving them toward the red part of the spectrum. This happens when an object moves away from us or when space expands as light travels through it.
Not to be confused with: Doppler Shifts
Redshift is the phenomenon where light or electromagnetic waves from an object increase in wavelength, shifting toward the red end of the spectrum. This occurs when the source moves away from the observer (Doppler effect) or when space itself expands during the light's travel (cosmological effect). It is a key tool in astronomy for measuring distances to galaxies and understanding the expansion of the universe.
What this means in real life
When an ambulance drives away from you, its siren pitch drops—light works the same way. As distant galaxies move away from Earth, their light stretches to longer wavelengths, appearing redder than it would if stationary.
What it isn’t
It's not the same as an object actually turning red. The light waves are stretched, not the object itself. A blue star moving away doesn't become a red star; its light just shifts toward longer wavelengths.
Commonly misused online
People often use 'redshift' to mean any distant or faint object appears reddish, when it actually refers to a specific wavelength shift measurable through spectroscopy, not visual redness.
Based on 2 reference sources, including Wikipedia. Last verified July 12, 2026.