What physical process causes the stretching of light wavelengths when a source recedes?
Answer
The Doppler effect applied to light, stretching wavelengths toward the redder end.
The stretching of light wavelengths when a source moves away from an observer is fundamentally governed by the Doppler effect as applied specifically to electromagnetic radiation. When a light source recedes, the intervals between successive wave crests emitted by that source increase relative to the observer. This increase in wavelength corresponds precisely to a shift toward the longer, redder portion of the electromagnetic spectrum, which is termed redshift. Conversely, if the source were approaching, the wavelengths would compress, resulting in a blueshift, which is the characteristic observed for gravitationally bound neighbors within the Local Group.
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