How cosmic expansion stretches light
As the universe expands, light travelling across it can be stretched to longer wavelengths. This cosmological redshift shifts recognisable spectral features towards the red end of the spectrum or beyond. It is distinct from simply passing through red dust, and local motion can also contribute to an observed wavelength shift.
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Compare a known pattern with its observed position
Atoms and molecules produce characteristic spectral features. Astronomers compare where those features appear in distant light with their wavelengths measured under familiar conditions.
For cosmological redshift, expansion during the light's travel increases its wavelength. The greater the expansion between emission and reception, the larger the shift.
Redshift describes a wavelength ratio, not a visible red coating. Light can be shifted from ultraviolet into visible wavelengths or from visible into infrared.
- Spectral patterns help identify the original wavelengths.
- Expansion changes their observed scale.
- Source motion and gravity can also affect wavelengths.
Interpreting a measured redshift therefore requires context. For distant galaxies, cosmological expansion is usually central, while nearby motions can be important when estimating distances from smaller shifts.