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Loading contentThe stretching of light to longer (redder) wavelengths as it travels through expanding space.
As space expands, the wavelength of light travelling through it is stretched, shifting features in a galaxy's spectrum toward the red. The greater the redshift, the more the Universe has expanded since the light was emitted — making redshift the primary yardstick of cosmic distance and time.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.
Southern-hemisphere observatory data and imagery (VLT, ALMA partner).