cosmological redshift

When you split the light of a distant galaxy into its colours, you find the familiar barcode of dark lines made by its atoms — but the whole pattern is shifted toward the red, longer-wavelength end of the spectrum. The farther the galaxy, the bigger the shift. This stretching of light to redder wavelengths is the cosmological redshift, and it is the single most important measurement in observational cosmology.

Here is the key, and it is subtle: cosmological redshift is not a Doppler shift caused by the galaxy speeding away through space. Instead, while the light was in flight across the universe — often for billions of years — space itself expanded, and that expansion stretched the light wave along with everything else, lengthening its wavelength. A wave emitted as blue arrives as red simply because the space it travelled through grew. The amount is captured by a number called redshift, written z and defined so that the observed wavelength is (1 + z) times the emitted wavelength. A galaxy at z = 1 has its light wavelengths doubled, meaning the universe was half its present size when that light set out; z = 9 means the light left when the universe was one-tenth its current scale.

Cosmological redshift is the cosmologist's universal yardstick and clock. Through Hubble's law it gives distance, and because the light left long ago, it also tells look-back time, letting us read the universe's history layer by layer like tree rings. The deepest galaxies seen by space telescopes sit beyond z = 10, their light emitted when the cosmos was a few hundred million years old. The honest caveat: at small distances the redshift is well approximated by an ordinary Doppler velocity, which is why textbooks often quote it as a recession speed, but at cosmic distances that picture breaks down and the correct story is stretched space.