Redshift

As light crosses an expanding universe, its wavelength is stretched along with space itself — shifted toward the red end of the spectrum. We measure it as

1 + z = \frac{\lambda_\text{observed}}{\lambda_\text{emitted}},

where $z$ is the redshift. Because farther light has traveled through more expansion, more distant objects show larger $z$ — the relationship behind Hubble's law. Redshift is thus both a speedometer and a clock: a galaxy's $z$ tells us how much the universe has grown since its light set out, via the scale factor.

0.00redshift z

Redshift z = 0.00

Each dark line is a fingerprint of a specific element in the source's light. Expansion stretches every wavelength by the same factor (1 + z), so the whole pattern slides toward the red — and at high z, lines that were visible (like Hα, rest 656 nm → 656 nm now) leave the visible window entirely and can only be caught in the infrared. Because the pattern is preserved, we can still identify the lines and read off exactly how much the universe has expanded since the light left.

The lines don't smear or scramble — the entire fingerprint shifts together — which is what lets us measure $z$ so precisely even for the faintest, most distant galaxies.