Redshift
Definition
The stretching of light to longer (redder) wavelengths from receding objects — how we know the universe is expanding.
Symbol: · Unit: dimensionless
Technical definition
Defined as z = (λ_observed − λ_emitted)/λ_emitted. Cosmological redshift from expanding space (Hubble's law: v = H₀d); also gravitational redshift near massive objects and Doppler redshift from motion. Distant galaxies show z > 1.
Example
A galaxy with z = 0.1 recedes at ~10% the speed of light — its light arrives stretched 10% longer than emitted.
Common confusion
Cosmological redshift is not ordinary Doppler motion through space — space itself expands, stretching the light's wavelength during its journey. At high z, the ‘recession velocity’ framing breaks down entirely.