What is redshift?
Redshift is the observation of light at longer wavelengths than it had at its source.
How it works and what the evidence shows
Core idea Recognizable spectral lines can be measured at longer wavelengths. Relative motion, gravity and the expansion of space are distinct physical causes of such shifts. Evidence and research Cosmological shifts in distant galaxy spectra inform studies of expansion. Matching several lines improves confidence; a cosmological model is needed to infer distance.
Uses and limitations
Why it matters Redshift helps investigate distant light and the expansion history of the universe. Blueshift means a shift toward shorter wavelengths. Common misunderstandings Not every redshift is merely a galaxy flying through space away from us. Dust reddening is also different from a coherent displacement of spectral lines.