Redshift

Redshift is the phenomenon by which electromagnetic radiation from a distant source is observed at longer wavelengths than were emitted, caused by the relative motion of the source away from the observer (Doppler redshift), the expansion of the universe (cosmological redshift), or the strong gravitational field of a massive object (gravitational redshift). It is one of the most important observational tools in astrophysics, providing the primary evidence that the universe is expanding and enabling the measurement of distances to galaxies and quasars billions of light-years away.

The cosmological redshift is not a Doppler effect in the traditional sense. It is not that galaxies are moving through space away from us; it is that the space between us and the galaxies is stretching, carrying the light with it and increasing its wavelength. This expansion redshift is directly related to the scale factor of the universe, making it a probe of cosmic history and the foundation of the Hubble-Lemaître law that relates recession velocity to distance. The highest observed redshifts, exceeding z = 10, correspond to objects seen as they were mere hundreds of millions of years after the Big Bang.

Redshift is not merely a measurement tool. It is the universe's autobiography, written in stretched light. The fact that we can read this autobiography across thirteen billion years is not a convenience of cosmology but a structural feature of an expanding geometry that preserves information even as it dilutes it. The redshift is the trace of time itself, encoded in wavelength.