What type of redshift is caused by the expansion of the universe?

Cosmological redshift is the type of redshift caused by the expansion of the universe. As light travels through the evolving universe, the wavelength measured by a later observer is longer than the wavelength emitted by a distant source.

In the standard cosmological description, the effect is expressed through the cosmic scale factor: 1 + z = a_now/a_then. If the universe expands by a factor of two between emission and observation, the light's wavelength is also stretched by a factor of two. This is why distant galaxies generally appear redder than nearby ones, and why redshift helps astronomers estimate cosmic distances and look back into earlier eras.

Cosmological redshift is often described as space stretching the photon wavelengths. More precisely, in general relativity it can also be understood as the accumulated effect of many infinitesimal Doppler shifts along the light's journey. It should not be confused with gravitational redshift, caused by changing gravitational potential, or relativistic Doppler shift, caused by local relative motion.

The relationship between redshift and distance is not perfectly linear at great distances because the universe's expansion rate has changed over time. Measurements of distant Type Ia supernovae helped reveal that cosmic expansion is accelerating.