redshift

Redshift is a change in the wavelength of electromagnetic radiation, observed when the source moves away from the observer or when space itself expands. It is quantified by the dimensionless parameter z = (λ_obs − λ_emit)/λ_emit, where λ_obs is the observed wavelength and λ_emit is the emitted wavelength. The counterpart, blueshift, occurs when wavelengths shorten.

Several physical causes can produce redshift. Doppler redshift arises from the relative motion of the source and observer. Cosmological redshift results from the expansion of space, stretching photons as they travel through the universe. Gravitational redshift is predicted by general relativity: photons climbing out of a gravitational well lose energy and increase in wavelength.

Redshift is measured by spectroscopy, identifying shifts in known spectral lines. For small redshifts, velocity can be approximated by v ≈ cz, linking redshift to recession speed. In cosmology, redshift tracks the expansion history of the universe, with the relation z = 1/a − 1, where a is the scale factor.

It provides a key tool for distance measurements and studies of cosmic structure. Objects with high redshifts reveal the distant past, such as galaxies and quasars. The cosmic microwave background has a redshift of about z ≈ 1100. Redshift observations also test gravitational theories through measurements of gravitational redshift in strong fields.