redshift: a standalone construct

Vesto M. Slipher's spectrographic surveys of spiral nebulae, summarized in his 1917 report, were the first systematic measurements of what is now called redshift: the displacement of features in an object's light toward longer, redder wavelengths, with most of the nebulae he observed showing their spectral lines shifted toward the red end. Edwin Hubble's 1929 paper then tied this shift to distance, finding that more distant galaxies show larger redshifts, the proportional relation now known as Hubble's law. Redshift is quantified as z, the fractional increase in wavelength, that is the observed wavelength minus the emitted wavelength divided by the emitted wavelength, so a larger z means a larger stretch. Astronomers separate the Doppler redshift, produced by an object's motion away from the observer through space, from the cosmological redshift, produced by the expansion of space itself stretching light while it travels, and it is this cosmological meaning that underlies the recession velocities and distance estimates used across cosmology. Light can also be shifted the other way, toward shorter wavelengths, a case called blueshift that arises for objects approaching the observer.[1][2]

Unit

A dimensionless fractional increase in wavelength, denoted z

Recovery means

Not applicable

Measured by

Measured from the displacement of identified spectral lines in an object's spectrum relative to their known rest wavelengths