When the light an object emits is displaced toward the red end of the spectrum it is said to be redshifted. In general, photons of light that are emitted at a source at one energy and detected by an observer at a lower energy are redshifted.
Often, the redshift of an object can be measured by examining atomic absorption or emission lines in its spectrum. Redshifts can be caused by the motion of a source away from an observer. For distant objects, redshifts can be caused by the expansion of the Universe.
The redshift equals z if the wavelength of light is z + 1 times the normal wavelength; thus a redshift of 0.40 means that the wavelength of the light is 40% longer than normal.
Using the Hubble space telescope, astronomers have measured redshifts greater than 5.0.