$\star$ Definition of the redshift

The redshift corresponds to a displacement towards the big wavelengths, therefore towards the red in the visible one, of the whole of the spectrum of the radiation emitted by a source which moves away from the observer (in the same way, a shift towards blue corresponds to a bringing together of the source). This shift, consequence of the Doppler effect, makes it possible to measure the radial speed of an object according to the line of sight.

The redshift is measured by comparing the position of certain lines in the spectrum of the source moving compared to their counterparts observed in laboratory. If $\Delta \lambda$ the shift in wavelength, the redshift is:

$$z = \frac{\Delta \lambda}{\lambda_e} =\frac{v}{c}$$ (1.4)

where the celerity of the light.