Cosmological wavelength-redshift relation
= Cosmological wavelength-redshift relation
{title2=$1+z=\lambda_{\rm obs}/\lambda_{\rm rest}$}
Cosmic expansion stretches an emitted or absorbed spectral feature's wavelength by $1+z$. A small observed wavelength interval at fixed transition wavelength gives a redshift interval by division by the rest wavelength. Cosmological radial distances then require the <Hubble parameter> and, for proper distance at that epoch, an extra factor $1/(1+z)$.