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The redshift: An observational quantity

In the first half of the twentieth century, optical astronomers noted that a source emitting in its rest frame at a wavelength \ensuremath{\lambda_\ensuremath{\mathrm{}}^{\ensuremath{\mathrm{rest}}}} will appear to emit at a larger wavelength \ensuremath{\lambda_\ensuremath{\mathrm{}}^{\ensuremath{\mathrm{obs}}}} in the observatory frame. To quantify this phenomenon, they introduced the observational notion of redshift, \ensuremath{z_{\ensuremath{\mathrm{}}}}, defined with

\begin{displaymath}
\ensuremath{\lambda_\ensuremath{\mathrm{}}^{\ensuremath{\ma...
...thrm{rest}}}}\, (1+\ensuremath{z_{\ensuremath{\mathrm{}}}}{}).
\end{displaymath} (20)

The redshift is a positive quantity that can become extremely large. It is straightforward to reexpress the redshift using frequency instead of wavelength, i.e.,
\begin{displaymath}
\ensuremath{f_\ensuremath{\mathrm{}}^{\ensuremath{\mathrm{o...
...mathrm{rest}}}}}{1+\ensuremath{z_{\ensuremath{\mathrm{}}}}{}}.
\end{displaymath} (21)

Some radio-astronomers used another definition of the redshift, called radio redshift, but this definition is now deprecated and the optical definition of the redshift is today universally used.

The redshift is of course linked to the Doppler effect. We however stress that the redshift is an observational quantity, independent of any mathematical expression of the Doppler effect. This is the key point to understand the solution of Gordon et al. (1992).


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Next: Splitting the change of Up: Interpreting the spectral axis Previous: Interpreting the spectral axis   Contents
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