Apr 4, 2018 · The 2dF redshift survey took place in limited regions in the sky. Galaxies can be labelled in terms of a coordinate consisting of a redshift, a right ascension and a declination . The galaxies here lie within a narrow strip in declination and the declination coordinate has been "collapsed" (i.e. ignored) to produce this plot (see #3).

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Because the Hubble expansion is the dominant factor in the relative velocity of distant objects, the spectra can be used to measure distances on cosmological scales. This is how the large 3d maps from surveys such as 2df and SDSS are made. $\endgroup$ –

The bandwidth theorem was first fully understood in an all-time fantastic paper: Gabor's Theory of Communication (1946).