How Universal is Hubble’s Law?

In 1929 Hubble [1] published his profound result that the spectra of extragalactic nebulae show ‘redshifts’ in their spectra, with the redshifts increasing as the nebulae get fainter and fainter in the sample. In a typical case a well known absorption line appeared with wavelength λ₀(I+z) instead of its laboratory wavelength λ₀, thus implying a redshift z. Taking faintness to be indicative of distance, Hubble’s findings implied a relation of the following kind.