Therefore, it is not possible to determine how much of an extragalactic object’s redshift is due to cosmic expansion and how much is due to Doppler motion. However, observationally it is impossible to distinguish between Doppler motion and cosmological redshift. Furthermore, cosmic redshift increases with increasing distance. Unlike Doppler motion, cosmic expansion always results in positive redshift.

Since this redshift is due to an effect of cosmology, we say that redshifts of this nature are cosmological. Where a 0 is current scale factor of the universe, and a is the scale factor at the time when light we are now receiving from distant objects was emitted. Hence, positive z is called a redshift, while a negative z is called a blueshift. However, when z is negative, an object’s distance is decreasing, and the shift in its spectrum is toward shorter wavelengths, corresponding to blue in the visible spectrum. 1 When z is positive, an object’s distance is increasing, and its wavelength is shifted toward longer wavelengths, corresponding to red in the visible spectrum. While one can express redshift in velocity, it is more common today to use the unitless quantity z = Δλ/λ, where λ is the rest wavelength of a spectral line and Δλ is its shift in wavelength.

The vertical axis is in terms of velocity (though Hubble omitted the “per second” in the units). Note that the horizontal axis is in terms of parsecs (pc), the preferred unit of distance outside the solar system.

1 for a reproduction of Hubble’s original 1929 plot). Technically, Hubble found the Hubble relation, a linear relationship between galaxy redshift and distance (see Fig. Keywords: AGNs, quasars, redshift, Hubble relation, Hubble constant, cosmological redshift, Lyman-alpha forests, quantized redshifts IntroductionĮdwin Hubble (1929) is credited with the discovery of the expansion of the universe.

Cosmological red shift