This value for the z parameter corresponds to a recession speed of. Reported in November 1994 in Monthly Notices of the Royal Astronomical Society is a galaxy with a measured red shift of z=4.25, a new record. With z=3, the density is 64 times that of the present density. The z parameter can also be used to assess the mass density compared to the mass density ρ 0 at the present time. At such large values of z, the redshift is mainly the cosmological redshift, and not a valid measure of the actual recessional velocity of the object with respect to us. The wavelength of the received radiation has expanded by a factor of four because space has expanded by a factor of four during its transit from the emitting object.
For example, if the redshift of an object is found to correspond to z=3, then R=1/4 and we imply that the universe has expanded by a factor of four since light left that object. The z parameter is related to R by the expression where the scale is taken as R 0 =1 at the present time. Measurements of >100 quasars have given a range of 0.16 to 3.53, corresponding to recession speeds of 0.15c to 0.91c.Īnother application of the z parameter is to imply the scale factor R of the universe at the time that light was emitted from a given observed object. The largest red shifts measured are those of the quasars. Where λ is understood to be λ emitted = λ rest, in the rest frame of the emitter.Ĭalculating v/c from this expression gives: It is common practice to express this velocity as a fraction of the speed of light and to use a parameter z defined by: Measured Doppler red shifts give the recession velocity of stars or galaxies, presuming that the Hubble law is valid. The largest measured z values are associated with the quasars. The measured red shifts are usually stated in terms of a z parameter. If this proportionality (called Hubble's Law) holds true, it can be used as a distance measuring tool itself. The building up of methods for measuring distance to stars and galaxies led Hubble to the fact that the red shift (recession speed) is proportional to distance. The red line of the spectrum below is the transition from n=3 to n=2 of hydrogen and is famous as the H-alpha line seen throughout all the universe. This is the kind of result one would expect for an expanding universe. Using the results from the nearer ones, it becomes evident that the more distant galaxies are moving away from us faster. This shift is apparently a Doppler shift and indicates that essentially all of the galaxies are moving away from us. When these spectra are examined, they are found to be shifted toward the red end of the spectrum. The light from distant stars and more distant galaxies is not featureless, but has distinct spectral features characteristic of the atoms in the gases around the stars.
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