Cosmological Redshifts

The light from a distant galaxy can exhibit a significant cosmological redshift. If the cosmological redshift is not a velocity redshift, what is its origin? Can the two effects be distinguished from each other by observing the spectrum of the galaxy or other light source?

Answer

There are three distinct causes for the spectral shift of light emitted (or absorbed) by a galaxy: the kinematical Doppler shift of the special theory of relativity (STR), the gravitational redshift of the general theory of relativity (GTR), and the cosmological redshift caused by the expansion of the universe. These three effects cannot be distinguished from one another by observing the spectrum of a single galaxy or other single light source. One can separate out the kinematical Doppler shift for a cluster of galaxies, however, via statistical methods.

The standard explanation of the cosmological redshift says that the coordinate system of the universe is expanding while the galaxies remain at their local coordinate values. One can use an expanding balloon to “mimic” this type of behavior. Inflate a balloon enough to enable you to draw a coordinate system on its surface. Place some galaxies on the balloon surface. Now inflate the balloon further. The galaxies are farther apart, but they maintain the same coordinate values. Or one can use Earth. If Earth began to expand, Philadelphia and Los Angeles would move apart, yet each would retain its present longitude and latitude. Distances are stretched, so wavelengths will become longer.

Perhaps another viewpoint will be helpful. One does not need to expand space in this view. According to S. Weinberg in the Chown reference below, simply accept the fact that “every bit of the universe is rushing away from every other bit” that is, “the galaxies are exploding away from each other, as any cloud of particles would do if they are set in motion away from each other.” The matter inside the individual galaxies does not take part in the general expansion because the local gravity holds the local matter together. The universe’s expansion appears just beyond the frontiers of the Local Group of galaxies, about 4 million light-years from the Local Group’s center of mass.

M. L. Bedran in the reference listed below compares the Doppler redshift to the cosmological redshift for a galaxy with z = 1, where 1 + z = exp [v/c], determining that a 2.4 percent difference between the two redshifts exists for this galaxy with an STR value of v/c = 0.6.