IV.

Redshift and Cosmology

Redshift and Hubble’s law are vital tools to scientists who study the structure, evolution, and age of the universe. This field is called cosmology. Redshift provides astronomers with a good idea of the general motion of matter in the universe. Observing objects that do not follow Hubble’s law enables astronomers to see the motion of individual galaxies and groups of galaxies and provides useful information about the structure of the universe.

The gravitational pull of nearby galaxies affects the motion of a galaxy. Measurements of redshift reveal that the Great Andromeda Spiral Galaxy, one of the Milky Way’s nearest neighbors, is actually moving toward the Milky Way at about 50 km/s (about 30 mi/s). Neighboring groups of galaxies also affect each other’s motion. The Milky Way and its neighbors are called the Local Group. The Local Group’s neighbor, the Virgo Cluster, is moving away at only about 80 percent of the velocity predicted by Hubble’s law. Deviations from Hubble’s law (also called the Hubble flow) provide one of the best means of calculating the total density of matter in the universe.

Astronomers can also use redshift to identify the oldest and most distant objects in the observable universe. Astronomers believe that quasars are the most distant objects in the universe, because they have some of the largest redshifts. Quasars are objects in space that strongly emit radio waves. Astronomers originally named these objects quasars, which stands for quasi-stellar (or starlike) radio source, because they appear as points of light, like stars, in photographs of the sky. When astronomers began studying quasars in radio and other wavelengths, however, they discovered that quasars are not really starlike at all. They emit far more radiation, especially radio-wavelength radiation, than stars do, and quasars have huge redshifts. Their redshifts are so large that the radiation they emit in the ultraviolet range (with wavelengths shorter than visible light) reaches Earth in the infrared range (with wavelengths longer than visible light). The redshift for some of the most distant quasars is about 5.0, meaning that the shift in wavelength is about five times greater than the wavelength itself. A quasar with a redshift of 5.0 would be between about 3000 Mpc and 6000 Mpc away from Earth—so far away that light from the quasar would take between 9 billion and 19 billion years to reach Earth. Astronomers believe that quasars may be huge black holes, or regions that are so dense that not even light can escape their gravitational pull, surrounded by swirling matter. The matter swirling around black holes is very hot and is moving very quickly. Under these conditions, matter can produce light. This may be the source of quasars’ radiation.

If Hubble’s law holds for most of the age of the universe, Hubble’s constant would give an accurate age of the universe. The universe’s age would be the inverse of the constant (1 divided by Hubble’s constant), or between 12 billion and 16 billion years. However, astronomers have evidence that Hubble’s constant probably is not really constant—that the rate of expansion of the universe has changed and will keep changing as the universe evolves. The estimated age of the universe is actually only about 14 billion years.