The Structure of Our Universe

Distances beyond our Galaxy

As dramatic as is the change in scale from Solar System distances to interstellar distances, an equally dramatic change in scale is found when we go from the scale of our Galaxy to the limits of the observable universe. While the scale of our Galaxy is thousands of parsecs, or kiloparsecs (kpc), the scale of the distances between the galaxies is millions of parsecs, or megaparsecs (Mpc). To measure the observable universe, we must expand our scale by another factor of one thousand to units of gigaparsec (Gpc). These distances are measured by observing the brightness of objects of known luminosity.

Our Galaxy is one of a small cluster of galaxies, which are collectively called the Local Group. The Local Group contains numerous dwarf galaxies and a few larger galaxies. The two closest galaxies to the Milky Way are the Large and Small Magellanic Clouds, which are dwarf irregular galaxies that are 50 and 60 kpc from Earth. These two galaxies are unusually close to the Milky Way. One of the more prominent galaxies in the local group is the Andromeda Galaxy (M31), which is a massive spiral galaxy 730 kpc from Earth. The other spiral galaxy in the Local Group is the Triangulum Galaxy (M33), which is 900 kpc away. The distances of other members of the Local Group are typically around 750 kpc, although several are farther than 1.5 Mpc away.

As we look beyond the Local Group, we see more galaxies. Most galaxies form gravitationally-bound groups and clusters, with the remaining galaxies spread out in the field. Our ability to see the most distant galaxies is limited by their motion away from us. Observations show that the galaxies in the universe are on average moving away from one another; this is the motivation for the Big Bang theory of cosmology. One consequence of this expansion is that the farther a galaxy is away from us, the faster it is moving away from us. At some point the effects of special relativity come into play, and the light from a galaxy is Doppler shifted out of the optical frequencies and into the infrared and radio frequencies. In tandem with this, the Doppler shift lowers the rate at which photons from a galaxy arrive at Earth. These two effects of special relativity set a bound on our ability to observed distant galaxies. When you read stories about the discovery of the most distant galaxy to date, you are reading about the bounds of the observable universe being pushed out with improvements in telescope technology.

The relationship between the redshift of a galaxy and its distance is set by the Hubble constant, which is the ratio of the velocity of a galaxy away from us to its distance. The best measurements put the Hubble constant at 60 km s^-1 Mpc^-1, although the error in this value is around 20%. The scale of the visible universe is then of order the speed of light divided by the Hubble constant. This scale is around 5,000 Mpc, or 5 gigaparsecs (Gpc). The scale of the observable universe is therefore 300 thousand times the scale of our Galaxy, which is comparable to the ratio of the distance to the nearest stars to the semimajor axis of Earth's orbit.