EXAMPLE 1 The Big Bang

How did the universe begin? One popular theory is known as the “Big Bang.” The universe began with a big bang and matter expanded outward, like a balloon inflating. If the Big Bang theory is correct, galaxies farthest away from the origin of the bang must be moving faster than those closest to the origin. This also means that galaxies close to the earth must be moving at a similar speed to that of earth, and galaxies far from earth must be moving at very different speeds from earth. Hence, relative to earth, the farther away a galaxy is, the faster it appears to be moving away from earth. Are data consistent with this theory? The answer is Yes.

In 1929, Edwin Hubble investigated the relationship between the distance from the earth and the recession velocity (the speed at which an object is moving away from an observer) of galaxies. Using data he had collected, Hubble estimated the distance, in megaparsecs, from the earth to 24 galaxies. One parsec equals 3.26 light-years (the distance light travels in one year), and a megaparsec is one million parsecs. The recession velocities, in kilometers per second, of the galaxies were also measured. Figure 14.2 is a scatterplot that shows how recession velocity is related to distance from the earth. We think that “distance from the earth” will help explain “recession velocity.” That is, “distance from the earth” is the explanatory variable, and “recession velocity” is the response variable. We want to see how recession velocity changes when distance from the earth changes, so we put distance from the earth (the explanatory variable) on the horizontal axis. We can then see that, as distance from the earth goes up, recession velocity goes up. Each point on the plot represents one galaxy. For example, the point with a different plotting symbol corresponds to a galaxy that is 1.7 megaparsecs from the earth and that has a recession velocity of 960 kilometers per second.

Hubble’s discovery turned out to be one of the most important discoveries in all of astronomy. The data helped establish Hubble’s law, which is recession velocity = H₀ × Distance, where H₀ is the value known as the Hubble constant. Hubble’s law says that the apparent recession velocities of galaxies are directly proportional to their distances. This relationship is the key evidence for the idea of the expanding universe, as suggested by the Big Bang.