Overview

In the first half of the twentieth century, astronomers demonstrated that the universe was expanding, thus providing support for the idea that it had arisen from a tremendous explosion billions of years ago. This explosion came to be known as the Big Bang. Depending on the methods used and the assumptions that are made, different values are obtained for the age of the universe. Today, scientists are coming closer to agreement on an age of 12-16 billion years.

Background

The nineteenth century invention of spectroscopy allowed astronomers to break up the light coming from distant stars and galaxies into its constituent wavelengths. Different types of stars and galaxies have characteristic patterns, or spectra. According to the Doppler effect, the spectra are shifted towards longer wavelengths, or red-shifted, when the...