To understand the universe, cosmologists first needed to know our place in it. About 400 years ago, most people believed that Earth was at the center of the universe. They thought the Moon, the Sun, and the other planets all traveled around Earth. Nicholas Copernicus, Galileo Galilei, and Isaac Newton helped us to learn the truth. We now know that the Moon travels around Earth. Earth and the other planets in our solar system travel around the Sun.

About 100 years ago, astronomer Edwin Hubble made observations that showed the universe was getting bigger and bigger! This led most cosmologists to believe in a theory called the Big Bang. The Big Bang theory says the universe began with a huge explosion ten to twenty billion years ago. Cosmologists think the universe did not exist before the Big Bang. Some cosmologists think the universe will keep getting bigger forever. Other cosmologists think the Universe will someday start getting smaller. They think it will shrink until it no longer exists. No one knows yet which opinion is correct.

Cosmologists use the data gathered by telescopes and astronomy satellites to help them understand the universe. They also use computers to model their ideas. Each new discovery helps them to understand more about the past, present, and future of our universe.

A Question Use one or more of the underlined words to help you fill in the blank. People used to believe the Sun, the other planets and the Moon traveled around Earth. Today we know that only the __________________ travels around Earth.