The Whole Universe
Cosmology deals with one of mankind's earliest questions: what is the nature of the world? Ancient civilizations believed that Earth was flat, and covered with a spherical dome on which the Sun, Moon, planets, and stars resided. In the second century A.D., the Ptolemaic system moved Earth to the universe's center. In the mid 1500s, the Copernican system placed the Sun at the universe's center.
Later, Galileo Galilei showed that the Milky Way is made of millions of stars. He argued that the Sun is itself a star, no different than the others. The Milky Way is now known to be a system composed of hundreds of billions of stars, organized in the shape of a flattened disk with a diameter of about 100,000 light-years. Our Sun is about 27,000 light-years from its center.
 Temperature variations in leftover radiation from the Big Bang mark sites where the first galaxies probably formed (red, yellow). Until the early twentieth century, astronomers thought the Milky Way was the sole constituent of the universe. Then they discovered that the spiral nebulae -- fuzzy objects found all over the night sky -- are large systems of stars.
Among the nebulae, they found a few special stars called "Cepheid variables," whose brightnesses regularly increase and decrease every few days. By comparing how much light these stars actually give off to how bright they appear from Earth, astronomers measured their distances. It was a stunning discovery: The nebulae containing these stars were at least one million light-years away, far beyond the edge of the Milky Way. The universe was much larger than originally thought.
In our modern picture, the nebulae are called galaxies, and there are at least 100 billion of them in the universe. The Milky Way is a galaxy just like these others.
In 1929, Edwin Hubble discovered that the galaxies are in motion. With few exceptions, all galaxies are moving away from the Milky Way. Furthermore, the farther any galaxy is from us, the faster it's moving.
This implies that the galaxies are moving not only away from us, but also from each other. The universe is not static, but evolving. It expands. Expansion implies that the galaxies were closer to one another in the past. The farther back we go, the closer together they were, until a time when they were all at the same place. This leads to the idea that the universe was created by a primordial explosion, or "Big Bang," which occurred 13 billion to 14 billion years ago. The galaxies today are moving away from each other because the universe itself is expanding.
With this discovery, cosmology acquires a new meaning. It is no longer concerned solely with the nature of the universe, but with its evolution as well.
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