Universe


Not so much a theory of the universe as a simple picture of the planet we call home, the flat-earth model proposed that Earth's surface was level. Although everyday experience makes this seem a reasonable assumption, direct observation of nature shows the real world isn't that simple. For instance, when a sailing ship heads into port, the first part that becomes visible is the crow's-nest, followed by the sails, and then the bow of the ship. If the Earth were flat, the entire ship would come into view at once as soon as it came close enough to shore.
The Greek philosopher Aristotle provided two more reasons why the Earth was round. First, he noted that Earth's shadow always took a circular bite out of the moon during a lunar eclipse, which would only be possible with a spherical Earth. (If the Earth were a disk, its shadow
would appear as an elongated ellipse at least during part of the eclipse.) Second, Aristotle knew that people who journeyed north saw the North Star ascend higher in the sky, while those heading south saw the North Star sink. On a flat Earth, the positions of the stars wouldn't vary with a person's location. Despite these arguments, which won over most of the world's educated citizens, belief in a flat Earth persisted among many others. Not until explorers first circumnavigated the globe in the 16th century did those
beliefs begin to die out.
Ptolemy, the last of the great Greek astronomers of antiquity, developed an effective system for mapping the
universe. Basing much of his theory on the work of his predecessor, Hipparchus, Ptolemy designed a geocentric, or Earth-centered, model that held sway for 1400 years. That Ptolemy could place Earth at the center of the universe and still predict the planets' positions adequately was a testament to his ability as a mathematician. That he could do so while maintaining the Greek belief that the heavens were perfect?and thus that each planet moved along a circular orbit at a constant speed?is nothing short of
remarkable.
Copernicus made a great leap forward by realizing that the motions of the planets could be explained by placing the Sun at the center of the universe instead of Earth. In his view, Earth was simply one of many planets orbiting the Sun, and the daily motion of the stars and planets were just a reflection of Earth spinning on its axis. Although the Greek astronomer Aristarchus developed the same hypothesis more than 1500 years earlier, Copernicus was the first person to argue its merits in modern times.
Despite the basic truth of his model, Copernicus did not prove that Earth moved around the Sun. That was left for later astronomers. The first direct evidence came from Newton's laws of motion, which say that when objects orbit one another, the lighter object moves more than the heavier one. Because the Sun has about 330,000 times more mass than Earth, our planet must be doing almost all the moving. A direct observation of Earth's motion came in 1838 when the German astronomer Friedrich Bessel measured the tiny displacement, or parallax, of a nearby star relative to the more distant stars. This minuscule displacement reflects our planet's changing vantage point as we orbit the Sun during the year.
How did the universe really begin? Most astronomers would say that the debate is now over: The universe started with a giant explosion, called the Big Bang. The big-bang theory got its start with the observations by Edwin Hubble that showed the universe to be expanding. If you imagine the history of the universe as a long-running movie, what happens when you show the movie in reverse? All the galaxies would move closer and closer together, until eventually they all get crushed together into one massive yet tiny sphere. It was just this sort of thinking that led to the concept of the Big Bang.
The Big Bang marks the instant at which the universe began, when space and time came into existence and all the matter in the cosmos started to expand. Amazingly, theorists have deduced the history of the universe dating back to just 1043 second (10 million trillion trillion trillionths of a second) after the Big Bang. Before this time all four fundamental forces?gravity, electromagnetism, and the