Cosmology
Cosmology is the study of the origin and evolution of the universe. In the last half of the twentieth century, astronomers made enormous progress in understanding cosmology. The discovery that the universe apparently began at a specific point in time and has continued to evolve ever since is one of the most revolutionary discoveries in science.
The History of the Universe: in the Beginning
The universe began in what astronomers dubbed the "Big Bang"—an initial event, after which the universe began to expand. Current estimates place the Big Bang at about 13 to 15 × 109 years ago. During the first seconds after the Big Bang, the universe was extremely hot and dense. The physics needed to understand the universe in these early stages is very speculative because it is impossible to recreate these conditions in an experiment today to check the predictions of the theory. Before 10-44 seconds after the Big Bang, the four fundamental forces of nature—gravity, the electromagnetic force, and the strong and weak nuclear forces—were unified into a single force. At 10-44 seconds, gravity separated from the others; at 10-34 seconds,the strong force became separated; and at 10-11 seconds, the weak force separated from the electromagnetic force.
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