Astronomy by Britannica Educational Publishing

Author:Britannica Educational Publishing
Language: eng
Format: epub
Publisher: Britannica Educational Publishing
Published: 2012-06-28T16:00:00+00:00

Chapter 4

THE UNIVERSE

Cosmology is the scientific inquiry into the nature, history, development, and fate of the universe. By making assumptions that are not contradicted by the behavior of the observable universe, scientists build models, or theories, that attempt to describe the universe as a whole, including its origin and its future. They use each model until something is found that contradicts it. Then the model must be modified or discarded.

A REVOLUTION IN COSMOLOGY

In 1905 Albert Einstein published his theory of special relativity, which showed that space and time can be seen as aspects of a deeper structure, space-time, and that mass and energy are really the same thing. In 1916 he followed this with his theory of general relativity, in which gravity is understood as a warping, or bending, of space-time by the presence of mass. This new theory of gravity, which has passed a number of experimental tests, paved the way for the modern scientific study of cosmology.

Einstein soon realized that his basic equations, in their simplest form, required that the universe be either expanding or contracting. Its matter—along with space itself—would be either flying apart or falling together. Einstein, like most astronomers at the time and much like Newton two centuries before, objected to such a conclusion. He favored instead the idea of a static universe, one essentially unchanging through infinite time. He realized that his equations could include a special term, called the “cosmological constant”, which could supply a sort of repulsive force, capable of balancing gravity and keeping the universe static. While it might be simpler to leave it out (by assigning it a value of zero), Einstein assigned it a positive value so that the universe would be essentially unchanging, as he expected.

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