A new view of the universe emerged during the early 1900's, chiefly from the work of the famous German-born physicist Albert Einstein. In 1905, Einstein proposed his special theory of relativity. According to this theory, nothing can travel faster than the speed of light. From this theory comes the idea that mass and energy are interchangeable and are related by the equation E equals m times c-squared. In this equation, E stands for energy, m for mass, and c-squared for the speed of light multiplied by itself. During the 1930's, astronomers discovered that stars get their energy through the transformation of mass into energy as described by Einstein's equation.
In 1915, Einstein announced his theory of gravitation, called the general theory of relativity. This theory links the three dimensions of space with a fourth dimension, time. In most cases, the results obtained by using Einstein's theory do not differ significantly from those obtained by using Isaac Newton's theories. However, the general theory of relativity must be used in studies of the universe as a whole or of events that occur in extremely strong gravitational fields. For example, the general theory explains how the mass of a black hole can affect space in such a way that not even light can escape.
The general theory of relativity implies that the universe is expanding. But in 1915, Einstein had no observational evidence to support this idea. He therefore changed his equations to describe a universe of constant size. In 1929, however, the American astronomer Edwin Hubble demonstrated that the universe is expanding. As a result, Einstein restored his original equations. Modern theories of cosmology are based on solutions to these equations.
Excerpt from the "Astronomy" article, The World Book Encyclopedia © 1999