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General relativity completely changed the discussion of the origin and fate of the universe. A static universe could have existed forever or could have been created in its present form at some time in the past. On the other hand, if galaxies are moving apart today, they must have been closer together in the past. About 15 billion years ago, they would all have been on top of one another and their density would have been infinite. According to the general theory, this Big Bang was the beginning of the universe and of time itself. So maybe Einstein deserves to be the person of a longer period than just the past 100 years.
General relativity also predicts that time comes to a stop inside black holes, regions of space-time that are so warped that light cannot escape them. But both the beginning and the end of time are places where the equations of general relativity fall apart. Thus the theory cannot predict what should emerge from the Big Bang. Some see this as an indication of God's freedom to start the universe off any way God wanted. Others (myself included) feel that the beginning of the universe should be governed by the same laws that hold at all other times. We have made some progress toward this goal, but we don't yet have a complete understanding of the origin of the universe.
The reason general relativity broke down at the Big Bang was that it was not compatible with quantum theory, the other great conceptual revolution of the early 20th century. The first step toward quantum theory came in 1900, when Max Planck, working in Berlin, discovered that the radiation from a body that was glowing red hot could be explained if light came only in packets of a certain size, called quanta. It was as if radiation were packaged like sugar; you cannot buy an arbitrary amount of loose sugar in a supermarket but can only buy it in 1-lb. bags. In one of his groundbreaking papers written in 1905, when he was still at the patent office, Einstein showed that Planck's quantum hypothesis could explain what is called the photoelectric effect, the way certain metals give off electrons when light falls on them. This is the basis of modern light detectors and television cameras, and it was for this work that Einstein was awarded the 1921 Nobel Prize in Physics.
Einstein continued to work on the quantum idea into the 1920s but was deeply disturbed by the work of Werner Heisenberg in Copenhagen, Paul Dirac in Cambridge and Erwin Schrodinger in Zurich, who developed a new picture of reality called quantum mechanics. No longer did tiny particles have a definite position and speed. On the contrary, the more accurately you determined the particle's position, the less accurately you could determine its speed, and vice versa.
Einstein was horrified by this random, unpredictable element in the basic laws and never fully accepted quantum mechanics. His feelings were expressed in his famous God-does-not-play-dice dictum. Most other scientists, however, accepted the validity of the new quantum laws because they showed excellent agreement with observations and because they seemed to explain a whole range of previously unaccounted-for phenomena. They are the basis of modern developments in chemistry, molecular biology and electronics and the foundation of the technology that has transformed the world in the past half-century.