The Theory of General Relativity and Its Main Solutions

The first cosmological revolutionCosmological revolution had three important and well-differentiated milestones. The first I have just explained: the finding that the Universe was millions of times larger than the Milky WayMilky Way. The second will be presented in this chapter: contrary to the prevailing view in the 1920s, supported by rigorous physical considerations that everyone, including Einstein, accepted at face value, the Universe was not static, but expanding. The third, which will be discussed in the next chapter, was to realize that our Universe had not existed forever: it had an origin, some fourteen billion years ago. Moreover, the cosmological revolution exactly coincided with the time when cosmology itself became a modern, a true science. And the observation of celestial objects, including the determination of their positions and speeds, and their evolution, all just correspond to the empirical part. These results must be given coherence, they must be explained on the basis of some fundamental law, which is expected to be predictive and tell us how these objects and the entire universe will evolve in the future, and how they evolved in the past; even before the astronomical data to confirm both these extremes are available. We will see that such fundamental law is none other than Albert Einstein's general theory of relativity. The whole chapter will be devoted to this theory.