PHYS195 NotesChapter16

PHYS195 NotesChapter16 - Chapter 16 Eects of Gravitation...

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Chapter 16 E ff ects of Gravitation 16.1 Curvature around a Massive Body 16.2 The Geometry and Evolution of the Universe 16.2.1 Background Ideas After 1916, Einstein and others applied the General Theory of Relativity, the modern theory of gravity to the entire universe. The basic ideas are so simple and compelling that it seems that they must be correct and most of the observational data are in complete concordance. Despite this simplicity, the history of the subject is full of surprising turns and it is worthwhile telling some of this history so that we can understand the context of our current understanding and why this is still an exciting and active research field – hardly a week goes by without some new article in the newspapers indicating some controversial measurement. Like all good science, cosmology is now being driven by new experimental results. It is important to realize that the current controversies in our understanding of the operation of the universe are all really at the interface of General Relativity and micro-physics. In this section, we will deal only with the broadly accepted aspects of the subject and leave the issues that emerge from the interaction of the large scale universe with microphysics to a later chapter, see Chapter 17. Because of this, in this chapter, we will treat the matter in universe very simply and accept forms of matter that are currently not understood. Einstein had a rather simple outlook on the nature of the universe and its origin. Like Descartes and others before him, he felt that that the uni- verse has always been present or at least reasonably stable. This desire was tempered though by the observation that, although the ages of the sun and 361
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362 CHAPTER 16. EFFECTS OF GRAVITATION planets were quite large, there were certainly dynamical processes taking place in the cosmos. This balance between perpetuity and evolution meant that he wanted solutions for the space-time structure of the universe that had stationary or at least quasi-stationary solutions, i. e. solutions that were stable over long periods of time. We should realize that the astronomy of the period was not nearly as advanced as it is today and the observational situation was that, at all distances, the night sky looked the same. Due to the fact that the speed of light is finite, looking at longer distances was the same as looking back in time. It is just that the distances that we being observed were small compared to what we now know are relevant to cosmo- logical questions. Also, we have been observing the universe seriously for only the last few hundred years and on the lifetime of stars and things like that this is but an instant. As they were originally proposed the equations for the evolution of space- time, the Einstein equations ?? , did not possess any stationary solutions; there were not enough dimensionful parameters to define a time. He realized that there was a simple way to modify the equations and he added the term now called the cosmological constant.
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