ch34 - 34. General relativity and cosmology 34.1 Why...

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34. General relativity and cosmology 34.1 Why general relativity? In the Newtonian description we assume that time and space are absolute. In cosmology, however, we discuss the greatest possible spans in space and time, and we therefore need general relativity to accurately describe the universe. It turns out, though, that we will arrive at equations very similar to those obtained in the Newtonian treatment. Part of the diFerence is the actual meaning of the variables. Problems with the Newtonian description can be easily seen, even if invoking special relativity. The light travel time from a distant galaxy is related to its velocity of recession. t = r c = 1 H 0 v c (34 . 1) Obviously the light cannot have been emitted before the big bang, and therefore the light travel time should be less than the age of the universe, t < t 0 . But as t approaches t 0 , the galaxy in question must have emitted the radiation when the universe was young and all objects were close to the location of the big bang. So there whould be a limit to the velocity of recession, that corresponds to the distance of objects at their locations at time t 0, which indeed is the location of the big bang. Measurements show no such limit up to v
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ch34 - 34. General relativity and cosmology 34.1 Why...

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