Notes on Relativity and Cosmology - Physics Department, UCSB

Chapter 10
Cosmology: The Study of
the Universe as a whole
Read Einstein, ch. 30-32
In the last few chapters we have been talking a lot about the geometry, or shape,
of spacetime. In the particular case of the spacetime near a Schwarzschild black
hole, we have gone into great detail on this subject. But what about the big
picture? What can we say about the shape of the Universe as a whole?
Einstein asked this question very early on. He was motivated by technical
problems with the description of the Universe as a whole in Newtonian gravity
and he wanted to see if his theory worked better. It did, but not quite in the
way that he expected....
10.1 The Copernican Principle and Relativity
Of course, in the early 1900’s people did not know all that much about the
universe, but they did have a few ideas on the subject. In particular, a certain
philosophical tradition ran strong in astronomy, dating back to Copernicus.
(Copernicus was the person who promoted the idea that the stars and planets
did not go around the earth, but that instead the planets go around the sun.)
This tradition held in high esteem the principle that “The earth is not at a
particularly special place in the Universe”. It was this idea which had freed
Copernicus from having to place the earth at the center of the Universe.
The idea was then generalized to say that, for example “The Sun is not a
particularly special star,” and then further to “There is no special place in the
Universe.” Or, said differently, the Copernican principle is that “Every place
in the universe is basically the same.”
So, on philosophical grounds, people believed that the stars were sprinkled more
or less evenly throughout the universe. Now, one might ask, is this really true?
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