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Cosmological solutions of the Einstein-Friedmann equations ...

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Cosmology<br />

Note: z = constant is <strong>the</strong> circle: (cT) 2 + W2 = constant. Homogeneity and Isotropy<br />

follow from <strong>the</strong> symmetry: S + 4 is invariant under S O(2, 3). The force is linear<br />

attractive F ∝ −1 3 Λ r. Causality problem: periodicity in time ct → ct + 2 π a. The<br />

circle in <strong>the</strong> (cT, W) plane is acausal. May be cured by mapping <strong>the</strong> circle to <strong>the</strong><br />

real line: which is <strong>the</strong> universal cover <strong>of</strong> <strong>the</strong> AdS space.<br />

For <strong>the</strong> cosmological <strong>solutions</strong> <strong>of</strong> <strong>the</strong> <strong>Einstein</strong>-<strong>Friedmann</strong> <strong>equations</strong> <strong>the</strong> RW-metric<br />

is only affected as far as S (t) solves a different dynamical equation.<br />

Exercise: find <strong>the</strong> model <strong>Einstein</strong> proposed: an eternal static solution. Show that<br />

<strong>Einstein</strong>’s GRT with vanishing cosmological constant has no such solution.<br />

Exercise: <strong>the</strong> observation <strong>of</strong> planetary motions constrains <strong>the</strong> cosmological<br />

constant to |Λ| ≪ 10 −42 cm −2 !. Show that this is compatible with a dark energy<br />

density, which has been determined to be ΩΛ ∼ 0.74 ± 0.03.<br />

Previous ≪❘ , next ❘≫ lecture.<br />

c○ 2009, F. Jegerlehner ≪❘ Lect. 7 ❘≫ 486

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