The Cosmological Constant Problems

@inproceedings{Weinberg2000TheCC,
  title={The Cosmological Constant Problems},
  author={Steven Weinberg},
  year={2000}
}
The old cosmological constant problem is to understand why the vacuum energy is so small; the new problem is to understand why it is comparable to the present mass density. Several approaches to these problems are reviewed. Quintessence does not help with either; anthropic considerations offer a possibility of solving both. In theories with a scalar field that takes random initial values, the anthropic principle may apply to the cosmological constant, but probably to nothing else. 
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