Published in "Measuring and Modeling the Universe",
Carnegie Observatories Astrophysics Series Vol. 2, ed. W. L. Freedman
For a PDF version of the article, click
For a PDF version of the article, click here.
Abstract. The universe appears to be accelerating, but the reason why is a complete mystery. The simplest explanation, a small vacuum energy (cosmological constant), raises three difficult issues: why the vacuum energy is so small, why it is not quite zero, and why it is comparable to the matter density today. I discuss these mysteries, some of their possible resolutions, and some issues confronting future observations.
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