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There is significant observational evidence for the detection of Einstein's cosmological constant, Lambda, or a component of the material content of the universe that varies only slowly with time and space and so acts like Lambda. We will use the term dark energy for Lambda or a component that acts like it. Detection of dark energy would be a new clue to an old puzzle, the gravitational effect of the zero-point energies of particles and fields. The total with other energies that are close to homogeneous and nearly independent of time act as dark energy. The puzzle has been that the value of the dark energy density has to be tiny compared to what is suggested by dimensional analysis; the startling new evidence is that it may be different from the only other natural value, zero.

The main question to consider now has to be whether to accept the evidence for detection of dark energy. We outline the nature of the case in this section. After reviewing the basic concepts of the relativistic world model, in Sec. II, and in Sec. III reviewing the history of ideas, we present in Sec. IV a more detailed assessment of the state of the cosmological tests and the evidence for detection of Lambda or its analog in dark energy.

There is little new to report on the big issue for physics -- why the dark energy density is so small -- since Weinberg's (1989) review in this Journal 1. But there have been analyses of a simpler idea: can we imagine the present dark energy density is evolving, perhaps approaching zero? Models are introduced in Secs. II.C and III.E, and recent work is summarized in more detail in the Appendix. Feasible advances in the cosmological tests could detect evolution of the dark energy density, and maybe its gravitational response to the large-scale fluctuations in the mass distribution. That would really drive the search for a more fundamental physics model for dark energy.

1 Sahni and Starobinsky (2000), Carroll (2001), Witten (2001), Weinberg (2001), and Ellwanger (2002) present more recent reviews. Back.

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