Recent astronomical observations have provided strong evidence that we live in an accelerating universe. By itself, acceleration is easy to understand in the context of general relativity and quantum field theory; however, the very small but nonzero energy scale seemingly implied by the observations is completely perplexing. In trying to understand the universe in which we apparently live, we are faced with a problem, a puzzle, and a scandal:
The cosmological constant problem: why is the energy of the vacuum so much smaller than we estimate it should be?
The dark energy (1) puzzle: what is the nature of the smoothly-distributed, persistent energy density which appears to dominate the universe?
The coincidence scandal: why is the dark energy density approximately equal to the matter density today?
Any one of these issues would represent a serious challenge to physicists and astronomers; taken together, they serve to remind us how far away we are from understanding one of the most basic features of the universe.
The goal of this article is to present a pedagogical (and necessarily superficial) introduction to the physics issues underlying these questions, rather than a comprehensive review; for more details and different points of view see Sahni and Starobinski (2000), Carroll (2001), Padmanabhan (2003), or Peebles and Ratra (2003). After a short discussion of the issues just mentioned, we will turn to mechanisms which might address any or all of them; we will pay special attention to the dark energy puzzle, only because there is more to say about that issue than the others. We will close with an idiosyncratic discussion of issues confronting observers studying dark energy.
1 "Dark energy" is not, strictly speaking, the most descriptive name for this substance; lots of things are dark, and everything has energy. The feature which distinguishes dark energy from ordinary matter is not the energy but the pressure, so "dark pressure" would be a better term. However, it is not the existence of the pressure, but the fact that it is negative - tension rather than ordinary pressure - that drives the acceleration of the universe, so "dark tension" would be better yet. And we would have detected it long ago if it had collected into potential wells rather than being smoothly distributed, so "smooth tension" would be the best term of all, not to mention sexier. I thank Evalyn Gates, John Beacom, and Timothy Ferris for conversations on this important point. Back.