Why is the Universe Accelerating?
Sean M. Carroll
Enrico Fermi Institute, Department of Physics, and Kavli Institute for Cosmological Physics, University of
Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
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.
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
The dark energy
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), 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.
“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
conversationson this important point.
This paper has previously appeared in the proceedings of (i) the 31st SLAC Summer Institute on Particle Physics: Cosmic Connection to Particle
Physics (SSI 2003), 28 Jul - 8 Aug 2003, and (ii) the Carnegie Observatories Centennial Symposium on Measuring and Modeling the Universe,
17-22 Nov 2002.
CP743, The New Cosmology: Conference on Strings and Cosmology and The Mitchell Symposium on Observational Cosmology
edited by R. E. Allen, D. V. Nanopoulos, and C. N. Pope
© 2004 American Institute of Physics 0-7354-0227-2/04/$22.00