Published in "Measuring and Modeling the Universe", Carnegie Observatories Astrophysics Series Vol. 2, ed. W. L. Freedman (2004).
astro-ph/0310342

For a PDF version of the article, click here.


WHY IS THE UNIVERSE ACCELERATING?

Sean M. Carroll

Enrico Fermi Institute, Department of Physics, and Center for Cosmological Physics, University of Chicago
Kavli Institute for Theoretical Physics, University of California, Santa Barbara
carroll@theory.uchicago.edu


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.


Table of Contents

INTRODUCTION

THE MYSTERIES
Classical vacuum energy
Quantum zero-point energy
The coincidence scandal

WHAT MIGHT BE GOING ON?
Are we misinterpreting the data?
Is general relativity breaking down?
Is dark energy dynamical?
Did we just get lucky?
Are we witnessing environmental selection?

OBSERVATIONAL ISSUES
Equation-of-state parameter
Direct detection of dark energy

CONCLUSIONS

REFERENCES

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