astro-ph/0207347

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THE COSMOLOGICAL CONSTANT AND DARK ENERGY

P.J.E. Peebles


Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544

Bharat Ratra


Department of Physics, Kansas State University, Manhattan, KS 66506


Abstract. Physics invites the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, Lambda; nowadays the concept is termed dark energy or quintessence. Physics also suggests the dark energy could be dynamical, allowing the arguably appealing picture that the dark energy density is evolving to its natural value, zero, and is small now because the expanding universe is old. This alleviates the classical problem of the curious energy scale of order a millielectronvolt associated with a constant Lambda. Dark energy may have been detected by recent advances in the cosmological tests. The tests establish a good scientific case for the context, in the relativistic Friedmann-Lemaître model, including the gravitational inverse square law applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one quarter of the critical Einstein-de Sitter value. The case for detection of dark energy is serious but not yet as convincing; we await more checks that may come out of work in progress. Planned observations might be capable of detecting evolution of the dark energy density; a positive result would be a considerable stimulus to attempts to understand the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.


Table of Contents

INTRODUCTION
The issues for observational cosmology
The opportunity for physics
Some explanations

BASIC CONCEPTS
The Friedmann-Lemaître model
The cosmological constant
Inflation and dark energy

HISTORICAL REMARKS
Einstein's thoughts
The development of ideas
Early indications of Lambda
The coincidences argument against Lambda
Vacuum energy and Lambda
Inflation
The scenario
Inflation in a low density universe
The cold dark matter model
Dark energy

THE COSMOLOGICAL TESTS
The theories
General relativity
The cold dark matter model for structure formation
The tests
The thermal cosmic microwave background radiation
Light element abundances
Expansion times
The redshift-angular size and redshift-magnitude relations
Galaxy counts
The gravitational lensing rate
Dynamics and the mean mass density
The baryon mass fraction in clusters of galaxies
The cluster mass function
Biasing and the development of nonlinear mass density fluctuations
The anisotropy of the cosmic microwave background radiation
The mass autocorrelation function and nonbaryonic matter
The gravitational inverse square law
The state of the cosmological tests

CONCLUDING REMARKS

APPENDIX: RECENT DARK ENERGY SCALAR FIELD RESEARCH

REFERENCES

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