Is the evidence for dark energy secure?

Abstract

Several kinds of astronomical observations, interpreted in the framework of the standard Friedmann-Robertson-Walker cosmology, have indicated that our universe is dominated by a Cosmological Constant. The dimming of distant Type Ia supernovae suggests that the expansion rate is accelerating, as if driven by vacuum energy, and this has been indirectly substantiated through studies of angular anisotropies in the cosmic microwave background (CMB) and of spatial correlations in the large-scale structure (LSS) of galaxies. However there is no compelling direct evidence yet for (the dynamical effects of) dark energy. The precision CMB data can be equally well fitted without dark energy if the spectrum of primordial density fluctuations is not quite scale-free and if the Hubble constant is lower globally than its locally measured value. The LSS data can also be satisfactorily fitted if there is a small component of hot dark matter, as would be provided by neutrinos of mass 0.5 eV. Although such an Einstein-de Sitter model cannot explain the SNe Ia Hubble diagram or the position of the `baryon acoustic oscillation' peak in the autocorrelation function of galaxies, it may be possible to do so e.g. in an inhomogeneous Lemaitre-Tolman-Bondi cosmology where we are located in a void which is expanding faster than the average. Such alternatives may seem contrived but this must be weighed against our lack of any fundamental understanding of the inferred tiny energy scale of the dark energy. It may well be an artifact of an oversimplified cosmological model, rather than having physical reality.

12 pages, 5 figures; to appear in a special issue of General Relativity and Gravitation, eds. G.F.R. Ellis et al; Changes: references reformatted in journal style - text unchanged

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• Viscous dark fluid universe
• Constraints on large scale inhomogeneities from WMAP-5 and SDSS: confrontation with recent observations
• Non-adiabatic dark fluid cosmology
• The Cosmic Microwave Background in an Inhomogeneous Universe - why void models of dark energy are only weakly constrained by the CMB
• Reconciling the local void with the CMB
• Peculiar motions, accelerated expansion and the cosmological axis
• Transient cosmic acceleration from interacting fluids
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• Dark energy as a mirage
• Model independent constraints on the cosmological expansion rate
• Tension in the Void: Cosmic Rulers Strain Inhomogeneous Cosmologies
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• On the role of shear in cosmological averaging
• Self-consistent models of triaxial galaxies in MOND gravity
• On the role of shear in cosmological averaging II: large voids, non-empty voids and a network of different voids
• High-energy density implications of a gravitoweak unification scenario
• Cosmological constraints from low-redshift data
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• Power-law solutions and accelerated expansion in scalar-tensor theories
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• Phenomenological constraints on Lemaitre-Tolman-Bondi cosmological inhomogeneities from solar system dynamics
• Power-law solutions for TeVeS
• A Null Test of the Metric Nature of the Cosmic Acceleration
• Cosmological constraints from supernova data set with corrected redshift
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