We provide preliminary quantitative evidence that a new solution to averaging the observed inhomogeneous structure of matter in the universe may lead to an observationally viable cosmology without exotic dark energy. We find parameters which simultaneously satisfy three independent tests: the match to the angular scale of the sound horizon detected in the cosmic microwave background anisotropy spectrum; the effective comoving baryon acoustic oscillation scale detected in galaxy clustering statistics; and Type Ia supernova luminosity distances. Independently of the supernova data, concordance is obtained for a value of the Hubble constant which agrees with the measurement of the and an age of the universe of billion years as measured by observers in galaxies. The mass ratio of ϩ0.714.7 Ϫ0.5 nonbaryonic dark matter to baryonic matter is , computed with a baryon-to-photon ratio that is in con-ϩ2.53.1 Ϫ2.4 cordance with primordial lithium abundances. Subject headings: cosmological parameters -cosmology: observations -cosmology: theory -dark matterlarge-scale structure of universe Online material: color figuresThe apparent acceleration in present cosmic expansion is usually attributed to a smooth "dark energy," whose nature poses a foundational mystery to physics. Our standard LCDM cosmology, with a cosmological constant L as dark energy, fits three independent observational tests: Type Ia supernovae (SNe Ia) luminosity distances; the angular scale of the Doppler peaks in the spectrum of cosmic microwave background (CMB) temperature anisotropies; and the baryon acoustic oscillation scale detected in galaxy clustering statistics. In this Letter we provide preliminary evidence that these same tests can all be satisfied in ordinary general relativity without exotic dark energy, within a model (Wiltshire 2007a(Wiltshire , 2007b which takes a new approach to averaging the observed structure of the universe, presently dominated by voids.Recently a number of cosmologists have questioned whether cosmic acceleration might in fact be an artifact of replacing the actual observed structure of the universe by a smooth featureless dust fluid in Einstein's equations (for a review see Buchert 2007). The specific solution to the averaging problem we investigate here (Wiltshire 2007a) realizes cosmic acceleration as an apparent effect that arises in the decoupling of bound systems from the global expansion of the universe. In particular, gradients in the kinetic energy of expansion, and more importantly, in the quasi-local energy associated with spatial curvature gradients between bound systems and a volume-average position in freely expanding space, can manifest themselves in a significant difference in clock rates between the two locations. This difference is negligible in the early universe when the assumption of homogeneity is valid, but becomes important after the transition to void dominance, making apparent acceleration a phenomenon registered by observers in galaxies at relatively late epochs.Galaxies and other objects...

Dark energy cosmologies with an equation of state parameter w less than −1 are often found to violate the null energy condition and show unstable behaviour. A solution to this problem may require the existence of a consistent effective theory that violates the null energy condition only momentarily and does not develop any instabilities or other pathological features for a late time cosmology. A model which incorporates a dynamical scalar field ϕ coupled to the quadratic Riemann invariant of the Gauss-Bonnet form is a viable proposal. Such an effective theory is shown to admit nonsingular cosmological evolutions for a wide range of scalar-Gauss-Bonnet coupling. We discuss the conditions for which our model yields observationally supported spectra of scalar and tensor fluctuations, under cosmological perturbations. The model can provide a reasonable explanation for the transition from matter dominance to dark energy regime and the late time cosmic acceleration, offering an interesting testing ground for investigations of the cosmological modified gravity.

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