From time to timescape - Einstein's unfinished revolution

I argue that Einstein overlooked an important aspect of the relativity of time in never quite realizing his quest to embody Mach's principle in his theory of gravity. As a step towards that goal, I broaden the Strong Equivalence Principle to a new principle of physics, the Cosmological Equivalence Principle, to account for the role of the evolving average regional density of the universe in the synchronisation of clocks and the relative calibration of inertial frames. In a universe dominated by voids of the size observed in large-scale structure surveys, the density contrasts of expanding regions are strong enough that a relative deceleration of the background between voids and the environment of galaxies, typically of order 10^{-10} m/s^2, must be accounted for. As a result one finds a universe whose present age varies by billions of years according to the position of the observer: a timescape. This model universe is observationally viable: it passes three critical independent tests, and makes additional predictions. Dark energy is revealed as a mis-identification of gravitational energy gradients and the resulting variance in clock rates. Understanding the biggest mystery in cosmology therefore involves a paradigm shift, but in an unexpected direction: the conceptual understanding of time and energy in Einstein's own theory is incomplete.Comment: 13 pages, 3 figures; A runner-up in the 2008 FQXi Essay Contest on the Nature of Time; Int. J. Mod. Phys. D 18, in pres

Gravitational energy as dark energy: Towards concordance cosmology without Lambda

I briefly outline a new physical interpretation to the average cosmological parameters for an inhomogeneous universe with backreaction. The variance in local geometry and gravitational energy between ideal isotropic observers in bound structures and isotropic observers at the volume average location in voids plays a crucial role. Fits of a model universe to observational data suggest the possibility of a new concordance cosmology, in which dark energy is revealed as a mis-identification of gravitational energy gradients that become important when voids grow at late epochs.Comment: 8 pages, 1 figure; in E. Pecontal, T. Buchert, Ph. Di Stefano and Y. Copin (eds), "Dark Energy and Dark Matter: Observations, Experiments and Theories", Proceedings, Lyon, 7-11 July, 200

Comment on "Hubble flow variations as a test for inhomogeneous cosmology"

Saulder et al [2019, A&A, 622, A83; arXiv:1811.11976] have performed a novel observational test of the local expansion of the Universe for the standard cosmology as compared to an alternative model with differential cosmic expansion. Their analysis employs mock galaxy samples from the Millennium Simulation, a Newtonian $N$-body simulation on a $\Lambda$CDM background. For the differential expansion case the simulation has been deformed in an attempt to incorporate features of a particular inhomogeneous cosmology: the timescape model. It is shown that key geometrical features of the timescape cosmology have been omitted in this rescaling. Consequently, the differential expansion model tested by Saulder et al (2019) cannot be considered to approximate the timescape cosmology.Comment: 4 pages; v2: small changes to match published version in A&

Dilaton black holes with a cosmological term

The properties of static spherically symmetric black holes, which carry electric and magnetic charges, and which are coupled to the dilaton in the presence of a cosmological term (Liouville-type potential, or cosmological constant) are reviewed.Comment: 20 pages, phyzzx, v2 contains expanded introduction, additional references etc to coincide with published versio

Viable inhomogeneous model universe without dark energy from primordial inflation

A new model of the observed universe, using solutions to the full Einstein equations, is developed from the hypothesis that our observable universe is an underdense bubble, with an internally inhomogeneous fractal bubble distribution of bound matter systems, in a spatially flat bulk universe. It is argued on the basis of primordial inflation and resulting structure formation, that the clocks of the isotropic observers in average galaxies coincide with clocks defined by the true surfaces of matter homogeneity of the bulk universe, rather than the comoving clocks at average spatial positions in the underdense bubble geometry, which are in voids. This understanding requires a systematic reanalysis of all observed quantities in cosmology. I begin such a reanalysis by giving a model of the average geometry of the universe, which depends on two measured parameters: the present matter density parameter, Omega_m, and the Hubble constant, H_0. The observable universe is not accelerating. Nonetheless, inferred luminosity distances are larger than naively expected, in accord with the evidence of distant type Ia supernovae. The predicted age of the universe is 15.3 +/-0.7 Gyr. The expansion age is larger than in competing models, and may account for observed structure formation at large redshifts.Comment: 4 pages, aastex, emualteapj5.sty. v5: Complete overhaul of notation and presentation to improve clarity. Corrected volume factor increases age of univers

Gravitational energy as dark energy: Average observational quantities

In the timescape scenario cosmic acceleration is understand as an apparent effect, due to gravitational energy gradients that grow when spatial curvature gradients become significant with the nonlinear growth of cosmic structure. This affects the calibratation of local geometry to the solutions of the volume-average evolution equations corrected by backreaction. In this paper I discuss recent work on defining observational tests for average geometric quantities which can distinguish the timescape model from a cosmological constant or other models of dark energy.Comment: 10 pages, 7 figures; submitted to the Proceedings of the Invisible Universe Conference, Paris, 29 June - 3 July, 2009; J.-M. Alimi (ed), AIP Conf. Proc., to appea