« Signe, culture et lien social à l'ère des réseaux », Degrés, n° 126-127, 2007

Célérier, Laure

doi:10.4000/lectures.470

Cited by 9 publications

(10 citation statements)

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“…This approach offers the possibility of solving the cosmological constant problems within the framework of general relativity and keeps the introduction of new physics to a minimum. 2 Several different ways in which inhomogenities could potentially mimic dark energy have been considered in the literature -for reviews see Celerier (2007); Buchert (2008); Enqvist (2008). In an inhomogeneous universe averaged quantities satisfy modified Friedmann equations which contain extra terms corresponding to 'backreaction' since the operations of spatial averaging and time evolution do not commute (Buchert 2000).…”

Section: Introductionmentioning

confidence: 99%

Constraints on large-scale inhomogeneities fromWMAP5 and SDSS: confrontation with recent observations

Hunt¹,

Sarkar²

2010

Monthly Notices of the Royal Astronomical Society

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Measurements of the Type Ia supernovae Hubble diagram which suggest that the Universe is accelerating due to the effect of dark energy may be biased because we are located in a 200-300 Mpc underdense 'void' which is expanding 20-30 per cent faster than the average rate. With the smaller global Hubble parameter, the Wilkinson Microwave Anisotropy Probe 5 data on cosmic microwave background (CMB) anisotropies can be fitted without requiring dark energy if there is some excess power in the spectrum of primordial perturbations on 100 Mpc scales. The Sloan Digital Sky Survey (SDSS) data on galaxy clustering can also be fitted if there is a small component of hot dark matter in the form of 0.5 eV mass neutrinos. We show however that if the primordial fluctuations are Gaussian, the expected variance of the Hubble parameter and the matter density are far too small to allow such a large local void. Nevertheless, many such large voids have been identified in the SDSS Luminous Red Galaxy survey in a search for the late integrated Sachs-Wolfe effect due to dark energy. The observed CMB temperature decrements imply that they are nearly empty, thus these real voids too are in gross conflict with the concordance cold dark matter model. The recently observed high peculiar velocity flow presents another challenge for the model. Therefore, whether a large local void actually exists must be tested through observations and cannot be dismissed a priori.

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Section: Introductionmentioning

confidence: 99%

Constraints on large-scale inhomogeneities fromWMAP5 and SDSS: confrontation with recent observations

Hunt¹,

Sarkar²

2010

Monthly Notices of the Royal Astronomical Society

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“…In one approach exact inhomogeneous cosmological models can be utilised. Indeed, it has been shown that the Lemaître-Tolman-Bondi (LTB) solution can be used to fit the observed data without the need of dark energy, although it may be necessary to place the observer at a preferred location [2].…”

Section: Introductionmentioning

confidence: 99%

Averaging in cosmological models using scalars

Coley¹

2010

Class. Quantum Grav.

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The averaging problem in cosmology is of considerable importance for the correct interpretation of cosmological data. A rigorous mathematical definition of averaging in a cosmological model is necessary. In general, a spacetime is completely characterized by its scalar curvature invariants, and this suggests a particular spacetime averaging scheme based entirely on scalars. We clearly identify the problems of averaging in a cosmological model. We then present a precise definition of a cosmological model, and based upon this definition, we propose an averaging scheme in terms of scalar curvature invariants. This scheme is illustrated in a simple static spherically symmetric perfect fluid cosmological spacetime, where the averaging scales are clearly identified.

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“…Célérier [23] considered a series expansion on the past null cone of an LTB model, and showed that the supernova data can be reproduced. Recently Célérier reviewed [24,25] the need for inhomogeneous models in relation to observations, concluding that the standard methods of fitting a homogeneous universe to the data are open to significant systematic errors, and noting that fitting observations with inhomogeneous models is a significant challenge. Bishop and Haines [26] approached the Kristian and Sachs program as a time-reversed characteristic initial value problem, but they didn't solve the problem of the maximum in the diameter distance, and they only tested their numerical code with Einstein-de Sitter data.…”

Section: Introductionmentioning

confidence: 99%

Determining the metric of the Cosmos: Stability, accuracy, and consistency

McClure

Hellaby

2008

Phys. Rev. D

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The ultimate application of Einstein's field equations is to empirically determine the geometry of the Universe from its matter content, rather than simply assuming the Universe can be represented by a homogeneous model on all scales. Choosing a Lemaître-Tolman-Bondi model as the most convenient inhomogeneous model for the early stages of development, a data reduction procedure was recently validated using perfect test data. Here, we simulate observational uncertainties and improve the previous numerical scheme to ensure that it will be usable with real data as soon as observational surveys are sufficiently deep and complete. Two regions require special treatment-the origin and the maximum in the areal radius. To minimize numerical errors near the origin, we use a Lemaître-Tolman-Bondi series expansion to provide the initial values for integrating the differential equations. We also use an improved method to match the numerical integration to the series expansion that bridges the region near the maximum in the areal radius. Because the mass enclosed within the maximum obeys a specific relationship, we show that it is possible to correct for a fixed systematic error in either the distance scale or the redshift-space mass density, such that the integrated values are consistent with the data at the maximum.

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« Signe, culture et lien social à l'ère des réseaux », Degrés, n° 126-127, 2007

Cited by 9 publications

References 0 publications

Constraints on large-scale inhomogeneities fromWMAP5 and SDSS: confrontation with recent observations

Constraints on large-scale inhomogeneities fromWMAP5 and SDSS: confrontation with recent observations

Averaging in cosmological models using scalars

Determining the metric of the Cosmos: Stability, accuracy, and consistency

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