On Backreaction in Newtonian cosmology

Buchert, Thomas

doi:10.1093/mnrasl/slx160

Cited by 36 publications

(40 citation statements)

References 26 publications

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“…Therefore, it is natural to assume that f (r) is singular as r → 0. 5 The problem of the influence of the global cosmological expansion on local dynamics is still being discussed [78] and is connected with another polemic issue, namely, back-reaction [79]. 6 Adding quantum mechanics, we have the universal mass density c 5 /(hG 2 ) ≃ 10 97 kg/m 3 (h is the Planck constant).…”

Section: Nonlinearity Chaos and Turbulence In Gravitational Clusteringmentioning

confidence: 99%

The Fractal Geometry of the Cosmic Web and Its Formation

Gaite

2019

Advances in Astronomy

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The cosmic web structure is studied with the concepts and methods of fractal geometry, employing the adhesion model of cosmological dynamics as a basic reference. The structures of matter clusters and cosmic voids in cosmological N-body simulations or the Sloan Digital Sky Survey are elucidated by means of multifractal geometry. A non-lacunar multifractal geometry can encompass three fundamental descriptions of the cosmic structure, namely, the web structure, hierarchical clustering, and halo distributions. Furthermore, it explains our present knowledge of cosmic voids. In this way, a unified theory of the large-scale structure of the universe seems to emerge. The multifractal spectrum that we obtain significantly differs from the one of the adhesion model and conforms better to the laws of gravity. The formation of the cosmic web is best modeled as a type of turbulent dynamics, generalizing the known methods of Burgers turbulence.The geometry of the cosmic web has been observed in galaxy surveys [12][13][14] and cosmological N-body simulations [15][16][17]. In addition, the fractal geometry of the large scale structure has been also studied in the distribution of galaxies [18][19][20] and in cosmological N-body simulations [21][22][23]. Recent N-body simulations have very good resolution and they clearly show both the morphological and self-similar aspects of the cosmic web. However, while there seems to be a consensus about the reality and importance of this type of structure, a comprehensive geometrical model is still missing. In particular, although our present knowledge supports a multifractal model, its spectrum of dimensions is only partially known, as will be discussed in this paper.We must also consider halo models [24], as models of large scale structure that are constructed from statistical rather than geometrical principles. In the basic halo model, the matter distribution is separated into a distribution within "halos", with given density profiles, and a distribution of halo centers in space. Halo models have gained popularity as models suitable for analyzing the results of cosmological N-body simulations and can even be employed for designing simulations [25]. Halo models are also employed for the study of the small scale problems of the Lambda-cold-dark-matter (LCDM) cosmology [26]. Remarkably, the basic halo model, with some modifications, can be combined with fractal models [27][28][29][30][31]. At any rate, halo models have questionable aspects and a careful analysis of the mass distribution within halos shows that it is too influenced by discreteness effects intrinsic to N-body simulations [31,32].We review in this paper a relevant portion of the efforts to understand the cosmic structure, with a bias towards methods of fractal geometry, as regards the description of the structure, and towards nonlinear methods of the theory of turbulence, as regards the formation of the structure. So this review does not have as broad a scope as, for example, Sahni and Coles' review of nonlinear gravitational ...

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Section: Nonlinearity Chaos and Turbulence In Gravitational Clusteringmentioning

confidence: 99%

The Fractal Geometry of the Cosmic Web and Its Formation

Gaite

2019

Advances in Astronomy

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“…the mean of an observable might not converge to that expected based on the background Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime. (Except when otherwise explicitly mentioned, cosmic backreaction [1][2][3][4] is not considered in the work presented here and accordingly a well-defined FLRW background is assumed to exist.) If such biases are not taken into account when interpreting observations, it can severely compromise parameter determinations in an era of precision cosmology and e.g.…”

Section: Introductionmentioning

confidence: 99%

Light path averages in spacetimes with nonvanishing average spatial curvature

Koksbang

2019

Phys. Rev. D

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Effects of inhomogeneities on observations have been vastly studied using both perturbative methods, N-body simulations and Swiss cheese solutions to the Einstein equations. In nearly all cases, such studied setups assume vanishing spatial background curvature. While a spatially flat Friedmann-Lemaitre-Robertson-Walker model is in accordance with observations, a non-vanishing curvature is not ruled out. It is therefore important to note that, as has been pointed out in the literature, 1 dimensional averages might not converge to volume averages in non-Euclidean space. If this is indeed the case, it will affect the interpretation of observations in spacetimes with non-vanishing average spatial curvature. This possibility is therefore studied here by computing the integrated expansion rate and shear, the accumulated density contrast, and fluctuations in the redshift-distance relation in Swiss cheese models with different background curvatures. It is found that differences in mean and dispersion of these quantities in the different models are small and naturally attributable to differences in background expansion rate and density contrasts. Thus, the study does not yield an indication that the relationship between 1 dimensional spatial averages and volume averages depends significantly on background curvature. arXiv:1909.00610v2 [astro-ph.CO]

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“…A background-free and fully relativistic approach shows that a physical cosmological model must account for the evolution of global properties of universe models due to the interaction of its average properties with fluctuations in matter and geometry. Whether cosmological backreaction can fully replace the need for dark energy and even dark matter is an open question that needs generic models whose architecture is not restricted as in Newtonian cosmology [21]. Since observations are interpreted within the standard model, the quantitative investigation of inhomogeneous models has to be accompanied by a re-interpretation of observational data in these new models to reach a conclusive answer.…”

Section: Where Do We Go From Here ?mentioning

confidence: 99%

Is Dark Energy Simulated by Structure Formation in the Universe ?

Buchert¹

2018

Proceedings of 2nd World Summit: Exploring the Dark Side of the Universe — PoS(EDSU2018)

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The standard model of cosmology assumes that the Universe can be described to hover around a homogeneous-isotropic solution of Einstein's general theory of relativity. This description needs (sometimes hidden) hypotheses that restrict the generality, and relaxing these restrictions is the headline of a new physical approach to cosmology that refurnishes the cosmological framework. Considering a homogeneous geometry as a template geometry for the in reality highly inhomogeneous Universe must be considered a strong idealization. Unveiling the limitations of the standard model opens the door to rich consequences of general relativity, giving rise to effective (i.e. spatially averaged) cosmological models that may even explain the longstanding problems of dark energy and dark matter. We explore in this talk the influence of structure formation on average properties of the Universe by discussing: (i) general thoughts on why considering average properties, on the key-issue of non-conserved curvature, and on the global gravitational instability of the standard model of cosmology; (ii) the general set of cosmological equations arising from averaging the scalar parts of Einstein's equations, the generic property of structure formation interacting with the average properties of the Universe in a scale-dependent way, and the description of cosmological backreaction in terms of an effective scalar field.Before we discuss a more general cosmological framework within which we can understand inhomogeneity effects affecting global properties of world models, so-called backreaction effects, we put our heads together on a couple of general thoughts that lie at the basis of this framework. 1 Why averaging ?Cosmology is built on models for the evolution of space. Within the four-dimensional framework of general relativity we need to split the equations into spatial variables that evolve in a timedirection. Having set up such a foliation of space-time, we may select one that admits a global cosmological time, which labels spatial hypersurfaces to define a cosmological model [1,5]. As we observe structures in the Universe we look along the light-cone implying the need to relate observables to spatial distributions, in order to interpret them within a cosmological model. Within the standard model of cosmology, spatial fluctuations are conceived to evolve on an assumed background geometry that belongs to the class of FLRW (Friedmann-Lemaître-Robertson-Walker) cosmologies being homogeneous-isotropic solutions of Einstein's equations. However, the description of fluctuations only makes sense with respect to their spatial average distribution and its evolution: employing the standard model we implicitly assume that it reliably describes the evolution of the universal average. However, by deriving the spatial average distribution, which is homogeneous by construction and may be large-scale isotropic to comply with constraints from the observable Universe, we cannot confirm this hypothesis: a priori, the average does, in general, not evolve ...

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On Backreaction in Newtonian cosmology

Cited by 36 publications

References 26 publications

The Fractal Geometry of the Cosmic Web and Its Formation

The Fractal Geometry of the Cosmic Web and Its Formation

Light path averages in spacetimes with nonvanishing average spatial curvature

Is Dark Energy Simulated by Structure Formation in the Universe ?

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