Light propagation in statistically homogeneous and isotropic universes with general matter content

Räsänen, Syksy

doi:10.1088/1475-7516/2010/03/018

Cited by 91 publications

(222 citation statements)

References 222 publications

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“…However, the Universe is not FLRW -at best, it is statistically isotropic and homogeneous on large enough scales. Bridging the gap between the exact result and the real Universe entails a set of unresolved complexities in cosmology [434,435]. How do we perform covariant averages in GR [436,437,438]?…”

Section: Testing the Foundational Assumptions Of λCdm Plenary Speakermentioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

459

210

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Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.Keywords: cosmology -dark energy -cosmological constant problem -modified gravitydark matter -early universe Cosmology has been both blessed and cursed by the establishment of a standard model: ΛCDM. On the one hand, the model has turned out to be extremely predictive, explanatory, and observationally robust, providing us with a substantial understanding of the formation of large-scale structure, the state of the early Universe, and the cosmic abundance of different types of matter and energy. It has also survived an impressive battery of precision observational tests -anomalies are few and far between, and their significance is contentious where they do arise -and its predictions are continually being vindicated through the discovery of new effects (B-mode polarization [1] and lensing [2,3] of the cosmic microwave background (CMB), and the kinetic Sunyaev-Zel'dovich effect [4] being some recent examples). These are the hallmarks of a good and valuable physical theory.On the other hand, the model suffers from profound theoretical difficulties. The two largest contributions to the energy content at late times -cold dark matter (CDM) and the cosmological constant (Λ) -have entirely mysterious physical origins. CDM has so far evaded direct detection by laboratory experiments, and so the particle field responsible for it -presumably a manifestation of "beyond the standard model" particle physics -is unknown. Curious discrepancies also appear to exist between the predicted clustering properties of CDM on small scales and observations. The cosmological constant is even more puzzling, giving rise to quite simply the biggest problem in all of fundamental physics: the question of why Λ appears to take such an unnatural value [5,6,7]. Inflation, the theory of the very early Universe, has also been criticized for being fine-tuned and under-predictive [8], and appears to leave many problems either unsolved or fundamentally unresolvable. These problems are indicative of a crisis.From January 14th-17th 2015, we held a conference in Oslo, Norway to surve...

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Section: Testing the Foundational Assumptions Of λCdm Plenary Speakermentioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

459

210

View full text Add to dashboard Cite

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“…In the recent years the problem has attracted attention of the researchers in the context of cosmology [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Obviously in the cosmological setting we assume the existence of a homogeneity scale, which may be as large as 2000Mpc [16], and a complicated, multiscale structure below that.…”

Section: Introductionmentioning

confidence: 99%

Backreaction and continuum limit in a closed universe filled with black holes

Korzyński

2014

Class. Quantum Grav.

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Abstract. We discuss the continuum limit of the initial data for a vacuum, closed cosmological model with black holes as the only sources of the gravitational field. The model we consider is an exact solution of the constraint equations and represents a vacuum universe with a number of black holes placed on a spatial slice of S 3 topology considered at the moment of its largest expansion when the black holes are momentary at rest. We explain how and under what conditions the FLRW metric arises as the continuum limit when the number of black holes contained in the model goes to infinity. We also discuss the relation between the effective cosmological parameters of the model, inferred from the large scale geometry of the spacetime, and the masses of individual black holes. In particular, we prove an estimate for the difference between the total effective mass of the system and the sum of the masses of all black holes, thus quantifying the effects of the inhomogeneities in the matter distribution or the cosmological backreaction.

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“…Further extensions that deal with general hypersurfaces tilted with respect to the fluid flow have been discussed by various authors [47]- [51].…”

Section: Spatial Averagesmentioning

confidence: 99%

What is dust?—Physical foundations of the averaging problem in cosmology

Wiltshire¹

2011

Class. Quantum Grav.

108

125

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Abstract. The problems of coarse-graining and averaging of inhomogeneous cosmologies, and their backreaction on average cosmic evolution, are reviewed from a physical viewpoint. A particular focus is placed on comparing different notions of average spatial homogeneity, and on the interpretation of observational results. Among the physical questions we consider are: the nature of an average Copernican principle, the role of Mach's principle, the issue of quasilocal gravitational energy and the different roles of spacetime, spatial and null cone averages. The observational interpretation of the timescape scenario is compared to other approaches to cosmological averaging, and outstanding questions are discussed.PACS numbers: 04.20.Cv, 98.80.Jk, 98.80.Es

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Light propagation in statistically homogeneous and isotropic universes with general matter content

Cited by 91 publications

References 222 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

Backreaction and continuum limit in a closed universe filled with black holes

What is dust?—Physical foundations of the averaging problem in cosmology

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