Cosmological perturbations and structure formation in nonlocal infrared modifications of general relativity

Dirian, Yves; Foffa, Stefano; Khosravi, Nima; Kunz, M.; Maggiore, Michele

doi:10.1088/1475-7516/2014/06/033

Cited by 91 publications

(246 citation statements)

References 103 publications

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“…The idea of degravitation or screening the vacuum energy may also be possible to implement within theories of non-local gravity, potentially resolving the old CC problem [86]. Although no consistent theory of non-local gravity that solves the old CC problem has been constructed so far, there are classes that can provide non-Λ self-accelerating solutions, with interesting phenomenologies [146,147,148,149] that can be observationally distinguishable from GR. There seem to be at least two obstacles that need to be resolved in order to have further progress with these theories, however: 1) there is no systematic way of constructing a model in an EFT framework, and 2) the generic theory seems to suffer from instabilities [150].…”

Section: Gravity In Cosmologymentioning

confidence: 99%

“…2.4). For example, a series of papers [189,148,149,190,49] has explored the viability -both theoretical and observational -of introducing non-local terms to the gravitational field equations, e.g. ∼ m 2 g µν −1 R T , where −1 is the inverse d'Alembertian, and R is the Ricci scalar (see also Refs.…”

Section: Gravity In Cosmologymentioning

confidence: 99%

“…We choose to defend this culture, however -theories that experience a rapid rise in popularity generally do so because they have feature(s) of particular appeal. They may incorporate a natural screening mechanism or be able to fit the expansion history of the Universe with the same number of free parameters as ΛCDM [149], for example. The fact that they are subsequently found to be non-viable is testament to the energy of this research community, and the machinery we have in place to rapidly take theories from their initial mathematical development to observational constraints.…”

Section: Gravity In Cosmologymentioning

confidence: 99%

See 2 more Smart Citations

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

441

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: Gravity In Cosmologymentioning

confidence: 99%

Section: Gravity In Cosmologymentioning

confidence: 99%

Section: Gravity In Cosmologymentioning

confidence: 99%

See 1 more Smart Citation

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

441

210

View full text Add to dashboard Cite

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“…. 2 Incidentally, since we consider no homogeneous solution in the definition of −1 , we have that −1 0 = 0.…”

Section: Introductionmentioning

confidence: 99%

Stability analysis and future singularity of them²R□^-2Rmodel of non-local gravity

Dirian¹,

Mitsou²

2014

J. Cosmol. Astropart. Phys.

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Abstract. We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term ∼ m 2 R −2 R to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding the analytic solution for the late-time behaviour of the scale factor, which leads to a big rip singularity, we argue that the linear perturbations are bounded in the future because of the domination of Hubble friction. In particular, this effect damps the scalar ghost perturbations which were responsible for destabilizing Minkowski space-time. Thus, the model remains phenomenologically viable.

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“…Many aspects of nonlocal gravity models have been considered, see e.g. [20,16,17,59,18,36] and references therein.…”

Section: Nonlocal Modified Gravitymentioning

confidence: 99%

On Nonlocal Modified Gravity and Its Cosmological Solutions

Dimitrijevic

Драгович

Stanković

et al. 2016

Springer Proceedings in Mathematics &Amp; Statistics

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During hundred years of General Relativity (GR), many significant gravitational phenomena have been predicted and discovered. General Relativity is still the best theory of gravity. Nevertheless, some (quantum) theoretical and (astrophysical and cosmological) phenomenological difficulties of modern gravity have been motivation to search more general theory of gravity than GR. As a result, many modifications of GR have been considered. One of promising recent investigations is Nonlocal Modified Gravity. In this article we present a brief review of some nonlocal gravity models with their cosmological solutions, in which nonlocality is expressed by an analytic function of the d'Alembert-Beltrami operator . Some new results are also presented.

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Cosmological perturbations and structure formation in nonlocal infrared modifications of general relativity

Cited by 91 publications

References 103 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

Stability analysis and future singularity of them²R□^-2Rmodel of non-local gravity

On Nonlocal Modified Gravity and Its Cosmological Solutions

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Cosmological perturbations and structure formation in nonlocal infrared modifications of general relativity

Cited by 91 publications

References 103 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

Stability analysis and future singularity of them2R□-2Rmodel of non-local gravity

On Nonlocal Modified Gravity and Its Cosmological Solutions

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Product

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Stability analysis and future singularity of them²R□^-2Rmodel of non-local gravity