The null energy condition and its violation

Рубаков, Валерий А.

doi:10.3367/ufne.0184.201402b.0137

Cited by 218 publications

(210 citation statements)

References 177 publications

(252 reference statements)

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“…They have also been used to violate the null energy condition [537,[544][545][546][547][548][549][550][551][552][553]. Covariantizing the galileons for such applications is subtle, it requires introducing non-minimal couplings to curvature, which generically destroys the shift symmetry [554][555][556][557][558] (for a construction which couples galileons covariantly to massive gravity while retaining galilean symmetry, see [559][560][561]).…”

Section: The Vainshtein Mechanism: Galileonsmentioning

confidence: 99%

Beyond the cosmological standard model

et al. 2015

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After a decade and a half of research motivated by the accelerating universe, theory and experiment have a reached a certain level of maturity. The development of theoretical models beyond Λ or smooth dark energy, often called modified gravity, has led to broader insights into a path forward, and a host of observational and experimental tests have been developed. In this review we present the current state of the field and describe a framework for anticipating developments in the next decade. We identify the guiding principles for rigorous and consistent modifications of the standard model, and discuss the prospects for empirical tests.We begin by reviewing recent attempts to consistently modify Einstein gravity in the infrared, focusing on the notion that additional degrees of freedom introduced by the modification must "screen" themselves from local tests of gravity. We categorize screening mechanisms into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives of the field become important, and those for which second derivatives of the field are important. Examples of the first class, such as f (R) gravity, employ the familiar chameleon or symmetron mechanisms, whereas examples of the last class are galileon and massive gravity theories, employing the Vainshtein mechanism. In each case, we describe the theories as effective theories and discuss prospects for completion in a more fundamental theory. We describe experimental tests of each class of theories, summarizing laboratory and solar system tests and describing in some detail astrophysical and cosmological tests. Finally, we discuss prospects for future tests which will be sensitive to different signatures of new physics in the gravitational sector.The review is structured so that those parts that are more relevant to theorists vs. observers/experimentalists are clearly indicated, in the hope that this will serve as a useful reference for both audiences, as well as helping those interested in bridging the gap between them.

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Section: The Vainshtein Mechanism: Galileonsmentioning

confidence: 99%

Beyond the cosmological standard model

et al. 2015

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“…See [355] for a recent review. Ghosts can be avoided in certain set-ups by employing, for example, a ghost condensate (higher derivatives) [356,357], conformal galileons [230] (conformally invariant scalar field theories with particular higher-derivative interactions) [238] or DBI conformal galileons (a 3-brane moving in an anti-de Sitter bulk, AdS 5 ), used, for example, in the inflationary models of DBI Genesis [358].…”

Section: Nec Violationmentioning

confidence: 99%

A critical review of classical bouncing cosmologies

2015

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Given the proliferation of bouncing models in recent years, we gather and critically assess these proposals in a comprehensive review. The PLANCK data shows an unmistakably red, quasi scaleinvariant, purely adiabatic primordial power spectrum and no primary non-Gaussianities. While these observations are consistent with inflationary predictions, bouncing cosmologies aspire to provide an alternative framework to explain them. Such models face many problems, both of the purely theoretical kind, such as the necessity of violating the NEC and instabilities, and at the cosmological application level, as exemplified by the possible presence of shear. We provide a pedagogical introduction to these problems and also assess the fitness of different proposals with respect to the data. For example, many models predict a slightly blue spectrum and must be fine-tuned to generate a red spectral index; as a side effect, large non-Gaussianities often result.We highlight several promising attempts to violate the NEC without introducing dangerous instabilities at the classical and/or quantum level. If primordial gravitational waves are observed, certain bouncing cosmologies, such as the cyclic scenario, are in trouble, while others remain valid. We conclude that, while most bouncing cosmologies are far from providing an alternative to the inflationary paradigm, a handful of interesting proposals have surfaced, which warrant further research. The constraints and lessons learned as laid out in this review might guide future research.

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“…One of the most exciting endeavors in this area is to build nonsingular cosmological models with the field theories which can violate the Null Energy Condition (NEC) [20]. Usually the violation of NEC may lead to the ghost instability [21,22].…”

Section: Introductionmentioning

confidence: 99%

The Effective Field Theory of nonsingular cosmology

Cai

Wan

et al. 2017

J. High Energ. Phys.

149

150

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In this paper, we explore the nonsingular cosmology within the framework of the Effective Field Theory (EFT) of cosmological perturbations. Due to the recently proved no-go theorem, any nonsingular cosmological models based on the cubic Galileon suffer from pathologies. We show how the EFT could help us clarify the origin of the no-go theorem, and offer us solutions to break the no-go. Particularly, we point out that the gradient instability can be removed by using some spatial derivative operators in EFT. Based on the EFT description, we obtain a realistic healthy nonsingular cosmological model, and show the perturbation spectrum can be consistent with the observations.

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The null energy condition and its violation

Cited by 218 publications

References 177 publications

Beyond the cosmological standard model

Beyond the cosmological standard model

A critical review of classical bouncing cosmologies

The Effective Field Theory of nonsingular cosmology

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