General Second-Order Scalar-Tensor Theory and Self-Tuning

Charmousis, Christos; Copeland, Edmund J.; Padilla, Antonio; Saffin, Paul M.

doi:10.1103/physrevlett.108.051101

Cited by 441 publications

(569 citation statements)

References 23 publications

(28 reference statements)

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“…Consistency with solar system tests puts an upper bound on the largest phenomenologically-acceptable value of Λ that can be degravitated in this way, and unfortunately the answer is a measly meV 4 . A similar restriction was found in the self-tuning theories of [771][772][773].…”

Section: Degravitationmentioning

confidence: 66%

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: Degravitationmentioning

confidence: 66%

Beyond the cosmological standard model

et al. 2015

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“…Recently, there has been much interest in general models that attempt to contain a number of individual models [9][10][11][12][13][14][15]. One of the motivations for these approaches is to be able to construct model-independent constraints, which will hopefully save time and effort analyzing individual models.…”

Section: Introductionmentioning

confidence: 99%

A simplified approach to general scalar-tensor theories

Bloomfield¹

2013

J. Cosmol. Astropart. Phys.

113

163

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The most general covariant action describing gravity coupled to a scalar field with only second order equations of motion, Horndeski's theory (also known as "Generalized Galileons"), provides an all-encompassing model in which single scalar dark energy models may be constrained. However, the generality of the model makes it cumbersome to manipulate. In this paper, we demonstrate that when considering linear perturbations about a Friedmann-Robertson-Walker background, the theory is completely specified by only six functions of time, two of which are constrained by the background evolution. We utilise the ideas of the Effective Field Theory of Inflation/Dark Energy to explicitly construct these six functions of time in terms of the free functions appearing in Horndeski's theory. These results are used to investigate the behavior of the theory in the quasistatic approximation. We find that only four functions of time are required to completely specify the linear behavior of the theory in this limit, which can further be reduced if the background evolution is fixed. This presents a significantly reduced parameter space from the original presentation of Horndeski's theory, giving hope to the possibility of constraining the parameter space. This work provides a cross-check for previous work on linear perturbations in this theory, and also generalizes it to include spatial curvature.

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“…Constraints on the allowed cosmology of Galileon theories can be obtained from a wide variety of observations, unveiling a very rich phenomenology [12,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Here we consider for the first time the constraints that current and near future observations of gravitational waves can place on these theories.…”

Section: Introductionmentioning

confidence: 99%

The speed of Galileon gravity

Brax

Burrage

Davis

2016

J. Cosmol. Astropart. Phys.

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Abstract:We analyse the speed of gravitational waves in coupled Galileon models with an equation of state ω φ = −1 now and a ghost-free Minkowski limit. We find that the gravitational waves propagate much faster than the speed of light unless these models are small perturbations of cubic Galileons and the Galileon energy density is sub-dominant to a dominant cosmological constant. In this case, the binary pulsar bounds on the speed of gravitational waves can be satisfied and the equation of state can be close to -1 when the coupling to matter and the coefficient of the cubic term of the Galileon Lagrangian are related. This severely restricts the allowed cosmological behaviour of Galileon models and we are forced to conclude that Galileons with a stable Minkowski limit cannot account for the observed acceleration of the expansion of the universe on their own. Moreover any sub-dominant Galileon component of our universe must be dominated by the cubic term. For such models with gravitons propagating faster than the speed of light, the gravitons become potentially unstable and could decay into photon pairs. They could also emit photons by Cerenkov radiation. We show that the decay rate of such speedy gravitons into photons and the Cerenkov radiation are in fact negligible. Moreover the time delay between the gravitational signal and light emitted by explosive astrophysical events could serve as a confirmation that a modification of gravity acts on the largest scales of the Universe.

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General Second-Order Scalar-Tensor Theory and Self-Tuning

Cited by 441 publications

References 23 publications

Beyond the cosmological standard model

Beyond the cosmological standard model

A simplified approach to general scalar-tensor theories

The speed of Galileon gravity

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