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20
Modified Gravity and Cosmology
, 2012
"... In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, ScalarTensor, EinsteinAether, and Bimetric theories, as well as TeVeS, f(R), general higherorder theo ..."
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In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, ScalarTensor, EinsteinAether, and Bimetric theories, as well as TeVeS, f(R), general higherorder theories, HořavaLifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including KaluzaKlein, RandallSundrum, DGP, and higher codimension braneworlds. We also review attempts to construct a Parameterised PostFriedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a selfcontained, comprehensive and uptodate introduction to the subject as a whole.
Relativistic Quantum Gravity at a Lifshitz Point
, 2009
"... We show that the Hoˇrava theory for the completion of General Relativity at UV scales can be interpreted as a gauge fixed TensorVector theory, and it can be extended to an invariant theory under the full group of fourdimensional diffeomorphisms. In this respect, although being fully relativistic, ..."
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We show that the Hoˇrava theory for the completion of General Relativity at UV scales can be interpreted as a gauge fixed TensorVector theory, and it can be extended to an invariant theory under the full group of fourdimensional diffeomorphisms. In this respect, although being fully relativistic, it results to be locally anisotropic in the timelike and spacelike directions defined by a family of irrotational observers. We show that this theory propagates generically three degrees of freedom: two of them are related to the fourdimensional diffeomorphism invariant graviton (the metric) and one is related to a propagating scalar mode of the vector. Finally, we note that in the present formulation, matter can be consistently coupled to gravity.
Caustic avoidance in HoravaLifshitz gravity
, 2009
"... There are at least four versions of HoravaLishitz gravity in the literature. We consider the version without the detailed balance condition with the projectability condition and address one aspect of the theory: avoidance of caustics for constant time hypersurfaces. We show that there is no caustic ..."
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There are at least four versions of HoravaLishitz gravity in the literature. We consider the version without the detailed balance condition with the projectability condition and address one aspect of the theory: avoidance of caustics for constant time hypersurfaces. We show that there is no caustic with plane symmetry in the absence of matter source if λ ̸ = 1. If λ = 1 is a stable IR fixed point of the renormalization group flow then λ is expected to deviate from 1 near wouldbe caustics, where the extrinsic curvature increases and highenergy corrections become important. Therefore, the absence of caustics with λ ̸ = 1 implies that caustics cannot form with this symmetry in the absence of matter source. We argue that inclusion of matter source will not change the conclusion. We also argue that caustics with codimension higher than one will not form because of repulsive gravity generated by nonlinear higher curvature terms. These arguments support our conjecture that there is no caustic for constant time hypersurfaces. Finally,
HoˇravaLifshitz f(R) Gravity
, 907
"... Preprint typeset in JHEP style HYPER VERSION hepth/0907.3566 ..."
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Preprint typeset in JHEP style HYPER VERSION hepth/0907.3566
CERNPHTH/2009087 On the Extra Mode and Inconsistency of Hoˇrava Gravity
, 906
"... We address the consistency of Hoˇrava’s proposal for a theory of quantum gravity from the lowenergy perspective. We uncover the additional scalar degree of freedom arising from the explicit breaking of the general covariance and study its properties. The analysis is performed both in the original f ..."
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We address the consistency of Hoˇrava’s proposal for a theory of quantum gravity from the lowenergy perspective. We uncover the additional scalar degree of freedom arising from the explicit breaking of the general covariance and study its properties. The analysis is performed both in the original formulation of the theory and in the Stückelberg picture. A peculiarity of the new mode is that it satisfies an equation of motion that is of first order in time derivatives. At linear level the mode is manifest only around spatially inhomogeneous and timedependent backgrounds. We find two serious problems associated with this mode. First, the mode develops very fast exponential instabilities at short distances. Second, it becomes strongly coupled at an extremely low cutoff scale. We also discuss the “projectable ” version of Hoˇrava’s proposal and argue that this version can be understood as a certain limit of the ghost condensate model. The theory is still problematic since the additional field generically forms caustics and, again, has a very low strong coupling scale. We clarify some subtleties that arise in the application of the Stückelberg formalism to Hoˇrava’s model due to its nonrelativistic nature. 1 1 Introduction and
Perturbative instabilities in Hoˇrava gravity
, 907
"... Abstract: We investigate the scalar and tensor perturbations in Hoˇrava gravity, with and without detailed balance, around a flat background. Once both types of perturbations are taken into account, it is revealed that the theory is plagued by ghostlike scalar instabilities in the range of paramete ..."
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Abstract: We investigate the scalar and tensor perturbations in Hoˇrava gravity, with and without detailed balance, around a flat background. Once both types of perturbations are taken into account, it is revealed that the theory is plagued by ghostlike scalar instabilities in the range of parameters which would render it powercounting renormalizable, that cannot be overcome by simple tricks such as analytic continuation. Implementing a consistent flow between the UV and IR limits seems thus more challenging than initially presumed, regardless of whether the theory approaches General Relativity at low energies or not. Even in the phenomenologically viable parameter space, the tensor sector leads to additional potential problems, such