Showing papers on "Gravitation published in 2010"
TL;DR: In this article, the authors present a review of the most important aspects of the different classes of modified gravity theories, including higher-order curvature invariants and metric affine.
Abstract: Modified gravity theories have received increased attention lately due to combined motivation coming from high-energy physics, cosmology, and astrophysics. Among numerous alternatives to Einstein's theory of gravity, theories that include higher-order curvature invariants, and specifically the particular class of $f(R)$ theories, have a long history. In the last five years there has been a new stimulus for their study, leading to a number of interesting results. Here $f(R)$ theories of gravity are reviewed in an attempt to comprehensively present their most important aspects and cover the largest possible portion of the relevant literature. All known formalisms are presented---metric, Palatini, and metric affine---and the following topics are discussed: motivation; actions, field equations, and theoretical aspects; equivalence with other theories; cosmological aspects and constraints; viability criteria; and astrophysical applications.
4,027 citations
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TL;DR: Various applications of f(R) theories to cosmology and gravity — such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds are reviewed.
Abstract: Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.
3,375 citations
TL;DR: In this paper, the structure and cosmological properties of modified theories, including traditional $F(R)$ and Hořava-Lifshitz$ gravity, scalar-tensor theory, string-inspired and Gauss-Bonnet theory, non-local gravity, nonminimally coupled models, and power-counting renormalizable covariant gravity are discussed.
Abstract: Classical generalization of general relativity is considered as gravitational alternative for unified description of the early-time inflation with late-time cosmic acceleration. The structure and cosmological properties of number of modified theories, including traditional $F(R)$ and Hořava-Lifshitz $F(R)$ gravity, scalar-tensor theory, string-inspired and Gauss-Bonnet theory, non-local gravity, non-minimally coupled models, and power-counting renormalizable covariant gravity are discussed. Different representations and relations between such theories are investigated. It is shown that some versions of above theories may be consistent with local tests and may provide qualitatively reasonable unified description of inflation with dark energy epoch. The cosmological reconstruction of different modified gravities is made in great detail. It is demonstrated that eventually any given universe evolution may be reconstructed for the theories under consideration: the explicit reconstruction is applied to accelerating spatially-flat FRW universe. Special attention is paid to Lagrange multiplier constrained and conventional $F(R)$ gravities, for last theory the effective $\Lambda$CDM era and phantom-divide crossing acceleration are obtained. The occurrence of Big Rip and other finite-time future singularities in modified gravity is reviewed as well as its curing via the addition of higher-derivative gravitational invariants.
2,031 citations
01 Mar 2010
TL;DR: In this paper, Weinberg et al. present a survey of the history of the field of quantum physics and its application to the theory of the Kerr metric and its peturbations.
Abstract: List of contributors Preface 1. An introductory survey S. W. Hawking and W. Israel 2. The confrontation between gravitation theory and experiment C. M. Will 3. Gravitational-radiation experiments D. H. Douglass and V. B. Braginsky 4. The initial value problem and the dynamical formulation of general relativity A. E. Fischer and J. E. Marsden 5. Global structure of spacetimes R. Geroch and G. T. Horowitz 6. The general theory of the mechanical, electromagnetic and thermodynamic properties of black holes B. Carter 7. An introduction to the theory of the Kerr metric and its peturbations S. Chandrasekhar 8. Black hole astrophysics R. D. Blandford and K. S. Thorne 9. The big bang cosmology - enigmas and nostrums R. H. Dicke and P. J. E. Peebles 10. Cosmology and the early universe Ya B. Zel'dovitch 11. Anisotropic and inhomogeneous relativistic cosmologies M. A. H. MacCallum 12. Singularities and time-asymmetry R. Penrose 13. Quantum field theory in curved spacetime G. W. Gibbons 14. Quantum gravity: the new synthesis B. S. DeWitt 15. The path-integral approach to quantum gravity S. W. Hawking 16. Ultraviolet divergences in quantum theories of gravitation S. Weinberg References Index.
1,911 citations
TL;DR: It is conjecture that this duality persists to all quantum loop orders and can thus be used to obtain multiloop gravity amplitudes easily from gauge-theory ones.
Abstract: In a previous paper we observed that (classical) tree-level gauge-theory amplitudes can be rearranged to display a duality between color and kinematics. Once this is imposed, gravity amplitudes are obtained using two copies of gauge-theory diagram numerators. Here we conjecture that this duality persists to all quantum loop orders and can thus be used to obtain multiloop gravity amplitudes easily from gauge-theory ones. As a nontrivial test, we show that the three-loop four-point amplitude of N=4 super-Yang-Mills theory can be arranged into a form satisfying the duality, and by taking double copies of the diagram numerators we obtain the corresponding amplitude of N=8 supergravity. We also remark on a nonsupersymmetric two-loop test based on pure Yang-Mills theory resulting in gravity coupled to an antisymmetric tensor and dilaton.
1,046 citations
TL;DR: In this paper, an extension of teleparallel, or torsion scalar T, gravity to f(T) theories is considered, where the field equations are naturally second order, avoiding pathologies.
Abstract: Spacetime curvature plays the primary role in general relativity but Einstein later considered a theory where torsion was the central quantity. Just as the Einstein-Hilbert action in the Ricci curvature scalar R can be generalized to f(R) gravity, we consider extensions of teleparallel, or torsion scalar T, gravity to f(T) theories. The field equations are naturally second order, avoiding pathologies, and can give rise to cosmic acceleration with unique features.
982 citations
Book•
03 Nov 2010
TL;DR: The Noether symmetry approach to f(R) gravity has been proposed in this article, where the weak-field limit of metric f(r) gravity is shown to be weak.
Abstract: 1. Extended gravity: a primer.- 2. Mathematical Tools.- 3. The landscape beyond Einstein gravity.- 4. Spherical symmetry.- 5. Weak-field limit.- 6. Qualitative analysis and exact solutions in cosmology.- 7. Cosmology.- 8. From the early to the present universe.- A. Physical constants and astrophysical and cosmological parameters.- B. The Noether symmetry approach to f(R) gravity.- C. The weak-field limit of metric f(R) gravity.- References.- Index.
857 citations
TL;DR: The fact that one can associate thermodynamic properties with horizons brings together principles of quantum theory, gravitation and thermodynamics and possibly offers a window to the nature of quantum geometry as mentioned in this paper.
Abstract: The fact that one can associate thermodynamic properties with horizons brings together principles of quantum theory, gravitation and thermodynamics and possibly offers a window to the nature of quantum geometry. This review discusses certain aspects of this topic, concentrating on new insights gained from some recent work. After a brief introduction of the overall perspective, sections 2 and 3 provide the pedagogical background on the geometrical features of bifurcation horizons, path integral derivation of horizon temperature, black hole evaporation, structure of Lanczos-Lovelock models, the concept of Noether charge and its relation to horizon entropy. Section 4 discusses several conceptual issues introduced by the existence of temperature and entropy of the horizons. In section 5 we take up the connection between horizon thermodynamics and gravitational dynamics and describe several peculiar features which have no simple interpretation in the conventional approach. The next two sections describe the recent progress achieved in an alternative perspective of gravity. In section 6 we provide a thermodynamic interpretation of the field equations of gravity in any diffeomorphism invariant theory and in section 7 we obtain the field equations of gravity from an entropy maximization principle. The last section provides a summary.
835 citations
TL;DR: A screening mechanism that allows a scalar field to mediate a long-range force of gravitational strength in the cosmos while satisfying local tests of gravity and predicting deviations from general relativity in the solar system that are within reach of next-generation experiments, as well as astrophysically observable violations of the equivalence principle.
Abstract: We present a screening mechanism that allows a scalar field to mediate a long-range ($\ensuremath{\sim}\mathrm{Mpc}$) force of gravitational strength in the cosmos while satisfying local tests of gravity. The mechanism hinges on local symmetry restoration in the presence of matter. In regions of sufficiently high matter density, the field is drawn towards $\ensuremath{\phi}=0$ where its coupling to matter vanishes and the $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{-}\ensuremath{\phi}$ symmetry is restored. In regions of low density, however, the symmetry is spontaneously broken, and the field couples to matter with gravitational strength. We predict deviations from general relativity in the solar system that are within reach of next-generation experiments, as well as astrophysically observable violations of the equivalence principle. The model can be distinguished experimentally from Brans-Dicke gravity, chameleon theories and brane-world modifications of gravity.
650 citations
[...]
TL;DR: In this paper, the authors review various applications of f(R) theories to cosmology and gravity, such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds.
Abstract: Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.
524 citations
TL;DR: There is a unique nonminimal derivative coupling of the standard model Higgs boson to gravity such that it propagates no more degrees of freedom than general relativity sourced by a scalar field, reproduces a successful inflating background within the standardmodel Higgs parameters, and finally does not suffer from dangerous quantum corrections.
Abstract: In this Letter we show that there is a unique nonminimal derivative coupling of the standard model Higgs boson to gravity such that it propagates no more degrees of freedom than general relativity sourced by a scalar field, reproduces a successful inflating background within the standard model Higgs parameters, and finally does not suffer from dangerous quantum corrections.
TL;DR: In a recent review as discussed by the authors, the lower bound on the photon Compton wavelength has been improved by four orders of magnitude, to about one astronomical unit, and rapid current progress in astronomy makes further advance likely.
Abstract: Efforts to place limits on deviations from canonical formulations of electromagnetism and gravity have probed length scales increasing dramatically over time. Historically, these studies have passed through three stages: (1) testing the power in the inverse-square laws of Newton and Coulomb, (2) seeking a nonzero value for the rest mass of photon or graviton, and (3) considering more degrees of freedom, allowing mass while preserving explicit gauge or general-coordinate invariance. Since the previous review the lower limit on the photon Compton wavelength has improved by four orders of magnitude, to about one astronomical unit, and rapid current progress in astronomy makes further advance likely. For gravity there have been vigorous debates about even the concept of graviton rest mass. Meanwhile there are striking observations of astronomical motions that do not fit Einstein gravity with visible sources. ''Cold dark matter'' (slow, invisible classical particles) fits well at large scales. ''Modified Newtonian dynamics'' provides the best phenomenology at galactic scales. Satisfying this phenomenology is a requirement if dark matter, perhaps as invisible classical fields, could be correct here too. ''Dark energy''might be explained by a graviton-mass-like effect, with associated Compton wavelength comparable to the radius of the visible universe. Significant mass limitsmore » are summarized in a table.« less
TL;DR: In this article, the authors studied the properties of the holographic CFT dual to Gauss-Bonnet gravity in general D(≥ 5) dimensions and established the AdS/CFT dictionary and in particular related the couplings of the gravitational theory to the universal couplings arising in correlators of the stress tensor of the dual CFT.
Abstract: We study the properties of the holographic CFT dual to Gauss-Bonnet gravity in general D(≥ 5) dimensions. We establish the AdS/CFT dictionary and in particular relate the couplings of the gravitational theory to the universal couplings arising in correlators of the stress tensor of the dual CFT. This allows us to examine constraints on the gravitational couplings by demanding consistency of the CFT. In particular, one can demand positive energy fluxes in scattering processes or the causal propagation of fluctuations. We also examine the holographic hydrodynamics, commenting on the shear viscosity as well as the relaxation time. The latter allows us to consider causality constraints arising from the second-order truncated theory of hydrodynamics.
TL;DR: For a positive gravity induced anomalous dimension A λ > 0, the running of the quartic scalar self interaction at scales beyond the Planck mass is determined by a fixed point at zero as mentioned in this paper.
TL;DR: In this paper, a frequency-domain matching of post-Newtonian inspiral waveforms with numerical relativity based binary black hole coalescence waveforms is proposed to quantify the various possible sources of systematic errors that arise in matching post-newtonian and numerical relativity waveforms, and use a matching criteria based on minimizing these errors.
Abstract: We present a new phenomenological gravitational waveform model for the inspiral and coalescence of nonprecessing spinning black hole binaries. Our approach is based on a frequency-domain matching of post-Newtonian inspiral waveforms with numerical relativity based binary black hole coalescence waveforms. We quantify the various possible sources of systematic errors that arise in matching post-Newtonian and numerical relativity waveforms, and we use a matching criteria based on minimizing these errors; we find that the dominant source of errors are those in the post-Newtonian waveforms near the merger. An analytical formula for the dominant mode of the gravitational radiation of nonprecessing black hole binaries is presented that captures the phenomenology of the hybrid waveforms. Its implementation in the current searches for gravitational waves should allow cross-checks of other inspiral-merger-ringdown waveform families and improve the reach of gravitational-wave searches.
TL;DR: A test of the applicability of general relativity on cosmological length scales using a quantity that combines measures of large-scale gravitational lensing, galaxy clustering and structure growth rate, reports that EG = 0.39 ± 0.06 on length scales of tens of megaparsecs, in agreement with the general relativistic prediction.
Abstract: Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Such a test has recently been proposed, using a quantity, EG, that combines measures of large-scale gravitational lensing, galaxy clustering and structure growth rate. The combination is insensitive to `galaxy bias' (the difference between the clustering of visible galaxies and invisible dark matter) and is thus robust to the uncertainty in this parameter. Modified theories of gravity generally predict values of EG different from the general relativistic prediction because, in these theories, the `gravitational slip' (the difference between the two potentials that describe perturbations in the gravitational metric) is non-zero, which leads to changes in the growth of structure and the strength of the gravitational lensing effect. Here we report that EG = 0.39+/-0.06 on length scales of tens of megaparsecs, in agreement with the general relativistic prediction of EG~0.4. The measured value excludes a model within the tensor-vector-scalar gravity theory, which modifies both Newtonian and Einstein gravity. However, the relatively large uncertainty still permits models within f(R) theory, which is an extension of general relativity. A fivefold decrease in uncertainty is needed to rule out these models.
TL;DR: In this paper, the authors provide a physical interpretation for the field equations of gravity based on a thermodynamic perspective, and the virtual degrees of freedom associated with the horizons, as perceived by the local Rindler observer, play a crucial role in this approach.
Abstract: It is possible to provide a physical interpretation for the field equations of gravity based on a thermodynamical perspective. The virtual degrees of freedom associated with the horizons, as perceived by the local Rindler observer, play a crucial role in this approach. In this context, the relation S = E/2T between the entropy (S), active gravitational mass (E) and temperature (T) — obtained previously in gr-qc/0308070 [CQG, 21, 4485 (2004)] — can be reinterpreted as the law of equipartition E = (1/2) nkBT where $n=\Delta A/L_{\rm P}^2$ is the number (density) of microscopic horizon degrees of freedom in an area ΔA. Conversely, one can use the equipartition argument to provide a thermodynamic interpretation of gravity, even in the nonrelativistic limit. These results emphasize the intrinsic quantum nature of all gravitational phenomena and diminishes the distinction between thermal phenomena associated with local Rindler horizons and the usual thermodynamics of macroscopic bodies in non-inertial frames. J...
TL;DR: In this paper, the creation and evolution of boost-invariant anisotropic, strongly-coupled N=4 supersymmetric Yang-Mills plasma was studied using gauge/gravity duality.
Abstract: Using gauge/gravity duality, we study the creation and evolution of boost-invariant anisotropic, strongly-coupled N=4 supersymmetric Yang-Mills plasma. In the dual gravitational description, this corresponds to horizon formation in a geometry driven to be anisotropic by a time-dependent change in boundary conditions.
TL;DR: It is argued that the modern formulation of Eddington's theory, Born-Infeld gravity, presents us with a novel, nonsingular description of the Universe.
Abstract: We resurrect Eddington's proposal for the gravitational action in the presence of a cosmological constant and extend it to include matter fields. We show that the Newton-Poisson equation is modified in the presence of sources and that charged black holes show great similarities with those arising in Born-Infeld electrodynamics coupled to gravity. When we consider homogeneous and isotropic space-times, we find that there is a minimum length (and maximum density) at early times, clearly pointing to an alternative theory of the big bang. We thus argue that the modern formulation of Eddington's theory, Born-Infeld gravity, presents us with a novel, nonsingular description of the Universe.
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TL;DR: In this article, the authors generalize the f(R) type gravity models by assuming that the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar R and of the matter Lagrangians L m m m − 1, and derive the equations of motion for test particles, which follow from the covariant divergence of the energy-momentum tensor.
Abstract: We generalize the f(R) type gravity models by assuming that the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar R and of the matter Lagrangian L
m
. We obtain the gravitational field equations in the metric formalism, as well as the equations of motion for test particles, which follow from the covariant divergence of the energy-momentum tensor. The equations of motion for test particles can also be derived from a variational principle in the particular case in which the Lagrangian density of the matter is an arbitrary function of the energy density of the matter only. Generally, the motion is non-geodesic, and it takes place in the presence of an extra force orthogonal to the four-velocity. The Newtonian limit of the equation of motion is also considered, and a procedure for obtaining the energy-momentum tensor of the matter is presented. The gravitational field equations and the equations of motion for a particular model in which the action of the gravitational field has an exponential dependence on the standard general relativistic Hilbert–Einstein Lagrange density are also derived.
Book•
16 Aug 2010TL;DR: In this article, a pedagogical introduction to numerical relativity is presented, with 300 exercises to help readers master new material as it is presented and numerous illustrations, many in color, assist in visualizing new geometric concepts and highlighting the results of computer simulations.
Abstract: Aimed at students and researchers entering the field, this pedagogical introduction to numerical relativity will also interest scientists seeking a broad survey of its challenges and achievements. Assuming only a basic knowledge of classical general relativity, the book develops the mathematical formalism from first principles, and then highlights some of the pioneering simulations involving black holes and neutron stars, gravitational collapse and gravitational waves. The book contains 300 exercises to help readers master new material as it is presented. Numerous illustrations, many in color, assist in visualizing new geometric concepts and highlighting the results of computer simulations. Summary boxes encapsulate some of the most important results for quick reference. Applications covered include calculations of coalescing binary black holes and binary neutron stars, rotating stars, colliding star clusters, gravitational and magnetorotational collapse, critical phenomena, the generation of gravitational waves, and other topics of current physical and astrophysical significance.
TL;DR: In this paper, the basic construction and cosmological implications of a power-counting renormalizable theory of gravitation, recently proposed by Hořava, are reviewed.
Abstract: Here we review the basic construction and cosmological implications of a power-counting renormalizable theory of gravitation, recently proposed by Hořava. We explain that (i) at low energy this theory does not exactly recover general relativity but instead mimics general relativity plus dark matter; (ii) higher spatial curvature terms allow bouncing and cyclic universes as regular solutions; (iii) the anisotropic scaling with the dynamical critical exponent z = 3 solves the horizon problem and leads to scale-invariant cosmological perturbations even without inflation. We also comment on issues related to an extra scalar degree of freedom called scalar graviton. In particular, for spherically-symmetric, static, vacuum configurations we prove non-perturbative continuity of the λ → 1 + 0 limit, where λ is a parameter in the kinetic action and general relativity has the value λ = 1. We also derive the condition under which linear instability of the scalar graviton does not show up.
TL;DR: In this paper, it was shown that the five-dimensional Maxwell theory with the Chern-Simons term is tachyonic in the presence of a constant electric field and the instability happens only at nonvanishing momenta, suggesting a spatially modulated phase in the holographically dual quantum field theory in (3+1) dimensions.
Abstract: We show that the five-dimensional Maxwell theory with the Chern-Simons term is tachyonic in the presence of a constant electric field. When coupled to gravity, a sufficiently large Chern-Simons coupling causes instability of the Reissner-Nordstrom black holes in anti-de Sitter space. The instability happens only at nonvanishing momenta, suggesting a spatially modulated phase in the holographically dual quantum field theory in (3+1) dimensions, with spontaneous current generation in a helical configuration. The three-charge extremal black hole in the type IIB superstring theory on AdS_5×S^5 barely satisfies the stability condition.
TL;DR: A field theory which lives in fractal spacetime and is argued to be Lorentz invariant, power-counting renormalizable, ultraviolet finite, and causal, and implications for quantum gravity, cosmology, and the cosmological constant are discussed.
Abstract: We propose a field theory which lives in fractal spacetime and is argued to be Lorentz invariant, power-counting renormalizable, ultraviolet finite, and causal. The system flows from an ultraviolet fixed point, where spacetime has Hausdorff dimension 2, to an infrared limit coinciding with a standard four-dimensional field theory. Classically, the fractal world where fields live exchanges energy momentum with the bulk with integer topological dimension. However, the total energy momentum is conserved. We consider the dynamics and the propagator of a scalar field. Implications for quantum gravity, cosmology, and the cosmological constant are discussed.
TL;DR: In this article, it was shown that the linearization of all exact solutions of classical chiral gravity around the vacuum have positive energy, while nonchiral and negative energy solutions of the linearized equations are infrared divergent at second order, and so are removed from the spectrum.
Abstract: We show that the linearization of all exact solutions of classical chiral gravity around the ${\mathrm{AdS}}_{3}$ vacuum have positive energy. Nonchiral and negative-energy solutions of the linearized equations are infrared divergent at second order, and so are removed from the spectrum. In other words, chirality is confined and the equations of motion have linearization instabilities. We prove that the only stationary, axially symmetric solutions of chiral gravity are BTZ black holes, which have positive energy. It is further shown that classical log gravity---the theory with logarithmically relaxed boundary conditions---has finite asymptotic symmetry generators but is not chiral and hence may be dual at the quantum level to a logarithmic conformal field theories (CFT). Moreover we show that log gravity contains chiral gravity within it as a decoupled charge superselection sector. We formally evaluate the Euclidean sum over geometries of chiral gravity and show that it gives precisely the holomorphic extremal CFT partition function. The modular invariance and integrality of the expansion coefficients of this partition function are consistent with the existence of an exact quantum theory of chiral gravity. We argue that the problem of quantizing chiral gravity is the holographic dual of the problem of constructing an extremal CFT, while quantizing log gravity is dual to the problem of constructing a logarithmic extremal CFT.
TL;DR: In this paper, a holographic description of four-dimensional single-scalar inflationary universes is proposed, where cosmological observables, such as the primordial power spectrum, are encoded in the correlation functions of a three-dimensional quantum field theory (QFT).
Abstract: We propose a holographic description of four-dimensional single-scalar inflationary universes, and show how cosmological observables, such as the primordial power spectrum, are encoded in the correlation functions of a three-dimensional quantum field theory (QFT). The holographic description correctly reproduces standard inflationary predictions in the regime where a perturbative quantization of fluctuations is justified. In the opposite regime, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative large N QFT. Initiating a holographic phenomenological approach, we show that models containing only two parameters, N and a dimensionful coupling constant, are capable of satisfying the current observational constraints.
TL;DR: In this article, a new gravitational action which includes cubic curvature interactions and which provides a useful toy model for the holographic study of a three parameter family of four-and higher-dimensional CFTs was constructed.
Abstract: We construct a new gravitational action which includes cubic curvature interactions and which provides a useful toy model for the holographic study of a three parameter family of four- and higher-dimensional CFT’s We also investigate the black hole solutions of this new gravity theory Further we examine the equations of motion of quasi-topological gravity While the full equations in a general background are fourth-order in derivatives, we show that the linearized equations describing gravitons propagating in the AdS vacua match precisely the second-order equations of Einstein gravity
TL;DR: In this article, the dynamical properties of the equations of motion for these theories of gravity in the presence of positive and negative cosmological constants and radiation were studied and an algorithm for tracking the super-Hubble perturbations during the bounce was presented.
Abstract: One of the greatest problems of standard cosmology is the Big Bang singularity. Previously it has been shown that non-local ghostfree higher-derivative modifications of Einstein gravity in the ultra-violet regime can admit non-singular bouncing solutions. In this paper we study in more details the dynamical properties of the equations of motion for these theories of gravity in presence of positive and negative cosmological constants and radiation. We find stable inflationary attractor solutions in the presence of a positive cosmological constant which renders inflation geodesically complete, while in the presence of a negative cosmological constant a cyclic universe emerges. We also provide an algorithm for tracking the super-Hubble perturbations during the bounce and show that the bouncing solutions are free from any perturbative instability.
TL;DR: In this article, a power-counting renormalizable theory of gravitation was proposed by Horava, which does not exactly recover general relativity but instead mimic general relativity plus dark matter, and higher spatial curvature terms allow bouncing and cyclic universes as regular solutions.
Abstract: This article reviews basic construction and cosmological implications of a power-counting renormalizable theory of gravitation recently proposed by Horava. We explain that (i) at low energy this theory does not exactly recover general relativity but instead mimic general relativity plus dark matter; that (ii) higher spatial curvature terms allow bouncing and cyclic universes as regular solutions; and that (iii) the anisotropic scaling with the dynamical critical exponent z=3 solves the horizon problem and leads to scale-invariant cosmological perturbations even without inflation. We also comment on issues related to an extra scalar degree of freedom called scalar graviton. In particular, for spherically-symmetric, static, vacuum configurations we prove non-perturbative continuity of the lambda->1+0 limit, where lambda is a parameter in the kinetic action and general relativity has the value lambda=1. We also derive the condition under which linear instability of the scalar graviton does not show up.
TL;DR: A three-slit experiment with photons was performed and bounded the magnitude of three- path interference to less than 10−2 of the expected two-path interference, thus ruling out third- and higher-order interference and providing a bound on the accuracy of Born’s rule.
Abstract: Quantum mechanics and gravitation are two pillars of modern physics. Despite their success in describing the physical world around us, they seem to be incompatible theories. There are suggestions that one of these theories must be generalized to achieve unification. For example, Born's rule--one of the axioms of quantum mechanics--could be violated. Born's rule predicts that quantum interference, as shown by a double-slit diffraction experiment, occurs from pairs of paths. A generalized version of quantum mechanics might allow multipath (i.e., higher-order) interference, thus leading to a deviation from the theory. We performed a three-slit experiment with photons and bounded the magnitude of three-path interference to less than 10(-2) of the expected two-path interference, thus ruling out third- and higher-order interference and providing a bound on the accuracy of Born's rule. Our experiment is consistent with the postulate both in semiclassical and quantum regimes.