Showing papers on "Gravitation published in 2011"

Resummation of Massive Gravity

Abstract: We construct four-dimensional covariant nonlinear theories of massive gravity which are ghost-free in the decoupling limit to all orders. These theories resum explicitly all the nonlinear terms of an effective field theory of massive gravity. We show that away from the decoupling limit the Hamiltonian constraint is maintained at least up to and including quartic order in nonlinearities, hence excluding the possibility of the Boulware-Deser ghost up to this order. We also show that the same remains true to all orders in a similar toy model.

On the origin of gravity and the laws of Newton

Abstract: Starting from first principles and general assumptions we present a heuristic argument that shows that Newton’s law of gravitation naturally arises in a theory in which space emerges through a holographic scenario. Gravity is identified with an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton’s law of inertia needs to be explained. The equivalence principle auggests that it is actually the law of inertia whose origin is entropic.

Loop Quantum Cosmology: A Status Report

Abstract: Loop quantum cosmology (LQC) is the result of applying principles of loop quantum gravity (LQG) to cosmological settings. The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of LQG. In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low spacetime curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction. In cosmological models, while Einstein's equations hold to an excellent degree of approximation at low curvature, they undergo major modifications in the Planck regime: for matter satisfying the usual energy conditions, any time a curvature invariant grows to the Planck scale, quantum geometry effects dilute it, thereby resolving singularities of general relativity. Quantum geometry corrections become more sophisticated as the models become richer. In particular, in anisotropic models, there are significant changes in the dynamics of shear potentials which tame their singular behavior in striking contrast to older results on anisotropies in bouncing models. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues—e.g. the 'horizon problem'—from a new perspective. Such conceptual issues as well as potential observational consequences of the new Planck scale physics are being explored, especially within the inflationary paradigm. These considerations have given rise to a burst of activity in LQC in recent years, with contributions from quantum gravity experts, mathematical physicists and cosmologists. The goal of this review is to provide an overview of the current state of the art in LQC for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general and cosmologists who wish to apply LQC to probe modifications in the standard paradigm of the early universe. In this review, effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without loss of continuity.

Holographic c-theorems in arbitrary dimensions

Abstract: We re-examine holographic versions of the c-theorem and entanglement entropy in the context of higher curvature gravity and the AdS/CFT correspondence. We select the gravity theories by tuning the gravitational couplings to eliminate non-unitary operators in the boundary theory and demonstrate that all of these theories obey a holographic c-theorem. In cases where the dual CFT is even-dimensional, we show that the quantity that flow is the central charge associated with the A-type trace anomaly. Here, unlike in conventional holographic constructions with Einstein gravity, we are able to distinguish this quantity from other central charges or the leading coefficient in the entropy density of a thermal bath. In general, we are also able to identify this quantity with the coefficient of a universal contribution to the entanglement entropy in a particular construction. Our results suggest that these coefficients appearing in entanglement entropy play the role of central charges in odd-dimensional CFT's. We conjecture a new c-theorem on the space of odd-dimensional field theories, which extends Cardy's proposal for even dimensions. Beyond holography, we were able to show that for any even-dimensional CFT, the universal coefficient appearing the entanglement entropy which we calculate is precisely the A-type central charge.

Varying Constants, Gravitation and Cosmology

Abstract: Fundamental constants are a cornerstone of our physical laws. Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. Thus, it is of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy. We detail the relations between the constants, the tests of the local position invariance and of the universality of free fall. We then review the main experimental and observational constraints that have been obtained from atomic clocks, the Oklo phenomenon, solar system observations, meteorite dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic microwave background and big bang nucleosynthesis. At each step we describe the basics of each system, its dependence with respect to the constants, the known systematic effects and the most recent constraints that have been obtained. We then describe the main theoretical frameworks in which the low-energy constants may actually be varying and we focus on the unification mechanisms and the relations between the variation of different constants. To finish, we discuss the more speculative possibility of understanding their numerical values and the apparent fine-tuning that they confront us with.

Sensitivity studies for third-generation gravitational wave observatories

Abstract: Advanced gravitational wave detectors, currently under construction, are expected to directly observe gravitational wave signals of astrophysical origin. The Einstein Telescope (ET), a third-generation gravitational wave detector, has been proposed in order to fully open up the emerging field of gravitational wave astronomy. In this paper we describe sensitivity models for ET and investigate potential limits imposed by fundamental noise sources. A special focus is set on evaluating the frequency band below 10 Hz where a complex mixture of seismic, gravity gradient, suspension thermal and radiation pressure noise dominates. We develop the most accurate sensitivity model, referred to as ET-D, for a third-generation detector so far, including the most relevant fundamental noise contributions.

Introduction to Holographic Superconductors

Abstract: These lectures give an introduction to the theory of holographic superconductors These are superconductors that have a dual gravitational description using gauge/gravity duality After introducing a suitable gravitational theory, we discuss its properties in various regimes: the probe limit, the effects of backreaction, the zero temperature limit, and the addition of magnetic fields Using the gauge/gravity dictionary, these properties reproduce many of the standard features of superconductors Some familiarity with gauge/gravity duality is assumed A list of open problems is included at the end

Millikelvin cooling of an optically trapped microsphere in vacuum

Abstract: Microscale resonators cooled so that their vibrational motion approaches the quantum limit enable the study of quantum effects in macroscopic systems. An approach that could probe the interface between quantum mechanics and general relativity is now demonstrated by using lasers to suspend a glass microsphere in a vacuum. Cooling of micromechanical resonators towards the quantum mechanical ground state in their centre-of-mass motion has advanced rapidly in recent years1,2,3,4,5,6,7,8. This work is an important step towards the creation of ‘Schrodinger cats’, quantum superpositions of macroscopic observables, and the study of their destruction by decoherence. Here we report optical trapping of glass microspheres in vacuum with high oscillation frequencies, and cooling of the centre-of-mass motion from room temperature to a minimum temperature of about 1.5 mK. This new system eliminates the physical contact inherent to clamped cantilevers, and can allow ground-state cooling from room temperature9,10,11,12,13,14,15. More importantly, the optical trap can be switched off, allowing a microsphere to undergo free-fall in vacuum after cooling15. This is ideal for studying the gravitational state reduction16,17,18,19, a manifestation of the apparent conflict between general relativity and quantum mechanics16,20. A cooled optically trapped object in vacuum can also be used to search for non-Newtonian gravity forces at small scales21, measure the impact of a single air molecule14 and even produce Schrodinger cats of living organisms9.

f(T) gravity and local Lorentz invariance

Abstract: We show that in theories of generalized teleparallel gravity, whose Lagrangians are algebraic functions of the usual teleparallel Lagrangian, the action and the field equations are not invariant under local Lorentz transformations. We also argue that these theories appear to have extra degrees of freedom with respect to general relativity. The usual teleparallel Lagrangian, which has been extensively studied and leads to a theory dynamically equivalent to general relativity, is an exception. Both of these facts appear to have been overlooked in the recent literature on f(T) gravity, but are crucial for assessing the viability of these theories as alternative explanations for the acceleration of the Universe.

Extended Theories of Gravity

Abstract: Extended Theories of Gravity can be considered a new paradigm to cure shortcomings of General Relativity at infrared and ultraviolet scales. They are an approach that, by preserving the undoubtedly positive results of Einstein's Theory, is aimed to address conceptual and experimental problems recently emerged in Astrophysics, Cosmology and High Energy Physics. In particular, the goal is to encompass, in a self-consistent scheme, problems like Inflation, Dark Energy, Dark Matter, Large Scale Structure and, first of all, to give at least an effective description of Quantum Gravity. We review the basic principles that any gravitational theory has to follow. The geometrical interpretation is discussed in a broad perspective in order to highlight the basic assumptions of General Relativity and its possible extensions in the general framework of gauge theories. Principles of such modifications are presented, focusing on specific classes of theories like f (R)-gravity and scalar-tensor gravity in the metric and Palatini approaches. The special role of torsion is also discussed. The conceptual features of these theories are fully explored and attention is payed to the issues of dynamical and conformal equivalence between them considering also the initial value problem. A number of viability criteria are presented considering the post-Newtonian and the post-Minkowskian limits. In particular, we discuss the problems of neutrino oscillations and gravitational waves in Extended Gravity. Finally, future perspectives of Extended Gravity are considered with possibility to go beyond a trial and error approach.

Matter Bounce Cosmology with the f(T) Gravity

Abstract: We show that the f(T) gravitational paradigm, in which gravity is described by an arbitrary function of the torsion scalar, can provide a mechanism for realizing bouncing cosmologies, thereby avoiding the Big Bang singularity. After constructing the simplest version of an f(T) matter bounce, we investigate the scalar and tensor modes of cosmological perturbations. Our results show that metric perturbations in the scalar sector lead to a background-dependent sound speed, which is a distinguishable feature from Einstein gravity. Additionally, we obtain a scale-invariant primordial power spectrum, which is consistent with cosmological observations, but suffers from the problem of a large tensor-to-scalar ratio. However, this can be avoided by introducing extra fields, such as a matter bounce curvaton.Communicated by P R L V Moniz

Cosmological perturbations in f(T) gravity

Abstract: We investigate the cosmological perturbations in $f(T)$ gravity. Examining the pure gravitational perturbations in the scalar sector using a diagonal vierbein, we extract the corresponding dispersion relation, which provides a constraint on the $f(T)$ Ans\"atze that lead to a theory free of instabilities. Additionally, upon inclusion of the matter perturbations, we derive the fully perturbed equations of motion, and we study the growth of matter overdensities. We show that $f(T)$ gravity with $f(T)$ constant coincides with General Relativity, both at the background as well as at the first-order perturbation level. Applying our formalism to the power-law model we find that on large subhorizon scales ($\mathcal{O}(100\text{ }\text{ }\mathrm{Mpc})$ or larger), the evolution of matter overdensity will differ from $\ensuremath{\Lambda}\mathrm{CDM}$ cosmology. Finally, examining the linear perturbations of the vector and tensor sectors, we find that (for the standard choice of vierbein) $f(T)$ gravity is free of massive gravitons.

On holographic entanglement entropy and higher curvature gravity

Abstract: We examine holographic entanglement entropy with higher curvature gravity in the bulk. We show that in general Wald’s formula for horizon entropy does not yield the correct entanglement entropy. However, for Lovelock gravity, there is an alternate prescription which involves only the intrinsic curvature of the bulk surface. We verify that this prescription correctly reproduces the universal contribution to the entanglement entropy for CFT’s in four and six dimensions. We also make further comments on gravitational theories with more general higher curvature interactions.

f(T) gravity mimicking dynamical dark energy. Background and perturbation analysis

Abstract: We investigate f(T) cosmology in both the background, as well as in the perturbation level, and we present the general formalism for reconstructing the equivalent one-parameter family of f(T) models for any given dynamical dark energy scenario Despite the completely indistinguishable background behavior, the perturbations break this degeneracy and the growth histories of all these models differ from one another As an application we reconstruct the f(T) equivalent for quintessence, and we show that the deviation of the matter overdensity evolution is strong for small scales and weak for large scales, while it is negligible for large redshifts

Holography and Colliding Gravitational Shock Waves in Asymptotically Ad S 5 Spacetime

Abstract: Using holography, we study the collision of planar shock waves in strongly coupled N=4 supersymmetric Yang-Mills theory. This requires the numerical solution of a dual gravitational initial value problem in asymptotically anti-de Sitter spacetime.

Holography and Colliding Gravitational Shock Waves in Asymptotically AdS(5) Spacetime

Abstract: Introduction.| The recognition that the quark-gluon plasma (QGP) produced in relativistic heavy ion collisions is strongly coupled [1], combined with the advent of gauge/gravity duality (or \holography") [2, 3], has prompted much work exploring both equilibrium and non-equilibrium properties of strongly coupledN = 4 supersymmetric Yang-Mills theory (SYM), which may be viewed as a theoretically tractable toy model for real QGP. Multiple authors have discussed collisions of innitely extended planar shock waves in SYM, which may be viewed as instructive caricatures of collisions of large, highly Lorentz-contracted nuclei. In the dual description of strongly coupled (and large Nc) SYM, this becomes a problem of colliding gravitational shock waves in asymptotically anti-de Sitter (AdS5) spacetime. Previous work has examined qualitative properties and trapped surfaces [4{7], possible early time behavior [8{10], and expected late time asymptotics [11, 12]. As no analytic solution is known for this gravitational problem, solving the gravitational initial value problem numerically is the only way to obtain quantitative results which properly connect early and late time behavior. In this letter, we report the results of such a calculation, and examine the evolution of the post-collision stress-energy tensor. Unlike previous work considering singular shocks with vanishing thickness [5, 9], or shocks driven by nonvanishing sources in the bulk [5, 6], we choose to study planar gravitational \shocks" which are regular, nonsingular, source-less solutions to Einstein’s equations. Equivalently, we study the evolution of initial states in SYM with nite energy density concentrated on two pla

Large-scale Structure in f(T) Gravity

Abstract: In this work we study the cosmology of the general f(T) gravity theory. We express the modified Einstein equations using covariant quantities, and derive the gauge-invariant perturbation equations in covariant form. We consider a specific choice of f(T), designed to explain the observed late-time accelerating cosmic expansion without including an exotic dark energy component. Our numerical solution shows that the extra degree of freedom of such f(T) gravity models generally decays as one goes to smaller scales, and consequently its effects on scales such as galaxies and galaxies clusters are small. But on large scales, this degree of freedom can produce large deviations from the standard �CDM scenario, leading to severe constraints on the f(T) gravity models as an explanation to the cosmic acceleration.

Matter-gravity couplings and Lorentz violation

Abstract: The gravitational couplings of matter are studied in the presence of Lorentz and $CPT$ violation. At leading order in the coefficients for Lorentz violation, the relativistic quantum Hamiltonian is derived from the gravitationally coupled minimal standard-model extension. For spin-independent effects, the nonrelativistic quantum Hamiltonian and the classical dynamics for test and source bodies are obtained. A systematic perturbative method is developed to treat small metric and coefficient fluctuations about a Lorentz-violating and Minkowski background. The post-Newtonian metric and the trajectory of a test body freely falling under gravity in the presence of Lorentz violation are established. An illustrative example is presented for a bumblebee model. The general methodology is used to identify observable signals of Lorentz and $CPT$ violation in a variety of gravitational experiments and observations, including gravimeter measurements, laboratory and satellite tests of the weak equivalence principle, antimatter studies, solar-system observations, and investigations of the gravitational properties of light. Numerous sensitivities to coefficients for Lorentz violation can be achieved in existing or near-future experiments at the level of parts in ${10}^{3}$ down to parts in ${10}^{15}$. Certain coefficients are uniquely detectable in gravitational searches and remain unmeasured to date.

On the Density Distribution in Star-forming Interstellar Clouds

Abstract: We use deep adaptive mesh refinement simulations of isothermal self-gravitating supersonic turbulence to study the imprints of gravity on the mass density distribution in molecular clouds. The simulations show that the density distribution in self-gravitating clouds develops an extended power-law tail at high densities on top of the usual lognormal. We associate the origin of the tail with self-similar collapse solutions and predict the power index values in the range from ?7/4 to ?3/2 that agree with both simulations and observations of star-forming molecular clouds.

Post-1-Newtonian tidal effects in the gravitational waveform from binary inspirals

Abstract: The gravitational wave signal from an inspiralling binary neutron star system will contain detailed information about tidal coupling in the system, and thus, about the internal physics of the neutron stars. To extract this information will require highly accurate models for the gravitational waveform. We present here a calculation of the gravitational wave signal from a binary with quadrupolar tidal interactions which includes all post-1-Newtonian–order effects in both the conservative dynamics and wave generation. We consider stars with adiabatically induced quadrupoles moving in circular orbits, and work to linear order in the stars’ quadrupole moments. We find that post-1-Newtonian corrections increase the tidal signal by approximately 20% at gravitational wave frequencies of 400 Hz.

Symmetron Cosmology

Abstract: The symmetron is a scalar field associated with the dark sector whose coupling to matter depends on the ambient matter density The symmetron is decoupled and screened in regions of high density, thereby satisfying local constraints from tests of gravity, but couples with gravitational strength in regions of low density, such as the cosmos In this paper we derive the cosmological expansion history in the presence of a symmetron field, tracking the evolution through the inflationary, radiation- and matter-dominated epochs, using a combination of analytical approximations and numerical integration For a broad range of initial conditions at the onset of inflation, the scalar field reaches its symmetry-breaking vacuum by the present epoch, as assumed in the local analysis of spherically-symmetric solutions and tests of gravity For the simplest form of the potential, the energy scale is too small for the symmetron to act as dark energy, hence we must add a cosmological constant to drive late-time cosmic acceleration We briefly discuss a class of generalized, non-renormalizable potentials that can have a greater impact on the late-time cosmology, though cosmic acceleration requires a delicate tuning of parameters in this case

Analytic solutions in nonlinear massive gravity.

Abstract: We study spherically symmetric solutions in a covariant massive gravity model, which is a candidate for a ghost-free nonlinear completion of the Fierz-Pauli theory. There is a branch of solutions that exhibits the Vainshtein mechanism, recovering general relativity below a Vainshtein radius given by (r{sub g}m{sup 2}){sup 1/3}, where m is the graviton mass and r{sub g} is the Schwarzschild radius of a matter source. Another branch of exact solutions exists, corresponding to de Sitter-Schwarzschild spacetimes where the curvature scale of de Sitter space is proportional to the mass squared of the graviton.

Electron stars for holographic metallic criticality

Abstract: We refer to the ground state of a gravitating, charged ideal fluid of fermions held at a finite chemical potential as an ``electron star.'' In a holographic setting, electron stars are candidate gravity duals for strongly interacting finite fermion density systems. We show how electron stars develop an emergent Lifshitz scaling at low energies. This IR scaling region is a consequence of the two-way interaction between emergent quantum critical bosonic modes and the finite density of fermions. By integrating from the IR region to an asymptotically $Ad{S}_{4}$ spacetime, we compute basic properties of the electron stars, including their electrical conductivity. We emphasize the challenge of connecting UV and IR physics in strongly interacting finite density systems.

Nonsingular exponential gravity: A simple theory for early- and late-time accelerated expansion

Abstract: A theory of exponential modified gravity which explains both early-time inflation and late-time acceleration, in a unified way, is proposed. The theory successfully passes the local tests and fulfills the cosmological bounds and, remarkably, the corresponding inflationary era is proven to be unstable. Numerical investigation of its late-time evolution leads to the conclusion that the corresponding dark energy epoch is not distinguishable from the one for the $\ensuremath{\Lambda}\mathrm{CDM}$ model. Several versions of this exponential gravity, sharing similar properties, are formulated. It is also shown that this theory is nonsingular, being protected against the formation of finite-time future singularities. As a result, the corresponding future universe evolution asymptotically tends, in a smooth way, to de Sitter space, which turns out to be the final attractor of the system.

Horizons, holography and condensed matter

Abstract: The holographic correspondence creates an interface between classical gravitational physics and the dynamics of strongly interacting quantum field theories. This chapter will relate the physics of charged, asymptotically Anti-de Sitter spacetimes to the phenomenology of low temperature critical phases of condensed matter. Common essential features will characterise both the gravitational and field theoretic systems. Firstly, an emergent scaling symmetry at the lowest energy scales appears as an emergent isometry in the interior, `near horizon' regime of the spacetime. Secondly, the field theoretic distinction between fractionalized and mesonic phases appears as the presence or absence of a charge-carrying horizon in the spacetime. A perspective grounded in these two characteristics allows a unified presentation of `holographic superconductors', `electron stars' and `charged dilatonic spacetimes'.

Wilsonian Approach to Fluid/Gravity Duality

Abstract: The problem of gravitational fluctuations confined inside a finite cutoff at radius r = r c outside the horizon in a general class of black hole geometries is considered. Consistent boundary conditions at both the cutoff surface and the horizon are found and the resulting modes analyzed. For general cutoff r c the dispersion relation is shown at long wavelengths to be that of a linearized Navier-Stokes fluid living on the cutoff surface. A cutoff-dependent line-integral formula for the diffusion constant D (r c ) is derived. The dependence on r c is interpreted as renormalization group (RG) flow in the fluid. Taking the cutoff to infinity in an asymptotically AdS context, the formula for D(∞) reproduces as a special case well-known results derived using AdS/CFT. Taking the cutoff to the horizon, the effective speed of sound goes to infinity, the fluid becomes incompressible and the Navier-Stokes dispersion relation becomes exact. The resulting universal formula for the diffusion constant D(horizon) reproduces old results from the membrane paradigm. Hence the old membrane paradigm results and new AdS/CFT results are related by RG flow. RG flow-invariance of the viscosity to entropy ratio $ \frac{\eta }{s} $ is shown to follow from the first law of thermodynamics together with isentropy of radial evolution in classical gravity. The ratio is expected to run when quantum gravitational corrections are included.

Growth factor in $f(T)$ gravity

Abstract: We derive the evolution equation of growth factor for the matter over-dense perturbation in f(T) gravity. For instance, we investigate its behavior in power law model at small redshift and compare it to the prediction of Lambda CDM and dark energy with the same equation of state in the framework of Einstein general relativity. We find that the perturbation in f (T) gravity grows slower than that in Einstein general relativity if partial derivative f/partial derivative T > 0 due to the effectively weakened gravity.