Matrix Gravity and Massive Colored Gravitons

We formulate a theory of gravity with a matrix-valued complex vierbein based on the SL(2N,C)xSL(2N,C) gauge symmetry. The theory is metric independent, and before symmetry breaking all fields are massless. The symmetry is broken spontaneously and all gravitons corresponding to the broken generators acquire masses. If the symmetry is broken to SL(2,C) then the spectrum would correspond to one massless graviton coupled to $2N^2 -1$ massive gravitons. A novel feature is the way the fields corresponding to non-compact generators acquire kinetic energies with correct signs. Equally surprising is the way Yang-Mills gauge fields acquire their correct kinetic energies through the coupling to the non-dynamical antisymmetric components of the vierbeins.Comment: One reference adde

Einstein and Jordan frames reconciled: a frame-invariant approach to scalar-tensor cosmology

Scalar-Tensor theories of gravity can be formulated in different frames, most notably, the Einstein and the Jordan one. While some debate still persists in the literature on the physical status of the different frames, a frame transformation in Scalar-Tensor theories amounts to a local redefinition of the metric, and then should not affect physical results. We analyze the issue in a cosmological context. In particular, we define all the relevant observables (redshift, distances, cross-sections, ...) in terms of frame-independent quantities. Then, we give a frame-independent formulation of the Boltzmann equation, and outline its use in relevant examples such as particle freeze-out and the evolution of the CMB photon distribution function. Finally, we derive the gravitational equations for the frame-independent quantities at first order in perturbation theory. From a practical point of view, the present approach allows the simultaneous implementation of the good aspects of the two frames in a clear and straightforward way.Comment: 15 pages, matches version to be published on Phys. Rev.

The recovery of General Relativity in massive gravity via the Vainshtein mechanism

We study in detail static spherically symmetric solutions of non linear Pauli-Fierz theory. We obtain a numerical solution with a constant density source. This solution shows a recovery of the corresponding solution of General Relativity via the Vainshtein mechanism. This result has already been presented by us in a recent letter, and we give here more detailed information on it as well as on the procedure used to obtain it. We give new analytic insights upon this problem, in particular for what concerns the question of the number of solutions at infinity. We also present a weak field limit which allows to capture all the salient features of the numerical solution, including the Vainshtein crossover and the Yukawa decay.Comment: 38 pages, 9 Figs, revtex

Ghosts, Strong Coupling and Accidental Symmetries in Massive Gravity

We show that the strong self-interaction of the scalar polarization of a massive graviton can be understood in terms of the propagation of an extra ghost-like degree of freedom, thus relating strong coupling to the sixth degree of freedom discussed by Boulware and Deser in their Hamiltonian analysis of massive gravity. This enables one to understand the Vainshtein recovery of solutions of massless gravity as being due to the effect of the exchange of this ghost which gets frozen at distances larger than the Vainshtein radius. Inside this region, we can trust the two-field Lagrangian perturbatively, while at larger distances one can use the higher derivative formulation. We also compare massive gravity with other models, namely deconstructed theories of gravity, as well as DGP model. In the latter case we argue that the Vainshtein recovery process is of different nature, not involving a ghost degree of freedom.Comment: 21 page

A note on the uniqueness of D=4 N=1 Supergravity

We investigate in 4 spacetime dimensions, all the consistent deformations of the lagrangian ${\cal L}_2+{\cal L}_{{3/2}}$, which is the sum of the Pauli-Fierz lagrangian ${\cal L}_2$ for a free massless spin 2 field and the Rarita-Schwinger lagrangian ${\cal L}_{{3/2}}$ for a free massless spin 3/2 field. Using BRST cohomogical techniques, we show, under the assumptions of locality, Poincar\'e invariance, conservation of the number of gauge symmetries and the number of derivatives on each fields, that N=1 D=4 supergravity is the only consistent interaction between a massless spin 2 and a massless spin 3/2 field. We do not assume general covariance. This follows automatically, as does supersymmetry invariance. Various cohomologies related to conservations laws are also given.Comment: 22+1 pages, LaTeX. References adde

Generation of Entanglement Outside of the Light Cone

The Feynman propagator has nonzero values outside of the forward light cone. That does not allow messages to be transmitted faster than the speed of light, but it is shown here that it does allow entanglement and mutual information to be generated at space-like separated points. These effects can be interpreted as being due to the propagation of virtual photons outside of the light cone or as a transfer of pre-existing entanglement from the quantum vacuum. The differences between these two interpretations are discussed.Comment: 25 pages, 7 figures. Additional references and figur

On the Energy-Momentum Tensor of the Scalar Field in Scalar--Tensor Theories of Gravity

We study the dynamical description of gravity, the appropriate definition of the scalar field energy-momentum tensor, and the interrelation between them in scalar-tensor theories of gravity. We show that the quantity which one would naively identify as the energy-momentum tensor of the scalar field is not appropriate because it is spoiled by a part of the dynamical description of gravity. A new connection can be defined in terms of which the full dynamical description of gravity is explicit, and the correct scalar field energy-momentum tensor can be immediately identified. Certain inequalities must be imposed on the two free functions (the coupling function and the potential) that define a particular scalar-tensor theory, to ensure that the scalar field energy density never becomes negative. The correct dynamical description leads naturally to the Einstein frame formulation of scalar-tensor gravity which is also studied in detail.Comment: Submitted to Phys. Rev D15, 10 pages. Uses ReVTeX macro

Finite Size Effects in Thermal Field Theory

We consider a neutral self-interacting massive scalar field defined in a d-dimensional Euclidean space. Assuming thermal equilibrium, we discuss the one-loop perturbative renormalization of this theory in the presence of rigid boundary surfaces (two parallel hyperplanes), which break translational symmetry. In order to identify the singular parts of the one-loop two-point and four-point Schwinger functions, we use a combination of dimensional and zeta-function analytic regularization procedures. The infinities which occur in both the regularized one-loop two-point and four-point Schwinger functions fall into two distinct classes: local divergences that could be renormalized with the introduction of the usual bulk counterterms, and surface divergences that demand countertems concentrated on the boundaries. We present the detailed form of the surface divergences and discuss different strategies that one can assume to solve the problem of the surface divergences. We also briefly mention how to overcome the difficulties generated by infrared divergences in the case of Neumann-Neumann boundary conditions.Comment: 31 pages, latex, to appear in J. Math. Phy

A precise response function for the magnetic component of Gravitational Waves in Scalar-Tensor Gravity

The important issue of the magnetic component of gravitational waves (GWs) has been considered in various papers in the literature. From such analyses, it resulted that such a magnetic component becomes particularly important in the high frequency portion of the frequency range of ground based interferometers for GWs which arises from standard General Theory of Relativity (GTR). Recently, such a magnetic component has been extended to GWs arising from Scalar-Tensor Gravity (STG) too. After a review of some important issues on GWs in STG, in this paper we re-analyse the magnetic component in the framework of STG from a different point of view, by correcting an error in a previous paper and by releasing a more precise response function. In this way, we also show that if one neglects the magnetic contribution considering only the low-frequency approximation of the electric contribution, an important part of the signal could be, in principle, lost. The determination of a more precise response function for the magnetic contribution is important also in the framework of the possibility to distinguish other gravitational theories from GTR. At the end of the paper an expansion of the main results is also shown in order to recall the presence of the magnetic component in GRT too.Comment: Accepted for publication in Physical Review D, to be published during 2011. 36 pages, in this second version typos have been corrected and references have been update

Effective Lagrangian from Higher Curvature Terms: Absence of vDVZ Discontinuity in AdS Space

We argue that the van Dam-Veltman-Zakharov discontinuity arising in the $M^2 \to 0$ limit of the massive graviton through an explicit Pauli-Fierz mass term could be absent in anti de Sitter space. This is possible if the graviton can acquire mass spontaneously from the higher curvature terms or/and the massless limit $M^2\to 0$ is attained faster than the cosmological constant $\Lambda \to 0$. We discuss the effects of higher-curvature couplings and of an explicit cosmological term ($\Lambda$) on stability of such continuity and of massive excitations.Comment: 23 pages, Latex, the version to appear in Class. Quant. Gra