arXiv subjects

G. Gabadadze

Publications and source records attributed to G. Gabadadze.

15 recordsLinked to original sources

arXiv2013

Mixed Galileons and Spherically Symmetric Solutions

It was previously found that in a certain parameter subspace of scalar-tensor theories emerging from massive gravity, the only stable field configuration created by static spherically symmetric sources was one with cosmological asymptotics. Moreover, these backgrounds were shown to be sub-luminal everywhere in the space; in contrast to the common believe that these theories are necessarily superluminal in the vicinity of a static source. In this work we complete that analysis by extending it to cover the whole parameter space of these scalar-tensor theories. We find that the stability argument renders the asymptotically flat backgrounds unrealizable, forcing once again for cosmological asymptotics. In the case of pressureless sources these backgrounds are stable. However, they get destabilized in the presence of positive pressure, larger than a critical density. Even on the self-accelerated background, on which the scalar mode decouples from sources, in the region occupied by the source it acquires an elliptic equation of motion. Therefore, we conclude that the only parameter space which is not ruled out, by solar system measurements, is the one considered in Berezhiani {\it et al.} (arXiv:1302.0549), namely the one for which the scalar and tensor modes can be diagonalized via local transformations. We also reinvestigate the scale at which perturbation theory breaks down in a general Galileon theory. We show that the Vainshtein mechanism successfully redresses the strong coupling scale to a small one, just like in the cubic Galileon, despite the cancellations occurring in the special spherically symmetric case. We emphasize that even if these tests were performed at scales at which perturbation theory broke down, these could not be interpreted as a lower bound for the graviton mass.

hep-th↗

arXiv2011

On Black Holes in Massive Gravity

In massive gravity the so-far-found black hole solutions on Minkowski space happen to convert horizons into a certain type of singularities. Here we explore whether these singularities can be avoided if space-time is not asymptotically Minkowskian. We find an exact analytic black hole (BH) solution which evades the above problem by a transition at large scales to self-induced de Sitter (dS) space-time, with the curvature scale set by the graviton mass. This solution is similar to the ones discovered by Koyama, Niz and Tasinato, and by Nieuwenhuizen, but differs in detail. The solution demonstrates that in massive GR, in the Schwarzschild coordinate system, a BH metric has to be accompanied by the Stückelberg fields with nontrivial backgrounds to prevent the horizons to convert into the singularities. We also find an analogous solution for a Reissner-Nordström BH on dS space. A limitation of our approach, is that we find the solutions only for specific values of the two free parameters of the theory, for which both the vector and scalar fluctuations loose their kinetic terms, however, we hope our solutions represent a broader class with better behaved perturbations.

hep-th↗

arXiv2011

Massive Cosmologies

We explore the cosmological solutions of a recently proposed extension of General Relativity with a Lorentz-invariant mass term. We show that the same constraint that removes the Boulware-Deser ghost in this theory also prohibits the existence of homogeneous and isotropic cosmological solutions. Nevertheless, within domains of the size of inverse graviton mass we find approximately homogeneous and isotropic solutions that can well describe the past and present of the Universe. At energy densities above a certain crossover value, these solutions approximate the standard FRW evolution with great accuracy. As the Universe evolves and density drops below the crossover value the inhomogeneities become more and more pronounced. In the low density regime each domain of the size of the inverse graviton mass has essentially non-FRW cosmology. This scenario imposes an upper bound on the graviton mass, which we roughly estimate to be an order of magnitude below the present-day value of the Hubble parameter. The bound becomes especially restrictive if one utilizes an exact self-accelerated solution that this theory offers. Although the above are robust predictions of massive gravity with an explicit mass term, we point out that if the mass parameter emerges from some additional scalar field condensation, the constraint no longer forbids the homogeneous and isotropic cosmologies. In the latter case, there will exist an extra light scalar field at cosmological scales, which is screened by the Vainshtein mechanism at shorter distances.

hep-th↗

arXiv2008

DUSEL Theory White Paper

The NSF has chosen the site for the Deep Underground Science and Engineering Laboratory (DUSEL) to be in Lead, South Dakota. In fact, the state of South Dakota has already stepped up to the plate and contributed its own funding for the proposed lab, see http://www.sanfordlaboratoryathomestake.org/index.html. The final decision by NSF for funding the Initial Suite of Experiments for DUSEL will be made early in 2009. At that time the NSF Science Board must make a decision. Of order 200 experimentalists have already expressed an interest in performing experiments at DUSEL. In order to assess the interest of the theoretical community, the Center for Cosmology and Astro-Particle Physics (CCAPP) at The Ohio State University (OSU) organized a 3-day DUSEL Theory Workshop in Columbus, Ohio from April 4 - 6, 2008. The workshop focused on the scientific case for six proposed experiments for DUSEL: long baseline neutrino oscillations, proton decay, dark matter, astrophysical neutrinos, neutrinoless double beta decay and N-Nbar oscillations. The outcome of this workshop is the DUSEL Theory White paper addressing the scientific case at a level which may be useful in the decision making process for policy makers at the NSF and in the U.S. Congress. In order to assess the physics interest in the DUSEL project we have posted the DUSEL Theory White paper on the following CCAPP link http://ccapp.osu.edu/whitepaper.html . Please read the white paper and, if you are interested, use the link to show your support by co-signing the white paper.

hep-ph↗

arXiv2003

Softly Massive Gravity

Large-distance modification of gravity may be the mechanism for solving the cosmological constant problem. A simple model of the large-distance modification -- four-dimensional (4D) gravity with the hard mass term-- is problematic from the theoretical standpoint. Here we discuss a different model, the brane-induced gravity, that effectively introduces a soft graviton mass. We study the issues of unitarity, analyticity and causality in this model in more than five dimensions. We show that a consistent prescription for the poles of the Green's function can be specified so that 4D unitarity is preserved. However, in certain instances 4D analyticity cannot be maintained when theory becomes higher dimensional. As a result, one has to sacrifice 4D causality at distances of the order of the present-day Hubble scale. This is a welcome feature for solving the cosmological constant problem, as was recently argued in the literature. We also show that, unlike the 4D massive gravity, the model has no strong-coupling problem at intermediate scales.

hep-th↗

arXiv2001

Comments on "A Supernova Brane Scan"

Recently we proposed that the acceleration of the Universe could be due to gravity leaking to extra dimensions. This scenario gives an alternative to the cosmological constant or quintessence. In [astro-ph/0106274] the authors claimed that this proposal is strongly disfavored if not ruled out by the existing data on Supernovae, Cosmic Microwave Background and clusters. We show that the work [astro-ph/0106274] contains incorrect statements in the theoretical part, and, moreover, we argue that the main conclusions of the work concerning the comparison with the data are premature.

astro-ph↗

arXiv2000

(Quasi)Localized Gauge Field on a Brane: Dissipating Cosmic Radiation to Extra Dimensions?

We propose a mechanism ensuring (quasi)localization of massless gauge fields on a brane. The mechanism does not rely on BPS properties of the brane and can be realized in any theory where charged particles are confined to the world-volume. The localized matter fluctuations induce a gauge kinetic term on the brane. At short distances the resulting propagator for the gauge field is {\it identical} to the four-dimensional propagator. The gauge theory on the brane is effectively four-dimensional at short distances; it becomes higher-dimensional on very large (cosmic) scales. The brane-bulk system exhibits the phenomenon of ``infrared transparency''. As a result, only very low frequency modes can escape into extra dimensions. In this framework the large wavelength cosmic radiation can dissipate in extra space at a rate that may be observable, in principle. We briefly discuss some astrophysical consequences of this scenario. The same mechanism of localization of gauge fields may work in Kaplan's framework for domain wall chiral fermions on lattices.

hep-th↗

arXiv2000

Vacuum Structure and the Axion Walls in Gluodynamics and QCD with Light Quarks

Large N gluodynamics was shown to have a set of metastable vacua with the gluonic domain walls interpolating between them. The walls may separate the genuine vacuum from an excited one, or two excited vacua which are unstable at finite N (here N is the number of colors). One may attempt to stabilize them by switching on the axion field. We study how the light quarks and the axion affect the structure of the domain walls. In pure gluodynamics (with the axion field) the axion walls acquire a very hard gluonic core. Thus, we deal with a wall "sandwich" which is stable at finite N. In the case of the minimal axion, the wall "sandwich" is in fact a "2-pi" wall, i.e., the corresponding field configuration interpolates between identical hadronic vacua. The same properties hold in QCD with three light quarks and very large number of colors. However, in the realistic case of three-color QCD the phase corresponding to the axion field profile in the axion wall is screened by a dynamical phase associated with the eta-prime, so that the gluon component of the wall is not excited. We propose a toy Lagrangian which models these properties and allows one to get exact solutions for the domain walls.

hep-ph↗

arXiv2000

A Comment on Brane Bending and Ghosts in Theories with Infinite Extra Dimensions

Theories with infinite volume extra dimensions open exciting opportunities for particle physics. We argued recently that along with attractive features there are phenomenological difficulties in this class of models. In fact, there is no graviton zero-mode in this case and 4D gravity is obtained by means of continuum bulk modes. These modes have additional degrees of freedom which do not decouple at low energies and lead to inconsistent predictions for light bending and the precession of Mercury's perihelion. In a recent papers, [hep-th/0003020] and [hep-th/0003045] the authors made use of brane bending in order to cancel the unwanted physical polarization of gravitons. In this note we point out that this mechanism does not solve the problem since it uses a {\it ghost} which cancels the extra degrees of freedom. In order to have a consistent model the ghost should be eliminated. As soon as this is done, 4D gravity becomes unconventional and contradicts General Relativity. New mechanisms are needed to cure these models. We also comment on the possible decoupling of the ghost at large distances due to an apparent flat-5D nature of space-time and on the link between the presence of ghosts and the violation of positive-energy conditions.

hep-th↗

arXiv2000

Metastable Gravitons and Infinite Volume Extra Dimensions

We address the issue of whether extra dimensions could have an infinite volume and yet reproduce the effects of observable four-dimensional gravity on a brane. There is no normalizable zero-mode graviton in this case, nevertheless correct Newton's law can be obtained by exchanging bulk gravitons. This can be interpreted as an exchange of a single {\it metastable} 4D graviton. Such theories have remarkable phenomenological signatures since the evolution of the Universe becomes high-dimensional at very large scales. Furthermore, the bulk supersymmetry in the infinite volume limit might be preserved while being completely broken on a brane. This gives rise to a possibility of controlling the value of the bulk cosmological constant. Unfortunately, these theories have difficulties in reproducing certain predictions of Einstein's theory related to relativistic sources. This is due to the van Dam-Veltman-Zakharov discontinuity in the propagator of a massive graviton. This suggests that all theories in which contributions to effective 4D gravity come predominantly from the bulk graviton exchange should encounter serious phenomenological difficulties.

hep-th↗

arXiv1999

D-Walls and Junctions in Supersymmetric Gluodynamics in the Large N Limit Suggest the Existence of Heavy Hadrons

A number of arguments exists that the "minimal" BPS wall width in large N supersymmetric gluodynamics vanishes as 1/N. There is a certain tension between this assertion and the fact that the mesons coupled to the gluino condensate have masses which do not scale with N. To reconcile these facts we argue that there should exist additional soliton-like states with masses scaling as N. The BPS walls must be "made" predominantly of these heavy states which are coupled to the gluino condensate stronger than the conventional mesons. The tension of the BPS wall junction scales as N^2, which serves as an additional argument in favor of the 1/N scaling of the wall width. The heavy states can be thought of as solitons of the corresponding closed string theory. They are related to certain fivebranes in the M-theory construction. We study the issue of the wall width in toy models which capture some features of supersymmetric gluodynamics. We speculate that special hadrons with mass scaling as N should also exist in the large N limit of non-supersymmetric gluodynamics.

hep-th↗

arXiv1998

The Spectrum and Effective Action of SUSY Gluodynamics

We study the low-energy spectrum of SUSY gluodynamics using the generating functional for Green's functions of composite fields. Two dual formulations of the generating functional approach are given. Masses of the bound states are calculated and mixing patterns are discussed. Mass splittings of pure gluonic states, in the case when supersymmetry is softly broken, are consistent with predictions of conventional Yang-Mills theory. The results can be tested in lattice simulations of the SUSY Yang-Mills model.

hep-ph↗

arXiv1998

The Discrete $Z_{2N_c}$ Symmetry And Effective Superpotential In SUSY Gluodynamics

We find an expression for the effective superpotential describing the $N_c$ vacua of $SU(N_c)$ SUSY gluodynamics. The superpotential reduces in some approximation to the Veneziano-Yankielowicz expression amended by the term restoring the discrete $Z_{2N_c}$ symmetry. Moreover, the superpotential, being restricted to one particular vacuum state, yields the expression which was derived recently to describe all the lowest-spin physical states of the theory. The corresponding scalar potential has no cusp singularities and can be used to study the domain walls interpolating between the chirally asymmetric vacua of the model.

hep-th↗

arXiv1998

The Spectrum of Softly Broken N=1 Supersymmetric Yang-Mills Theory

We study the spectrum of the softly broken generalized Veneziano-Yankielowicz effective action for N=1 SUSY Yang-Mills theory. Two dual formulations of the effective action are given. The spurion method is used for the soft SUSY breaking. Masses of the bound states are calculated and mixing patterns are discussed. Mass splittings of pure gluonic states are consistent with predictions of conventional Yang-Mills theory. The results can be tested in lattice simulations of the SUSY Yang-Mills model.

hep-th↗

arXiv1997

On the Effective Action of N=1 Supersymmetric Yang-Mills Theory

We propose a generalization of the Veneziano-Yankielowicz effective low-energy action for N=1 SUSY Yang-Mills theory which includes composite operators interpolating pure gluonic bound states. The chiral supermultiplet of anomalies is embedded in a larger three-form multiplet and an extra term in the effective action is introduced. The mass spectrum and mixing of the lowest-spin bound states are studied within the effective Lagrangian approach. The physical mass eigenstates form two multiplets, each containing a scalar, pseudoscalar and Weyl fermion. The multiplet containing the states which are most closely related to glueballs is the lighter one.

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