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Zurab Kakushadze

Publications and source records attributed to Zurab Kakushadze.

At least 19 recordsLinked to original sources
arXiv2014

Non-perturbative Massive de Sitter Solutions

We discuss non-perturbative dynamics of massive gravity in de Sitter space via gravitational Higgs mechanism. We argue that enhanced local symmetry and null (ghost) state at (below) the perturbative Higuchi bound are mere artifacts of not only linearization but also assuming the Fierz-Pauli mass term. We point out that, besides de Sitter, there are vacuum solutions where the space asymptotically is de Sitter both in the past and in the future, the space first contracts, this contraction slows down, and then reverses into expansion, so there is an epoch where the space appears to be (nearly) flat, even though the vacuum energy density is non-vanishing. We confirm this by constructing a closed-form exact solution to full non-perturbative equations of motion for a "special" massive de Sitter case. We give a formula for the "critical" mass above which such solutions apparently do not exist. For the Fierz-Pauli mass term this "critical" mass coincides with the perturbative Higuchi bound, and the former serves as the non-perturbative reinterpretation of the latter. We argue that, notwithstanding the perturbative ghost, non-perturbatively there is no "instability". Instead, there are additional vacuum solutions that may have interesting cosmological implications, which we briefly speculate on.

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arXiv2014

No vDVZ Discontinuity in Non-Fierz-Pauli Theories

In theories of massive gravity with Fierz-Pauli mass term at the linearized level, perturbative radially symmetric asymptotic solutions are singular in the zero mass limit, hence van Dam-Veltman-Zakharov (vDVZ) discontinuity. In this note, in the context of gravitational Higgs mechanism, we argue that in non-Fierz-Pauli theories, which non-perturbatively are unitary, perturbative radially symmetric asymptotic solutions have a smooth massless limit, hence no vDVZ discontinuity. Perturbative vDVZ discontinuity as an artifact of the Fierz-Pauli mass term becomes evident in the language of constrained gravity, which is the massless limit of gravitational Higgs mechanism.

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arXiv2014

Asymmetric Orbifolds and Wilson Lines

We generalize the rules for the free fermionic string construction to include other asymmetric orbifolds. Examples are given to illustrate the use of these rules.

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arXiv2014

Massive Gravity in Extra Dimensions

We discuss a Brane World scenario where we live on a 3-brane with massive gravity in infinite-volume bulk. The bulk graviton can be much heavier than the inverse Hubble size, as heavy as the bulk Planck scale, whose lower bound is roughly the inverse of 0.1 mm. The 4D Einstein-Hilbert term on the brane shields the brane matter from both strong bulk gravity and large bulk graviton mass. Gravity on the brane does not become higher-dimensional at large distances. Instead, at distance scales above the bulk Planck length, gravity on the brane behaves as 4D gravity with small graviton mass roughly of order or below the inverse Hubble size. Unlike the massless case, with massive gravity in the bulk one can have: i) 4D tensor structure on a codimension-1 brane; and ii) no infrared tachyon for smoothed-out higher codimension branes. The effects of the brane dynamics on the bulk are exponentially suppressed away from the brane. One consequence is that there are no "self-accelerated" solutions. In codimension-2 cases there exist nonsingular solutions with a flat 3-brane for a continuous positive range of the brane tension. In higher codimension cases, as in the massless case, higher curvature terms are required to obtain such solutions.

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arXiv2013

Non-perturbative Massive Solutions in Gravitational Higgs Mechanism

We construct exact non-perturbative massive solutions in the gravitational Higgs mechanism. They confirm the conclusions of arXiv:1102.4991, which are based on non-perturbative Hamiltonian analysis for the relevant metric degrees of freedom, that while perturbatively unitarity may not be evident, no negative norm state is present in the full nonlinear theory. The non-perturbative massive solutions do not appear to exhibit instabilities and describe vacuum configurations which are periodic in time, including purely longitudinal solutions with isotropic periodically expanding and contracting spatial dimensions, "cosmological strings" with only one periodically expanding and contracting spatial dimension, and also purely non-longitudinal ("traceless") periodically expanding and contracting solutions with constant spatial volume. As an aside we also discuss massive solutions in New Massive Gravity. While such solutions are present in the linearized theory, we argue that already at the next-to-linear (quadratic) order in the equations of motion (and, more generally, for weak-field configurations) there are no massive solutions.

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arXiv2011

Non-perturbative Unitarity of Gravitational Higgs Mechanism

In this paper we discuss massive gravity in Minkowski space via gravitational Higgs mechanism, which provides a non-perturbative definition thereof. Using this non-perturbative definition, we address the issue of unitarity by studying the full nonlinear Hamiltonian for the relevant metric degrees of freedom. While perturbatively unitarity is not evident, we argue that no negative norm state is present in the full nonlinear theory.

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arXiv2010

Massive Gravity in de Sitter Space via Gravitational Higgs Mechanism

In this paper we discuss massive gravity in de Sitter space via gravitational Higgs mechanism, which provides a nonlinear definition thereof. The Higgs scalars are described by a nonlinear sigma model, which includes higher derivative terms required to obtain the Fierz-Pauli mass term. Using the aforesaid non-perturbative definition, we address appearance of an enhanced local symmetry and a null norm state in the linearized massive gravity in de Sitter space at the special value of the graviton mass to the Hubble parameter ratio. By studying full non-perturbative equations of motion, we argue that there is no enhanced symmetry in the full nonlinear theory. We then argue that in the full nonlinear theory no null norm state is expected to arise at the aforesaid special value. This suggests that no ghost might be present for lower graviton mass values and the full nonlinear theory might be unitary for all values of the graviton mass and the Hubble parameter with no van Dam-Veltman-Zakharov discontinuity. We argue that this is indeed the case by studying full nonlinear Hamiltonian for the relevant conformal and helicity-0 longitudinal modes. In particular, we argue that no negative norm state is present in the full nonlinear theory.

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arXiv2007

Tachyon-Free Non-Supersymmetric Strings on Orbifolds

We discuss tachyon-free examples of (Type IIB on) non-compact non-supersymmetric orbifolds. Tachyons are projected out by discrete torsion between orbifold twists, while supersymmetry is broken by a Scherk-Schwarz phase (+1/-1 when acting on space-time bosons/fermions) accompanying some even order twists. The absence of tachyons is encouraging for constructing non-supersymmetric D3-brane gauge theories with stable infrared fixed points. The D3-brane gauge theories in our orbifold backgrounds have chiral N = 1 supersymmetric spectra, but non-supersymmetric interactions.

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arXiv2007

Massive Gravity in Minkowski Space via Gravitational Higgs Mechanism

In gravitational Higgs mechanism graviton components acquire mass in the process of spontaneous breaking of diffeomorphisms (via scalar vacuum expectation values). Recently, gravitational Higgs mechanism was discussed in the context of obtaining massive gravity directly in four dimensions. We discuss a setup, with all diffeomorphisms spontaneously broken by scalars, where gravitational Higgs mechanism results in massive gravity in Minkowski space with no non-unitary propagating modes. This is achieved by including higher derivative terms in the scalar sector. Decoupling the non-unitary mode (trace of the graviton) then requires tuning a single parameter, namely, the cosmological constant. We also discuss a setup where only spatial diffeomorphisms are broken. In this case we obtain massive gravity in Minkowski space with only transverse modes propagating, so the mode count is the same as for a massless graviton.

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arXiv2007

Massless Limit of Gravitational Higgs Mechanism

In gravitational Higgs mechanism graviton components acquire mass via spontaneous diffeomorphism breaking by scalar vacuum expectation values. We point out that in the massless limit the resulting theory is not Einstein-Hilbert gravity (EHG) but constrained gravity (CG). Consequently, massive solutions in the massless limit must be compared to those in CG (as opposed to EHG). We discuss spherically symmetric solutions in this context. The Schwarzschild solution in EHG can be coordinate-transformed such that it is also a solution in CG. The resulting solutions are non-perturbative in the asymptotic regime, and are reproduced in the massless limit of asymptotic massive solutions, hence no van Dam-Veltman-Zakharov discontinuity. We point out that higher curvature terms must be included to obtain non-singular spherically symmetric massive solutions and discuss a suitable framework.

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arXiv2007

Gravitational Higgs Mechanism and Massive Gravity

In arXiv:hep-th/0011245, in the context of domain wall backgrounds, it was shown that spontaneous breaking of diffeomorphism invariance results in gravitational Higgs mechanism. Recently in arXiv:0708.3184 't Hooft discussed gravitational Higgs mechanism in the context of obtaining massive gravity directly in four dimensions, and pointed out some subtleties with unitarity. We attribute these subtleties to breaking time-like diffeomorphisms, and discuss gravitational Higgs mechanism with all but time-like diffeomorphisms spontaneously broken. The resulting background is no longer flat but exhibits expansion, which is linear in time. For space-time dimensions D <= 10 the background is stable and has no non-unitary propagating modes. The absence of non-unitary modes is due to the unbroken time-like diffeomorphism invariance. The physical states correspond to those of a massive graviton. The effective mass squared of the graviton is positive for D < 10, and vanishes for D = 10. For D > 10 the graviton modes become effectively tachyonic. The special value of D=10, which coincides with the critical dimension of superstring theory, arises in our setup completely classically.

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arXiv2003

A Novel Approach to the Cosmological Constant Problem

We propose a novel infinite-volume brane world scenario where we live on a non-inflating spherical 3-brane, whose radius is somewhat larger than the present Hubble size, embedded in higher dimensional bulk. Once we include higher curvature terms in the bulk, we find completely smooth solutions with the property that the 3-brane world-volume is non-inflating for a continuous range of positive values of the brane tension, that is, without fine-tuning. In particular, our solution, which is a near-BPS background with supersymmetry broken on the brane around TeV, is controlled by a single integration constant.

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arXiv2002

Toward Solving the Cosmological Constant Problem?

We discuss the cosmological constant problem in the context of higher codimension brane world scenarios with infinite-volume extra dimensions. In particular, by adding higher curvature terms in the bulk action we are able to find smooth solutions with the property that the 4-dimensional part of the brane world-volume is flat for a range of positive values of the brane tension.

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arXiv2002

A Remark on Smoothing Out Higher Codimension Branes

We discuss some issues arising in studying (linearized) gravity on non-BPS higher codimension branes in an infinite-volume bulk. In particular, such backgrounds are badly singular for codimension-3 and higher delta-function-like branes with non-zero tension. As we discuss in this note, non-trivial issues arise in smoothing out such singularities. Thus, adding higher curvature terms might be necessary in this context.

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arXiv2002

On Gauge Dynamics and SUSY Breaking in Orientiworld

In the Orientiworld framework the Standard Model fields are localized on D3-branes sitting on top of an orientifold 3-plane. The transverse 6-dimensional space is a non-compact orbifold (or a more general conifold). The 4-dimensional gravity on D3-branes is reproduced due to the 4-dimensional Einstein-Hilbert term induced at the quantum level. The orientifold 3-plane plays a crucial role, in particular, without it the D3-brane world-volume theories would be conformal due to the tadpole cancellation. We study non-perturbative gauge dynamics in various N=1 supersymmetric orientiworld models based on the Z_3 as well as Z_5 and Z_7 orbifold groups. Our discussions illustrate that there is a rich variety of supersymmetry preserving dynamics in some of these models. On the other hand, we also find some models with dynamical supersymmetry breaking.

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arXiv2001

Orientiworld

We propose a consistent string theory framework for embedding brane world scenarios with infinite-volume extra dimensions. In this framework the Standard Model fields are localized on D3-branes sitting on top of an orientifold 3-plane. The transverse 6-dimensional space is a non-compact orbifold or a more general conifold. The 4-dimensional gravity on D3-branes is reproduced due to the 4-dimensional Einstein-Hilbert term induced at the quantum level. The orientifold 3-plane plays a crucial role, in particular, without it the D3-brane world-volume theories would be conformal due to the tadpole cancellation. We point out that in some cases the 4-dimensional Planck scale is controlled by the size of certain relevant (as opposed to marginal) orbifold blow-ups. We can then have a scenario with the desirable 4-dimensional Planck scale, the string scale of order TeV, and the cross-over to 10-dimensional gravity around the present Hubble size. We discuss some general features as well as concrete models in this ``Orientiworld'' framework, including those with D7-branes. We point out that the D7-brane gauge symmetry at the quantum level becomes part of the 4-dimensional gauge symmetry. We present an N=1 supersymmetric model with 3 chiral generations of quarks and leptons, where the original gauge group (which contains an SU(6) subgroup) can be Higgsed to obtain a Pati-Salam model with 3 chiral generations, the Pati-Salam Higgs fields required to break the gauge group further to that of the Standard Model, as well as the desired electroweak Higgs doublets. The superpotential in this model is such that we have precisely one heavy (top-like) generation.

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arXiv2001

Gravity on a 3-brane in 6D Bulk

We study gravity in codimension-2 brane world scenarios with infinite volume extra dimensions. In particular, we consider the case where the brane has non-zero tension. The extra space then is a two-dimensional ``wedge'' with a deficit angle. In such backgrounds we can effectively have the Einstein-Hilbert term on the brane at the classical level if we include higher curvature (Gauss-Bonnet) terms in the bulk. Alternatively, such a term would be generated at the quantum level if the brane matter is not conformal. We study (linearized) gravity in the presence of the Einstein-Hilbert term on the brane in such backgrounds. We find that, just as in the original codimension-2 Dvali-Gabadadze model with a tensionless brane, gravity is almost completely localized on the brane with ultra-light modes penetrating into the bulk.

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