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.Comment: 9 LaTex pages; One reference and a comment adde

Special Massive Spin-2 on de Sitter Space

The theory of a massive spin-2 state on the de Sitter space -- with the mass squared equal to one sixth of the curvature -- is special for two reasons: (i) it exhibits an enhanced local symmetry; (ii) it emerges as a part of the model that gives rise to the self-accelerated Universe. The known problems of this theory are: either it cannot be coupled to a non-conformal conserved stress-tensor because of the enhanced symmetry, or it propagates a ghost-like state when the symmetry is constrained by the Lagrange multiplier method. Here we propose a solution to these problems in the linearized approximation.Comment: 9 pages, reference added, JCAP versio

Ultralight Scalars and Spiral Galaxies

We study some possible astrophysical implications of a very weakly coupled ultralight dilaton-type scalar field. Such a field may develop an (approximately stable) network of domain walls. The domain wall thickness is assumed to be comparable with the thickness of the luminous part of the spiral galaxies. The walls provide trapping for galactic matter. This is used to motivate the very existence of the spiral galaxies. A zero mode existing on the domain wall is a massless scalar particle confined to 1+2 dimensions. At distances much larger than the galaxy/wall thickness, the zero-mode exchange generates a logarithmic potential, acting as an additional term with respect to Newton's gravity. The logarithmic term naturally leads to constant rotational velocities at the periphery. We estimate the scalar field coupling to the matter energy-momentum tensor needed to fit the observable flat rotational curves of the spiral galaxies. The value of this coupling turns out to be reasonable -- we find no contradiction with the existing data.Comment: 19 pages, 2 eps figures; extra references and two important Comments adde

Braneworld Flattening by a Cosmological Constant

We present a model with an infinite volume bulk in which a braneworld with a cosmological constant evolves to a static, 4-dimensional Minkowski spacetime. This evolution occurs for a generic class of initial conditions with positive energy densities. The metric everywhere outside the brane is that of a 5-dimensional Minkowski spacetime, where the effect of the brane is the creation of a frame with a varying speed of light. This fact is encoded in the structure of the 4-dimensional graviton propagator on the braneworld, which may lead to some interesting Lorentz symmetry violating effects. In our framework the cosmological constant problem takes a different meaning since the flatness of the Universe is guaranteed for an arbitrary negative cosmological constant. Instead constraints on the model come from different concerns which we discuss in detail.Comment: 18 pages, 3 figures RevTe

Graviton Mass or Cosmological Constant?

To describe a massive graviton in 4D Minkowski space-time one introduces a quadratic term in the Lagrangian. This term, however, can lead to a readjustment or instability of the background instead of describing a massive graviton on flat space. We show that for all local Lorentz-invariant mass terms Minkowski space is unstable. We start with the Pauli-Fierz (PF) term that is the only local mass term with no ghosts in the linearized approximation. We show that nonlinear completions of the PF Lagrangian give rise to instability of Minkowski space. We continue with the mass terms that are not of a PF type. Although these models are known to have ghosts in the linearized approximations, nonlinear interactions can lead to background change due to which the ghosts are eliminated. In the latter case, however, the graviton perturbations on the new background are not massive. We argue that a consistent theory of a massive graviton on flat space can be formulated in theories with extra dimensions. They require an infinite number of fields or non-local description from a 4D point of view.Comment: 16 pages; references and comments adde

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.Comment: 22 pages Latex, no figure

The Power of Brane-Induced Gravity

We study the role of the brane-induced graviton kinetic term in theories with large extra dimensions. In five dimensions we construct a model with a TeV-scale fundamental Planck mass and a {\it flat} extra dimension the size of which can be astronomically large. 4D gravity on the brane is mediated by a massless zero-mode, whereas the couplings of the heavy Kaluza-Klein modes to ordinary matter are suppressed. The model can manifest itself through the predicted deviations from Einstein theory in long distance precision measurements of the planetary orbits. The bulk states can be a rather exotic form of dark matter, which at sub-solar distances interact via strong 5D gravitational force. We show that the induced term changes dramatically the phenomenology of sub-millimeter extra dimensions. For instance, high-energy constraints from star cooling or cosmology can be substantially relaxed.Comment: 24 pages, 4 eps figures; v2 typos corrected; v3 1 ref. added; PRD versio

A short review of "DGP Specteroscopy"

In this paper we provide a short review of the main results developed in hep-th/0604086. We focus on linearised vacuum perturbations about the self-accelerating branch of solutions in the DGP model. These are shown to contain a ghost in the spectrum for any value of the brane tension. We also comment on hep-th/0607099, where some counter arguments have been presented.Comment: Minor typos correcte

Solitonic D-branes and brane annihilation

We point out some intriguing analogies between field theoretic solitons (topological defects) and D-branes. Annihilating soliton-antisoliton pairs can produce stable solitons of lower dimensionality. Solitons that localize massless gauge fields in their world volume automatically imply the existence of open flux tubes ending on them and closed flux tubes propagating in the bulk. We discuss some aspects of this localization on explicit examples of unstable wall-anti-wall systems. The annihilation of these walls can be described in terms of tachyon condensation which renders the world-volume gauge field non-dynamical. During this condensation the world volume gauge fields (open string states) are resonantly excited. These can later decay into closed strings, or get squeezed into a network flux tubes similar to a network of cosmic strings formed at a cosmological phase transition. Although, as in the $D$-brane case, perturbatively one can find exact time-dependent solutions, when the energy of the system stays localized in the plane of the original soliton, such solutions are unstable with respect to decay into open and closed string states. Thus, when a pair of such walls annihilates, the energy is carried away (at least) by closed string excitations (``glueballs''), which are the lowest energy excitations about the bulk vacuum. Suggested analogies can be useful for the understanding of the complicated D-brane dynamics and of the production of topological defects and reheating during brane collision in the early universe.Comment: a typo correcte