Self-Protection of Massive Cosmological Gravitons

Relevant deformations of gravity present an exciting window of opportunity to probe the rigidity of gravity on cosmological scales. For a single-graviton theory, the leading relevant deformation constitutes a graviton mass term. In this paper, we investigate the classical and quantum stability of massive cosmological gravitons on generic Friedman backgrounds. For a Universe expanding towards a de Sitter epoch, we find that massive cosmological gravitons are self-protected against unitarity violations by a strong coupling phenomenon.Comment: 1+11 pages, v2: references adde

Consistency of Relevant Cosmological Deformations on all Scales

Using cosmological perturbation theory we show that the most relevant defor- mation of gravity is consistent at the linear level. In particular, we prove the absence of uni- tarity violating negative norm states in the weak coupling regime from sub- to super-Hubble scales. This demonstrates that the recently proposed classical self-protection mechanism of deformed gravity extends to the entire kinematical domain.Comment: 22 pages, 4 figure

Geometry of percolating monopole clusters

We perform detailed measurements of the geometrical characteristics of the percolating cluster of the magnetic monopole currents in the confining phase of the lattice SU(2) gluodynamics. The Maximal Abelian projection is used to define the monopoles. The use of the geometrical language is motivated by recent observations that the full non-Abelian action associated with the monopoles corresponds to point-like particles on the currently available lattices. Scaling behavior of various quantities is observed.Comment: 3 pages, 4 figures, Lattice2002(topology

Superluminality in DGP

We reconsider the issue of superluminal propagation in the DGP model of infrared modified gravity. Superluminality was argued to exist in certain otherwise physical backgrounds by using a particular, physically relevant scaling limit of the theory. In this paper, we exhibit explicit five-dimensional solutions of the full theory that are stable against small fluctuations and that indeed support superluminal excitations. The scaling limit is neither needed nor invoked in deriving the solutions or in the analysis of its small fluctuations. To be certain that the superluminality found here is physical, we analyze the retarded Green's function of the scalar excitations, finding that it is causal and stable, but has support on a widened light-cone. We propose to use absence of superluminal propagation as a method to constrain the parameters of the DGP model. As a first application of the method, we find that whenever the 4D energy density is a pure cosmological constant and a hierarchy of scales exists between the 4D and 5D Planck masses, superluminal propagation unavoidably occurs.Comment: 23 pages. Minor corrections. Version to appear in JHE

On Non Perturbative Corrections to the Potential for Heavy Quarks

We discuss non perturbative corrections to the Coulomb-like potential of heavy quarks at short distances. We consider both the standard framework provided by infrared renormalons and the assumption that confinement does not allow weak fields to penetrate the vacuum. In the former case the leading correction at short distances turns out to be quadratic in r for static quarks. In the latter case we find a potential which is proportional to r as r rightarrow 0. We point out that similar effects arise due to a new kind of non perturbative correction proportional to 1/Q^2, which is unaccounted for by the operator product expansion and which was recently discussed within a different framework. Phenomenological implications of the linear correction to the potential are briefly reviewed.Comment: 13 pages, latex, 2 figures, uses eps

Self-tuning of the P-vortices

We observe that on the currently available lattices the non-Abelian action associated with the P-vortices is ultraviolet divergent. On the other hand, the total area of the vortices scales in physical units. Since both the ultraviolet and infrared scales are manifested and there is no parameter to tune, the observed phenomenon can be called self tuning.Comment: Lattice2003(topology

On the Universality of the Leading, 1/Q1/Q Power Corrections in QCD

We discuss $1/Q$ corrections to hard processes in QCD where $Q$ is a large mass parameter like the total energy in $e^+e^-$ annihilation. The main problem we address ourselves to is whether these corrections to different processes (concentrating for definiteness on the Thrust and the Drell-Yan cross section) can be related to each other in a reliable way so that the phenomenology of the $1/Q$ corrections can be developed. We derive first the relation valid to lowest order using both the renormalon and finite-gauge-boson mass techniques to check its independence on the infrared cut- off procedure. We then argue that the $1/Q$ corrections are due to soft gluons which factorize into a universal factor such that the lowest order relations are preserved in higher orders.Comment: 12 page

Variational principle for frozen-in vortex structures interacting with sound waves

General properties of conservative hydrodynamic-type models are treated from positions of the canonical formalism adopted for liquid continuous media, with applications to the compressible Eulerian hydrodynamics, special- and general-relativistic fluid dynamics, and two-fluid plasma model including the Hall-magnetohydrodynamics. A variational formulation is found for motion and interaction of frozen-in localized vortex structures and acoustic waves in a special description where dynamical variables are, besides the Eulerian fields of the fluid density and the potential component of the canonical momentum, also the shapes of frozen-in lines of the generalized vorticity. This variational principle can serve as a basis for approximate dynamical models with reduced number of degrees of freedom.Comment: 7 pages, revtex4, no figure

The Vainshtein mechanism in the Decoupling Limit of massive gravity

We investigate static spherically symmetric solutions of nonlinear massive gravities. We first identify, in an ansatz appropriate to the study of those solutions, the analog of the decoupling limit (DL) that has been used in the Goldstone picture description. We show that the system of equations left over in the DL has regular solutions featuring a Vainshtein-like recovery of solutions of General Relativity (GR). Hence, the singularities found to arise integrating the full nonlinear system of equations are not present in the DL, despite the fact those singularities are usually thought to be due to a negative energy mode also seen in this limit. Moreover, we show that the scaling conjectured by Vainshtein at small radius is only a limiting case in an infinite family of non singular solutions each showing a Vainshtein recovery of GR solutions below the Vainshtein radius but a different common scaling at small distances. This new scaling is shown to be associated with a zero mode of the nonlinearities left over in the DL. We also show that, in the DL, this scaling allows for a recovery of GR solutions even for potentials where the original Vainshtein mechanism is not working. Our results imply either that the DL misses some important features of nonlinear massive gravities or that important features of the solutions of the full nonlinear theory have been overlooked. They could also have interesting outcomes for the DGP model and related proposals.Comment: 52 pages, 7 figures; v3: minor typos corrected, discussion of the validity of the Decoupling Limit extended; accepted for publication in JHE

A Property of Recombination in Polarized Hadronic Targets

The triple gluon-ladder vertex is shown to project the outgoing gluon in either polarization state with equal probability up to the leading double-ln(x)ln($Q^2$) approximation. This implies that the $Q^2$-evolution of $\Delta G (x, Q^2)$ is free from recombination effects to this level of approximation.Comment: latex, 6 pages AZPH-TH/92-29 Phys. Rev D 50 in press 199