Alberto Iglesias

Schwarzschild solution in brane induced gravity

The metric of a Schwarzschild solution in brane induced gravity in five dimensions is studied. We find a nonperturbative solution for which an exact expression on the brane is obtained. We also find a linearized solution in the bulk and argue that a nonsingular exact solution in the entire space should exist. The exact solution on the brane is highly nontrivial as it interpolates between different distance scales. This part of the metric is enough to deduce an important property--the Arnowitt-Deser-Misner canonical formalism (ADM) mass of the solution is suppressed compared to the bare mass of a static source. This screening of the mass is due to nonlinear interactions which give rise to a nonzero curvature outside the source. The curvature extends away from the source to a certain macroscopic distance that coincides with the would-be strong interaction scale. The very same curvature shields the source from strong coupling effects. The four-dimensional law of gravity, including the correct tensorial structure, is recovered at observable distances. We find that the solution has no van Dam-Veltman-Zakharov discontinuity and show that the gravitational field on the brane is always weak, in spite of the fact that the solution is nonperturbative.

Perturbations of the self-accelerated Universe

We discuss small perturbations on the self-accelerated solution of the Dvali–Gabadadze–Porrati model, and argue that claims of instability of the solution that are based on linearized calculations are unwarranted because of the following. (1) Small perturbations of an empty self-accelerated background can be quantized consistently without yielding ghosts. (2) Conformal sources, such as radiation, do not give rise to instabilities either. (3) A typical non-conformal source could introduce ghosts in the linearized approximation and become unstable; however, it also invalidates the approximation itself. Such a source creates a halo of variable curvature that locally dominates over the self-accelerated background and extends over a domain in which the linearization breaks down. Perturbations that are valid outside the halo may not continue inside, as is suggested by some non-perturbative solutions. (4) In the Euclidean continuation of the theory, with arbitrary sources, we derive certain constraints imposed by the second order equations on first order perturbations, thus restricting the linearized solutions that could be continued to the full non-linear theory. Naive linearized solutions fail to satisfy the above constraints. (5) Finally, we clarify in detail subtleties associated with the boundary conditions and analytic properties of the Greens functions.

How (not) to use the Palatini formulation of scalar-tensor gravity

We revisit the problem of defining non-minimal gravity in the first order formalism. Specializing to scalar-tensor theories, which may be disguised as ‘higher-derivative’ models with the gravitational Lagrangians that depend only on the Ricci scalar, we show how to recast these theories as Palatini-like gravities. The correct formulation utilizes the Lagrange multiplier method, which preserves the canonical structure of the theory, and yields the conventional metric scalar-tensor gravity. We explain the discrepancies between the näıve Palatini and the Lagrange multiplier approach, showing that the näıve Palatini approach really swaps the theory for another. The differences disappear only in the limit of ordinary General Relativity, where an accidental redundancy ensures that the näıve Palatini works there. We outline the correct decoupling limits and the strong coupling regimes. As a corollary we find that the so-called ‘Modified Source Gravity’ models suffer from strong coupling problems at very low scales, and hence cannot be a realistic approximation of our universe. We also comment on a method to decouple the extra scalar using the chameleon mechanism. iglesias@physics.ucdavis.edu kaloper@physics.ucdavis.edu antonio.padilla@nottingham.ac.uk park@physics.ucdavis.edu

Gravity on a 3-brane in 6D bulk

Abstract 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.

Short distance non-perturbative effects of large distance modified gravity

In a model of large distance modified gravity we compare the non-perturbative Schwarzschild solution of [G. Gabadadze, A. Iglesias, Phys. Rev. D 72 (2005) 084024, hep-th/0407049] to approximate solutions obtained previously. In the regions where there is a good qualitative agreement between the two, the non-perturbative solution yields effects that could have observational significance. These effects reduce, by a factor of a few, the predictions for the additional precession of the orbits in the Solar system, still rendering them in an observationally interesting range. The very same effects lead to a mild anomalous scaling of the additional scale-invariant precession rate found by Lue and Starkman.

De)coupling Limit of DGP

We investigate the decoupling limit in the DGP model of gravity by studying its non-linear equations of motion. We show that, unlike 4D massive gravity, the limiting theory does not reduce to a sigma model of a single scalar field: Non-linear mixing terms of the scalar with a tensor also survive. Because of these terms physics of DGP is different from that of the scalar sigma model. We show that the static spherically-symmetric solution of the scalar model found in [A. Nicolis, R. Rattazzi, JHEP 0406 (2004) 059, hep-th/0404159], is not a solution of the full set of non-linear equations. As a consequence of this, the interesting result on hidden superluminality uncovered recently in the scalar model in [A. Adams, N. Arkani-Hamed, S. Dubovsky, A. Nicolis, R. Rattazzi, hep-th/0602178], is not applicable to the DGP model of gravity. While the sigma model violates positivity constraints imposed by analyticity and the Froissart bound, the latter cannot be applied here because of the long-range tensor interactions that survive in the decoupling limit. We discuss further the properties of the Schwarzschild solution that exhibits the gravitational mass-screening phenomenon.

Fundamental Plasmid Strings and Black Rings

We construct excited states of fundamental strings that admit a semiclassical description as rotating circular loops of string. We identify them with the supergravity solutions for rotating dipole rings. The identification involves a precise match of the mass, radius and angular momentum of the two systems. Moreover, the degeneracy of the string state reproduces the parametric dependence of the entropy in the supergravity description. When the solutions possess two macroscopic angular momenta, they are better described as toroidal configurations (tubular loops) instead of loops of string. We argue that the decay of the string state can be interpreted as superradiant emission of quanta from the ergoregion of the rotating ring.

Strings at the bottom of the deformed conifold

We present solutions of the equations of motion of macroscopic F and D strings extending along the non compact 4D sections of the conifold geometry and winding around the internal directions. The effect of the Goldstone modes associated with the position of the strings on the internal manifold can be seen as a current on the string that prevents it from collapsing and allows the possibility of static 4D loops. Its relevance in recent models of brane inflation is discussed.

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 four-dimensional part of the brane world-volume is flat for a range of positive values of the brane tension.

Special massive spin-2 on the 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.