Antonios Papazoglou

The m→0 limit for massive graviton in dS4 and AdS4 — How to circumvent the van Dam–Veltman–Zakharov discontinuity

Abstract We show that, by considering physics in dS 4 or AdS 4 spacetime, one can circumvent the van Dam–Veltman–Zakharov theorem which requires that the extra polarization states of a massive graviton do not decouple in the massless limit. It is shown that the smoothness of the m →0 limit is ensured if the H (“Hubble”) parameter, associated with the horizon of the dS 4 or AdS 4 space, tends to zero slower than the mass of the graviton m .

A three three-brane universe: new phenomenology for the new millennium?

Abstract We consider an extension of the Randall–Sundrum model with three parallel 3-branes in a 5-dimensional spacetime. This new construction, apart from providing a solution to the Planck hierarchy problem, has the advantage that the SM fields are confined on a positive tension brane. The study of the phenomenology of this model reveals an anomalous first KK state which is generally much lighter than the remaining tower and also much more strongly coupled to matter. Bounds on the parameter space of the model can be placed by comparison of specific processes with the SM background as well as by the latest Cavendish experiments. The model suggests a further exotic possibility if one drops the requirement of solving the hierarchy problem. In this case gravity may result from the exchange of the ordinary graviton plus an ultralight KK state and modifications of gravity may occur at both small and extremely large scales.

A new bigravity model with exclusively positive branes

Abstract We propose a new “bigravity” model with two positive tension AdS 4 branes in AdS 5 bulk and no negative tension branes. The bounce of the “warp” factor mimics the effect of a negative brane and thus gives rise to an anomalously light graviton KK mode. This configuration satisfies the weak energy condition and has no ghost state. In addition, the extra polarization states of the massive graviton practically decouple and thus it does not contradict to Einsteinian gravity. However, the model has certain phenomenological difficulties associated with the presence of a negative cosmological constant on the branes.

Multibrane worlds and modification of gravity at large scales

Abstract We discuss the implications of multibrane constructions involving combinations of positive and negative tension brane and show how anomalously light KK states emerge when negative tension “ − ” branes are sandwiched between “ + ” branes. We present a detailed study of a “ +−−+ ” brane assignment which interpolates between two models that have been previously proposed in which gravity is modified at large scales due to the anomalously light states. We show that it has the peculiar characteristic that gravity changes from four-dimensional (4D) to 5D at large distances and returns to 4D at even larger scales. We also consider a crystalline universe which leads to a similar structure for gravity. The problems associated with intermediate negative tension branes are discussed and a possible resolution suggested.

Multigravity in six dimensions: Generating bounces with flat positive tension branes

We present a generalization of the five dimensional multigravity models to six dimensions. The key characteristic of these constructions is that that we obtain solutions which do not have any negative tension branes while at the same time the branes are kept flat. This is due to the fact that in six dimensions the internal space is not trivial and its curvature allows bounce configurations with the above feature. These constructions give for the first time a theoretically and phenomenologically viable realization of multigravity.

Multi-localization in multi-brane worlds

Abstract We study bulk fields in various multi-brane models with localized gravity. In the case of spin-0, 1 2 and 3 2 fields, the non-trivial background geometry itself can induce (multi-)localization of the zero modes along the extra dimension. The addition of appropriate mass terms controls the strength or/and the position of this localization and can induce (multi-)localization of spin-1 fields as well. We show that multi-brane models may also give rise to anomalously light KK modes which are also localized. The possibility of multi-localization in the context of supersymmetric brane world models in AdS5 spacetime is also discussed.

Radion in multibrane world

Abstract The radion dynamics related to the presence of moving branes with both positive or negative tensions is studied in the linearized approximation. The radion effective Lagrangian is computed for a compact system with three branes and in particular we examine the decompactification limit when one brane is sent to infinity. In the non-compact case we calculate the coupling of the gravitational modes (graviton, dilaton and radion) to matter on the branes. The character of gravity on the two branes for all possible combinations of brane tensions is also discussed. It turns out that one can have a normalizable dilaton mode even in the non-compact case. Finally, we speculate on the role of moving branes as a possible source of radion emission.

Dilaton tadpoles and mass in warped models

Abstract We review the brane world sum rules of Gibbons et al. for compact five-dimensional warped models with identical four-geometries and bulk dynamics involving scalar fields with generic potential. We show that the absence of dilaton tadpoles in the action functional of the theory is linked to one of these sum rules. Moreover, we calculate the dilaton mass term and derive the condition that is necessary for stabilizing the system.

Anisotropic Extra Dimensions

We consider the scenario where in a five-dimensional theory, the extra spatial dimension has different scaling than the other four dimensions. We find background maximally symmetric solutions, when the bulk is filled with a cosmological constant and at the same time it has a three-brane embedded in it. These background solutions are reminiscent of Randall-Sundrum warped metrics, with bulk curvature depending on the parameters of the breaking of diffeomorphism invariance. Subsequently, we consider the scalar perturbation sector of the theory and show that it has certain pathologies and the striking feature that in the limit where the diffeomorphism invariance is restored, there remain ghost scalar mode(s) in the spectrum.