Narrow Results

We study a minimally coupled tachyon field rolling down to its ground state on the FRW brane. We construct tachyonic potential which can implement power law inflation in the brane world cosmology. The potential turns out to be V₀ϕ^-1 on the brane and reduces to inverse square potential at late times when brane corrections to the Friedmann equation become negligible. We also do similar exercise with a normal scalar field and discover that the inverse square potential on the brane leads to power law inflation.

Gravitational theories including negative cosmological constant, dilaton and gauge fields are explored in higher dimensions, in which black hole solutions are shown to exist and their asymptotic behaviors are obtained. Based on these solutions, effective Randall–Sundrum brane worlds are constructed. In the framework of gauge/gravity duality, effects from cosmological constant on the spectra of standard model fields on the branes are perturbatively calculated.

In this paper, we extend the previous work by the authors, and elaborate further on the structure of the general solution to the graviton and dilaton equations of motion in brane world scenaria, in the context of five-dimensional effective actions with higher-curvature corrections, compatible with bulk string-amplitude calculations. We consider (multi)brane scenaria, dividing the bulk space into regions, in which one matches two classes of general solutions, a linear Randall–Sundrum solution and a (logarithmic) dilatonic domain wall (bulk naked singularity). We pay particular attention to examining the possibility of resolving the mass hierarchy problem together with the vanishing of the vacuum energy on the observable world, which is taken to be a positive tension brane. The appearance of naked dilatonic domain walls provides a dynamical restriction of the bulk space–time. Of particular interest is a dilatonic-wall solution, which after appropriate coordinate transformation, results in a linear dilaton conformal field theory. The latter may provide a holographic resolution of the naked singularity problem. All the string-inspired models involved have the generic feature that the brane tensions are proportional to the string coupling g_s; it remains a challenge for string theory, therefore, to show whether microscopic models respecting this feature can be constructed.

We introduce a numerical procedure to investigate the spectrum of massive modes and its contribution for gravity localization on thick branes. After considering a model with an analytically known Schrödinger potential, we present the method and discuss its applicability. With this procedure we can study several models even when the Schrödinger potential is not known analytically. We discuss both the occurrence of localization of gravity and the correction to the Newtonian potential given by the massive modes.

Within the framework of the modified geodetic brane gravity, conformed by the Regge–Teitelboim model and enhanced with a linear term in the extrinsic curvature of the brane, the possibility that under an FRW geometry this theory emulates the so-called dark energy is discussed. The cosmological behavior of this model displays a self-(non-self)-accelerated expansion of this universe which is caused by a combination of usual matter and gravitational geometric effects controlled by a $\beta$ parameter that accompanies the correction K term. Indeed, the self-accelerated branch, provided by the trace K model raises the question of whether the extrinsic curvature correction terms might be suitable for dark energy candidates. We discuss the analytical expression obtained for ${\rho }_{\mathrm{\text{dark}}}$ in addition to the main cosmological parameters such as the state parameter ${\omega }_{\mathrm{\text{eff}}}$ and the deceleration parameter q. Moreover, when we call for the contribution of dark radiation-like energy to be switched off, ${\mathrm{\Omega }}_{dr,0}\to 0$, we find the same acceleration behavior, as well as the same dark energy content provided by the DGP theory. The relationship of our findings to the analysis for ${\rho }_{\mathrm{\text{dark}}}$ performed by Davidson and Gurwich within the unified brane cosmology is briefly discussed.

Several boundary conditions for the universe have been hitherto suggested, basing on different philosophical approaches. In particular, one may choose between the notions that the universe was created either from nothing or by itself. The quantum state of a universe created from nothing has been already formulated under distinct standpoints by Hartle, Hawking, Vilenkin and others. In this paper we have concentrated on deriving a quantum theory for a self-created universe. Thus, we have first considered the spacetime structure of a six-dimensional de Sitter space with a multiply connected region and, by using a cutting and pasting procedure, we have then been able to show that one can introduce a four-brane in such a spacetime whose evolution can also be considered within the context of ekpyrotic and cyclic universes, and quantized in terms of the sum-over-histories formulation, according to the rules of the generalized quantum theory developed by Hartle.

A black string generalization of the Myers–Perry N-dimensional rotating black hole is considered in an (N + 1)-dimensional Randall–Sundrum brane world. The black string intercepts the (N - 1) brane in a N-dimensional rotating black hole. We examine the diverse cases arising for various non-zero rotation components and obtain the geodesic equations for these space–times. The causal structure and asymptotics of the resulting brane world geometries are analyzed.

We present the "classical" Nielsen–Olesen vortex solution on a warped 5-dimensional spacetime, where we solved the effective 4-dimensional equations from the 5-dimensional equations together with the junction and boundary conditions. 4-dimensional cosmic strings show some serious problems concerning the mechanism of string smoothing related to the string mass per unit length, Gμ ≤ 10^-6. Moreover, there is no observational evidence of axially symmetric lensing effect caused by cosmic strings. Also super-massive cosmic strings (Gμ ≳ 1), predicted by superstring theory, possess some problems. They are studied because the universe may have undergone phase transitions at scales much higher than the GUT scale. But Gμ ≳ 1 is far above observational bounds, so one needs an inflationary scenario to smooth them out. Further, it is believed that these super-massive strings never extended to macroscopic size. Brane world models could overcome these problems. Gμ could be warped down to GUT scale, even if its value was at the Planck scale.

In our warped cosmic string model, where the string mass per unit length in the bulk can be of order of the Planck scale, we find that the 4-dimensional brane spacetime is exponential warped down. Moreover, asymptotically the induced 4-dimensional spacetime does not show conical behavior. So there is no angle deficit compared to its value in the bulk and the spacetime seems to be unphysical, at least under fairly weak assumptions on the stress–energy tensor and without a positive brane tension. The results are confirmed by numerical solutions of the field equations.

We present the vacuum polarisation of a massless, conformally coupled scalar field on the brane for a Schwarzschild–Tangherlini black hole in a bulk of zero to seven additional dimensions.