Narrow Results

We discuss some issues arising in studying (linearized) gravity on non-BPS higher co-dimension branes in an infinite-volume bulk. In particular, such backgrounds are badly singular for codimension-3 and higher δ-function-like branes with nonzero tension. As we discuss in this note, nontrivial issues arise in smoothing out such singularities. Thus, adding higher curvature terms might be necessary in this context.

Gravitational field of cosmic strings in theories with extra spatial dimensions differs significantly from that in the 4D Einstein's theory. This means that all gravity induced properties of cosmic strings need to be revised. Here we consider the effect of vacuum polarization outside a straight infinitely thin cosmic string embedded in RS2 brane world.

We consider the stabilization of de Sitter brane-world. The scalar field bulk-brane theory produces the nontrivial minimum of modulus potential where temporal radion is realized. The hierarchy problem (between Planck and electroweak scales) may be solved. However, the interpretation of radion is not so clear as in AdS brane-world. In particular, the introduction of two times physics or pair-creation of bulk spaces or identification of one of spatial coordinates with imaginary time (nonzero temperature) may be required.

A five-dimensional rotating black string in a Randall–Sundrum brane world is considered. The black string intercepts the three-brane in a four-dimensional rotating black hole. The geodesic equations and the asymptotics in this background are discussed.

We discuss the cosmological constant problem in the context of higher codimension brane world scenarios with infinite-volume extra dimensions.

We discuss several possibilities of probing string theory through cosmology, e.g., the imprints of short distance physics on CMB anisotropies, the signatures of brane inflationary models, and the cosmological constraints on D-matter.

We show that the vacuum polarization of quantum fields in an anti-de Sitter space can naturally give rise to a small but nonzero cosmological constant in a brane world living in it. To explain the extremely small ratio of mass density in the cosmological constant to the Planck mass density in our universe (≈10^-123) as suggested by cosmological observations, all we need is a four-dimensional brane world (our universe) living in a five-dimensional anti-de Sitter space with a curvature radius r₀~10^-3cm and a fundamental Planck energy M_P~10⁹GeV, and a scalar field with a mass . Probing gravity down to a scale ~10^-3 cm, which is attainable in the near future, will provide a test of the model.

We study a gravity model where a tensionful codimension-one three-brane is embedded in a bulk with infinite transverse length. We find that 4D gravity is induced on the brane already at the classical level if we include higher-curvature (Gauss–Bonnet) terms in the bulk. Consistency conditions appear to require a negative brane tension as well as a negative coupling for the higher-curvature terms.

We construct a static solution for (4 + 1)-dimensional bulk such that the (3 + 1)-dimensional world has a linear warp factor and describes the Schwarzschild–dS₄ black hole. For m = 0 this four-dimensional universe and Friedmann–Robertson–Walker universe are related with an explicit coordinate transformation. We emphasize that for linear warp factors the effect of bulk on the brane world shows up as the dS₄ background which is favored by the big bang cosmology.

An FRW brane model embedded in a conformally flat bulk is considered to study gravitational collapse for matter cloud in non-interacting combination of dark matter and dark energy. The form of dark energy is chosen as the decaying vacuum energy. Both expanding and contracting model of the universe are found for different time intervals with a time interval in between, which is a forbidden region.

In the light of ϕ-mapping topological current theory, the structure of cosmic strings are obtained from the Abelian Higgs model, which is an effective description to the brane world cosmic string system. In this topological description of the cosmic string, combining the result of decomposition of U(1) gauge potential, we analytically reach the familiar conclusions that in the brane world scenario the magnetic flux of the cosmic string is quantized and the RR charge of it is screened.

In the scenario of an optimal consideration that is, homogeneous and flat spacetime, we study the Black Hole (BH) formation from the gravitational collapse of a spherical symmetric clump of matter in the case of the specific Dark Matter (DM) model interacting with Dark Energy (DE) in the context of the brane world regime. This clump of matter constituted of DM, ${\rho }_M$ and DE, $\rho$. In the present model, we consider anisotropic pressure in the energy–momentum tensor with a polytropic equation of state (EoS), $p_r=k\rho$ and $p_t=\omega \rho$, $(k+2\omega <-1)$. Our results show that the gravitational collapse of an interacting combination of DM and DE leads to the formation of BH in the presence of brane tension. Recent work provides the generalization of isotropic pressure to an-isotropic pressure in the energy–momentum tensor for the specific interacting combination model of DM and DE in a brane world regime.

In the orientiworld framework the Standard Model fields are localized on D3-branes sitting on top of an orientifold three-plane. The transverse six-dimensional space is a noncompact orbifold (or a more general conifold). The four-dimensional gravity on D3-branes is reproduced due to the four-dimensional Einstein–Hilbert term induced at the quantum level. The orientifold three-plane plays a crucial role, in particular, without it the D3-brane world-volume theories would be conformal due to the tadpole cancellation. We study nonperturbative gauge dynamics in various supersymmetric orientiworld models based on the Z₃ as well as Z₅ and Z₇ orbifold groups. Our discussions illustrate that there is a rich variety of supersymmetry preserving dynamics in some of these models. On the other hand, we also find some models with dynamical supersymmetry breaking.

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.

We propose a novel infinite-volume brane world scenario where we live on a noninflating spherical 3-brane, whose radius is somewhat larger than the present Hubble size, embedded in higher dimensional bulk. Once we include higher curvature terms in the bulk, we find completely smooth solutions with the property that the 3-brane world-volume is noninflating for a continuous range of positive values of the brane tension, that is, without fine-tuning. In particular, our solution, which is a near-BPS background with supersymmetry broken on the brane around TeV, is controlled by a single integration constant.

We review some recent results concerning the properties of static, spherically symmetric configurations in RS2 type brane worlds: 1) Solving the trace of the effective 4D gravity equations, we obtain general classes of black hole and wormhole metrics, including families which unify symmetric wormholes, extremal black holes and non-extremal non-singular black holes. 2) Assuming (where is a part of the 5D Weyl tensor contributing to the 4D gravity equations), we show that material scalar fields confined on the brane may violate the no-hair theorem, support wormholes or form particlelike objects, but only at super-nuclear matter densities in the strong field region. 3) Solutions to the bulk field equations are found in case , among them solutions with a nonzero 4D cosmological constant, generalizing the so-called "black string".

We have examined brane world solutions for the low energy effective string gravity action coupled with a quadratic dilaton potential in string frame. For a negative bulk cosmological constant, static 3-brane solutions exist. Their properties are analyzed depending on the quadratic potential parameters. It is found that a brane solution in a curved bulk space in Einstein frame corresponds to a dilaton solution in flat space in string frame.

An earlier paper points out that a quantum treatment of the string landscape is necessary. It suggests that the wave function of the universe is mobile in the landscape until the universe reaches a meta-stable site with its cosmological constant Λ₀ smaller than the critical value Λ_c, where Λ_c is estimated to be exponentially small compared to the Planck scale. Since this site has an exponentially long lifetime, it may well be today's universe. We investigate specific scenarios based on this quantum diffusion property of the cosmic landscape and find a plausible scenario for the early universe. In the last fast tunneling to the Λ₀(<Λ_c) site in this scenario, all energies are stored in the nucleation bubble walls, which are released to radiation only after bubble collisions and thermalization. So the Λ₀ site is chosen even if Λ₀ plus radiation is larger than Λ_c, as long as the radiation does not destabilize the Λ₀ vacuum. A consequence is that inflation must happen before this last fast tunneling, so the inflationary scenario that emerges naturally is extended brane inflation, where the brane motion includes a combination of rolling, fast tunnelings, slow-roll, hopping and percolation in the landscape. We point out that, in the brane world, radiation during nucleosynthesis are mostly on the standard model branes (brane radiation, as opposed to radiation in the bulk). This distinction may lead to interesting dynamics. We consider this paper as a road map for future investigations.

Studies of multidimensional models with different sources (models with S-branes, thin and thick brane worlds, Kaluza-Klein type models in curvature-nonlinear multidimensional gravity etc.) and their application to the cosmological constant, cosmological singularity, hierarchy and coincidence problems are presented. Their observational predictions: variations of fundamental physical constants, new types of black holes and wormholes are discussed.

We consider charged rotating black holes localized on a three-brane in the DGP model. Assuming a Z₂-symmetry across the brane and with a stationary and axisymmetric metric ansatz on the brane, a particular solution is obtained in the Kerr–Schild form. This solution belongs to the accelerated branch of the DGP model and has the characteristic of the Kerr–Newman–de Sitter-type solution in general relativity. Using a modified version of Boyer–Lindquist coordinates we examine the structures of the horizon and ergosphere.