M. C. B. Abdalla, M. E. X. Guimaraes, J. M. Hoff da Silva
We derive an one-parameter family of consistence conditions to braneworlds in the Brans-Dicke gravity. The sum rules are constructed in a completely general frame and they reproduce the conditions already obtained in General Relativity theory just by using a right limit of the Brans-Dicke parameter.
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We obtain, via the Gauss-Codazzi formalism, the expression of the effective Einstein-Brans-Dicke projected equations in a non-$\mathbb{Z}_{2}$ symmetric braneworld scenario which presents hybrid compactification. It is shown that the functional form of such equations resembles the one in the Einstein's case, except by the fact that they bring extra informations in the context of exotic compactifications.
In this work we study a scenario with a warped 5D smooth braneworld with 4D Minkowski geometry builded from bulk scalar matter non-minimally coupled to gravity with an additional Gauss-Bonnet term. We present exact solutions for the full braneworld configuration in contrast to previous results where only approximate solutions were constructed due to the highly non-linear character of the relevant differential equations. These solutions allow us to study the necessary conditions for the finiteness of the 4D Planck mass and additionally, enables us to perform a more rigorous analysis of 4D gravi…
Cosmic microwave background (CMB) observations provide in principle a high-precision test of models which are motivated by M theory. We set out the framework of a program to compute the tensor anisotropies in the CMB that are generated in braneworld models. In the simplest approximation, we show the braneworld imprint as a correction to the power spectra for standard temperature and polarization anisotropies.
Tensor type perturbations in the expanding brane world of the Randall Sundrum type are investigated. We consider a model composed of slow-roll inflation phase and the succeeding radiation phase. The effect of the presence of an extra dimension through the transition to the radiation phase is studied, giving an analytic formula for leading order corrections.
We investigate scalar and tensor fields in the brane model solution for the 5D space-time with standing gravitational waves in the bulk and show that even in the case of increasing warp factor there exist normalizable zero modes localized on the brane.
Inspired by the peculiarities of the effective geometry of crystalline structures, we reconsider thick brane scenarios from a metric-affine perspective. We show that for a rather general family of theories of gravity, whose Lagrangian is an arbitrary function of the metric and the Ricci tensor, the background and scalar field equations can be written in first-order form, and tensorial perturbations have a non negative definite spectrum, which makes them stable under linear perturbations regardless of the form of the gravity Lagrangian. We find, in particular, that the tensorial zero modes are …
We apply the Gauss-Codacci formalism to brane worlds scenario within the framework of Brans-Dicke gravity. The compactification is taken from six to five dimensions in order to formalize brane-world models with hybrid compactification in scalar-tensor theories.
A braneworld model in a massive gravity
该研究在洛伦兹破缺有质量引力的5维扩展中构建类Randall-Sundrum膜世界模型，解决规范等级问题，分析微扰稳定性与Kaluza-Klein约化后的质谱，引入Goldberger-Wise机制得到含近无质量引力子的4维有效理论。
We study the properties of gravity and bulk fields living in a torsion warped braneworld. The torsion is driven by a background vector whose norm provides a source for the bulk cosmological constant. For a vector as the derivative of a scalar field, we find new isotropic and anisotropic thick brane geometries. We analyse the features of bosonic and fermionic fields in this isotropic and in standing wave scenarios. The background vector provides nonminimal coupling between the field and the geometry leading to modifications in the Kaluza-Klein states. The spinor connection is modified by the to…
This work deals with braneworld models in the presence of scalar fields in a five-dimensional warped geometry with a single extra dimension of infinite extent. We consider generalized models in the presence of the Ricci scalar, the trace of the stress-energy tensor and the cuscuton contribution. The models describe novel braneworld scenarios and the investigations consider distinct possibilities, from which we show how the brane may change to become thinner, although keeping gravitational stability and the gravity zero mode under strict control. Moreover, we did not identify any split behavior…
Exact holography for cosmological branes in an AdS-Schwarzschild bulk was first introduced in hep-th/0204218. We extend this notion to include all co-dimension one branes moving in non-trivial bulk spacetimes. We use a covariant approach, and show that the bulk Weyl tensor projected on to the brane can always be traded in for "holographic" energy-momentum on the brane. More precisely, a brane moving in a non-maximally symmetric bulk has exactly the same geometry as a brane moving in a maximally symmetric bulk, so long as we include the holographic fields on the brane. This correspondence is ex…
This thesis is devoted to studying two important aspects of braneworld physics: their cosmology and their holography. We examine the Einstein equations induced on a general $(n-2)$-brane of arbitrary tension, embedded in some $n$-dimensional bulk. The brane energy-momentum tensor enters these equations both linearly and quadratically. From the point of view of a homogeneous and isotropic brane we see quadratic deviations from the FRW equations of the standard cosmology. There is also a contribution from a bulk Weyl tensor. We study this in detail when the bulk is AdS-Schwarzschild or Reissner-…
We present a linearized gravity investigation of the bent braneworld, where an AdS_4 brane is embedded in AdS_5. While we focus on static spherically symmetric mass distributions on the brane, much of the analysis continues to hold for more general configurations. In addition to the identification of the massive Karch-Randall graviton and a tower of Kaluza-Klein gravitons, we find a radion mode that couples to the trace of the energy-momentum tensor on the brane. The Karch-Randall radion arises as a property of the embedding of the brane in the bulk space, even in the context of a single brane…
Braneworld inflation is a phenomenology related to string theory that describes high-energy modifications to general relativistic inflation. The observable universe is a braneworld embedded in 5-dimensional anti de Sitter spacetime. Whe the 5-dimensional action is Einstein-Hilbert, we have a Randall-Sundrum type braneworld. The amplitude of tensor and scalar perturbations from inflation is strongly increased relative to the standard results, although the ratio of tensor to scalar amplitudes still obeys the standard consistency relation. If a Gauss-Bonnet term is included in the action, as a hi…
In this paper my prime objective is to explain the generation of large tensor-to-scalar ratio from the single field sub-Planckian inflationary paradigm within Randall Sundrum (RS) single braneworld scenario in a model independent fashion. By explicit computation I have shown that the effective field theory prescription of brane inflation within RS single brane setup is consistent with sub- Planckian excursion of the inflaton field, which will further generate large value of tensor-to-scalar ratio, provided the energy density for inflaton degrees of freedom is high enough compared to the brane …
The stability of Einstein static universe against homogeneous scalar perturbations in the context of braneworld scenario is investigated. The stability regions are obtained in terms of the constant geometric linear equation of state parameter $ω_{extr}=p_{extr}/ρ_{extr}$ and are studied for each evolutionary era of the universe. The results are discussed for the case of closed, open or flat universe in each era under the obtained restricting conditions. We also briefly investigate the stability against vector and tensor perturbations. Contrary to the classical general relativity, it is found t…
In this paper, we investigate the braneworld scenario in $f(T)$ gravity with a $K$-field as the background field. We consider various different specific forms of $f(T)$ gravity and $K$-field, and find a general way to construct the braneworld model. Based on our solutions, the split of branes is investigated. Besides, the stability of the braneworld is studied by investigating the tensor perturbation of the vielbein.
Braneworld sum rules and positive tension branes in a massive gravity
论文在 Lorentz 破缺大质量引力的五维推广中利用膜世界求和规则，构造仅含正张力膜的平坦3-膜构型，并分析其微扰谱与现象学等价性。
Linear Perturbations and Stability Analysis in $f(T)$ Braneworld Scenario
本文系统分解$f(T)$膜世界引力的线性扰动并推导各模式二次作用量，证明平坦背景下无额外自由度，且$f(T)=T+\alpha T^2$厚膜模型稳定。
We study braneworlds in six-dimensional Einstein-Gauss-Bonnet gravity. The Gauss-Bonnet term is crucial for the equations to be well-posed in six dimensions when non-trivial matter on the brane is included (the also involved induced gravity term is not significant for their structure), and the matching conditions of the braneworld are known. We show that the energy-momentum of the brane is always conserved, independently of any regular bulk energy-momentum tensor, contrary to the situation of the five-dimensional case.
Localization of Spinor Fields in Higher-Dimensional Braneworlds
针对六维及以上膜世界中费米子无法经最小引力耦合局域化的问题，本文提出保持 $SO(n,1)$ 对称性的张量耦合，并给出可实现左右手征解耦及旋量局域化的特殊形式。
We derive the low energy effective action for the dilatonic braneworld. In the case of the single-brane model, we find the effective theory is described by the Einstein-scalar theory coupled to the dark radiation. Remarkably, the dark radiation is not conserved in general due to a coupling to the bulk scalar field. The effective action incorporating Kaluza-Klein (KK) corrections is obtained and the role of the AdS/CFT correspondence in the dilatonic braneworld is revealed. In particular, it is shown that CFT matter would not be confined to the braneworld in the presence of the bulk scalar fiel…
In the framework of a general scalar-tensor theory, where the scalar field is non-minimally coupled to the five-dimensional Ricci scalar curvature, we investigate the emergence of complete brane-world solutions. By assuming a variety of forms for the coupling function, we solve the field equations in the bulk, and determine in an analytic way the form of the gravitational background and scalar field in each case. The solutions are always characterized by a regular scalar field, a finite energy-momentum tensor, and an exponentially decaying warp factor even in the absence of a negative bulk cos…
Super-exponential warp factors have been proposed in the context of two-brane models to solve the gauge hierarchy problem of scales by using a compactification scale of the same order of the fundamental Planck scale, completely eliminating the hierarchy between the electroweak and the Planck scales. However, most of the so far studied families of braneworlds with super-exponential warp factors do not localize 4D gravity when one of the branes is sent to infinity. Recently a braneworld model generated by a canonical scalar field $ϕ$ minimally coupled to 5D gravity with a bulk cosmological const…
The general procedure for analyzing the localization of matter fields in Brane models is by integrating, in the action, its zero mode solutions over the extra dimensions. If this is finite, the field is said to be localized. However, the zero mode solutions must also satisfy the Einstein equations. With this in mind, we obtain stringent constraints on a general energy-momentum tensor by analyzing the Einstein's equations. These {\it consistency conditions} must be satisfied for any Braneworld model. We apply it for some fields of the Standard Model. For a free massless scalar field, the zero-m…
We consider a Vaidya-type radiating spacetime in Einstein gravity with the Gauss-Bonnet combination of quadratic curvature terms. Simply generalizing the known static black hole solutions in Einstein-Gauss-Bonnet gravity, we present an exact solution in arbitrary dimensions with the energy-momentum tensor given by a null fluid form. As an application, we derive an evolution equation for the ``dark radiation'' in the Gauss-Bonnet braneworld.
Scalar and tensor perturbations arising in an inflationary braneworld scenario driven by a single scalar field are considered, where the bulk on either side of the brane corresponds to Anti-de Sitter spaces with different cosmological constants. A consistency relation between the two spectra is derived and found to have an identical form to that arising in standard single-field inflation based on conventional Einstein gravity. The dS/CFT correspondence may provide further insight into the origin of this degeneracy. Possible ways of lifting such a degeneracy are discussed.
The modified gravitational equations to describe a four-dimensional braneworld in the case with the Lorentz invariant violation in a bulk spacetime is presented. It contains a trace part of the brane energy-momentum tensor and the coefficients of all terms describe the Lorentz violation effects from the bulk spacetime. As an application, we apply this formalism to study cosmology. In respect to standard effective Friedmann equations on the brane, Lorentz invariance violation in the bulk causes a modification of this equations that can lead to significant physical consequences. In particular, t…
In the context of the Randall-Sundrum (RS) single-brane scenario, we discuss the bulk geometry and dynamics of a cosmological brane in terms of the local energy conservation law which exists for the bulk that allows slicing with a maximally symmetric 3-space. This conservation law enables us to define a local mass in the bulk. We show that there is a unique generalization of the dark radiation on the brane, which is given by the local mass. We find there also exists a conserved current associated with the Weyl tensor, and the corresponding local charge, which we call the Weyl charge, is given …
We discuss the flow equations in the context of general braneworld cosmologies with a modified Friedmann equation, for either an ordinary scalar field or a Dirac-Born-Infeld tachyon as inflaton candidates. The 4D, Randall-Sundrum, and Gauss-Bonnet cases are compared, using the patch formalism which provides a unified description of these models. The inflationary dynamics is described by a tower of flow parameters that can be evolved in time to select a particular subset of points in the space of cosmological observables. We analyze the stability of the fixed points in all the cosmologies (our …
We generalize the Randall Sundrum warped braneworld model in six and higher dimension and propose a resolution to the mass hierarchy among the standard model fermions. The fine tuning problem in connection with the scalar mass however is shown to reappear in a new guise in five dimensional warped model when the two form antisymmetric tensor fields, a massless string mode, propagates in the bulk. Finally the issue of modulus stabilization is re-examined in presence of a bulk scalar by considering it's back-reaction on the background geometry. The role of higher derivative term of the bulk scala…
We consider the static domain wall braneworld scenario constructed from the Palatini formalism $f(\mathcal{R})$ theory. We check the self-consistency under scalar perturbations. By using the scalar-tensor formalism we avoid dealing with the higher-order equations. We develop the techniques to deal with the coupled system. We show that under some conditions, the scalar perturbation simply oscillates with time, which guarantees the stability. We also discuss the localization condition of the scalar mode by analyzing the effective potential and the fifth dimensional profile of the scalar mode. We…
Linear stability of braneworld models constructed with multi-scalar fields is very different from that of single-scalar field models. It is well known that both the tensor and scalar perturbation equations of the later can always be written as a supersymmetric Schrödinger equation, so it can be shown that the perturbations are stable at linear level. However, in general it is not true for multi-scalar field models and especially there is no effective method to deal with the stability problem of the scalar perturbations for braneworld models constructed with non-minimally coupled multi-scalar f…
We study cosmological braneworld models with a single timelike extra dimension. Such models admit the intriguing possibility that a contracting braneworld experiences a natural bounce without ever reaching a singular state. This feature persists in the case of anisotropic braneworlds under some additional and not very restrictive assumptions. Generalizing our study to braneworld models containing an induced brane curvature term, we find that a FRW-type singularity is once again absent if the bulk extra dimension is timelike. In this case, the universe either has a non-singular origin or commen…
We investigate the generation of primordial gravitational waves from inflation in braneworld cosmologies with extra dimensions. Advantage of using primordial gravitational waves to probe extra dimensions is that their theory depends only on the geometry, not on the microscopic models of inflation and stabilization. D(D-3)/2 degrees of freedom of the free bulk gravitons are projected onto the 3d brane as tensor, vector and scalar modes. We found the following no-go results for a generic geometry of a five (or D) dimensional warped metric with four dimensional de Sitter (inflationary) slices and…
We calculate the spectral index and tensor-to-scalar ratio for patch inflation arisen from the Gauss-Bonnet braneworld scenario. The patch cosmological models consist of Gauss-Bonnet (GB), Randall-Sundrum (RS), and 4D general relativistic (GR) cases. In order to compare with the observation data, we perform leading-order calculations for all patch models by choosing large-field, small-field, and hybrid potentials. We show that the large-field potentials are sensitive to a given patch model, while the small-field and hybrid potentials are insensitive to a given patch model. It is easier to disc…
Constant-roll inflation with tachyon field in the holographic braneworld
本研究在全息膜世界框架下，假设第二慢滚参数$η$为常数，研究快子场驱动的常滚暴胀，推导求解哈勃膨胀率方程，计算宇宙学参数并结合观测约束模型，还探讨了相关沼泽地猜想。
We propose a braneworld scenario in a modified symmetric teleparallel gravitational theory, where the dynamics for the gravitational field is encoded in the nonmetricity tensor rather than in the curvature. Assuming a single real scalar field with a sine-Gordon self-interaction, the generalized quadratic nonmetricity invariant $\mathbb{Q}$ controls the brane width while keeping the shape of the energy density. By considering power corrections of the invariant $\mathbb{Q}$ in the gravitational Lagrangian, the sine-Gordon potential is modified exhibiting new barriers and false vacuum. As a resul…
We present a consistent derivation of the recently proposed 5D anisotropic standing wave braneworld generated by gravity coupled to a phantom-like scalar field. We explicitly solve the corresponding junction conditions, a fact that enables us to give a physical interpretation to the anisotropic energy-momentum tensor components on the brane. So matter on the brane represents an oscillating fluid which emits anisotropic waves into the bulk. We also analyze the Sturm-Liouville problem associated to the correct localization condition of the transverse to the brane metric and scalar fields. It is …
We investigate the Kaluza-Klein braneworld cosmology from the point of view of observers on the brane. We first generalize the Shiromizu-Maeda-Sasaki (SMS) equations to higher dimensions. As an application, we study a (4+n)-dimensional brane with n dimensions compactified on the brane, in a (5+n)-dimensional bulk. By assuming that the size of the internal space is static, that the bulk energy-momentum tensor can be neglected, we determine the effect of the bulk geometry on the Kaluza-Klein braneworld. Then we derive the effective Friedmann equation on the brane. It turns out that the Friedmann…
We propose a cosmological braneworld scenario in which two branes collide and emerge as reborn branes whose tensions have signs opposite to the original tensions of the respective branes. In this scenario, gravity on each of the branes is described by a scalar-tensor-type theory in which the radion plays the role of the gravitational scalar, and the branes are assumed to be inflating. However, the whole dynamics is different from those of the usual inflation, due to the non-trivial dynamics of the radion field. Transforming the conformal frame to the Einstein frame, this born-again scenario re…
We show how to construct low energy solutions to the Randall Sundrum II (RSII) model using an associated AdS_5-CFT_4 problem. The RSII solution is given in terms of a perturbation of the AdS-CFT solution, with the perturbation parameter being the radius of curvature of the brane metric compared to the AdS length \ell. The brane metric is then a specific perturbation of the AdS-CFT boundary metric. For low curvatures the RSII solution reproduces 4d GR on the brane. The leading correction is from local higher derivative curvature terms. The subleading correction is derived from similar terms and…
We review the results of investigations for brane-induced effects on the local properties of quantum vacuum in background of AdS spacetime. Two geometries are considered: a brane parallel to the AdS boundary and a brane intersecting the AdS boundary. For both these cases the contribution in the vacuum expectation value (VEV) of the energy-momentum tensor is separated explicitly and its behavior in various asymptotic regions of the parameters is studied. It is shown that the influence of the gravitational field on the local properties of the quantum vacuum is essential at distance from the bran…
We consider the case of a generic braneworld geometry in the presence of one or more moduli fields (e.g., the dilaton) that vary throughout the bulk spacetime. Working in an arbitrary conformal frame, using the generalized junction conditions of gr-qc/0008008 and the Gauss--Codazzi equations, we derive the effective ``induced'' on-brane gravitational equations. As usual in braneworld scenarios, these equations do not form a closed system in that the bulk can exchange both information and stress-energy with the braneworld. We work with an arbitrary number of moduli fields described by an arbitr…
We discuss the dynamics of linear, scalar perturbations in an almost Friedmann-Robertson-Walker braneworld cosmology of Randall-Sundrum type II using the 1+3 covariant approach. We derive a complete set of frame-independent equations for the total matter variables, and a partial set of equations for the non-local variables which arise from the projection of the Weyl tensor in the bulk. The latter equations are incomplete since there is no propagation equation for the non-local anisotropic stress. We supplement the equations for the total matter variables with equations for the independent cons…
We reformulate boundary conditions for axisymmetric codimension-2 braneworlds in a way which is applicable to linear perturbation with various gauge conditions. Our interest is in the thin brane limit and thus this scheme assumes that the perturbations are also axisymmetric and that the surface energy-momentum tensor of the brane is proportional to its induced metric. An advantage of our scheme is that it allows much more freedom for convenient coordinate choices than the other methods. This is because in our scheme, the coordinate system in the bulk and that on the brane are completely disent…
We evaluate the correlators for the vector potential and for the field strength tensor of the electromagnetic field in the geometry of two parallel planar plates in AdS spacetime. Two types of boundary conditions are considered on the plates. The first one is a generalization of perfect conductor boundary condition and the second one corresponds to the confining boundary conditions. By using the expressions for the correlators, the vacuum expectation values (VEVs) of the photon condensate and of the electric and magnetic fields squared are investigated. As another important local characteristi…
Motivated by the idea that quantum gravity corrections usually suppress the power of the scalar primordial spectrum (E-mode) more than the power of the tensor primordial spectrum (B-mode), in this paper we construct a concrete gravitational theory in five-dimensions for which $V(ϕ)\propto ϕ^n$-type inflation ($n\ge 1$) generates an appropriate tensor-to-scalar ratio that may be compatible with the BICEP2 and Planck data together. The true nature of gravity is five-dimensional and described by the action $S = \int d^5{x} \sqrt{|g|} M^3 (- 6λM^2 + R + αM^{-2} {\cal R}^2)$ where $M$ is the five-d…
We compute the boundary terms and junction conditions for Horndeski's panoptic class of scalar-tensor theories, and write the bulk and boundary equations of motion in explicitly second order form. We consider a number of special subclasses, including galileon theories, and present the corresponding formulae. Our analysis opens up of the possibility of studying tunnelling between vacua in generalized scalar-tensor theories, and braneworld dynamics. The latter follows because our results are independent of spacetime dimension.
This work deals with thick braneworld models, in an environment where the Ricci scalar is changed to accommodate the addition of two extra terms, one depending on the Ricci scalar itself, and the other, which takes into account the trace of the energy-momentum tensor of the scalar field that sources the braneworld scenario. We suppose that the scalar field engenders standard kinematics, and we show explicitly that the gravity sector of this new braneworld scenario is linearly stable. We illustrate the general results investigating two distinct models, focusing on how the brane profile is chang…
A note on supergravity inflation in braneworld
本文研究膜世界宇宙学中一类最小超引力暴涨模型，利用破缺平移对称性规避η问题，并给出谱指数和张标比的数值与解析结果，发现ρ²/λ修正可使暴涨场位移保持亚普朗克尺度。
In this paper we investigate, from the dynamical systems perspective, the evolution of an scalar field with arbitrary potential trapped in a Randall-Sundrum's Braneworld of type 2. We consider an homogeneous but anisotropic Bianchi I (BI) brane filled also with a perfect fluid. We also consider the effect of the projection of the five-dimensional Weyl tensor onto the three-brane in the form of a positive Dark Radiation term. Using the center manifold theory we obtain sufficient conditions for the asymptotic stability of de Sitter solution with standard 4D behavior. We also prove that there are…
We consider warped five-dimensional thick braneworlds with four-dimensional Poincaré invariance originated from bulk scalar matter non-minimally coupled to gravity plus a Gauss-Bonnet term. The background field equations as well as the perturbed equations are investigated. A relationship between 4D and 5D Planck masses is studied in general terms. By imposing finiteness of the 4D Planck mass and regularity of the geometry, the localization properties of the tensor modes of the perturbed geometry are analyzed to first order, for a wide class of solutions. In order to explore the gravity localiz…
Recently, a new scalar-tensor braneworld model was presented in [Phys. Rev. \textbf{D 86} (2012) 127502]. It not only solves the gauge hierarchy problem but also reproduces a correct Friedmann-like equation on the brane. In this new model, there are two different brane solutions, for which the mass spectra of gravity on the brane are the same. In this paper, we investigate localization and mass spectra of various bulk matter fields (i.e., scalar, vector, Kalb-Ramond, and fermion fields) on the brane. It is shown that the zero modes of all the matter fields can be localized on the positive tens…
We show that within tachyonic braneworld models,"super-exponential" warp factors of the form $e^{-2f} \sim e^{-2c_1e^{c_2 |σ|}}$ are problematic when dealing with both the finiteness of the effective four-dimensional (4d) Planck mass and the localization of 4d gravity, which can be stated by the requirement that $\int e^{-2f(σ)}dσ< \infty$, because this condition necessarily implies that c_1 and c_2 should be positive. As a consequence of this fact the tachyonic field $T$ turns out to be complex in contradiction with the real nature of the starting action for the tachyonic braneworld. Converse…
In the context of higher dimensional braneworld scenario, we have argued that the occurrence of horizonless exotic compact objects, as an alternative to classical black holes, are more natural. These exotic compact objects carry a distinctive signature of the higher dimension, namely a tidal charge parameter, inherited from the projection of the higher dimensional Weyl tensor onto the four dimensional spacetime we live in. Due to the absence of any horizon, rotating exotic compact objects are often unstable because of superradiance. Interestingly, these higher dimensional exotic compact object…
Tachyonic (In)stability in Randall-Sundrum Braneworld Scenarios
本文研究了在布兰奇世界场景中反粒子不稳定性与自发标量化现象的可能性，探讨了在不同场景下反粒子场的稳定性，并分析了反粒子场在普朗克布兰奇上的条件。
Observable strong field effects of extra spacetime dimension in the braneworld black hole
研究DMPR±黑洞中潮汐电荷Υ对强场透镜观测量和准正则模频率的影响，发现Υ可显著改变Schwarzschild透镜和QNM，且DMPR−黑洞的临界指数更低，表明其吸积盘更亮。
We consider a braneworld model in the scalar-tensor gravity. In order to solve the gauge hierarchy problem in this model, our world should be confined on the positive tension brane rather than on the negative one. This is crucial to reproduce a correct Friedmann-like equation on the brane. Interestingly, it is found that the spacing of mass spectrum in this scenario is very tiny, but the light gravitons cannot be observed individually in colliders because of their sufficiently weak interaction with matter fields on the visible brane.
Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w < -1 as well as w > -1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future `Big Rip' singularity which is usually encountered by Phantom models. (ii) For a class of Braneworld models the acceleration of the universe can be a transient phenomenon. In this case the current acceleration of the universe is sandwiched between two matter dominated epochs. Such a…
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