Brane Scale Research Articles

Bouncing cosmology from warped extra dimensional scenario

From the perspective of four dimensional effective theory on a two brane warped geometry model, we examine the possibility of “bouncing phenomena”on our visible brane. Our results reveal that the presence of a warped extra dimension lead to a non-singular bounce on the brane scale factor and hence can remove the “big-bang singularity”. We also examine the possible parametric regions for which this bouncing is possible.

Inflationary scenario from higher curvature warped spacetime

We consider a five dimensional warped spacetime, in presence of the higher curvature term like F(R) = R + alpha R^2 in the bulk, in the context of the two-brane model. Our universe is identified with the TeV scale brane and emerges as a four dimensional effective theory. From the perspective of this effective theory, we examine the possibility of “inflationary scenario” by considering the on-brane metric ansatz as an FRW one. Our results reveal that the higher curvature term in the five dimensional bulk spacetime generates a potential term for the radion field. Due to the presence of radion potential, the very early universe undergoes a stage of accelerated expansion and, moreover, the accelerating period of the universe terminates in a finite time. We also find the spectral index of curvature perturbation (n_s) and the tensor to scalar ratio (r) in the present context, which match with the observational results based on the observations of Planck (Astron. Astrophys. 594, A20, 2016).

Radion cosmology and stabilization

We solve the Einstein's equation in five-dimensional spacetime for Randall Sundrum Brane world model with time dependent radion field to study variation of brane scale factor with time. We have shown that as radion field decreases with time compactifying the extra dimension, the scale factor increases exponentially with time leading to inflationary scenario. We have also proposed a time dependent generalization of Goldberger-Wise moduli stabilization mechanism to explain the time evolution of the radion field to reach a stable value after which the scale factor on the brane exits from inflationary expansion.

Little Randall-Sundrum model and a multiply warped spacetime

A recent work has investigated the possibility that the mass scale for the ultraviolet (UV) brane in the Randall-Sundrum (RS) model is of the order ${10}^{3}\text{ }\text{ }\mathrm{TeV}$. In this so called ``little Randall-Sundrum'' (LRS) model the bounds on the gauge sector are less severe, permitting a lower Kaluza-Klein scale and cleaner discovery channels. However employing a low UV scale nullifies one major appeal of the RS model, namely, the elegant explanation of the hierarchy between the Planck and weak scales. In this work we show that by localizing the gauge, fermion, and scalar sector of the LRS model on a five dimensional slice of a doubly warped spacetime one may obtain the low UV brane scale employed in the LRS model and motivate the weak-Planck hierarchy. We also consider the generalization to an $n$-warped spacetime.

Regularized braneworlds of arbitrary codimension

We consider a thick p-brane embedded in an n-dimensional spacetime possessing radial symmetry in the directions orthogonal to the brane. We first consider a static brane, and find a general fine-tuning relationship between the brane and bulk parameters required for the brane to be flat. We then consider the cosmology of a time-dependent brane in a static bulk, and find the Friedmann equation for the brane scale factor a(t). The singularities that would ordinarily arise when considering arbitrary codimensions are avoided by regularizing the brane, giving it a finite profile in the transverse dimensions. However, since we consider the brane to be a strictly local defect, we find that the transverse dimensions must have infinite volume, and hence gravity cannot be localized on the brane without resorting to some infrared cutoff.

Dynamics of scalar-tensor cosmology from a Randall-Sundrum two-brane model

We consider a Randall-Sundrum two-brane cosmological model in the low energy gradient expansion approximation by Kanno and Soda. It is a scalar-tensor theory with a specific coupling function and a specific potential. Upon introducing the Friedmann-Lema\^{\i}tre-Robertson-WalkerFLRW metric and perfect fluid matter on both branes in the Jordan frame, the effective dynamical equation for the $A$-brane (our Universe) scale factor decouples from the scalar field and $B$-brane matter leaving only a nonvanishing integration constant (the dark radiation term). We find exact solutions for the $A$-brane scale factor for four types of matter: cosmological constant, radiation, dust, and cosmological constant plus radiation. We perform a complementary analysis of the dynamics of the scalar field (radion) using phase space methods and examine convergence towards the limit of general relativity. In particular, we find that radion stabilizes at a certain finite value for suitable negative matter densities on the $B$-brane. Observational constraints from solar system experiments (PPN) and primordial nucleosynthesis (BBN) are also briefly discussed.

Gauge coupling and fermion mass relations in low string scale brane models

We analyze the gauge coupling evolution in brane inspired models with $U(3) \times U(2) \times U(1)^N$ symmetry at the string scale. We restrict to the case of brane configurations with two and three abelian factors (N=2,3) and where only one Higgs doublet is coupled to down quarks and leptons and only one to the up quarks. We find that the correct hypercharge assignment of the Standard Model particles is reproduced for six viable models distinguished by different brane configurations. We investigate the third generation fermion mass relations and find that the correct low energy $m_b/m_{\tau}$ ratio can be obtained for $b-\tau$ Yukawa coupling equality at a string scale as low as $M_S \sim 10^3$ TeV.

Spin and a running radius in the Randall-Sundrum model 1

We develop a renormalization group formalism for the compactified Randall-Sundrum scenario wherein the extra-dimensional radius serves as the scaling parameter. Couplings on the hidden brane scale as we move within local effective field theories with varying size of the warped extra dimension. We consider this RG approach applied to U(1) gauge theories and gravity. We use this method to derive a low energy effective theory.

Brane worlds, string cosmology, and AdS/CFT

Using the thin-shell formalism we discuss the motion of domain walls in de Sitter and anti--de Sitter (AdS) time-dependent bulks. This motion results in a dynamics for the brane scale factor. We show that in the case of a clean brane the scale factor describes both singular and nonsingular universes, with phases of contraction and expansion. These phases can be understood as quotients of AdS spacetime by a discrete symmetry group. We discuss this effect in some detail, and suggest how the AdS/CFT correspondence could be applied to obtain a nonperturbative description of brane-world string cosmology.

The μ and soft masses from the intermediate scale brane with non-factorizable geometry

The proton decay problem and the negative brane tension problem in the original Randall–Sundrum model can be resolved by interpreting the Planck scale brane as the visible sector brane. The hierarchy problem is resolved with supersymmetry, and the TeV scales for soft masses and μ in supersymmetric models are generated by the physics at the intermediate scale (∼10 11–13 GeV) brane.

Cosmology of Randall–Sundrum models with an extra dimension stabilized by balancing bulk matter

We provide the cosmological solutions for Randall–Sundrum models with the bulk energy-momentum T ̂ 5 5 incorporated. It alters the Friedmann equation for the brane scale factor. We make a specific choice of T ̂ 5 5 which is adjusted to stabilize the extra dimension. This makes it possible to compactify the extra dimension with a single positive tension brane, and this model provides a RS-type solution to the cosmological constant problem. When the same idea is applied to the RS model with two branes, the wrong sign of Friedmann equation for the negative tension brane can be resolved, but a specific correlation between energy densities on two branes is still required to reproduce the usual FRW universe.

Brane cosmological evolution in a bulk with cosmological constant

We consider the cosmology of a “3-brane universe” in a five dimensional (bulk) space-time with a cosmological constant. We show that Einstein's equations admit a first integral, analogous to the first Friedmann equation, which governs the evolution of the metric in the brane, whatever the time evolution of the metric along the fifth dimension. We thus obtain the cosmological evolution in the brane for any equation of state describing the matter in the brane, without needing the dependence of the metric on the fifth dimension. In the particular case p= wρ ( w= constant), we give explicit expressions for the time evolution of the brane scale factor, which show that standard cosmological evolution can be obtained (after an early non conventional phase) in a scenario à la Randall and Sundrum, where a brane tension compensates the bulk cosmological constant. We also show that a tiny deviation from exact compensation leads to an effective cosmological constant at late time. Moreover, when the metric along the fifth dimension is static, we are able to extend the solution found on the brane to the whole spacetime.