Volume stabilization in a warped flux compactification model

We investigate the stability of the extra dimensions in a warped, codimension two braneworld that is based upon an Einstein-Maxwell-dilaton theory with a non-vanishing scalar field potential. The braneworld solution has two 3-branes, which are located at the positions of the conical singularities. For this type of brane solution the relative positions of the branes (the shape modulus) is determined via the tension-deficit relations, if the brane tensions are fixed. However, the volume of the extra dimensions (the volume modulus) is not fixed in the context of the classical theory, implying we should take quantum corrections into account. Hence, we discuss the one-loop effective potential of the volume modulus for a massless, minimally coupled scalar field.Comment: 25 pages, 8 figures, typos correcte

A Model of Habitability Within the Milky Way Galaxy

We present a model of the Galactic Habitable Zone (GHZ), described in terms of the spatial and temporal dimensions of the Galaxy that may favour the development of complex life. The Milky Way galaxy is modelled using a computational approach by populating stars and their planetary systems on an individual basis using Monte-Carlo methods. We begin with well-established properties of the disk of the Milky Way, such as the stellar number density distribution, the initial mass function, the star formation history, and the metallicity gradient as a function of radial position and time. We vary some of these properties, creating four models to test the sensitivity of our assumptions. To assess habitability on the Galactic scale, we model supernova rates, planet formation, and the time required for complex life to evolve. Our study improves on other literature on the GHZ by populating stars on an individual basis and by modelling SNII and SNIa sterilizations by selecting their progenitors from within this preexisting stellar population. Furthermore, we consider habitability on tidally locked and non-tidally locked planets separately, and study habitability as a function of height above and below the Galactic midplane. In the model that most accurately reproduces the properties of the Galaxy, the results indicate that an individual SNIa is ~5.6 \times more lethal than an individual SNII on average. In addition, we predict that ~1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy. Of those stars with a habitable planet, ~75% of planets are predicted to be in a tidally locked configuration with their host star. The majority of these planets that may support complex life are found towards the inner Galaxy, distributed within, and significantly above and below, the Galactic midplane.Comment: Accepted for publication in Astrobiology. 40 pages, 12 figures, 3 table

3-Form Flux Compactification of Salam-Sezgin Supergravity

The compactification of 6 dimensional Salam-Sezgin model in the presence of 3-form flux H is investigated. We find a torus topology for this compactification with two cusps which are the places of branes, while at the limit of large size L of the compact direction we also obtain sphere topology. This resembles the Randall-Sundrum I,II model. The branes at one of the cusps can be chosen to be 3- and 4-branes which fill our 4-dimensional space together with the fact that H=0 at this position restores the Lorentz symmetry. This compactification also provides an example for the so-called `time warp' solution, [0812.5107 [hep-th]]. According to a no-go theorem in $d\ne 6$, the time warp compactification violates the null energy condition. While the theorem is quiet for d=6, our model gives a time warp compactification which satisfies the null energy condition. We also derive the four dimensional effective Planck mass which is not obvious due to the time warp nature of the solution.Comment: 19 pages, 5 fig

A self-tuning mechanism in (3+p)d gravity-scalar theory

We present a new type of self-tuning mechanism for ($3+p$)d brane world models in the framework of gravity-scalar theory. This new type of self-tuning mechanism exhibits a remarkable feature. In the limit $g_s \to 0$, $g_s$ being the string coupling, the geometry of bulk spacetime remains virtually unchanged by an introduction of the Standard Model(SM)-brane, and consequently it is virtually unaffected by quantum fluctuations of SM fields with support on the SM-brane. Such a feature can be obtained by introducing Neveu-Schwarz(NS)-brane as a background brane on which our SM-brane is to be set. Indeed, field equations naturally suggest the existence of the background NS-brane. Among the given such models, of the most interest is the case with $\Lambda=0$, where $\Lambda$ represents the bulk cosmological constant. This model contains a pair of coincident branes (of the SM- and the NS-branes), one of which is a codimension-2 brane placed at the origin of 2d transverse space ($\equiv \Sigma_2$), another a codimension-1 brane placed at the edge of $\Sigma_2$. These two branes are (anti) T-duals of each other, and one of them may be identified as our SM-brane plus the background NS-brane. In the presence of the background NS-brane (and in the absence of $\Lambda$), the 2d transverse space $\Sigma_2$ becomes an orbifold $R_2 /Z_n$ with an appropriate deficit angle. But this is only possible if the ($3+p$)d Planck scale $M_{3+p}$ and the string scale $M_s$($\equiv 1/\sqrt{\alpha^{\prime}}$) are of the same order, which accords with the hierarchy assumption \cite{1,2,3} that the electroweak scale $m_{EW}$ is the only short distance scale existing in nature

The General Warped Solution with Conical Branes in Six-dimensional Supergravity

We present the general regular warped solution with 4D Minkowski spacetime in six-dimensional gauged supergravity. In this framework, we can easily embed multiple conical branes into the warped geometry by choosing an undetermined holomorphic function. As an example, for the holomorphic function with many zeroes, we find warped solutions with multi-branes and discuss the generalized flux quantization in this case.Comment: 1+19 pages, no figure, JHEP style, version to appear in JHE

General Axisymmetric Solutions and Self-Tuning in 6D Chiral Gauged Supergravity

We re-examine the properties of the axially-symmetric solutions to chiral gauged 6D supergravity, recently found in refs. hep-th/0307238 and hep-th/0308064. Ref. hep-th/0307238 finds the most general solutions having two singularities which are maximally-symmetric in the large 4 dimensions and which are axially-symmetric in the internal dimensions. We show that not all of these solutions have purely conical singularities at the brane positions, and that not all singularities can be interpreted as being the bulk geometry sourced by neutral 3-branes. The subset of solutions for which the metric singularities are conical precisely agree with the solutions of ref. hep-th/0308064. Establishing this connection between the solutions of these two references resolves a minor conflict concerning whether or not the tensions of the resulting branes must be negative. The tensions can be both negative and positive depending on the choice of parameters. We discuss the physical interpretation of the non-conical solutions, including their significance for the proposal for using 6-dimensional self-tuning to understand the small size of the observed vacuum energy. In passing we briefly comment on a recent paper by Garriga and Porrati which criticizes the realization of self-tuning in 6D supergravity.Comment: 27 pages, 1 figure; JHEP3 style; Some references added, and discussion of tension constraints and unwarped solutions made more explici

Supersymmetric codimension-two branes and U(1)_R mediation in 6D gauged supergravity

We construct a consistent supersymmetric action for brane chiral and vector multiplets in a six-dimensional chiral gauged supergravity. A nonzero brane tension can be accommodated by allowing for a brane-localized Fayet-Iliopoulos term proportional to the brane tension. When the brane chiral multiplet is charged under the bulk U(1)_R, we obtain a nontrivial coupling to the extra component of the U(1)_R gauge field strength as well as a singular scalar self-interaction term. Dimensionally reducing to 4D on a football supersymmetric solution, we discuss the implication of such interactions for obtaining the U(1)_R D-term in the 4D effective supergravity. By assuming the bulk gaugino condensates and nonzero brane F- and/or D-term for the uplifting potential, we have all the moduli stabilized with a vanishing cosmological constant. The brane scalar with nonzero R charge then gets a soft mass of order the gravitino mass. The overall sign of the soft mass squared depends on the sign of the R charge as well as whether the brane F- or D-term dominates.Comment: 28 pages, no figures, version to appear in JHE

4d-Flat Compactifications With Brane Vorticities

We present solutions in six-dimensional gravity coupled to a sigma model, in the presence of three-brane sources. The space transverse to the branes is a compact non-singular manifold. The example of O(3) sigma model in the presence of two three-branes is worked out in detail. We show that the four-dimensional flatness is obtained with a single condition involving the brane tensions, which are in general different and may be both positive, and another characteristic of the branes, vorticity. We speculate that the adjustment of the effective four-dimensional cosmological constant may occur through the exchange of vorticity between the branes. We then give exact instanton type solutions for sigma models targeted on a general K\"ahler manifold, and elaborate in this framework on multi-instantons of the O(3) sigma model. The latter have branes, possibly with vorticities, at the instanton positions, thus generalizing our two-brane solution.Comment: 8 pages. New references added and minor typos are correcte

Non-Vacuum Bianchi Types I and V in f(R) Gravity

In a recent paper \cite{1}, we have studied the vacuum solutions of Bianchi types I and V spacetimes in the framework of metric f(R) gravity. Here we extend this work to perfect fluid solutions. For this purpose, we take stiff matter to find energy density and pressure of the universe. In particular, we find two exact solutions in each case which correspond to two models of the universe. The first solution gives a singular model while the second solution provides a non-singular model. The physical behavior of these models has been discussed using some physical quantities. Also, the function of the Ricci scalar is evaluated.Comment: 15 pages, accepted for publication in Gen. Realtiv. Gravi

Gravity on codimension 2 brane worlds

We compute the matching conditions for a general thick codimension 2 brane, a necessary previous step towards the investigation of gravitational phenomena in codimension 2 braneworlds. We show that, provided the brane is weakly curved, they are specified by the integral in the extra dimensions of the brane energy-momentum, independently of its detailed internal structure. These general matching conditions can then be used as boundary conditions for the bulk solution. By evaluating Einstein equations at the brane boundary we are able to write an evolution equation for the induced metric on the brane depending only on physical brane parameters and the bulk energy-momentum tensor. We particularise to a cosmological metric and show that a realistic cosmology can be obtained in the simplest case of having just a non-zero cosmological constant in the bulk. We point out several parallelisms between this case and the codimension 1 brane worlds in an AdS space.Comment: 24 page