Supergravity on R4 x S1/Z2 and singular Calabi-Yaus

We discuss the moduli space singularities that are generally present in five-dimensional vector-coupled supergravity on a spactime of the form R4 x S1/Z2, with vector fields surviving on the Z2 fixed planes. The framework of supergravity is necessarily ambiguous when it comes to the non-singular embedding theory, so we focus on those models coming from Calabi-Yau three-folds with wrapped membranes.Comment: 13 p

The effects of inhomogeneities on the cosmology of type IIB conifold transitions

In this paper we examine the evolution of the effective field theory describing a conifold transition in type IIB string theory. Previous studies have considered such dynamics starting from the cosmological approximation of homogeneous fields, here we include the effects of inhomogeneities by using a real-time lattice field theory simulation. By including spatial variations we are able to simulate the effect of currents and the gauge fields which they source. We identify two different regimes where the inhomogeneities have opposite effects, one where they aid the system to complete the conifold transition and another where they hinder it. The existence of quantized fluxes in related systems has lead to the speculation that (unstable) string solutions could exist, using our simulations we give strong evidence that these string-like defects do not form.Comment: 11 pages, 2 figures. Published versio

Accelerating cosmologies from compactification with a twist

It is demonstrated by explicit solutions of the (4+n)-dimensional vacuum Einstein equations that accelerating cosmologies in the Einstein conformal frame can be obtained by a time-dependent compactification of string/M-theory, even in the case that internal dimensions are Ricci-flat, provided one includes one or more geometric twists. Such acceleration is transient. When both compact hyperbolic internal spaces and geometric twists are included, however, the period of accelerated expansion may be made arbitrarily large.Comment: 5 pages, 1 figure, RevTeX

Acceleration from M theory and Fine-tuning

The compactification of M theory with time dependent hyperbolic internal space gives an effective scalar field with exponential potential which provides a transient acceleration in Einstein frame in four dimensions. Ordinary matter and radiation are present in addition to the scalar field coming from compactification. We find that we have to fine-tune the initial conditions of the scalar field so that our Universe experiences acceleration now. During the evolution of our Universe, the volume of the internal space increases about 12 times. The time variation of the internal space results in a large time variation of the fine structure constant which violates the observational constraint on the variation of the fine structure constant. The large variation of the fine structure constant is a generic feature of transient acceleration models.Comment: 9 pages, 3 figures, use iopart, v2; references updated, accepted for publication in Class. Quantum Gra

N=2 supergravity in five dimensions revisited

We construct matter-coupled N=2 supergravity in five dimensions, using the superconformal approach. For the matter sector we take an arbitrary number of vector-, tensor- and hyper-multiplets. By allowing off-diagonal vector-tensor couplings we find more general results than currently known in the literature. Our results provide the appropriate starting point for a systematic search for BPS solutions, and for applications of M-theory compactifications on Calabi-Yau manifolds with fluxes.Comment: 35 pages; v.2: A sign changed in a bilinear fermion term in (5.7

Singular manifolds, topology change and the dynamics of compactification

We investigate the dynamics of the geometric transitions associated to compactified spacetimes. By including the dynamics of gravity we are able to follow the evolution of collapsing cycles as they attempt to undergo a topology changing transition. Rather than achieving this singular geometry we find that one of two scenarios occur, depending on the initial conditions. Either a horizon forms, shielding a curvature singularity, or the cycle re-expands after an initial contraction phase. For the case where a horizon forms we identify the final state with a known analytic black-hole solution. We also show use our results to demonstate a novel compactification mechanism, owing to the asymptotic structure of this black-hole solution

Relativistic D-brane Scattering is Extremely Inelastic

We study the effects of quantum production of open strings on the relativistic scattering of D-branes. We find strong corrections to the brane trajectory from copious production of highly-excited open strings, whose typical oscillator level is proportional to the square of the rapidity. In the corrected trajectory, the branes rapidly coincide and remain trapped in a configuration with enhanced symmetry. This is a purely stringy effect which makes relativistic brane collisions exceptionally inelastic. We trace this effect to velocity-dependent corrections to the open-string mass, which render open strings between relativistic D-branes surprisingly light. We observe that pair-creation of open strings could play an important role in cosmological scenarios in which branes approach each other at very high speeds.Comment: 30 pages; added references and a comment about velocity-dependent masse

Exact Analysis of Scaling and Dominant Attractors Beyond the Exponential Potential

By considering the potential parameter $\Gamma$ as a function of another potential parameter $\lambda$[47], We successfully extend the analysis of two-dimensional autonomous dynamical system of quintessence scalar field model to the analysis of three-dimension, which makes us be able to research the critical points of a large number of potentials beyond the exponential potential exactly. We find that there are ten critical points in all, three points $P_{3, 5, 6}$} are general points which are possessed by all quintessence models regardless of the form of potentials and the rest points are closely connected to the concrete potentials. It is quite surprising that, apart from the exponential potential, there are a large number of potentials which can give the scaling solution when the function $f(\lambda)(=\Gamma(\lambda)-1)$ equals zero for one or some values of $\lambda_{*}$ and if the parameter $\lambda_{*}$ also satisfies the condition Eq.(16) or Eq.(17) at the same time. We give the differential equations to derive these potentials $V(\phi)$ from $f(\lambda)$. We also find that, if some conditions are satisfied, the de-Sitter-like dominant point $P_4$ and the scaling solution point $P_9$(or $P_{10}$) can be stable simultaneously but $P_9$ and $P_{10}$ can not be stable simultaneity. Although we survey scaling solutions beyond the exponential potential for ordinary quintessence models in standard general relativity, this method can be applied to other extensively scaling solution models studied in literature[46] including coupled quintessence, (coupled-)phantom scalar field, k-essence and even beyond the general relativity case $H^2 \propto\rho_T^n$. we also discuss the disadvantage of our approach.Comment: 16 pages,no figure, this new revision has taken the suggestions from CQG referees and has been accepted for publication in Classical and Quantum Gravit

Unified N=2 Maxwell-Einstein and Yang-Mills-Einstein Supergravity Theories in Four Dimensions

We study unified N=2 Maxwell-Einstein supergravity theories (MESGTs) and unified Yang-Mills Einstein supergravity theories (YMESGTs) in four dimensions. As their defining property, these theories admit the action of a global or local symmetry group that is (i) simple, and (ii) acts irreducibly on all the vector fields of the theory, including the ``graviphoton''. Restricting ourselves to the theories that originate from five dimensions via dimensional reduction, we find that the generic Jordan family of MESGTs with the scalar manifolds [SU(1,1)/U(1)] X [SO(2,n)/SO(2)X SO(n)] are all unified in four dimensions with the unifying global symmetry group SO(2,n). Of these theories only one can be gauged so as to obtain a unified YMESGT with the gauge group SO(2,1). Three of the four magical supergravity theories defined by simple Euclidean Jordan algebras of degree 3 are unified MESGTs in four dimensions. Two of these can furthermore be gauged so as to obtain 4D unified YMESGTs with gauge groups SO(3,2) and SO(6,2), respectively. The generic non-Jordan family and the theories whose scalar manifolds are homogeneous but not symmetric do not lead to unified MESGTs in four dimensions. The three infinite families of unified five-dimensional MESGTs defined by simple Lorentzian Jordan algebras, whose scalar manifolds are non-homogeneous, do not lead directly to unified MESGTs in four dimensions under dimensional reduction. However, since their manifolds are non-homogeneous we are not able to completely rule out the existence of symplectic sections in which these theories become unified in four dimensions.Comment: 47 pages; latex fil

A deformation of AdS_5 x S^5

We analyse a one parameter family of supersymmetric solutions of type IIB supergravity that includes AdS_5 x S^5. For small values of the parameter the solutions are causally well-behaved, but beyond a critical value closed timelike curves (CTC's) appear. The solutions are holographically dual to N=4 supersymmetric Yang-Mills theory on a non-conformally flat background with non-vanishing R-currents. We compute the holographic energy-momentum tensor for the spacetime and show that it remains finite even when the CTC's appear. The solutions, as well as the uplift of some recently discovered AdS_5 black hole solutions, are shown to preserve precisely two supersymmetries.Comment: 16 pages, v2: typos corrected and references adde