Topology and Signature Changes in Braneworlds

It has been believed that topology and signature change of the universe can only happen accompanied by singularities, in classical, or instantons, in quantum, gravity. In this note, we point out however that in the braneworld context, such an event can be understood as a classical, smooth event. We supply some explicit examples of such cases, starting from the Dirac-Born-Infeld action. Topology change of the brane universe can be realised by allowing self-intersecting branes. Signature change in a braneworld is made possible in an everywhere Lorentzian bulk spacetime. In our examples, the boundary of the signature change is a curvature singularity from the brane point of view, but nevertheless that event can be described in a completely smooth manner from the bulk point of view.Comment: 26 pages, 8 figures, references and comments are added, minor revisions and a number of additional footnotes added, error corrected, minor corrections, to appear in Class. Quant. Gra

Ghosts in the self-accelerating universe

The self-accelerating universe realizes the accelerated expansion of the universe at late times by large-distance modification of general relativity without a cosmological constant. The Dvali-Gabadadze-Porrati (DGP) braneworld model provides an explicit example of the self-accelerating universe. Recently, the DGP model becomes very popular to study the observational consequences of the modified gravity models as an alternative to dark energy models in GR. However, it has been shown that the self-accelerating universe in the DGP model contains a ghost at the linearized level. The ghost carries negative energy densities and it leads to the instability of the spacetime. In this article, we review the origin of the ghost in the self-accelerating universe and explore the physical implication of the existence of the ghost.Comment: Invited topical review for Classical and Quantum Gravity, 20 pages, 4 figure

Strong Brane Gravity and the Radion at Low Energies

For the 2-brane Randall-Sundrum model, we calculate the bulk geometry for strong gravity, in the low matter density regime, for slowly varying matter sources. This is relevant for astrophysical or cosmological applications. The warped compactification means the radion can not be written as a homogeneous mode in the orbifold coordinate, and we introduce it by extending the coordinate patch approach of the linear theory to the non-linear case. The negative tension brane is taken to be in vacuum. For conformally invariant matter on the positive tension brane, we solve the bulk geometry as a derivative expansion, formally summing the `Kaluza-Klein' contributions to all orders. For general matter we compute the Einstein equations to leading order, finding a scalar-tensor theory with $\omega(\Psi) \propto \Psi / (1 - \Psi)$, and geometrically interpret the radion. We comment that this radion scalar may become large in the context of strong gravity with low density matter. Equations of state allowing $(\rho - 3 P)$ to be negative, can exhibit behavior where the matter decreases the distance between the 2 branes, which we illustrate numerically for static star solutions using an incompressible fluid. For increasing stellar density, the branes become close before the upper mass limit, but after violation of the dominant energy condition. This raises the interesting question of whether astrophysically reasonable matter, and initial data, could cause branes to collide at low energy, such as in dynamical collapse.Comment: 24 pages, 3 figure

Exact braneworld cosmology induced from bulk black holes

We use a new, exact approach in calculating the energy density measured by an observer living on a brane embedded in a charged black hole spacetime. We find that the bulk Weyl tensor gives rise to non-linear terms in the energy density and pressure in the FRW equations for the brane. Remarkably, these take exactly the same form as the ``unconventional'' terms found in the cosmology of branes embedded in pure AdS, with extra matter living on the brane. Black hole driven cosmologies have the benefit that there is no ambiguity in splitting the braneword energy momentum into tension and additional matter. We propose a new, enlarged relationship between the two descriptions of braneworld cosmology. We also study the exact thermodynamics of the field theory and present a generalised Cardy-Verlinde formula in this set up.Comment: 17 pages, no figures; v3: Minor change, References added, Version to appear in CQ

Cosmology of intersecting brane world models in Gauss-Bonnet gravity

We study the cosmological properties of a codimension two brane world that sits at the intersection between two four branes, in the framework of six dimensional Einstein-Gauss-Bonnet gravity. Due to contributions of the Gauss-Bonnet terms, the junction conditions require the presence of localized energy density on the codimension two defect. The induced metric on this surface assumes a FRW form, with a scale factor associated to the position of the brane in the background; we can embed on the codimension two defect the preferred form of energy density. We present the cosmological evolution equations for the three brane, showing that, for the case of pure AdS$_6$ backgrounds, they acquire the same form of the ones for the Randall-Sundrum II model. When the background is different from pure AdS$_6$, the cosmological behavior is potentially modified in respect to the typical one of codimension one brane worlds. We discuss, in a particular model embedded in an AdS$_6$ black hole, the conditions one should satisfy in order to obtain standard cosmology at late epochs.Comment: 19 pages, no figures, JHEP style. v2: Typos corrected and references adde

The Epidemiology of Stargardt Disease in the United Kingdom

The authors thank the British Ophthalmological Surveillance Unit (BOSU) for the support received, as well as Mr Barnaby Foot, research coordinator for BOSU, for his help and advice on this project. The authors thank the following ophthalmologists who assisted with data collection for this study: N. Acharya, S. Anwar, V. Bansal, P.N. Bishop, D. Byles, J.S. Chawla, A. Churchill, M. Clarke, B. Dhillon, M. Ekstein, S. George, J. Gillian, J.T. Gillow, D. Gilmour, R. Gray, P.T.S. Gregory, R. Gupta, S.P. Kelly, I.C. Lloyd, A. Lotery, M. McKibbin, R. MacLaren, G. Menon, A.T. Moore, A. Mulvihill, Y. Osoba, R. Pilling, H. Porooshani, A. Raghu Ram, T. Rimmer, I. Russell-Eggitt, M. Sarhan, R. Savides, S. Shafquat, A. Smith, A. Tekriwal, P. Tesha, P. Watts.Peer reviewedPublisher PD

Instabilities of Black Strings and Branes

We review recent progress on the instabilities of black strings and branes both for pure Einstein gravity as well as supergravity theories which are relevant for string theory. We focus mainly on Gregory-Laflamme instabilities. In the first part of the review we provide a detailed discussion of the classical gravitational instability of the neutral uniform black string in higher dimensional gravity. The uniform black string is part of a larger phase diagram of Kaluza-Klein black holes which will be discussed thoroughly. This phase diagram exhibits many interesting features including new phases, non-uniqueness and horizon-topology changing transitions. In the second part, we turn to charged black branes in supergravity and show how the Gregory-Laflamme instability of the neutral black string implies via a boost/U-duality map similar instabilities for non- and near-extremal smeared branes in string theory. We also comment on instabilities of D-brane bound states. The connection between classical and thermodynamic stability, known as the correlated stability conjecture, is also reviewed and illustrated with examples. Finally, we examine the holographic implications of the Gregory-Laflamme instability for a number of non-gravitational theories including Yang-Mills theories and Little String Theory.Comment: 119 pages, 16 figures. Invited review for Classical and Quantum Gravit

Cosmic acceleration from asymmetric branes

We consider a single 3-brane sitting in between two different five dimensional spacetimes. On each side of the brane, the bulk is a solution to Gauss-Bonnet gravity, although the bare cosmological constant, funda mental Planck scale, and Gauss-Bonnet coupling can differ. This asymmetry leads to weighted junction conditions across the brane and interesting brane cosmology. We focus on two special cases: a generalized Randall-Sundrum model without any Gauss-Bonnet terms, and a stringy model, without any bare cosmological constants, and positive Gauss-Bonnet coupling. Even though we assume there is no vacuum energy on the brane, we find late time de Sitter cosmologies can occur. Remarkably, in certain parameter regions, this acceleration is preceded by a period of matter/radiation domination, with $H^2 \propto \rho$, all the way back to nucleosynthesis.Comment: Version appearing in CQ

Compactification, topology change and surgery theory

We study the process of compactification as a topology change. It is shown how the mediating spacetime topology, or cobordism, may be simplified through surgery. Within the causal Lorentzian approach to quantum gravity, it is shown that any topology change in dimensions $\geq 5$ may be achieved via a causally continuous cobordism. This extends the known result for 4 dimensions. Therefore, there is no selection rule for compactification at the level of causal continuity. Theorems from surgery theory and handle theory are seen to be very relevant for understanding topology change in higher dimensions. Compactification via parallelisable cobordisms is particularly amenable to study with these tools.Comment: 1+19 pages. LaTeX. 9 associated eps files. Discussion of disconnected case adde

Completely localized gravity with higher curvature terms

In the intersecting braneworld models, higher curvature corrections to the Einstein action are necessary to provide a non-trivial geometry (brane tension) at the brane junctions. By introducing such terms in a Gauss-Bonnet form, we give an effective description of localized gravity on the singular delta-function branes. There exists a non-vanishing brane tension at the four-dimensional brane intersection of two 4-branes. Importantly, we give explicit expressions of the graviton propagator and show that the Randall-Sundrum single-brane model with a Gauss-Bonnet term in the bulk correctly gives a massless graviton on the brane as for the RS model. We explore some crucial features of completely localized gravity in the solitonic braneworld solutions obtained with a choice (\xi=1) of solutions. The no-go theorem known for Einstein's theory may not apply to the \xi=1 solution. As complementary discussions, we provide an effective description of the power-law corrections to Newtonian gravity on the branes or at the common intersection thereof.Comment: 19 pages, LaTeX, Revised/Published Versio