30,517 research outputs found
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 ,
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 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
backgrounds, they acquire the same form of the ones for the Randall-Sundrum II
model. When the background is different from pure AdS, 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
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 , 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 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
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