arXiv subjects

Ruth Gregory

Publications and source records attributed to Ruth Gregory.

At least 91 records · Page 5Linked to original sources

arXiv2008

Spinflation

We study the cosmological implications of including angular motion in the DBI brane inflation scenario. The non-canonical kinetic terms of the Dirac-Born-Infeld action give an interesting alternative to slow roll inflation, and cycling branes can drive periods of accelerated expansion in the Universe. We present explicit numerical solutions demonstrating brane inflation in the Klebanov-Strassler throat. We find that demanding sufficient inflation takes place in the throat is in conflict with keeping the brane's total energy low enough so that local gravitational backreaction on the Calabi-Yau manifold can be safely ignored. We deduce that spinflation (brane inflation with angular momentum) can ease this tension by providing extra e-foldings at the start of inflation. Cosmological expansion rapidly damps the angular momentum causing an exit to a more conventional brane inflation scenario. Finally, we set up a general framework for cosmological perturbation theory in this scenario, where we have multi-field non-standard kinetic term inflation.

hep-th↗

arXiv2008

Classical and quantum gravity of brane black holes

We test the holographic conjecture for brane black holes: that a full classical 5D solution will correspond to a quantum corrected 4D black hole. We show that a Schwarzschild-AdS black string in the bulk can be consistently interpreted as a quantum-corrected black hole on the brane, but the form of the quantum corrections is unlike what we would expect. The stress tensor extracted from the bulk solution does not have a thermal component corresponding to Hawking radiation outside the black hole. We compare this strong coupling prediction to a weak coupling calculation to study the differences in detail. We comment on implications for asymptotically flat black holes and for black holes localised in the extra dimension.

hep-th↗

arXiv2008

The three burials of Melquiades DGP

In this talk I review three fatal flaws of the DGP braneworld model, which has been put forward as a possible model for late time acceleration without a cosmological constant: Ghosts, Cosmological Crashes, and Instability of the 5D vacuum. The talk is based on work in collaboration with Charmousis, Kaloper, Myers and Padilla.

hep-th↗

arXiv2007

A New Perspective on DGP Gravity

We examine brane induced gravity on codimension-1 branes, a.k.a DGP gravity, as a theory of five-dimensional gravity containing a certain class four-dimensional branes. From this perspective, the model suffers from a number of pathologies which went unnoticed before. By generalizing the 5D geometry from Minkowski to Schwarzschild, we find that when the bulk mass is large enough, the brane hits a pressure singularity at finite radius. Further, on the self-accelerating branch, the five-dimensional energy is unbounded from below, implying that the self-accelerating backgrounds are unstable. Even in an empty Minkowski bulk, standard Euclidean techniques suggest that the spontaneous nucleation of self-accelerating branes is unsuppressed. If so, quantum effects will strongly modify any classical intuition about the theory. We also note that unless considered as Z_2-orbifold boundaries, self-accelerating branes correspond to `wormhole' configurations, which introduces the usual problematic issues associated with wormholes. Altogether these pathologies present a serious challenge that any proposed UV completion of the DGP model must overcome.

hep-th↗

arXiv2007

Cycling in the Throat

We analyse the dynamics of a probe D3-(anti-)brane propagating in a warped string compactification, making use of the Dirac-Born-Infeld action approximation. We also examine the time dependent expansion of such moving branes from the ``mirage cosmology'' perspective, where cosmology is induced by the brane motion in the background spacetime. A range of physically interesting backgrounds are considered: AdS5, Klebanov-Tseytlin and Klebanov-Strassler. Our focus is on exploring what new phenomenology is obtained from giving the brane angular momentum in the extra dimensions. We find that in general, angular momentum creates a centrifugal barrier, causing bouncing cosmologies. More unexpected, and more interesting, is the existence of bound orbits, corresponding to cyclic universes.

hep-th↗

arXiv2007

Stealth Acceleration and Modified Gravity

We show how to construct consistent braneworld models which exhibit late time acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our models have the standard Minkowski vacuum and accelerate only in the presence of matter, which we dub ``stealth-acceleration''. We use an effective action for the brane which includes an induced gravity term, and allow for an asymmetric set-up. We study the linear stability of flat brane vacua and find the regions of parameter space where the set-up is stable. The 4-dimensional graviton is only quasi-localised in this set-up and as a result gravity is modified at late times. One of the two regions is strongly coupled and the scalar mode is eaten up by an extra symmetry that arises in this limit. Having filtered the well-defined theories we then focus on their cosmology. When the graviton is quasi-localised we find two main examples of acceleration. In each case, we provide an illustrative model and compare it to LambdaCDM.

hep-th↗

arXiv2006

Braneworld stars and black holes

We look for spherically symmetric star or black hole solutions on a Randall-Sundrum brane from the perspective of the bulk. We take a known bulk solution, and analyse possible braneworld trajectories within it that correspond, from the braneworld point of view, to solutions of the brane Tolman-Oppenheimer-Volkoff equations. Our solutions are therefore embedded consistently into a full bulk solution. We find the full set of static gravitating matter sources on a brane in a range of bulk spacetimes, analyzing which can correspond to physically sensible sources. Finally, we look at time-dependent trajectories in a Schwarzschild--anti de Sitter spacetime as possible descriptions of time-dependent braneworld black holes, highlighting some of the general features one might expect, as well as some of the difficulties involved in getting a full solution to the question.

hep-th↗

arXiv2006

DGP Specteroscopy

We systematically explore the spectrum of gravitational perturbations in codimension-1 DGP braneworlds, and find a 4D ghost on the self-accelerating branch of solutions. The ghost appears for any value of the brane tension, although depending on the sign of the tension it is either the helicity-0 component of the lightest localized massive tensor of mass $0<m^2 < 2H^2$ for positive tension, the scalar `radion' for negative tension, or their admixture for vanishing tension. Because the ghost is gravitationally coupled to the brane-localized matter, the self-accelerating solutions are not a reliable benchmark for cosmic acceleration driven by gravity modified in the IR. In contrast, the normal branch of solutions is ghost-free, and so these solutions are perturbatively safe at large distance scales. We further find that when the $\mathbb{Z}_2$ orbifold symmetry is broken, new tachyonic instabilities, which are much milder than the ghosts, appear on the self-accelerating branch. Finally, using exact gravitational shock waves we analyze what happens if we relax boundary conditions at infinity. We find that non-normalizable bulk modes, if interpreted as 4D phenomena, may open the door to new ghost-like excitations.

hep-th↗

arXiv2004

Axisymmetric metrics in arbitrary dimensions

We consider axially symmetric static metrics in arbitrary dimension, both with and without a cosmological constant. The most obvious such solutions have an SO(n) group of Killing vectors representing the axial symmetry, although one can also consider abelian groups which represent a flat `internal space'. We relate such metrics to lower dimensional dilatonic cosmological metrics with a Liouville potential. We also develop a duality relation between vacuum solutions with internal curvature and those with zero internal curvature but a cosmological constant. This duality relation gives a solution generating technique permitting the mapping of different spacetimes. We give a large class of solutions to the vacuum or cosmological constant spacetimes. We comment on the extension of the C-metric to higher dimensions and provide a novel solution for a braneworld black hole.

gr-qc↗

arXiv2004

Observing braneworld black holes

Spacetime in the vicinity of an event horizon can be probed using observations which explore the dynamics of the accretion disc. Many high energy theories of gravity lead to modifications of the near horizon regime, potentially providing a testing ground for these theories. In this paper, we explore the impact of braneworld gravity on this region by formulating a method of deriving the general behaviour of the as yet unknown braneworld black hole solution. We use simple bounds to constrain the solution close to the horizon.

hep-th↗

arXiv2004

Einstein gravity on the codimension 2 brane?

We look at general braneworlds in six-dimensional Einstein-Gauss-Bonnet gravity. We find the general matching conditions for the Einstein-Gauss-Bonnet braneworld, which remarkably turn out to give precisely the four-dimensional Einstein equations for the induced metric and matter on the brane, even when the extra dimensions are non-compact and have infinite volume. We also show that relaxing regularity of the curvature in the vicinity of the brane, or alternatively having a finite width brane, gives rise to an additional possible correction to the Einstein equations, which contains information on the brane's embedding in the bulk and cannot be determined from knowledge of the braneworld alone. We comment on the advantages and disadvantages of each possibility, and the relevance of these results regarding a possible solution of the cosmological constant problem.

hep-th↗

arXiv2003

Spacetime decay of cones at strong coupling

We study time dependent solutions in dilaton gravity which correspond to the decay of conical spacetimes. In string theory this can be interpreted as a strong coupling limit of the decay of a non-supersymmetric orbifold spacetime with localized tachyons.

hep-th↗

arXiv2003

Inflating p-branes

We look for solutions in Einstein gravity corresponding to inflating braneworlds of arbitrary dimension and co-dimension. These solutions correspond to isolated sources (no long range fields). Using dynamical systems techniques, we show that there exists a unique solution corresponding to a black $p$-brane with a regular horizon at the location of the brane. The solution is {\it not} however asymptotically flat, but has global deficit angles.

hep-th↗

arXiv2002

Spacetime structure of the global vortex

We analyse the spacetime structure of the global vortex and its maximal analytic extension in an arbitrary number of spacetime dimensions. We find that the vortex compactifies space on the scale of the Hubble expansion of its worldvolume, in a manner reminiscent of that of the domain wall. We calculate the effective volume of this compactification and remark on its relevance to hierarchy resolution with extra dimensions. We also consider strongly gravitating vortices and derive bounds on the existence of a global vortex solution.

hep-th↗

arXiv2002

Macroscopic open strings and gravitational confinement

We consider classical solutions for strings ending on magnetically charged black holes in four-dimensional Kaluza-Klein theory. We examine the classical superstring and the global vortex, which can be viewed as a nonsingular model for the superstring. We show how both of these can end on a Kaluza-Klein monopole in the absence of self-gravity. Including gravitational back-reaction gives rise to a confinement mechanism of the magnetic flux of the black hole along the direction of the string. We discuss the relation of this work to localized solutions in ten dimensional supergravity.

hep-th↗

arXiv2001

Braneworld Instantons

We derive the general equations of motion for a braneworld containing a (localized) domain wall. Euclideanization gives the geometry for braneworld false vacuum decay. We explicitly derive these instantons and compute the amplitude for false vacuum decay. We also show how to construct a toy ekpyrotic instanton. Finally, we comment on braneworlds with a compact spatial direction and the adS soliton.

hep-th↗

arXiv2001

Nested braneworlds and strong brane gravity

We find the gravitational field of a `nested' domain wall living entirely within a brane-universe, or, a localised vortex within a wall. For a vortex living on a critical Randall-Sundrum brane universe, we show that the induced gravitational field on the brane is identical to that of an (n-1)-dimensional vacuum domain wall. We also describe how to set-up a nested Randall-Sundrum scenario using a flat critical vortex living on a subcritical (adS) brane universe.

hep-th↗

arXiv2001

Self-gravity of Brane Worlds: A New Hierarchy Twist

We examine the inclusion of brane self-gravity in brane-world scenarios with three or more compact extra dimensions. If the brane is a thin, localized one, then we find that the geometry in its vicinity is warped in such a way that gravity on the brane can become very weak, independently of the volume of the extra dimensions. As a consequence, self-gravity can make the brane structure enter into the determination of the hierarchy between the Planck scale and a lower fundamental scale. In an extreme case, one can obtain a novel reformulation of the hierarchy problem in brane worlds, without the need for large-size extra dimensions; the hierarchy would be generated when the ratio between the scales of brane tension and brane thickness is large. In a sense, such a scenario is half-way between the one of Arkani-Hamed et al.(ADD) (although with TeV-mass Kaluza-Klein states) and that of Randall and Sundrum (RS1) (but with only a TeV brane, and of positive tension). We discuss in detail the propagation of fields in the background of this geometry, and find that no problems appear even if the brane is taken to be very thin. We also discuss the presence of black branes and black holes in this setting, and the possibility of having a Planck brane.

hep-th↗