Caroline Santos

Black holes on thick branes

The interplay between topological defects ~branes! and black holes has been a subject of recent study, motivated in part by interest in brane-world scenarios. In this paper we analyze in detail the description of a black hole bound to a domain wall ~a two-brane in four dimensions!, for which an exact description in the limit of zero wall thickness has been given recently. We show how to smooth this singular solution with a thick domain wall. We also show that charged extremal black holes of a size ~roughly! smaller than the brane thickness expel the wall, thereby extending the phenomenon of flux expulsion. Finally, we analyze the process of black hole nucleation on a domain wall, and argue that it is preferred over a previously studied mechanism of black hole nucleation away from the wall.

Cosmic strings in dilaton gravity

We examine the metric of an isolated self-gravitating abelian-Higgs vortex in dilatonic gravity for arbitrary coupling of the vortex fields to the dilaton. We look for solutions in both massless and massive dilaton gravity. We compare our results to existing metrics for strings in Einstein and Jordan-Brans-Dicke theory. We explore the generalization of Bogomolnyi arguments for our vortices and comment on the effects on test particles.

Cosmology in Brans-Dicke theory with a scalar potential

We consider the general behaviour of cosmologies in Brans–Dicke theory where the dilaton is self-interacting via a potentialV(Φ). We show that the general radiation universe is a two-dimensional dynamical system, whereas the dust or false vacuum universe is three- dimensional. This is in contrast to the non-interacting dilaton which has uniformly a two-dimensional phase space. We find the phase spaces in each case and the general behaviour of the cosmologies.

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.

Evolution of the fine-structure constant in the non-linear regime

We study the evolution of the fine-structure constant, α, induced by non-linear density perturbations in the context of the simplest class of quintessence models with a non-minimal coupling to the electromagnetic field, in which the gauge kinetic function is Taylor-expanded up to linear order. We show that the results obtained using the spherical infall model for an infinite wavelength inhomogeneity are inconsistent with the results of a local linearized gravity study and we argue in favour of the second approach. We also discuss recent claims that the value of α inside virialized regions could be significantly different from the background one on the basis of these findings.

Cosmic strings in Bekenstein-type models

We study static cosmic string solutions in the context of Bekenstein-type models. We show that there is a class of models of this type for which the classical Nielsen–Olesen vortex is still a valid solution. However, in general, static string solutions in Bekenstein-type models strongly depart from the standard Nielsen–Olesen solution with the electromagnetic energy concentrated along the string core seeding spatial variations of the fine structure constant, α. We consider models with a generic gauge kinetic function and show that the equivalence principle constraints impose tight limits on the allowed variations of α induced by string networks on cosmological scales.

Cosmic strings in axionic-dilatonic gravity

We first consider local cosmic strings in dilaton-axion gravity and show that they are singular solutions. Then we take a supermassive Higgs limit and present expressions for the fields at far distances from the core by applying a Pecci-Quinn and a duality transformation to the dilatonic Melvins magnetic universe.

Gravitational Effects of Varying Alpha Strings

We study spatial variations of the fine-structure constant in the presence of static straight cosmic strings in the weak-field approximation in Einstein gravity. We work in the context of a generic Bekenstein-type model and consider a gauge kinetic function linear in the scalar field. We determine an analytical form for the scalar field and the string metric at large distances from the core. We show that the gravitational effects of α-varying strings can be seen as a combination of the gravitational effects of global and local strings. We also verify that at large distances to the core the space–time metric is similar to that of a global string. We study the motion of test particles approaching from infinity and show that photons are scattered to infinity while massive particles are trapped in bounded trajectories. We also calculate an overall limit on the magnitude of the variation of α for a GUT string, by considering suitable cosmological constraints coming from the Equivalence Principle.

Topological defects: A problem for cyclic universes?

We study the behavior of cosmic string networks in contracting universes, and discuss some of their possible consequences. We note that there is a fundamental time asymmetry between defect network evolution for an expanding universe and a contracting universe. A string network with negligible loop production and small-scale structure will asymptotically behave during the collapse phase as a radiation fluid. In realistic networks these two effects are important, making this solution only approximate. We derive new scaling solutions describing this effect, and test them against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation background it has generated, can significantly affect the dynamics of the universe both locally and globally. The network can be an important source of radiation, entropy and inhomogeneity. We discuss the possible implications of these findings for bouncing and cyclic cosmological models.

Vortices and black holes in dilatonic gravity

We study analytically black holes pierced by a thin vortex in dilatonic gravity for an arbitrary coupling of the vortex to the dilaton in an arbitrary frame. We show that the horizon of the charged black hole supports the long-range fields of the Nielsen-Olesen vortex that can be considered as black hole hair for both massive and massless dilatons. We also prove that extremal black holes exhibit a flux expulsion phenomenon for a sufficiently thick vortex. We consider the gravitational back-reaction of the thin vortex on the spacetime geometry and dilaton, and discuss under what circumstances the vortex can be used to smooth out the singularities in the dilatonic C-metrics. The effect of the vortex on the massless dilaton is to generate an additional dilaton flux across the horizon.