On the gravitational stability of D1-D5-P black holes

We examine the stability of the nonextremal D1-D5-P black hole solutions. In particular, we look for the appearance of a superradiant instability for the spinning black holes but we find no evidence of such an instability. We compare this situation with that for the smooth soliton geometries, which were recently observed to suffer from an ergoregion instability, and consider the implications for the fuzzball proposal.Comment: 18 pages, 3 figures. Minor comments added to match published versio

New gravitational solutions via a Riemann-Hilbert approach

We consider the Riemann-Hilbert factorization approach to solving the field equations of dimensionally reduced gravity theories. First we prove that functions belonging to a certain class possess a canonical factorization due to properties of the underlying spectral curve. Then we use this result, together with appropriate matricial decompositions, to study the canonical factorization of non-meromorphic monodromy matrices that describe deformations of seed monodromy matrices associated with known solutions. This results in new solutions, with unusual features, to the field equations.Comment: 29 pages, 2 figures; v2: reference added, matches published versio

The return of the membrane paradigm? Black holes and strings in the water tap

Several general arguments indicate that the event horizon behaves as a stretched membrane. We propose using this relation to understand gravity and dynamics of black objects in higher dimensions. We provide evidence that (i) the gravitational Gregory-Laflamme instability has a classical counterpart in the Rayleigh-Plateau instability of fluids. Each known feature of the gravitational instability can be accounted for in the fluid model. These features include threshold mode, dispersion relation, time evolution and critical dimension of certain phase transitions. Thus, we argue that black strings break in much the same way as water from a faucet breaks up into small droplets. (ii) General rotating black holes can also be understood with this analogy. In particular, instability and bifurcation diagrams for black objects can easily be inferred. This correspondence can and should be used as a guiding tool to understand and explore physics of gravity in higher dimensions.Comment: This essay received an honorable mention in the Gravity Research Foundation Essay Competition, 2007. v2: Published versio

Gravitational signature of Schwarzschild black holes in dynamical Chern-Simons gravity

Dynamical Chern-Simons gravity is an extension of General Relativity in which the gravitational field is coupled to a scalar field through a parity-violating Chern-Simons term. In this framework, we study perturbations of spherically symmetric black hole spacetimes, assuming that the background scalar field vanishes. Our results suggest that these spacetimes are stable, and small perturbations die away as a ringdown. However, in contrast to standard General Relativity, the gravitational waveforms are also driven by the scalar field. Thus, the gravitational oscillation modes of black holes carry imprints of the coupling to the scalar field. This is a smoking gun for Chern-Simons theory and could be tested with gravitational-wave detectors, such as LIGO or LISA. For negative values of the coupling constant, ghosts are known to arise, and we explicitly verify their appearance numerically. Our results are validated using both time evolution and frequency domain methods.Comment: RevTex4, 12 pages, 8 figures, 3 Tables. v2: minor typos corrected and references added. Published versio

Investigating how educational technologies can enhance learning experiences by assisting different learning activities

Current educational technology implementations are done independently and not necessarily linked, neither with key elements of the pedagogic model, nor accommodating different learning styles. This work focuses on developing a framework that would standardise the use of assistive technologies in education. In particular, the focus of this work is on how social media, computer-assisted assessment, augmented and mixed reality can be used to improve the learning experience in certain educational contexts. This research study is based on a combination of grounded theory that included a literature review on the following relevant areas, covering key topics that correspond to the dimensions of the proposed framework: i) communication in education; ii) assessment; iii) and feedback. This stage provided a review of the learning activity spectrum that can be affected by educational technologies. The deliverable of this stage was a detailed literature review with distinct links to the action research in the form of specific pilot studies. The following stage provides a discussion on the impact of educational technologies on learning activities. The main deliverable is a review of current technologies with emphasis on how they affect specific learning activities The research also included an element of action research in the form of six pilot studies: i) Google Glass: Student Experience; ii) Google Glass: Presentation Feedback; iii) Google Glass: Feedback on Feedback; iv) Google Glass: Voting System; v) Social Media: Facebook/Twitter/LinkedIn; vi) Student Observable Behaviour. The pilots were conducted and analysed in order to provide sufficient evidence supporting the proposed framework guidelines. The framework proposed consists of four dimensions: i) content; ii) communication; iii) assessment; iv) feedback. This framework is a framework of good practice. It can be used to support academics who wishes to deploy educational technologies in support of a range of learning activities. Furthermore, the framework has the flexibility of applying different educational technologies for different scenarios without missing a standardised evaluation criteria