Gravity and Scalar Fields

Gravity theories with non-minimally coupled scalar fields are used as characteristic examples in order to demonstrate the challenges, pitfalls and future perspectives of considering alternatives to general relativity. These lecture notes can be seen as an illustration of concepts, subtleties and techniques present in all alternative theories, but they also provide a brief review of generalised scalar-tensor theories.Comment: 21 pages, based on a lecture given at the Seventh Aegean Summer School "Beyond Einstein's Theory of Gravity

The PORTOLE Project: Supporting e-Learning

The PORTOLE (Providing Online Resources To Online Learning Environments) Project was a JISC-funded project which sought to produce a range of tools for tutors which could be used to enable them to discover information resources and to embed these into their course modules from within a University Virtual Learning Environment (VLE). The VLE in use at Universities of Leeds and Oxford is the Bodington system. A key deliverable of the Project was to produce tools that were designed with ease of incorporation into other VLE environments in mind. This paper discusses the background to the project and the key outcomes. Aworking service has been developed and is now being tested and evaluated with academic staff

Curvature scalar instability in f(R) gravity

An instability in the presence of matter in theories of gravity which include a 1/R correction in the gravitational action has been found by Dolgov and Kawasaki. In the present paper this instability is discussed for f(R) gravity in general. We focus on the Palatini formalism of the theory and it is shown that no such instability occurs in this version of f(R) gravity. The reasons for the appearance of the instability in the metric but not in the Palatini formalism are fully investigated.Comment: typos corrected, replaced to match published versio

Geodesic properties in terms of multipole moments in scalar-tensor theories of gravity

The formalism for describing a metric and the corresponding scalar in terms of multipole moments has recently been developed for scalar-tensor theories. We take advantage of this formalism in order to obtain expressions for the observables that characterise geodesics in terms of the moments. These expressions provide some insight into how the structure of a scalarized compact object affects observables. They can also be used to understand how deviations from general relativity are imprinted on the observables.Comment: 16 page

Strong coupling in extended Horava-Lifshitz gravity

An extension of Horava-Lifshitz gravity was recently proposed in order to address the pathological behavior of the scalar mode all previous versions of the theory exhibit. We show that even in this new extension the strong coupling persists, casting doubts on whether such a model can constitute an interesting alternative to general relativity (GR).Comment: 4 pages; v2: minor changes and references added; v3: clarification regarding the strong coupling scale added; v4: version accepted for publication in PLB, addresses issues raised in arXiv:0912.055

Horava-Lifshitz gravity with detailed balance

Horava-Lifshitz gravity with "detailed balance" but without the projectability assumption is discussed. It is shown that detailed balance is quite efficient in limiting the proliferation of couplings in Horava-Lifshitz gravity, and that its implementation without the projectability assumption leads to a theory with sensible dynamics. However, the (bare) cosmological constant is restricted to be large and negative.Comment: Contribution to the proceedings of NEB 15 conference, Chania, 20-23 June 2012 (talk given by D.V.