5,292 research outputs found
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.
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