945 research outputs found
Three-manifold invariants and their relation with the fundamental group
We consider the 3-manifold invariant I(M) which is defined by means of the
Chern-Simons quantum field theory and which coincides with the
Reshetikhin-Turaev invariant. We present some arguments and numerical results
supporting the conjecture that, for nonvanishing I(M), the absolute value |
I(M) | only depends on the fundamental group \pi_1 (M) of the manifold M. For
lens spaces, the conjecture is proved when the gauge group is SU(2). In the
case in which the gauge group is SU(3), we present numerical computations
confirming the conjecture.Comment: 22 pages, Latex document, two eps figure
FRW Cosmological Perturbations in Massive Bigravity
Cosmological perturbations of FRW solutions in ghost free massive bigravity,
including also a second matter sector, are studied in detail. At early time, we
find that sub horizon exponential instabilities are unavoidable and they lead
to a premature departure from the perturbative regime of cosmological
perturbations.Comment: latex, 14 page
Disformal invariance of continuous media with linear equation of state
We show that the effective theory describing single component continuous
media with a linear and constant equation of state of the form is
invariant under a 1-parameter family of continuous disformal transformations.
In the special case of (ultrarelativistic gas), such a family reduces
to conformal transformations. As examples, perfect fluids, homogeneous and
isotropic solids are discussed.Comment: latex, 7 page
The Fate of the Radion in Models with Metastable Graviton
We clarify some general issues in models where gravity is localized at
intermediate distances. We introduce the radion mode, which is usually
neglected, and we point out that its role in the model is crucial. We show that
the brane bending effects discussed in the literature can be obtained in a
formalism where the physical origin is manifest. The model violates positivity
of energy due to a negative tension brane, which induces a negative kinetic
term for the radion. The very same effect that violates positivity is
responsible for the recovery of conventional Einstein gravity at intermediate
distances.Comment: Latex file, 13 page
Graviton loops and brane observables
We discuss how to consistently perform effective Lagrangian computations in
quantum gravity with branes in compact extra dimensions. A reparametrization
invariant and infrared finite result is obtained in a non trivial way. It is
crucial to properly account for brane fluctuations and to correctly identify
physical observables. Our results correct some confusing claims in the
literature. We discuss the implications of graviton loops on electroweak
precision observables and on the muon g-2 in models with large extra
dimensions. We model the leading effects, not controlled by effective field
theory, by introducing a hard momentum cut-off.Comment: 9 pages + 4 eps figures, JHEP style latex document. The paper is
composed by a theoretical part, followed (after page 21) by a
phenomenological part. v2: version published in JHEP, few typos corrected.
v3: few additional typos corrected in the Appendi
Living with ghosts in Horava-Lifshitz gravity
We consider the branch of the projectable Horava-Lifshitz model which
exhibits ghost instabilities in the low energy limit. It turns out that, due to
the Lorentz violating structure of the model and to the presence of a finite
strong coupling scale, the vacuum decay rate into photons is tiny in a wide
range of phenomenologically acceptable parameters. The strong coupling scale,
understood as a cutoff on ghosts' spatial momenta, can be raised up to TeV. At lower momenta, the projectable Horava-Lifshitz gravity is
equivalent to General Relativity supplemented by a fluid with a small positive
sound speed squared () , that could
be a promising candidate for the Dark Matter. Despite these advantages, the
unavoidable presence of the strong coupling obscures the implementation of the
original Horava's proposal on quantum gravity. Apart from the Horava-Lifshitz
model, conclusions of the present work hold also for the mimetic matter
scenario, where the analogue of the projectability condition is achieved by a
non-invertible conformal transformation of the metric.Comment: 33 pages, 1 figure. The proof of an equivalence between the IR limit
of the projectable Horava-Lifshitz gravity and the mimetic matter scenario is
given in Appendix A. Version accepted for publication in JHE
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