2,938 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
Formalism and Conserved Currents in Cosmology
The formalism, based on the counting of the number of e-folds
during inflation in different local patches of the Universe, has been
introduced several years ago as a simple and physically intuitive approach to
calculate (non-linear) curvature perturbations from inflation on large sales,
without resorting to the full machinery of (higher-order) perturbation theory.
Later on, it was claimed the equivalence with the results found by introducing
a conserved fully non-linear current , thereby allowing to directly
connect perturbations during inflation to late-Universe observables. We discus
some issues arising from the choice of the initial hyper-surface in the formalism. By using a novel exact expression for , valid for any
barotropic fluid, we find that it is not in general related to the standard
uniform density curvature perturbation ; such a result conflicts with
the claimed equivalence with formalism. Moreover, a similar analysis
is done for the proposed non-perturbative generalization of the
comoving curvature perturbation .Comment: 19 pages, 1 figures. Final version accepted for publication in JCAP.
Title slightly changed to avoid confusion with existing literature. Expanded
content with the same conclusion
Weak Massive Gravity
We find a new class of theories of massive gravity with five propagating
degrees of freedom where only rotations are preserved. Our results are based on
a non-perturbative and background-independent Hamiltonian analysis. In these
theories the weak field approximation is well behaved and the static
gravitational potential is typically screened \`a la Yukawa at large distances,
while at short distances no vDVZ discontinuity is found and there is no need to
rely on nonlinear effects to pass the solar system tests. The effective field
theory analysis shows that the ultraviolet cutoff is (m M_PL)^1/2 ~ 1/\mu m,
the highest possible. Thus, these theories can be studied in weak-field regime
at all the phenomenologically interesting scales, and are candidates for a
calculable large-distance modified gravity.Comment: 5 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
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