518 research outputs found
Bigravity and Lorentz-violating Massive Gravity
Bigravity is a natural arena where a non-linear theory of massive gravity can
be formulated. If the interaction between the metrics and is
non-derivative, spherically symmetric exact solutions can be found. At large
distances from the origin, these are generically Lorentz-breaking bi-flat
solutions (provided that the corresponding vacuum energies are adjusted
appropriately). The spectrum of linearized perturbations around such
backgrounds contains a massless as well as a massive graviton, with {\em two}
physical polarizations each. There are no propagating vectors or scalars, and
the theory is ghost free (as happens with certain massive gravities with
explicit breaking of Lorentz invariance). At the linearized level, corrections
to GR are proportional to the square of the graviton mass, and so there is no
vDVZ discontinuity. Surprisingly, the solution of linear theory for a static
spherically symmetric source does {\em not} agree with the linearization of any
of the known exact solutions. The latter coincide with the standard
Schwarzschild-(A)dS solutions of General Relativity, with no corrections at
all. Another interesting class of solutions is obtained where and are
proportional to each other. The case of bi-de Sitter solutions is analyzed in
some detail.Comment: 25 pages. v3 Typos corrected, references added. v4 Introduction
extende
Generalized sine-Gordon/massive Thirring models and soliton/particle correspondences
We consider a real Lagrangian off-critical submodel describing the soliton
sector of the so-called conformal affine Toda model coupled to
matter fields (CATM). The theory is treated as a constrained system in the
context of Faddeev-Jackiw and the symplectic schemes. We exhibit the parent
Lagrangian nature of the model from which generalizations of the sine-Gordon
(GSG) or the massive Thirring (GMT) models are derivable. The dual description
of the model is further emphasized by providing the relationships between
bilinears of GMT spinors and relevant expressions of the GSG fields. In this
way we exhibit the strong/weak coupling phases and the (generalized)
soliton/particle correspondences of the model. The case is also
outlined.Comment: 22 pages, LaTex, some comments and references added, conclusions
unchanged, to appear in J. Math. Phy
Three-dimensional massive gravity and the bigravity black hole
We study three-dimensional massive gravity formulated as a theory with two
dynamical metrics, like the f-g theories of Isham-Salam and Strathdee. The
action is parity preserving and has no higher derivative terms. The spectrum
contains a single massive graviton. This theory has several features discussed
recently in TMG and NMG. We find warped black holes, a critical point, and
generalized Brown-Henneaux boundary conditions.Comment: 8 pages, Revtex. Minor change. References adde
Transverse Symmetry and Spin-3/2 Fields
We study the possible covariant Lagrangians that describe the propagation of
pure spin-3/2 particles. We show that, apart from the well-known
Rarita-Schwinger Lagrangian, there is another possibility where the field is
described by a gamma-traceless combination and that both Lagrangians yield the
same physical predictions for the interaction of conserved sources. We also
prove that for the case when the spin-2 field is described by a traceless
field, there is no possible spin-3/2 action that makes the system
supersymmetric. Nevertheless, the interaction between this field andthe
spin-3/2 field may be possible.Comment: 12 pages. To appear in Peyresq XII proceeding
The Vainshtein mechanism in the Decoupling Limit of massive gravity
We investigate static spherically symmetric solutions of nonlinear massive
gravities. We first identify, in an ansatz appropriate to the study of those
solutions, the analog of the decoupling limit (DL) that has been used in the
Goldstone picture description. We show that the system of equations left over
in the DL has regular solutions featuring a Vainshtein-like recovery of
solutions of General Relativity (GR). Hence, the singularities found to arise
integrating the full nonlinear system of equations are not present in the DL,
despite the fact those singularities are usually thought to be due to a
negative energy mode also seen in this limit. Moreover, we show that the
scaling conjectured by Vainshtein at small radius is only a limiting case in an
infinite family of non singular solutions each showing a Vainshtein recovery of
GR solutions below the Vainshtein radius but a different common scaling at
small distances. This new scaling is shown to be associated with a zero mode of
the nonlinearities left over in the DL. We also show that, in the DL, this
scaling allows for a recovery of GR solutions even for potentials where the
original Vainshtein mechanism is not working. Our results imply either that the
DL misses some important features of nonlinear massive gravities or that
important features of the solutions of the full nonlinear theory have been
overlooked. They could also have interesting outcomes for the DGP model and
related proposals.Comment: 52 pages, 7 figures; v3: minor typos corrected, discussion of the
validity of the Decoupling Limit extended; accepted for publication in JHE
Remarks on the Scalar Graviton Decoupling and Consistency of Horava Gravity
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. But there have been
confusions regarding the extra scalar graviton mode and the consistency of the
Horava model. I reconsider these problems and show that, in the Minkowski
vacuum background, the scalar graviton mode can be consistency decoupled from
the usual tensor graviton modes by imposing the (local) Hamiltonian as well as
the momentum constraints.Comment: Some clarifications regarding the projectable case added, Typos
corrected, Comments (Footnote No.9, Note Added) added, References updated,
Accepted in CQ
CMB photons shedding light on dark matter
The annihilation or decay of Dark Matter (DM) particles could affect the
thermal history of the universe and leave an observable signature in Cosmic
Microwave Background (CMB) anisotropies. We update constraints on the
annihilation rate of DM particles in the smooth cosmological background, using
WMAP7 and recent small-scale CMB data. With a systematic analysis based on the
Press-Schechter formalism, we also show that DM annihilation in halos at small
redshift may explain entirely the reionization patterns observed in the CMB,
under reasonable assumptions concerning the concentration and formation
redshift of halos. We find that a mixed reionization model based on DM
annihilation in halos as well as star formation at a redshift z~6.5 could
simultaneously account for CMB observations and satisfy constraints inferred
from the Gunn-Peterson effect. However, these models tend to reheat the
inter-galactic medium (IGM) well above observational bounds: by including a
realistic prior on the IGM temperature at low redshift, we find stronger
cosmological bounds on the annihilation cross-section than with the CMB alone.Comment: 35 pages, 14 figures; version accepted in JCAP after minor revision
Instability and Spatiotemporal Dynamics of Alternans in Paced Cardiac Tissue
We derive an equation that governs the spatiotemporal dynamics of small
amplitude alternans in paced cardiac tissue. We show that a pattern-forming
linear instability leads to the spontaneous formation of stationary or
traveling waves whose nodes divide the tissue into regions with opposite phase
of oscillation of action potential duration. This instability is important
because it creates dynamically an heterogeneous electrical substrate for
inducing fibrillation if the tissue size exceeds a fraction of the pattern
wavelength. We compute this wavelength analytically as a function of three
basic length scales characterizing dispersion and inter-cellular electrical
coupling.Comment: 4 pages, 3 figures, submitted to PR
Positive self-evaluation versus negative other-evaluation in the political genre of pre-election debates.
The present study explores the language of evaluation in a sub-genre of political discourse, pre-electoral debates, and its potential persuasive function for gaining voters via a contraposition of positive self-evaluation and negative evaluation of the other candidate. A further aim of this research is to check whether the candidate¿s ideology has a bearing on the entities that get evaluated. After a brief examination of the characteristics of the sub-genre at hand, specifically in the Spanish context, we present the results of an evaluation analysis carried out in a corpus of 19,849 words, which is the extension of the most recent pre-electoral debate held in Spain between the candidates of the two main political parties. Taking into account Van Dijk¿s CDA framework (2005) for parliamentary debates as global semantic strategies of positive self-presentation and negative other-presentation, Martin and White¿s (2005) method was adopted as an analytical tool. The results showed that, although each candidate had different preferences in the choice of evaluative devices, they both used them as a strategy to win electoral votes while deprecating the opposing party and, therefore, minimizing their chances of winning the elections. On the other hand, and despite their opposing ideology, they both seem to defend those policies that are more widely accepted in order not to risk losing voters: public services and egalitarian social policies
Patterns of Soft Masses from General Semi-Direct Gauge Mediation
We give a general formulation of semi-direct gauge mediation of supersymmetry
breaking where the messengers interact with the hidden sector only through a
weakly gauged group. Using this general formulation, we provide an explicit
proof that the MSSM gaugino masses are vanishing to leading order in the gauge
couplings. On the other hand, the MSSM sfermion masses have, generically, a
non-vanishing leading contribution. We discuss how such a mechanism can
successfully be combined with other mediation schemes which give tachyonic
sfermions, such as sequestered anomaly mediation and some direct gauge
mediation models.Comment: 26 pages, 5 figures; v2: refs added; v3: minor corrections,
clarifications added, mainly in section
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