2,408 research outputs found
On Unitarity of Massive Gravity in Three Dimensions
We examine a unitarity of a particular higher-derivative extension of general
relativity in three space-time dimensions, which has been recently shown to be
equivalent to the Pauli-Fierz massive gravity at the linearized approximation
level, and explore a possibility of generalizing the model to higher space-time
dimensions. We find that the model in three dimensions is indeed unitary in the
tree-level, but the corresponding model in higher dimensions is not so due to
the appearance of non-unitary massless spin-2 modes.Comment: 10 pages, references adde
DECIGO/BBO as a probe to constrain alternative theories of gravity
We calculate how strongly one can constrain the alternative theories of
gravity with deci-Hz gravitational wave interferometers such as DECIGO and BBO.
Here we discuss Brans-Dicke theory and massive graviton theories as typical
examples. We consider the inspiral of compact binaries composed of a neutron
star (NS) and an intermediate mass black hole (IMBH) for Brans-Dicke (BD)
theory and those composed of a super massive black hole (SMBH) and a black hole
(SMBH) for massive graviton theories. Using the restricted 2PN waveforms
including spin effects and taking the spin precession into account, we perform
the Monte Carlo simulations of binaries to estimate the determination
accuracy of binary parameters including the Brans-Dicke parameter
and the graviton Compton length . Assuming a
NS/BH binary of SNR=, the constraint on
is obtained as ,
which is 300 times stronger than the estimated constraint from LISA
observation. Furthermore, we find that, due to the expected large merger rate
of NS/BH binaries of yr, a statistical analysis yields
, which is 4 orders of magnitude stronger
than the current strongest bound obtained from the solar system experiment. For
massive graviton theories, assuming a BH/BH binary at
3Gpc, one can put a constraint cm, on average.
This is three orders of magnitude stronger than the one obtained from the solar
system experiment. From these results, it is understood that DECIGO/BBO is a
very powerful tool for constraining alternative theories of gravity, too.Comment: 4 pages, 3 figures; Accepted to Prog. Theor. Phys. Letters; Many
interpretations and some references have been added; Some Coding errors being
corrected and the final constraints came out stronge
Particle production in models with helicity-0 graviton ghost in de Sitter spacetime
We revisit the problem of the helicity-0 ghost mode of massive graviton in
the de Sitter background. In general, the presence of a ghost particle, which
has negative energy, drives the vacuum to be unstable through pair production
of ghost particles and ordinary particles. In the case that the vacuum state
preserves the de Sitter invariance, the number density created by the pair
production inevitably diverges due to unsuppressed ultra-violet(UV)
contributions. In such cases one can immediately conclude that the model is not
viable. However, in the massive gravity theory we cannot construct a vacuum
state which respects the de Sitter invariance. Therefore the presence of a
ghost does not immediately mean the breakdown of the model. Explicitly
estimating the number density and the energy density of particles created by
the pair production of two conformal scalar particles and one helicity-0 ghost
graviton, we find that these densities both diverge. However, since models with
helicity-0 ghost graviton have no de Sitter invariant vacuum state, it is
rather natural to consider a UV cutoff scale in the three-dimensional momentum
space. Then, even if we take the cutoff scale as large as the Planck scale, the
created number density and energy density are well suppressed. In many models
the cutoff scale is smaller than the Planck scale. In such models the created
number density and the energy density are negligiblly small as long as only the
physics below the cutoff scale is concerned.Comment: 7 pages, 1 figur
No de Sitter invariant vacuum in massive gravity theory with ghost
In this letter we point out that the massive gravity theory with a graviton
ghost mode in de Sitter background cannot possess a de Sitter invariant vacuum
state. In order to avoid a negative norm state, we must associate the creation
operator of the ghost mode with a negative-energy mode function instead of a
positive-energy one as the mode function. Namely, we have to adopt a different
procedure of quantization for a ghost. When a theory has a symmetry mixing a
ghost mode with ordinary non-ghost modes, the choice of a ghost mode is not
unique. However, quantization of a ghost is impossible without specifying a
choice of ghost mode, which breaks the symmetry. For this reason, the vacuum
state cannot respect the symmetry. In the massive gravity theory with a
graviton ghost mode in de Sitter background, the ghost is the helicity-0 mode
of the graviton. This ghost mode is mixed with the other helicity graviton
modes under the action of de Sitter symmetry. Therefore, there is no de Sitter
invariant vacuum in such models. This leads to an interesting possibility that
non-covariant cutoff of the low energy effective theory may naturally arise. As
a result, the instability due to the pair production of a ghost and normal
non-ghost particles gets much milder and that the model may escape from being
rejected.Comment: 5 page
Aspects of a supersymmetric Brans-Dicke theory
We consider a locally supersymmetric theory where the Planck mass is replaced
by a dynamical superfield. This model can be thought of as the Minimal
Supersymmetric extension of the Brans-Dicke theory (MSBD). The motivation that
underlies this analysis is the research of possible connections between Dark
Energy models based on Brans-Dicke-like theories and supersymmetric Dark Matter
scenarios. We find that the phenomenology associated with the MSBD model is
very different compared to the one of the original Brans-Dicke theory: the
gravitational sector does not couple to the matter sector in a universal metric
way. This feature could make the minimal supersymmetric extension of the BD
idea phenomenologically inconsistent.Comment: 6 pages, one section is adde
Helicity-1/2 Mode as a Probe of Interactions of Massive Rarita-Schwinger Field
We consider the electromagnetic and gravitational interactions of a massive
Rarita-Schwinger field. Stueckelberg analysis of the system, when coupled to
electromagnetism in flat space or to gravity, reveals in either case that the
effective field theory has a model-independent upper bound on its UV cutoff,
which is finite but parametrically larger than the particle's mass. It is the
helicity-1/2 mode that becomes strongly coupled at the cutoff scale. If the
interactions are inconsistent, the same mode becomes a telltale sign of
pathologies. Alternatively, consistent interactions are those that propagate
this mode within the light cone. Studying its dynamics not only sheds light on
the Velo-Zwanziger acausality, but also elucidates why supergravity and other
known consistent models are pathology-free.Comment: 18 pages, cutoff analysis improved, to appear in PR
The mass of the graviton and the cosmological constant
We show that the graviton acquires a mass in a de Sitter background given by
This is precisely the fine-tuning value required for
the perturbed gravitational field to mantain its two degrees of freedom.Comment: Title changed and few details added, without any changes in the
conclusio
L'utilisation d'édifices privés pour la protection de la population civile contre l'action de la guerre chimique
Il y a tout lieu d'admettre qu'au cours d'une guerre future, les moyens chimiques de combat que l'on désigne de façon générale sous le nom de gaz de guerre, seront de nouveau utilisés. En effet, il n'existe pas actuellement de bases certaines permettant d'assurer que les méthodes chimiques de combat ne seront employées à aucun prix. La question se complique encore étant donné qu'il existe toute une série de substances chimiques utilisables pour des buts de guerre mais que l'industrie emploie en temps de paix sur une large échelle et qui servent à la fabrication de produits nécessaires aux besoins de la population civile. On ne saurait nier le danger qui résulte du fait que l'on dispose de manière permanente en temps de paix du matériel utilisable en temps de guerre et que par conséquent un belligérant se trouve tenté d'avoir recours à la guerre chimiqu
Exact Solution to the "Auxiliary Extra Dimension" Model of Massive Gravity
The "Auxiliary Extra Dimension" model was proposed in order to provide a
geometrical interpretation to modifications of general relativity, in
particular to non-linear massive gravity. In this context, the theory was shown
to be ghost free to third order in perturbations, in the decoupling limit. In
this work, we exactly solve the equation of motion in the extra dimension, to
obtain a purely 4-dimensional theory. Using this solution, it is shown that the
ghost appears at the fourth order and beyond. We explore potential
modifications to address the ghost issue and find that their consistent
implementation requires going beyond the present framework.Comment: v3:8 pages, LaTex, a comment and a reference added, version to appear
in PL
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