734 research outputs found
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
An equivalence principle for scalar forces
The equivalence of inertial and gravitational masses is a defining feature of
general relativity. Here, we clarify the status of the equivalence principle
for interactions mediated by a universally coupled scalar, motivated partly by
recent attempts to modify gravity at cosmological distances. Although a
universal scalar-matter coupling is not mandatory, once postulated, it is
stable against classical and quantum renormalizations in the matter sector. The
coupling strength itself is subject to renormalization of course. The scalar
equivalence principle is violated only for objects for which either the
graviton self-interaction or the scalar self-interaction is important---the
first applies to black holes, while the second type of violation is avoided if
the scalar is Galilean-symmetric.Comment: 4 pages, 1 figur
Time-dependent spherically symmetric covariant Galileons
We study spherically symmetric solutions of the cubic covariant Galileon
model in curved spacetime in presence of a matter source, in the test scalar
field approximation. We show that a cosmological time evolution of the Galileon
field gives rise to an induced matter-scalar coupling, due to the
Galileon-graviton kinetic braiding, therefore the solution for the Galileon
field is non trivial even if the bare matter-scalar coupling constant is set to
zero. The local solution crucially depends on the asymptotic boundary
conditions, and in particular, Minkowski and de Sitter asymptotics correspond
to different branches of the solution. We study the stability of these
solutions, namely, the well-posedness of the Cauchy problem and the positivity
of energy for scalar and tensor perturbations, by diagonalizing the kinetic
terms of the spin-2 and spin-0 degrees of freedom. In addition, we find that in
presence of a cosmological time evolution of the Galileon field, its kinetic
mixing with the graviton leads to a friction force, resulting to efficient
damping of scalar perturbations within matter.Comment: 20 pages, no figure, RevTeX4 format; v2: minor changes reflecting the
published version in PR
An Exotic Theory of Massless Spin-Two Fields in Three Dimensions
It is a general belief that the only possible way to consistently deform the
Pauli-Fierz action, changing also the gauge algebra, is general relativity.
Here we show that a different type of deformation exists in three dimensions if
one allows for PT non-invariant terms. The new gauge algebra is different from
that of diffeomorphisms. Furthermore, this deformation can be generalized to
the case of a collection of massless spin-two fields. In this case it describes
a consistent interaction among them.Comment: 21+1 pages. Minor corrections and reference adde
Constraints on Shift-Symmetric Scalar-Tensor Theories with a Vainshtein Mechanism from Bounds on the Time Variation of G
We show that the current bounds on the time variation of the Newton constant
G can put severe constraints on many interesting scalar-tensor theories which
possess a shift symmetry and a nonminimal matter-scalar coupling. This
includes, in particular, Galileon-like models with a Vainshtein screening
mechanism. We underline that this mechanism, if efficient to hide the effects
of the scalar field at short distance and in the static approximation, can in
general not alter the cosmological time evolution of the scalar field. This
results in a locally measured time variation of G which is too large when the
matter-scalar coupling is of order one.Comment: RevTeX4 format; v.2: 5 pages, title changed, matches published
versio
The recovery of General Relativity in massive gravity via the Vainshtein mechanism
We study in detail static spherically symmetric solutions of non linear
Pauli-Fierz theory. We obtain a numerical solution with a constant density
source. This solution shows a recovery of the corresponding solution of General
Relativity via the Vainshtein mechanism. This result has already been presented
by us in a recent letter, and we give here more detailed information on it as
well as on the procedure used to obtain it. We give new analytic insights upon
this problem, in particular for what concerns the question of the number of
solutions at infinity. We also present a weak field limit which allows to
capture all the salient features of the numerical solution, including the
Vainshtein crossover and the Yukawa decay.Comment: 38 pages, 9 Figs, revtex
Gauge-Fixing and Residual Symmetries in Gauge/Gravity Theories with Extra Dimensions
We study compactified pure gauge/gravitational theories with gauge-fixing
terms and show that these theories possess quantum mechanical SUSY-like
symmetries between unphysical degrees of freedom. These residual symmetries are
global symmetries and generated by quantum mechanical N=2 supercharges. Also,
we establish new one-parameter family of gauge choices for higher-dimensional
gravity, and calculate as a check of its validity one graviton exchange
amplitude in the lowest tree-level approximation. We confirm that the result is
indeed -independent and the cancellation of the -dependence is
ensured by the residual symmetries. We also give a simple interpretation of the
vDVZ-discontinuity, which arises in the lowest tree-level approximation, from
the supersymmetric point of view.Comment: REVTeX4, 17 pages, 1 figur
Ghosts, Strong Coupling and Accidental Symmetries in Massive Gravity
We show that the strong self-interaction of the scalar polarization of a
massive graviton can be understood in terms of the propagation of an extra
ghost-like degree of freedom, thus relating strong coupling to the sixth degree
of freedom discussed by Boulware and Deser in their Hamiltonian analysis of
massive gravity. This enables one to understand the Vainshtein recovery of
solutions of massless gravity as being due to the effect of the exchange of
this ghost which gets frozen at distances larger than the Vainshtein radius.
Inside this region, we can trust the two-field Lagrangian perturbatively, while
at larger distances one can use the higher derivative formulation. We also
compare massive gravity with other models, namely deconstructed theories of
gravity, as well as DGP model. In the latter case we argue that the Vainshtein
recovery process is of different nature, not involving a ghost degree of
freedom.Comment: 21 page
Probing Strong-Field Scalar-Tensor Gravity with Gravitational Wave Asteroseismology
We present an alternative way of tracing the existence of a scalar field
based on the analysis of the gravitational wave spectrum of a vibrating neutron
star. Scalar-tensor theories in strong-field gravity can potentially introduce
much greater differences in the parameters of a neutron star than the
uncertainties introduced by the various equations of state. The detection of
gravitational waves from neutron stars can set constraints on the existence and
the strength of scalar fields. We show that the oscillation spectrum is
dramatically affected by the presence of a scalar field, and can provide unique
confirmation of its existence.Comment: 14 pages, 7 figure
Electromagnetic Properties for Arbitrary Spin Particles: Part 2 Natural Moments and Transverse Charge Densities
In a set of two papers, we propose to study an old-standing problem, namely
the electromagnetic interaction for particles of arbitrary spin. Based on the
assumption that light-cone helicity at tree level and should be
conserved non-trivially by the electromagnetic interaction, we are able to
derive \emph{all} the natural electromagnetic moments for a pointlike particle
of \emph{any} spin. In this second paper, we give explicit expressions for the
light-cone helicity amplitudes in terms of covariant vertex functions, leading
to the natural electromagnetic moments at . As an application of our
results, we generalize the discussion of quark transverse charge densities to
particles with arbitrary spin.Comment: 12 pages, 1 tabl
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