34 research outputs found
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
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
Dimensional regularization of the third post-Newtonian gravitational wave generation from two point masses
Dimensional regularization is applied to the computation of the gravitational
wave field generated by compact binaries at the third post-Newtonian (3PN)
approximation. We generalize the wave generation formalism from isolated
post-Newtonian matter systems to d spatial dimensions, and apply it to point
masses (without spins), modelled by delta-function singularities. We find that
the quadrupole moment of point-particle binaries in harmonic coordinates
contains a pole when epsilon = d-3 -> 0 at the 3PN order. It is proved that the
pole can be renormalized away by means of the same shifts of the particle
world-lines as in our recent derivation of the 3PN equations of motion. The
resulting renormalized (finite when epsilon -> 0) quadrupole moment leads to
unique values for the ambiguity parameters xi, kappa and zeta, which were
introduced in previous computations using Hadamard's regularization. Several
checks of these values are presented. These results complete the derivation of
the gravitational waves emitted by inspiralling compact binaries up to the
3.5PN level of accuracy which is needed for detection and analysis of the
signals in the gravitational-wave antennas LIGO/VIRGO and LISA.Comment: 60 pages, LaTeX 2e, REVTeX 4, 10 PostScript files (1 figure and 9
Young tableaux used in the text
Vector theories in cosmology
This article provides a general study of the Hamiltonian stability and the
hyperbolicity of vector field models involving both a general function of the
Faraday tensor and its dual, , as well as a Proca potential
for the vector field, . In particular it is demonstrated that theories
involving only do not satisfy the hyperbolicity conditions. It is then
shown that in this class of models, the cosmological dynamics always dilutes
the vector field. In the case of a nonminimal coupling to gravity, it is
established that theories involving or are generically
pathologic. To finish, we exhibit a model where the vector field is not diluted
during the cosmological evolution, because of a nonminimal vector
field-curvature coupling which maintains second-order field equations. The
relevance of such models for cosmology is discussed.Comment: 17 pages, no figur
Light deflection by gravitational waves from localized sources
We study the deflection of light (and the redshift, or integrated time delay)
caused by the time-dependent gravitational field generated by a localized
material source lying close to the line of sight. Our calculation explicitly
takes into account the full, near-zone, plus intermediate-zone, plus wave-zone,
retarded gravitational field. Contrary to several recent claims in the
literature, we find that the deflections due to both the wave-zone 1/r
gravitational wave and the intermediate-zone 1/r^2 retarded fields vanish
exactly. The leading total time-dependent deflection caused by a localized
material source, such as a binary system, is proven to be given by the
quasi-static, near-zone quadrupolar piece of the gravitational field, and
therefore to fall off as the inverse cube of the impact parameter.Comment: 12 pages, REVTeX 3.0, no figur
Orbital Tests of Relativistic Gravity using Artificial Satellites
We reexamine non-Einsteinian effects observable in the orbital motion of
low-orbit artificial Earth satellites. The motivations for doing so are
twofold: (i) recent theoretical studies suggest that the correct theory of
gravity might contain a scalar contribution which has been reduced to a small
value by the effect of the cosmological expansion; (ii) presently developed
space technologies should soon give access to a new generation of satellites
endowed with drag-free systems and tracked in three dimensions at the
centimeter level. Our analysis suggests that such data could measure two
independent combinations of the Eddington parameters (beta - 1) and (gamma - 1)
at the 10^-4 level and probe the time variability of Newton's "constant" at the
d(ln G)/dt ~ 10^-13 yr^-1 level. These tests would provide well-needed
complements to the results of the Lunar Laser Ranging experiment, and of the
presently planned experiments aiming at measuring (gamma -1). In view of the
strong demands they make on the level of non- gravitational perturbations,
these tests might require a dedicated mission consisting of an optimized
passive drag-free satellite.Comment: 17 pages, IHES/P/94/22 and CPT-94/P.E.302
Tensor-scalar gravity and binary-pulsar experiments
Some recently discovered nonperturbative strong-field effects in
tensor-scalar theories of gravitation are interpreted as a scalar analog of
ferromagnetism: "spontaneous scalarization". This phenomenon leads to very
significant deviations from general relativity in conditions involving strong
gravitational fields, notably binary-pulsar experiments. Contrary to
solar-system experiments, these deviations do not necessarily vanish when the
weak-field scalar coupling tends to zero. We compute the scalar "form factors"
measuring these deviations, and notably a parameter entering the pulsar timing
observable gamma through scalar-field-induced variations of the inertia moment
of the pulsar. An exploratory investigation of the confrontation between
tensor-scalar theories and binary-pulsar experiments shows that nonperturbative
scalar field effects are already very tightly constrained by published data on
three binary-pulsar systems. We contrast the probing power of pulsar
experiments with that of solar-system ones by plotting the regions they exclude
in a generic two-dimensional plane of tensor-scalar theories.Comment: 35 pages, REVTeX 3.0, uses epsf.tex to include 9 Postscript figure
Gravitational-wave versus binary-pulsar tests of strong-field gravity
Binary systems comprising at least one neutron star contain strong
gravitational field regions and thereby provide a testing ground for
strong-field gravity. Two types of data can be used to test the law of gravity
in compact binaries: binary pulsar observations, or forthcoming
gravitational-wave observations of inspiralling binaries. We compare the
probing power of these two types of observations within a generic two-parameter
family of tensor-scalar gravitational theories. Our analysis generalizes
previous work (by us) on binary-pulsar tests by using a sample of realistic
equations of state for nuclear matter (instead of a polytrope), and goes beyond
a previous study (by C.M. Will) of gravitational-wave tests by considering more
general tensor-scalar theories than the one-parameter Jordan-Fierz-Brans-Dicke
one. Finite-size effects in tensor-scalar gravity are also discussed.Comment: 23 pages, REVTeX 3.0, uses epsf.tex to include 5 postscript figures
(2 paragraphs and a 5th figure added at the end of section IV + minor
changes
Dimensional regularization of the third post-Newtonian dynamics of point particles in harmonic coordinates
Dimensional regularization is used to derive the equations of motion of two
point masses in harmonic coordinates. At the third post-Newtonian (3PN)
approximation, it is found that the dimensionally regularized equations of
motion contain a pole part [proportional to 1/(d-3)] which diverges as the
space dimension d tends to 3. It is proven that the pole part can be
renormalized away by introducing suitable shifts of the two world-lines
representing the point masses, and that the same shifts renormalize away the
pole part of the "bulk" metric tensor g_munu(x). The ensuing, finite
renormalized equations of motion are then found to belong to the general
parametric equations of motion derived by an extended Hadamard regularization
method, and to uniquely determine the heretofore unknown 3PN parameter lambda
to be: lambda = - 1987/3080. This value is fully consistent with the recent
determination of the equivalent 3PN static ambiguity parameter, omega_s = 0, by
a dimensional-regularization derivation of the Hamiltonian in
Arnowitt-Deser-Misner coordinates. Our work provides a new, powerful check of
the consistency of the dimensional regularization method within the context of
the classical gravitational interaction of point particles.Comment: 82 pages, LaTeX 2e, REVTeX 4, 8 PostScript figures, minor changes to
reflect Phys. Rev. D versio