171 research outputs found
Tests of scalar-tensor gravity
The best motivated alternatives to general relativity are scalar-tensor
theories, in which the gravitational interaction is mediated by one or several
scalar fields together with the usual graviton. The analysis of their various
experimental constraints allows us to understand better which features of the
models have actually been tested, and to suggest new observations able to
discriminate between them. This talk reviews three classes of constraints on
such theories, which are qualitatively different from each other: (i)
solar-system experiments; (ii) binary-pulsar tests and future detections of
gravitational waves from inspiralling binaries; (iii) cosmological
observations. While classes (i) and (ii) impose precise bounds respectively on
the first and second derivatives of the matter-scalar coupling function, (iii)
a priori allows us to reconstruct the full shapes of the functions of the
scalar field defining the theory, but obviously with more uncertainties and/or
more theoretical hypotheses needed. Simple arguments such as the absence of
ghosts (to guarantee the stability of the field theory) nevertheless suffice to
rule out a wide class of scalar-tensor models. Some of them can be probed only
if one takes simultaneously into account solar-system and cosmological
observations.Comment: 18 pages, 6 figures, invited talk at the workshop "Phi in the Sky:
The Quest for Cosmological Scalar Fields", Porto, 8-10 July 200
Binary-pulsar tests of strong-field gravity and gravitational radiation damping
This talk reviews the constraints imposed by binary-pulsar data on gravity
theories, focusing on ``tensor-scalar'' ones which are the best motivated
alternatives to general relativity. We recall that binary-pulsar tests are
qualitatively different from solar-system experiments, because of
nonperturbative strong-field effects which can occur in compact objects like
neutron stars, and because one can observe the effect of gravitational
radiation damping. Some theories which are strictly indistinguishable from
general relativity in the solar system are ruled out by binary-pulsar
observations. During the last months, several impressive new experimental data
have been published. Today, the most constraining binary pulsar is no longer
the celebrated (Hulse-Taylor) PSR B1913+16, but the neutron star-white dwarf
system PSR J1141-6545. In particular, in a region of the ``theory space'',
solar-system tests were known to give the tightest constraints; PSR J1141-6545
is now almost as powerful. We also comment on the possible scalar-field effects
for the detection of gravitational waves with future interferometers. The
presence of a scalar partner to the graviton might be detectable with the LISA
space experiment, but we already know that it would have a negligible effect
for LIGO and VIRGO, so that the general relativistic wave templates can be used
securely for these ground interferometers.Comment: 20 pages, LaTeX 2e, 7 postscript figures, contribution to 10th Marcel
Grossmann Meeting, 20-26 July 2003, Rio de Janeiro, Brazi
Field-theoretical formulations of MOND-like gravity
Modified Newtonian dynamics (MOND) is a possible way to explain the flat
galaxy rotation curves without invoking the existence of dark matter. It is
however quite difficult to predict such a phenomenology in a consistent field
theory, free of instabilities and admitting a well-posed Cauchy problem. We
examine critically various proposals of the literature, and underline their
successes and failures both from the experimental and the field-theoretical
viewpoints. We exhibit new difficulties in both cases, and point out the hidden
fine tuning of some models. On the other hand, we show that several published
no-go theorems are based on hypotheses which may be unnecessary, so that the
space of possible models is a priori larger. We examine a new route to
reproduce the MOND physics, in which the field equations are particularly
simple outside matter. However, the analysis of the field equations within
matter (a crucial point which is often forgotten in the literature) exhibits a
deadly problem, namely that they do not remain always hyperbolic. Incidentally,
we prove that the same theoretical framework provides a stable and well-posed
model able to reproduce the Pioneer anomaly without spoiling any of the
precision tests of general relativity. Our conclusion is that all MOND-like
models proposed in the literature, including the new ones examined in this
paper, present serious difficulties: Not only they are unnaturally fine tuned,
but they also fail to reproduce some experimental facts or are unstable or
inconsistent as field theories. However, some frameworks, notably the
tensor-vector-scalar (TeVeS) one of Bekenstein and Sanders, seem more promising
than others, and our discussion underlines in which directions one should try
to improve them.Comment: 66 pages, 6 figures, RevTeX4 format, version reflecting the changes
in the published pape
Arbitrary p-form Galileons
We show that scalar, 0-form, Galileon actions --models whose field equations
contain only second derivatives-- can be generalized to arbitrary even p-forms.
More generally, they need not even depend on a single form, but may involve
mixed p combinations, including equal p multiplets, where odd p-fields are also
permitted: We construct, for given dimension D, general actions depending on
scalars, vectors and higher p-form field strengths, whose field equations are
of exactly second derivative order. We also discuss and illustrate their
curved-space generalizations, especially the delicate non-minimal couplings
required to maintain this order. Concrete examples of pure and mixed actions,
field equations and their curved space extensions are presented.Comment: 8 pages, no figure, RevTeX4 format, v2: minor editorial changes
reflecting the published version in PRD Rapid Communication
Improving relativistic MOND with Galileon k-mouflage
We propose a simple field theory reproducing the MOND phenomenology at galaxy
scale, while predicting negligible deviations from general relativity at small
scales thanks to an extended Vainshtein ("k-mouflage") mechanism induced by a
covariant Galileon-type Lagrangian. The model passes solar-system tests at the
post-Newtonian order, including those of local Lorentz invariance, and its
anomalous forces in binary-pulsar systems are orders of magnitude smaller than
the tightest experimental constraints. The large-distance behavior is obtained
as in Bekenstein's tensor-vector-scalar (TeVeS) model, but with several
simplifications. In particular, no fine-tuned function is needed to interpolate
between the MOND and Newtonian regimes, and no dynamics needs to be defined for
the vector field because preferred-frame effects are negligible at small
distances. The field equations depend on second (and lower) derivatives, and
avoid thus the generic instabilities related to higher derivatives. Their
perturbative solution around a Schwarzschild background is remarkably simple to
derive. We also underline why the proposed model is particularly efficient
within the class of covariant Galileons.Comment: 6 pages, 1 figure, RevTeX4 forma
Covariant Galileon
We consider the recently introduced "galileon" field in a dynamical
spacetime. When the galileon is assumed to be minimally coupled to the metric,
we underline that both field equations of the galileon and the metric involve
up to third-order derivatives. We show that a unique nonminimal coupling of the
galileon to curvature eliminates all higher derivatives in all field equations,
hence yielding second-order equations, without any extra propagating degree of
freedom. The resulting theory breaks the generalized "Galilean" invariance of
the original model.Comment: 10 pages, no figure, RevTeX4 format; v2 adds footnote 1, Ref. [12],
reformats the link in Ref. [14], and corrects very minor typo
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