36 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
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
Field equations and cosmology for a class of nonlocal metric models of MOND
We consider a class of nonlocal, pure-metric modified gravity models which
were developed to reproduce the Tully-Fisher relation without dark matter and
without changing the amount of weak lensing predicted by general relativity.
Previous work gave only the weak field limiting form of the field equations
specialized to a static and spherically symmetric geometry. Here we derive the
full field equations and specialize them to a homogeneous, isotropic and
spatially flat geometry. We also discuss the problem of fitting the free
function to reproduce the expansion history. Results are derived for models in
which the MOND acceleration a_0 ~ 1.2 x 10^{-10} m/s^{2} is a fundamental
constant and for the more phenomenologically interesting case in which the MOND
acceleration changes with the cosmological expansion rate.Comment: 15 pages, no figures, uses revtex4, dedicated to Stanley Deser on the
occasion of his 83rd birthda
Counting the degrees of freedom of generalized Galileons
We consider Galileon models on curved spacetime, as well as the counterterms
introduced to maintain the second-order nature of the field equations of these
models when both the metric and the scalar are made dynamical. Working in a
gauge invariant framework, we first show how all the third-order time
derivatives appearing in the field equations -- both metric and scalar -- of a
Galileon model or one defined by a given counterterm can be eliminated to leave
field equations which contain at most second-order time derivatives of the
metric and of the scalar. The same is shown to hold for arbitrary linear
combinations of such models, as well as their k-essence-like/Horndeski
generalizations. This supports the claim that the number of degrees of freedom
in these models is only 3, counting 2 for the graviton and 1 for the scalar. We
comment on the arguments given previously in support of this claim. We then
prove that this number of degrees of freedom is strictly less that 4 in one
particular such model by carrying out a full-fledged Hamiltonian analysis. In
contrast to previous results, our analyses do not assume any particular gauge
choice of restricted applicability.Comment: 27 pages, no figure; v2: short explanation added below Eq. (42),
improved Sec. II.B.
Gravitational radiation from inspiralling compact binaries completed at the third post-Newtonian order
The gravitational radiation from point particle binaries is computed at the
third post-Newtonian (3PN) approximation of general relativity. Three
previously introduced ambiguity parameters, coming from the Hadamard self-field
regularization of the 3PN source-type mass quadrupole moment, are consistently
determined by means of dimensional regularization, and proved to have the
values xi = -9871/9240, kappa = 0 and zeta = -7/33. These results complete the
derivation of the general relativistic prediction for compact binary inspiral
up to 3.5PN order, and should be of use for searching and deciphering the
signals in the current network of gravitational wave detectors.Comment: 4 pages in 2-column format, LaTeX 2e, REVTeX 4, no figur
Vaccine breakthrough hypoxemic COVID-19 pneumonia in patients with auto-Abs neutralizing type I IFNs
Life-threatening `breakthrough' cases of critical COVID-19 are attributed to poor or waning antibody response to the SARS- CoV-2 vaccine in individuals already at risk. Pre-existing autoantibodies (auto-Abs) neutralizing type I IFNs underlie at least 15% of critical COVID-19 pneumonia cases in unvaccinated individuals; however, their contribution to hypoxemic breakthrough cases in vaccinated people remains unknown. Here, we studied a cohort of 48 individuals ( age 20-86 years) who received 2 doses of an mRNA vaccine and developed a breakthrough infection with hypoxemic COVID-19 pneumonia 2 weeks to 4 months later. Antibody levels to the vaccine, neutralization of the virus, and auto- Abs to type I IFNs were measured in the plasma. Forty-two individuals had no known deficiency of B cell immunity and a normal antibody response to the vaccine. Among them, ten (24%) had auto-Abs neutralizing type I IFNs (aged 43-86 years). Eight of these ten patients had auto-Abs neutralizing both IFN-a2 and IFN-., while two neutralized IFN-omega only. No patient neutralized IFN-ss. Seven neutralized 10 ng/mL of type I IFNs, and three 100 pg/mL only. Seven patients neutralized SARS-CoV-2 D614G and the Delta variant (B.1.617.2) efficiently, while one patient neutralized Delta slightly less efficiently. Two of the three patients neutralizing only 100 pg/mL of type I IFNs neutralized both D61G and Delta less efficiently. Despite two mRNA vaccine inoculations and the presence of circulating antibodies capable of neutralizing SARS-CoV-2, auto-Abs neutralizing type I IFNs may underlie a significant proportion of hypoxemic COVID-19 pneumonia cases, highlighting the importance of this particularly vulnerable population