A generic class of theories where gravity is mediated by one tensor field together with an arbitrary number of scalar fields is considered. The predictions of these theories are worked out in four different observationally relevant regimes: (i) quasi-stationary weak fields (solar system conditions); (ii) rapidly varying weak fields (gravitational wave experiments); (iii) quasi-stationary strong fields (motion of systems of compact bodies, i.e. neutron stars or black holes); and (iv) the mixing of strong and radiative field effects in the gravitational radiation of N-compact-body systems. Moreover, the authors derive several significant relations between the theoretical quantities entering these predictions. They show how strong-field-gravity effects in the motion and gravitational radiation of N-compact-body systems can be parametrized by a set of theory parameters that generalize the usual post-Newtonian parameters ( gamma , beta ,. . .) introduced in the context of quasi-stationary weak gravitational fields. These new parameters ( beta 2, beta ', beta 3, beta ",. . .) provide a chart for the yet essentially unexplored domain of strong-gravitational-field effects, and thereby suggest new directions for testing relativistic gravity. This is illustrated by studying in detail a specific two-parameter tensor-bi-scalar theory T( beta ', beta ") which has the same post-Newtonian limit as general relativity but leads to new nonEinsteinian predictions for the various observables that can be extracted from binary pulsar data.

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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

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

The present acceleration of the Universe strongly indicated by recent observational data can be modeled in the scope of a scalar-tensor theory of gravity. We show that it is possible to determine the structure of this theory (the scalar field potential and the functional form of the scalar-gravity coupling) along with the present density of dustlike matter from the following two observable cosmological functions: the luminosity distance and the linear density perturbation in the dustlike matter component as functions of redshift. Explicit results are presented in the first order in the small inverse Brans-Dicke parameter ω −1 .PACS numbers: 98.80. Cq, 04.50.+h Recent observational data on type Ia supernovae explosions at high redshifts z ≡ a(t0) a(t) −1 ∼ 1 obtained independently by two groups [1,2], as well as numerous previous arguments (see the recent reviews [3,4]), strongly support the existence of a new kind of matter in the Universe whose energy density not only is positive but also dominates the energy densities of all previously known forms of matter [here a(t) is the scale factor of the FriedmannRobertson-Walker (FRW) isotropic cosmological model, and t 0 is the present time]. This form of matter has a strongly negative pressure and remains unclustered at all scales where gravitational clustering of baryons and (nonbaryonic) cold dark matter (CDM) is seen. Its gravity results in the present acceleration of the expansion of the Universe:ä(t 0 ) > 0. In a first approximation, this kind of matter may be described by a constant Λ-term in the gravity equations as first introduced by Einstein. However, a Λ-term could also be slowly varying with time. If so, this will be soon determined from observational data. In particular, if we use the simplest model of a variable Λ-term (also called quintessence in [5]) borrowed from the inflationary scenario of the early Universe, namely an effective scalar field Φ with some self-interaction potential U (Φ) minimally coupled to gravity, then the functional form of U (Φ) can be determined from observational cosmological functions: either from the luminosity distance D L (z) [6,7], or from the linear density perturbation in the dustlike component of matter in the Universe δ m (z) for a fixed comoving smoothing radius [6]. However, this model cannot account for any future observational data, in particular, for any functional form of D L (z). This happens because a variable Λ-term in this model should satisfy the weak-energy condition ε Λ +p Λ ≥ 0. In terms of the observable quantity H(z) ≡ȧ(t)/a(t) describing the evolution of the expanding Universe at recent epochs, the following inequality should be satisfied [4]Here, H 0 = H(z = 0) is the Hubble constant, Ω m,0 is the present energy density of the dustlike (CDM+baryons) matter component in terms of the critical density ε crit = 3H 2 0 /8πG (c =h = 1, and an index 0 stands for the present value of the corresponding quantity). Note that the inequality (1) saturates when the Λ-term is exactly constant. It is not clear fr...

We report the results of a 10‐year timing campaign on PSR J1738+0333, a 5.85‐ms pulsar in a low‐eccentricity 8.5‐h orbit with a low‐mass white dwarf companion. We obtained 17 376 pulse times of arrival with a stated uncertainty smaller than s and weighted residual rms of s. The large number and precision of these measurements allow highly significant estimates of the proper motion μα, δ= (+7.037 ± 0.005, +5.073 ± 0.012) mas yr−1, parallax πx = (0.68 ± 0.05) mas and a measurement of the apparent orbital decay, (all 1σ uncertainties). The measurements of μα, δ and πx allow for a precise subtraction of the kinematic contribution to the observed orbital decay; this results in a significant measurement of the intrinsic orbital decay: . This is consistent with the orbital decay from the emission of gravitational waves predicted by general relativity, , i.e. general relativity passes the test represented by the orbital decay of this system. This agreement introduces a tight upper limit on dipolar gravitational wave emission, a prediction of most alternative theories of gravity for asymmetric binary systems such as this. We use this limit to derive the most stringent constraints ever on a wide class of gravity theories, where gravity involves a scalar‐field contribution. When considering general scalar–tensor theories of gravity, our new bounds are more stringent than the best current Solar system limits over most of the parameter space, and constrain the matter–scalar coupling constant to be below the 10−5 level. For the special case of the Jordan–Fierz–Brans–Dicke, we obtain the 1σ bound , which is within a factor of 2 of the Cassini limit. We also use our limit on dipolar gravitational wave emission to constrain a wide class of theories of gravity which are based on a generalization of Bekenstein’s Tensor–Vector–Scalar gravity, a relativistic formulation of modified Newtonian dynamics.

We extend to curved backgrounds all flat-space scalar field models that obey purely secondorder equations, while maintaining their second-order dependence on both field and metric. This extension simultaneously restores to second order the, originally higher derivative, stress tensors as well. The process is transparent and uniform for all dimensions.

We consider scalar-tensor theories of gravity in an accelerating universe. The equations for the background evolution and the perturbations are given in full generality for any parametrization of the Lagrangian, and we stress that apparent singularities are sometimes artifacts of a pathological choice of variables. Adopting a phenomenological viewpoint, i.e., from the observations back to the theory, we show that the knowledge of the luminosity distance as a function of redshift up to z ∼ (1 − 2), which is expected in the near future, severely constrains the viable subclasses of scalar-tensor theories. This is due to the requirement of positive energy for both the graviton and the scalar partner. Assuming a particular form for the Hubble diagram, consistent with present experimental data, we reconstruct the microscopic Lagrangian for various scalar-tensor models, and find that the most natural ones are obtained if the universe is (marginally) closed.