On the cosmic convergence mechanism of the massless dilaton

The converging mechanism discussed in [Damour & Nordtvedt, Physical Review Letters,70,15] for scalar-tensor theories has been applied to dilaton-like theories in several subsequent papers. In the present communication, we show that an unfortunate assumption in those studies led to a scalar-field equation unsuitable for the study of the dilaton field. The corrected scalar-field equation turns to change the numerical outcome of those studies in general, but even sometimes their qualitative aftermath. Therefore, the present result call for new investigations of the subject. On the other hand, our result shows that the string-inspired theory presented in [Minazzoli & Hees, Physical Review D,88,4] is naturally solution to the problem of the effective constancy of the fundamental coupling constants at late cosmic times, while it requires less fine-tuning than other massless dilaton or usual stalar-tensor theories.Comment: 4 pages -- accepted for publication in Physics Letters

New derivation of the Lagrangian of a perfect fluid with a barotropic equation of state

In this paper we give a simple proof that when the particle number is conserved, the Lagrangian of a barotropic perfect fluid is $\mathcal{L}_m=-\rho [c^2 +\int P(\rho)/\rho^2 d\rho]$, where $\rho$ is the \textit{rest mass} density and $P(\rho)$ is the pressure. To prove this result nor additional fields neither Lagrange multipliers are needed. Besides, the result is applicable to a wide range of theories of gravitation. The only assumptions used in the derivation are: 1) the matter part of the Lagrangian does not depend on the derivatives of the metric, and 2) the particle number of the fluid is conserved ($\nabla_\sigma (\rho u^\sigma)=0$)

Scalar-tensor propagation of light in the inner solar system at the millimetric level

In a recent paper [1], motivated by forthcoming space experiments involving propagation of light in the Solar System, we have proposed an extention of the IAU metric equations at the c-4 level in General Relativity. However, scalar-tensor theories may induce corrections numerically comparable to the c-4 general relativistic terms. Accordingly, one first proposes in this paper an extension of [1] to the scalar-tensor case. The case of a hierarchized system (such as the Solar system) is emphasized. In this case, the relevant metric solution is proposed. Then, the geodesic solution relevant for propagation of light in the inner solar system at the millimetric level is given in explicit form.Comment: 18 pages, This article can be regarded as an extension of "eprint arXiv:1003.1889

On dilatons with intrinsic decouplings

In this paper, we show that there exists a class of dilaton models with non-trivial scalar-Ricci and scalar-matter couplings that strongly reduces observational deviations from general relativity in the dust limit. Essentially, depending on the coupling between the dilaton and the fundamental matter fields, various strengths of decoupling can appear. They range from no decoupling at all to a total decoupling state. In this latter case, the theory becomes indistinguishable from general relativity (in the dust limit), as all dilatonic effects can be re-absorbed through a simple change of unit. Furthermore, for particular decouplings, we show that the phenomenology used to constrain theories from universality of free fall observations is significantly different from what is commonly used. Finally, from a fundamental perspective, the class of non-dynamical decouplings proposed in this paper might play a role in the current non-observation of any deviation from general relativity (in both tests of the equivalence principle and of the parametrized post-Newtonian formalism).Comment: 7 pages, comments welcom