arXiv subjects

Gilles Esposito-Farèse

Publications and source records attributed to Gilles Esposito-Farèse.

3 recordsLinked to original sources

arXiv2025

Large black-hole scalar charges induced by cosmology in Horndeski theories

The regularity of black hole solutions, embedded in an expanding Universe, is studied in a subclass of Horndeski theories, namely the sum of the simplest quadratic, cubic and quintic actions. We find that in presence of a time derivative of the scalar field, driven by the cosmological expansion, this regularity generically imposes large scalar charges for black holes, even when assuming strictly no direct coupling of matter to the scalar field. Such charges cause a significant accretion of the scalar field by the black holes, driving its local time derivative to a small value. This phenomenon, together with the Vainshtein screening typical of these theories, strongly suppresses observable scalar effects. We show that this full class of models is consistent with LIGO/Virgo detections of gravitational waves, but that the LISA mission should be able to constrain the coefficient of the quintic term at the $10^{-30}$ level in a self-acceleration scenario, an improvement by 16 orders of magnitude with respect to what is imposed by the speed of gravitational waves.

gr-qc↗

arXiv2012

The relativistic pulsar-white dwarf binary PSR J1738+0333 II. The most stringent test of scalar-tensor gravity

(abridged) 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-hour orbit with a low-mass white dwarf companion (...) The measurements of proper motion and parallax 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: (-25.9 +/- 3.2) \times 10^{-15} s/s. This is consistent with the orbital decay from the emission of gravitational waves predicted by general relativity, (-27.7 +1.5/-1.9) \times 10^{-15} s/s (...). 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 α_0^2 to be below the 10^{-5} level. For the special case of the Jordan-Fierz-Brans-Dicke, we obtain the one-sigma bound α_0^2 < 2 \times 10^{-5}, which is within a factor two 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 (TeVeS), a relativistic formulation of Modified Newtonian Dynamics (MOND).

astro-ph.GA↗

arXiv2011

Universality of high-energy absorption cross sections for black holes

We consider the absorption problem for a massless scalar field propagating in static and spherically symmetric black holes of arbitrary dimension endowed with a photon sphere. For this wide class of black holes, we show that the fluctuations of the high-energy absorption cross section are totally and very simply described from the properties (dispersion relation and damping) of the waves trapped near the photon sphere and therefore, in the eikonal regime, from the characteristics (orbital period and Lyapunov exponent) of the null unstable geodesics lying on the photon sphere. This is achieved by using Regge pole techniques. They permit us to make an elegant and powerful resummation of the absorption cross section and to extract then all the physical information encoded in the sum over the partial wave contributions. Our analysis induces moreover some consequences concerning Hawking radiation which we briefly report.

gr-qc↗