arXiv subjects

Etienne Ligout

Publications and source records attributed to Etienne Ligout.

4 recordsLinked to original sources

arXiv2026

Innermost stable circular orbit (ISCO) of arbitrary-mass compact binaries with spins at the fourth post-Newtonian order

We compute, up to 4th post-Newtonian (4PN) order, the gauge-invariant stability criterion determining the innermost stable circular orbit (ISCO) for arbitrary-mass compact binaries with spins aligned with the orbital angular momentum. Our calculation, a perturbation analysis, incorporates spin-orbit and spin-spin contributions, both up to next-to-next-to-leading order, as well as leading-order cubic and quartic spin contributions, all calculated in prior works within the effective-field-theory (EFT) Hamiltonian formalism. In the test-mass limit, we recover the Kerr ISCO truncated at 4PN order. Estimating the ISCO shift at first order in the mass ratio and comparing our results with numerical gravitational self-force (GSF) calculations, we find good agreement for retrograde spin (the smallest discrepancy being obtained for maximal retrograde spin) and moderate prograde spin. For nearly maximal prograde spin, the PN approach fails to determine the ISCO, for it is close to the black hole (BH) horizon. We also estimate the ISCO shift due to the test-particle spin and compare our result with the exact prediction from the Mathisson-Papapetrou-Dixon (MPD) equations for a spinning particle orbiting a Kerr black hole. Finally, we discuss the stability and the ISCO of corotating black-hole binaries.

gr-qc↗

arXiv2026

Neutron stars with primary scalar hair

We investigate static and spherically symmetric neutron star solutions endowed with primary scalar hair in a subfamily of Degenerate-Higher-Order-Scalar-Tensor (DHOST) theories of gravity. By solving the modified Tolman-Oppenheimer-Volkoff (TOV) equations, we construct equilibrium configurations for polytropic and realistic equations of state and analyse the impact of the scalar hair on the stellar structure. We examine the resulting metric and scalar field profiles as well as the mass-radius relation, showing deviations from the predictions of General Relativity (GR). Positive scalar charges lead to more compact stars than in GR and, above a critical threshold, to singularities. Observations could therefore put stringent constraints on the parameters characterising the beyond-GR effects in these theories and their potential scalar hair.

gr-qc↗

arXiv2025

In Praise of Slowness

While speed is an ubiquitous concept in physics, its inverse - known as slowness - sometimes proves more relevant. We discuss some case studies within classical physics where such a notion is fruitful, before exploring how it can be realized within the relativistic and thermodynamical frameworks. This leads us to a generalized slowness, not specifically defined in relation with speed but as characterizing the evolution of any physical parameter. We advocate the pedagogical relevance of slowness, by analogy with other couples of quantities, inverse of each other, commonly used in physics.

physics.ed-ph↗

arXiv2025

Innermost stable circular orbit of arbitrary-mass compact binaries at fourth post-Newtonian order

We compute by means of post-Newtonian (PN) methods the innermost stable circular orbit (ISCO) of arbitrary-mass (in particular, comparable-mass) compact binaries. Two methods are used with equivalent results: dynamical perturbation of the conservative equations of motion in harmonic coordinates, and dynamical perturbation of the conservative Hamiltonian in ADM coordinates. The perturbation of the non-local tail term at 4PN order in both approaches is carefully investigated. Our final gauge invariant result for the location of the ISCO at 4PN order is close to the numerical value of the ISCO shift computed by the gravitational self-force (GSF) approach in the small mass-ratio limit, and is also in good agreement with the full numerical-relativity calculation in the case of equal masses. The PN method followed here is considered in standard Taylor-expanded form, without any resummation techniques applied. As a complement, we also compute explicitly the gauge transformation from harmonic coordinates to ADM coordinates up to 4PN order, including the tail contribution therein.

gr-qc↗