arXiv subjects

Thomas P. Sotiriou

Publications and source records attributed to Thomas P. Sotiriou.

At least 19 recordsLinked to original sources

arXiv2026

Adiabatic evolution of asymmetric binaries on generic orbits with new fundamental fields I: characterization of gravitational wave fluxes

We investigate the dynamics of asymmetric binaries in extensions of General Relativity featuring a massless scalar field non-minimally coupled to gravity, focusing on the interplay between eccentricity and inclination in fully generic bound orbits. Building on an effective field theory framework tailored to extreme- and intermediate-mass-ratio inspirals, we compute scalar-field perturbations using a new arbitrary-precision C++ code capable of evolving perturbations along generic Kerr geodesics, STORM. We investigate the complete set of scalar fluxes at infinity and through the horizon across the relevant parameter space and analyze their harmonic structure as a function of orbital geometry and black-hole spin. Our results advance ongoing efforts to construct accurate waveform models for asymmetric binaries beyond GR and lay the groundwork for precision tests of fundamental physics with next-generation gravitational-wave detectors.

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arXiv2026

Nonlinear evolution in Galileon EFTs: Regularization and screening

In a scalar effective field theory (EFT) with Galileon symmetry, we distinguish the strong-field expansion from the derivative expansion and study spherically symmetric classical nonlinear dynamics. More concretely, we consider two models related via perturbative field redefinitions: one with second-order equations that can also exhibit screening, and one that includes a higher-derivative term that introduces a propagating ghost. The latter term is of the type that can ``regularise'' the equations to render them well-posed as an initial value problem. For initial data that respect the derivative expansion, we find that the first model (ghost-free, unregularised) develops ill-posed regions during evolution only when the derivative expansion breaks down. This result persists in the strong-field regime. We further find that the regularized model reproduces the evolution of the unregularised one when the latter remains well-posed, while it exhibits tachyonic behaviour when the unregularised one becomes ill-posed. We also consider initial data that corresponds to stationary states that exhibit screening in the unregularised, ghost-free theory. In this case, we find that the regularised theory can remain well-posed for initial data that the unregularised one developed ill-posed regions. However, the ghost term naturally dominates over the Galileon term in the screening regime, contrary to common expectation.

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arXiv2026

The significance of first post-adiabatic contributions for scalar charge measurements with intermediate and extreme mass ratio inspirals

We present the first self-force-based beyond-GR waveform model incorporating post-adiabatic orbital evolution for intermediate- and extreme-mass-ratio inspirals in theories of gravity with additional scalar fields. Focusing on quasi-circular inspirals into a non-spinning primary, we combine a first post-adiabatic (1PA) gravitational sector with leading-order scalar field effects and use Bayesian injection-recovery studies to assess the impact of waveform systematics on the inference of scalar charges with LISA. We find that neglecting 1PA effects in the gravitational sector can bias the inference of intrinsic binary parameters, while scalar-charge measurements remain robust across a wide range of mass ratios. In contrast, analysing signals from binaries in which the secondary carries a scalar charge using pure-GR templates leads to significant biases and underestimated uncertainties due to unmodelled correlations between the scalar charge and the binary parameters. We also investigate the role of secondary spin and find no significant correlation between the secondary spin and the scalar charge. Notably, up to a mass ratio of $10^{-4}$, the secondary spin itself remains unconstrained even in the pure-GR case, in contrast with previous claims in the literature. Finally, we show that modelling scalar emission with a leading-order dipolar post-Newtonian approximation -- for quasi-circular inspirals into a non-spinning primary -- introduces negligible systematic errors relative to fully relativistic scalar fluxes.

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arXiv2026

Beyond black hole spectroscopy: Quasinormal mode contamination by massless scalars

Testing General Relativity (GR) with black hole ringdowns has conventionally focused on attempting to detect shifts away from the quasinormal mode (QNM) frequencies of the Kerr metric. It has recently been argued, however, that the ringdown signal will also be contaminated with the QNM frequencies of any new fields that are present in a beyond-GR scenario, provided that they couple nonminimally to gravity. We study black hole perturbations for the shift-symmetric Horndeski action, which includes all interactions between a massless scalar and gravity that lead to second order equations upon variation. We perturb linearly in the field and also employ a perturbative expansion in the scalar charge per unit black hole mass, $q$. Assuming that the scalar amplitude is suppressed by $q$, we demonstrate that, to order $q^2$, the coupling between the scalar and the Gauss-Bonnet invariant is the only term that contributes to both frequency shifts and contamination, and that the two effects appear at the same perturbative order. If the assumption about the suppression of the scalar amplitude is relaxed, contamination can appear at leading order in $q$, and hence dominate over frequency shifts. In this case, contamination also receives subleading corrections from an additional coupling

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arXiv2026

Minimum mass, maximum charge and hyperbolicity in scalar Gauss-Bonnet gravity

We study the loss of hyperbolicity of perturbation equations for black hole solutions of scalar Gauss-Bonnet gravity. We consider a class of coupling functions allowing for static black hole solutions with arbitrary small masses. For masses below a minimum value, such solutions become unphysical, because the perturbation equations become elliptic; this arguably corresponds to the loss of validity of the effective field theory. We analyse the dependence of this minimum mass on the parameters of the theory, finding that with an appropriate choice of the coupling function, such mass can be chosen arbitrarily small. However, this does not correspond to larger deviations from general relativity, since observable quantities like the black hole scalar charge are bounded by above.

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arXiv2026

From mergers to collapse: scalarisation dynamics in neutron star binaries

We present the first fully non-linear evolutions of binary neutron star mergers in a moving-punctures approach in Einstein-scalar-Gauss-Bonnet gravity. We study both linear and quadratic-type couplings between the scalar and the Gauss-Bonnet invariant, and uncover new post-merger phenomena. These include an enhancement of the prompt collapse of a long-lived hyper-massive neutron star remnant and cases where the remnant develops a scalar configuration due to different scalarisation instabilities. This study initiates the exploration of beyond-General-Relativistic effects enhanced by the non-linear dynamics of the neutron star's fluid.

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arXiv2026

Probing fundamental physics with Extreme Mass Ratio Inspirals: a full Bayesian inference for scalar charge

Extreme Mass Ratio Inspirals (EMRIs) are key sources for the future space-based gravitational wave detector LISA, and are considered promising probes of fundamental physics. Here, we present the first complete Bayesian analysis of EMRI signals in theories with an additional massless scalar, which could arise in an extension of General Relativity or of the Standard Model of Particle Physics. We develop a waveform model accurate at adiabatic order for equatorial eccentric orbits around spinning black holes. Using full Bayesian inference, we forecast LISA's ability to probe the presence of new fundamental fields with EMRI observations.

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arXiv2025

A well-posed BSSN-type formulation for scalar-tensor theories of gravity with second-order field equations

Recent developments in the modified harmonic and modified puncture gauges have opened new possibilities for performing stable numerical evolutions beyond General Relativity. In this work, we utilise techniques developed in the aforementioned formalisms to derive a BSSN-type formalism compatible with certain classes of modified gravity theories. As an intermediate step, we also derived modified versions of the Z4 and Z3 formalisms, thereby completing the connection between these formalisms beyond General Relativity. We then test the robustness of the new modified BSSN formalism by simulating the dynamics of black hole systems and benchmarking the results against the modified CCZ4 formulation. These developments enable the exploration of theories beyond General Relativity in many well-known Numerical Relativity codes that use different versions of the puncture gauge approach.

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arXiv2025

Screening of dipolar emission in two-scale Gauss-Bonnet gravity

We study black holes in shift-symmetric scalar Gauss-Bonnet gravity extended by a cubic Galileon interaction with a distinct energy scale. Introducing this hierarchy profoundly modifies the theory's phenomenology. The cubic interaction allows for smaller black holes, and can generate a screening mechanism near the horizon, making large Gauss-Bonnet couplings consistent with gravitational-wave bounds. Observable quantities such as the scalar charge, the innermost stable circular orbit, and its frequency are most affected for small black holes. The resulting multi-scale effective field theory remains technically natural and offers new avenues to probe gravity in the strong-field regime.

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arXiv2025

Frequency contamination from new fundamental fields in black hole ringdowns

We revisit the modelling of black hole ringdown beyond General Relativity (GR), emphasizing the limitations of approaches that rely solely on shifted quasinormal mode (QNM) frequencies. Starting from modified Teukolsky equations in such scenarios, we classify the distinct types of deviations that can arise -- those shifting QNM frequencies, and those introducing additional frequencies associated with extra fields. We then construct the most general ansatz for metric perturbations in this context and discuss its implications for QNM modelling and theory-agnostic tests of GR using gravitational wave data.

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arXiv2025

Supermassive black hole scalarization and effective field theory

A model in which black hole scalarization occurs for supermassive black holes, while their less massive counterparts remain unscalarized, has been recently proposed. We explore whether this model can emerge from an effective field theory obtained by integrating out a heavy second scalar field. We show that the resulting EFT does not have the right coupling sign or the right hierarchy of scales. We then consider whether supermassive black hole scalarization could occur in theories with two scalars. We show that, although they can violate black hole uniqueness through curvature- and spin-induced scalarization, they do not naturally produce scalarization exclusively for supermassive black holes.

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arXiv2025

Bayesian Search of Massive Scalar Fields from LIGO-Virgo-KAGRA Binaries

Massive scalar fields are promising candidates to address many unresolved problems in fundamental physics. We report the first model-agnostic Bayesian search of massive scalar fields in LIGO/Virgo/KAGRA gravitational-wave data. We find no evidence for such fields and place the most stringent upper limits on their coupling for scalar masses $\lesssim 2\times10^{-12}\,{\rm eV}$. We exemplify the strength of these bounds by applying them to massive scalar-Gauss-Bonnet gravity, finding the tightest constraints on the coupling constant to date, $\sqrt{α_{\rm GB}}\lesssim 1\,{\rm km}$ for scalar masses $\lesssim 10^{-13}\,{\rm eV}$ to 90% confidence.

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arXiv2025

Neutron stars and the cosmological constant problem

Phase transitions can play an important role in the cosmological constant problem, allowing the underlying vacuum energy, and therefore the value of the cosmological constant, to change. Deep within the core of neutron stars, the local pressure may be sufficiently high to trigger the QCD phase transition, thus generating a shift in the value of the cosmological constant. The gravitational effects of such a transition should then be imprinted on the properties of the star. Working in the framework of General Relativity, we provide a new model of the stellar interior, allowing for a QCD and a vacuum energy phase transition. We determine the impact of a vacuum energy jump on mass-radius relations, tidal deformability-radius relations, I-Love-Q relations and on the combined tidal deformability measured in neutron star binaries.

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arXiv2024

Hyperbolicity in scalar-Gauss-Bonnet gravity: a gauge invariant study for spherical evolution

We study spherical evolution in scalar-Gauss-Bonnet gravity with additional Ricci coupling and use the gauge-invariant approach of Ref.~\cite{Reall:2021voz} to track well-posedness. Our results show that loss of hyperbolicity when it occurs, is due to the behaviour of physical degrees of freedom. They provide further support to the idea that this behaviour can be tamed by additional interactions of scalar. We also point out a limitation of this gauge-invariant approach: the fact that field redefinitions can change the character of the evolution equations.

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arXiv2024

The dynamics of spherically symmetric black holes in scalar-Gauss-Bonnet gravity with a Ricci coupling

We study the dynamics of spherically symmetric black holes in scalar Gauss-Bonnet gravity with an additional coupling between the scalar field and the Ricci scalar using non-linear simulations that employ excision. In this class of theories, black holes possess hair if they lie in a specific mass range, in which case they exhibit a finite-area singularity, unlike general relativity. Our results show that the Ricci coupling can mitigate the loss of hyperbolicity in spherical evolution with black hole initial data. Using excision can enlarge the parameter space for which the system remains well-posed, as one can excise the elliptic region that forms inside the horizon. Furthermore, we explore a possible relation between the loss of hyperbolicity and the formation of the finite-area singularity inside the horizon. We find that the location of the singularity extracted from the static analysis matches the location of the sonic line well. Finally, when possible, we extract the monopolar quasi-normal modes and the time scale of the linear tachyonic instability associated with scalarization. We also check our results by utilizing a continued fraction analysis and supposing linear perturbations of the static solutions.

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arXiv2024

Spontaneous scalarization

Scalarization is a mechanism that endows strongly self-gravitating bodies, such as neutron stars and black holes, with a scalar-field configuration. It resembles a phase transition in that the scalar configuration appears only when a certain quantity that characterizes the compact object, for example, its compactness or spin, is beyond a threshold. A critical and comprehensive review of scalarization, including the mechanism itself, theories that exhibit it, its manifestation in neutron stars, black holes and their binaries, potential extension to other fields, and a thorough discussion of future perspectives, is provided.

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arXiv2024

Ringdowns for black holes with scalar hair: the large mass case

Deviations from General Relativity can alter the quasi-normal mode (QNM) ringdown of perturbed black holes. It is known that a shift-symmetric (hence massless) scalar can only introduce black hole hair if it couples to the Gauss-Bonnet invariant, in which case the scalar charge is fixed with respect to the black hole mass and controlled by the strength of that coupling. The charge per unit mass decreases with the mass and can, therefore, be used as a perturbative parameter for black holes that are sufficiently large with respect to the scale suppressing the deviation from General Relativity or the Standard model. We construct an effective field theory scheme for QNMs using this perturbative parameter to capture deviations from Kerr for both the background and the perturbations. We demonstrate that up to second order in the charge per unit mass, QNMs can be calculated by solving standard linearised perturbation equations for the Kerr metric with sources depending on solutions of the same equations up to first order. It follows that corrections to the QNM frequencies are heavily suppressed for sufficiently massive black holes, meaning that LISA is very unlikely to detect any evidence of scalar hair in ringdown signals.

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arXiv2024

Exotic compact objects and light bosonic fields

In this note, we discuss the effect of light, non-gauge, bosonic degrees of freedom on the exterior spacetime of an exotic compact object. We show that such fields generically introduce large deviations from black hole spacetimes of General Relativity near and outside the surfaces of ultra-compact exotic objects unless one assumes they totally decouple from the standard model or new heavy fields. Hence, using black hole spacetimes of General Relativity to model ultra-compact exotic objects and their perturbations relies implicitly on this assumption or on the absence of such fields.

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