arXiv subjects

Christos Charmousis

Publications and source records attributed to Christos Charmousis.

At least 19 recordsLinked to original sources

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↗

arXiv2026

Radial Perturbations of Black Holes in DHOST Theories

We study radial perturbations of static black holes with primary hair in a subfamily of degenerate higher-order scalar-tensor (DHOST) theories. We recast the equation of motion for the monopole degree of freedom into a flat radial wave equation and show that the associated operator can be extended, through appropriate boundary conditions, to a positive self-adjoint operator which ensures the stability of the radial mode. Remarkably, the coordinate choice that leads to the flat wave equation corresponds to the unitary gauge, in which the scalar field is uniform. As a result, the radial coordinate extends beyond the event horizon, into the black hole interior, in contrast with the tortoise coordinate in General Relativity. The same wave equation with the same coordinate choice applies to all solutions that are connected by disformal transformations. We also examine stealth black hole solutions, with either a constant or non constant kinetic term. In the former case, we find, to linear order, the absence of a propagating degree of freedom. In the latter case, we identify a stable radial degree of freedom, except for special values of the theory coupling constants.

gr-qc↗

arXiv2026

An effective cosmological constant as black hole primary hair

We study Generalized Proca theories inspired by the recent regularised Proca theory of four-dimensional Gauss-Bonnet gravity. By abandoning the rigid constraints typically imposed by specific regularization schemes, we treat the coefficients of the terms in the action as free parameters. This approach uncovers a broader solution space that admits static and spherically symmetric black hole solutions characterized by primary hair, where, surprisingly, the cosmological constant arises naturally as a constant of integration even in the absence of a bare cosmological term.

gr-qc↗

arXiv2025

Theoretical filters for shift-symmetric Horndeski gravities

We investigate the structure of nontrivial maximally symmetric vacua and compact-object solutions in shift-symmetric scalar-tensor theories. Focusing on Horndeski gravity, we derive consistency conditions directly from the field equations to identify the subclasses that admit Minkowski and de Sitter vacua with a nontrivial scalar field. In doing so, we obtain a filtering mechanism that operates independently of observational data. In this context, we introduce the notion of stealth vacua, where the scalar field remains active without altering the vacuum. Following this, we examine the theoretical framework of Horndeski theories that admit homogeneous geometries and we extract the implicit form of the solution pertaining to the entire family of theories. Building upon these frameworks, we construct exact solutions in beyond-Horndeski gravity by applying a linear disformal transformation to the regularized Einstein-Gauss-Bonnet black hole. This procedure yields solitonic spacetimes with scalar hair as well as black holes carrying primary scalar hair, demonstrating how disformal maps can qualitatively modify solution properties. We delineate the parameter space in which the transformation is well-defined and analyze the solutions. Our results provide both a principled criterion for selecting viable Horndeski models and a framework for exploring rich solution spaces in beyond-Horndeski gravity.

gr-qc↗

arXiv2025

Proca theory of four-dimensional regularized Gauss-Bonnet gravity and black holes with primary hair

We introduce a novel, well-defined four-dimensional regularized Gauss-Bonnet theory of gravity by applying a dimensional regularization procedure. The resulting theory is a vector-tensor theory within the generalized Proca class. We then consider the static spherically symmetric solutions of this theory and find black hole solutions that acquire primary hair. Notably, one of the integration constants associated with the Proca field is not manifest in the original metric, but under a disformal transformation of the seed solution, it emerges as a second, independent primary hair. This additional hair acts as an effective cosmological constant in the disformed geometry, even in the absence of a bare cosmological constant term. We further generalize these black hole solutions to include electromagnetic charges and effects related to the scalar-tensor counterparts of the regularized Gauss-Bonnet theory. We discuss the implications of our findings to observations.

gr-qc↗

arXiv2025

Axial perturbations of black holes with primary scalar hair

We study axial perturbations of static black holes with primary hair in a family of degenerate higher-order scalar-tensor (DHOST) theories. These solutions possess a scalar charge, fully independent of the mass, leading to a continuous one-parameter deformation of the standard Schwarzschild black hole. Starting from these solutions, we also construct new black holes, solutions of other DHOST theories, obtained via disformal transformations of the metric. In particular, we investigate two specific types of disformal transformations: the first leading to a theory where gravitational waves propagate at the speed of light, the second to a Horndeski theory, where the equations of motion remain second order. The dynamics of axial perturbations can be formally related to the general relativistic equations of motion of axial perturbations in an effective metric. The causal structure of the effective metric differs from that of the background metric, leading to distinct gravitational and luminous horizons. Using a WKB approximation, we compute the quasi-normal modes for the Schr\"odinger-like equation associated with the effective metric outside the gravitational horizon.

gr-qc↗

arXiv2025

Rotating spacetimes with a free scalar field in four and five dimensions

We construct explicit rotating solutions in Einstein's theory of relativity with a minimally coupled free scalar field rederiving and finding solutions in four or five spacetime dimensions. These spacetimes describe, in particular, the back-reaction of a free scalar field evolving in a Kerr spacetime. Adapting the general integrability result obtained many years ago from Eri\c{s}-G\"{u}rses to simpler spherical coordinates, we present a method for rederiving the four-dimensional Bogush-Gal'tsov solution. Furthermore, we find the five-dimensional spacetime featuring a free scalar with two distinct angular momenta. In the static limit, these five-dimensional geometries provide higher-dimensional extensions of the Zipoy-Voorhees spacetime. Last but not least, we obtain the four-dimensional version of a Kerr-Newman-NUT spacetime endowed with a free scalar, where the scalar field's radial profile is extended to incorporate dependence on the polar angular coordinate. Our results offer a comprehensive analysis of several recently proposed four-dimensional static solutions with scalar multipolar hair, representing a unified study of spacetimes with a free scalar field in both four and five dimensions under the general integrability result of Eri\c{s}-G\"{u}rses.

gr-qc↗

arXiv2024

Horndeski speed tests with scalar-photon couplings

We revisit multi-messenger constraints from neutron star mergers on the speed of propagation of gravitational and electromagnetic waves in Horndeski and beyond Horndeski theories. By considering non-trivial couplings between the dark energy field and the electromagnetic sector, the electromagnetic wave can propagate through the cosmological background at non-unit speed, altering the phenomenological constraints on its gravitational counterpart. In particular, we show that recent models derived from a Kaluza-Klein compactification of higher dimensional Horndeski models fall into a broader class of theories disformally related to those whose gravitational waves propagate with unit speed. This disformal equivalence can, however, be broken by the gravitational couplings to other sectors with interesting phenomenological consequences. We also consider higher order couplings between the scalar and the photon with second order field equations, and show that they are not compatible with constraints coming from multi-messenger speed tests and the decay of the gravitational wave.

gr-qc↗

arXiv2024

Global conformal symmetry in scalar-tensor theories

We study a subclass of Horndeski gravity which has both global conformal and shift symmetries. Global symmetries are characterised by the presence of a conserved current which has been shown to be of particular importance for the integrability features of the theory at hand yielding numerous compact object solutions. We find the general conserved current associated to global conformal symmetry of Horndeski theories. We discuss some of its properties, how it can be conveniently broken by physically relevant terms and, show how it is related to that of shift symmetry when shift symmetry is present. Given our results, we consider a particular theory and demonstrate how the presence of symmetries provides integrability for the given black hole solution. We then find the charged extension of the solution thanks to the conformal invariance of the Maxwell action.

hep-th↗

arXiv2024

Testing disformal non-circular deformation of Kerr black holes with LISA

There is strong observational evidence that almost every large galaxy has a supermassive black hole at its center. It is of fundamental importance to know whether such black holes are described by the standard Kerr solution in General Relativity (GR) or by another black hole solution. An interesting alternative is the so-called disformal Kerr black holes which exist within the framework of degenerate higher-order scalar-tensor (DHOST) theories of gravity. The departure from the standard Kerr black hole spacetime is parametrized by a parameter $D$, called $\textit{disformal parameter}$. In the present work, we discuss the capability of LISA to detect the disformal parameter. For this purpose, we study Extreme Mass Ratio Inspirals (EMRI's) around disformal Kerr black holes within the framework of the quadrupole hybrid formalism. Even when the disformal parameter is very small, its effect on the globally accumulated phase of the gravitational waveform of an EMRI can be significant due to the large number of cycles in the LISA band made by the small compact object. We show that LISA will in principle be able to detect and measure extremely small values of the disformal parameter which in turn, can be seen as an assessment of LISA's ability to detect very small deviations from the Kerr geometry.

gr-qc↗

arXiv2023

Black holes with primary scalar hair

We present explicit black holes endowed with primary scalar hair within the shift-symmetric subclass of Beyond Horndeski theories. These solutions depend, in addition to the conventional mass parameter, on a second free parameter encoding primary scalar hair. The properties and characteristics of the solutions at hand are analyzed with varying scalar charge. We observe that when the scalar hair parameter is close to zero or relatively small in comparison to the black hole mass, the solutions closely resemble the Schwarzschild spacetime. As the scalar hair increases, the metric solutions gradually depart from General Relativity. Notably, for a particular relation between mass and scalar hair, the central singularity completely disappears, resulting in the formation of regular black holes or solitons. The scalar field accompanying the solutions is always found to be regular at future or past horizon(s), defining a distinct time direction for each. As a final byproduct of our analysis, we demonstrate the existence of a stealth Schwarschild black hole in Horndeski theory with a non-trivial kinetic term.

gr-qc↗

arXiv2023

Exact black hole solutions in higher-order scalar-tensor theories

In this chapter, we discuss explicit black hole solutions in higher-order scalar-tensor theories. After a brief recap of no-hair theorems, we start our discussion by so-called stealth solutions present in theories with parity and shift symmetry. Stealth solutions are such that their metric are Ricci flat General Relativity solutions, but they are accompanied by a non-trivial scalar field, in both spherically-symmetric and rotating cases. The stealth metrics then enable to construct an analytic stationary solution of scalar-tensor theory which is called disformed Kerr metric. This solution constitutes a measurable departure from the usual Kerr geometry of GR. We discuss within parity and shift symmetric theories several non-stealth solutions. We then consider scalar-tensor theories stemming from a Kaluza-Klein reduction of a higher-dimensional Lovelock theory. These theories encompass all Horndeski functionals and hence go beyond parity and shift symmetry. Reduction and singular limits allow one to obtain non-stealth black holes with differing interesting properties which are not Ricci flat metrics. We analyse the solutions obtained and classify them with respect to the geometry of the internal space according to their Kaluza-Klein origin.

gr-qc↗

arXiv2023

Rotating black holes embedded in a cosmological background for scalar-tensor theories

We present solutions of DHOST theories describing a rotating black hole embedded in an expanding universe. The solution is constructed by conformal transformation of a stealth Kerr(-de Sitter) black hole. The conformal factor depends explicitly on the scalar field -- but not on its derivative -- and defines the new theory. The scalar field of the stealth Kerr(-de Sitter) solution depends on time, leading to the time-dependence of the obtained conformal metric, with cosmological asymptotics at large distances. We study the properties of the obtained metric by considering regular null geodesic congruences, and identify trapping black hole and cosmological horizons.

gr-qc↗

arXiv2023

Selecting Horndeski theories without apparent symmetries and their black hole solutions

Starting from a generalised Kaluza-Klein action including arbitrary Horndeski potentials, we establish integrability and compatibility conditions that solve the generic field equations for spherical symmetry. The resulting theories can be identified as general Horndeski theories having no apparent symmetries in four dimensions or as effective string theory actions with an IR logarithmic running for the dilaton, higher order corrections and generalised Liouville type potentials. For such actions, we then find black holes with secondary hair parameterized by two coupling constants essentially characterising the theories at hand. One is related to an action which is conformally coupled in five dimensions while the second is related to a Kaluza-Klein reduction of Lovelock theory. We show that the full action can also be interpreted as a sum of conformally coupled actions in differing dimensions. Known solutions are mapped within the general chart of the found theories and novel general black holes are discussed, focusing on their important properties and some of their observational constraints.

gr-qc↗

arXiv2023

Conformally coupled scalar in Lovelock theory

In arbitrary higher dimension, we consider the combination of Lovelock gravity alongside a scalar-tensor action built out of higher order operators and Euler densities. The latter action is constructed in such a way as to ensure conformal invariance for the scalar field. For the combined version of these theories, we show the existence of black hole solutions interpreted as stealth configurations within Lovelock gravity theory. The scalar field solutions are endowed with an integration constant that may be identified as a scalar charge. In particular, we show that these stealth solutions can be extended to include a time-dependent scalar field despite the underlying theory being non shift-symmetric. Finally, we present a procedure to obtain a non-conformally invariant action in even dimensions from the considered theory. For the target theory, the scalar field is not conformally coupled to gravity although the scalar field equation itself is conformally invariant. By means of this procedure, the black hole stealth configurations are converted into non-stealth black hole solutions, as discovered recently in four dimensions.

gr-qc↗

arXiv2022

Compact objects in gravity theories

We briefly discuss explicit compact object solutions in higher-order scalar-tensor theories. We start by so-called stealth solutions, whose metric are General Relativity (GR) solutions, but accompanied by a non-trivial scalar field, in both spherically-symmetric and rotating cases. The latter then enables to construct an analytic stationary solution of scalar tensor theory which is called disformed Kerr metric. This solution constitutes a measurable departure from the usual Kerr geometry of GR. We finally consider a scalar-tensor theory stemming from a Kaluza-Klein reduction of a higher-dimensional Lovelock theory, and which enables to obtain non-stealth black holes, highly compact neutron stars and finally wormhole solutions.

gr-qc↗

arXiv2022

Compact objects of spherical symmetry in beyond Horndeski theories

We analyse in all generality beyond Horndeski theories of shift symmetry in a static and spherically symmetric spacetime. By introducing four auxiliary functions, we write the field equations in a particularly compact form. We show that assuming additionally parity symmetry renders the system directly integrable giving multiple families of black-hole solutions. These have typically an asymptotically-flat Reissner-Nordstrom behaviour, and emerge in the presence of a canonical kinetic term for the scalar field. In the absence of parity symmetry, we present a general method which allows us to integrate the field equations by choosing the form of only one coupling function and an auxiliary quantity. This method leads to asymptotically flat and AdS black hole solutions with differing properties. We finally discuss disformal transformations within this context as a means of obtaining wormhole and black hole solutions in different theories.

gr-qc↗

arXiv2022

Conformally coupled theories and their deformed compact objects: from black holes, radiating spacetimes to eternal wormholes

We study a higher order conformally coupled scalar tensor theory endowed with a covariant geometric constraint relating the scalar curvature with the Gauss-Bonnet scalar. It is a particular Horndeski theory including a canonical kinetic term but without shift or parity symmetry for the scalar. The theory also stems from a Kaluza-Klein reduction of a well defined higher dimensional metric theory. Properties of an asymptotically flat spherically symmetric black hole are analyzed, and new slowly rotating and radiating extensions are found. Through disformal transformations of the static configurations, gravitating monopole-like solutions and eternal wormholes are presented. The latter are shown to extract from spacetime possible naked singularities, yielding completely regular and asymptotically flat spacetimes.

gr-qc↗