In the context of higher order scalar tensor theories we will find explicit solutions with primary scalar charge evading classical no hair theorems. The large class of theories at hand are ultra violet departures from GR and will be given by specific analytic functions and will have certain symmetries. The scalar charge will be shown to be related to a conserved Noether charge associated to...
I will discuss the current understanding of the landscape of bosonic stars in simple GR models, the hairy black holes that are associated with them, and the corresponding dynamics. Some applications to both phenomenological and conceptual questions will be addressed.
I will present developments and recent applications of the excision technique in the case of the Fully Constrained Formalism. I will focus on spherically symmetric spacetimes representing the collapse of a neutron star to a black hole. I will also present a more general set up of boundary conditions to be imposed at the excised surface, an arbitrary coordinate sphere inside the apparent...
Gravitational wave radiation is only unambiguously defined at future null infinity -- the "location" where light rays arrive and where global properties of spacetimes can be measured. Reaching future null infinity is thus crucial for extracting correct waveforms from numerical relativity simulations of compact binaries. Hyperboloidal slices extend to null infinity while being spacelike and...
I will provide a quick overview of the status of our understanding concerning neutron star matter, relevant to test gravity with the densest objects available in the present-day universe, and some steps which we are taking so that quantum computing may one day help.
We perform a Bayesian analysis of the equation of state (EOS) constraints using recent observational data, including pulsar masses, radii, and tidal deformabilities. Our focus is on a class of hybrid neutron star EOS that incorporates color superconducting quark matter, based on a recently developed nonlocal chiral quark model. The nuclear matter phase is described using a relativistic density...
Teleparallel gravity employs independent connection that is curvature free but can be characterised by torsion and nonmetricity. While it is possible to formulate families of teleparallel theories that have field equations equivalent to general relativity and only differ from it by the boundary term in the action, the extensions like scalar-tensor start to stand apart from their respective...
We study rotating hybrid stars, with a particular emphasis on the effect of a deconfinement phase transition on their properties at high spin. Our analysis is based on a hybrid equation of state with a phase transition from hypernuclear matter to color-superconducting quark matter, where both phases are described within a relativistic density functional approach. By varying the vector meson...
First order relativistic viscous theories had, until recently, been believed to not be well-behaved (i.e. causal, stable and strongly hyperbolic). Because of this, relativistic viscosity in astrophysical contexts has remained understudied. Recently, the Bemfica-Disconzi-Noronha-Kovtun (BDNK) theory has been shown to be causal, stable and strongly hyperbolic, which makes it a well-suited model...
Kaluza and Klein proposed a theory with a compactified extra dimension, which may appear in high-energy phenomena, such as nuclear reactions, strong gravitational effects, or in the presence of superdense matter. In this work, I show how astrophysical observables will be modified in the presence of extra compactified dimensions.
The interior of a compact star is modelled as a...
Numerical relativity plays many important roles in astrophysics and general relativity, e.g., in understanding mergers of black holes and neutron stars, formation processes of black holes for a wide variety of stellar collapse, and launching mechanisms of jets. In this talk, I will summarize our current understanding of the merger and post-processes for neutron-star binaries introducing our...
Numerical relativity has been spectacularly successful during the past
decades, with numerous contributions toward gravitational wave
astrophysics for binary black holes and neutron stars. Despite all
accomplishments, the increasing sensitivity of gravitational wave
detectors and the broader bandwidth of future detectors require
significant further improvements of the numerical codes,...
This talks reviews challenges in the modelling of the waveforms of coalescing compact binaries as the GW community prepares for the next generation of GW observatories, and the current status and future prospects for the phenomenological waveforms program. The talk reports in particular recent progress in modelling eccentric binaries, spin precession, the memory effect, and the LISA response.
Sparse dictionary learning (SDL) techniques have demonstrated strong potential for extracting astrophysical signals from noise in gravitational-wave (GW) astronomy. In this talk, I will provide an overview of SDL algorithms and discuss their application to a range of GW data analysis challenges. Examples will include glitch mitigation, classification of the equation of state in binary neutron...
The dynamical interactions of compact objects in N-body clusters are of great interest for understanding the formation of black holes (BHs) in the upper mass gap, as well as intermediate and supermassive BHs. These systems are potential sources of gravitational waves (GWs) detectable by both current and future observatories. We present, for the first time, a fully general relativistic...
After the first detections of gravitational waves from the collision of compact merging binaries, including both black holes and neutron stars, the next great new discovery from LIGO-Virgo-KAGRA detectors could be associated with the collapse of massive stars. With a galactic event rate of about 2-3 per century, core-collapse supernovae (CCSNe) are a primary candidate for gravitational wave...
Gravitational wave observations of neutron star collisions offer a unique avenue into a regime where gravity and matter entangle strongly and evolve dynamically. As these waves travel essentially undistorted through the cosmos to reach the detectors, they constitute the most promising messenger to probe into nuclear physics at extreme densities and low temperatures. In this talk, we will focus...
Although the Penrose-Hawking singularity theorems leave little room for the fate of collapsing matter, since the 1960s there has been great interest in the possibility of deriving black hole solutions with a regular center. In this talk, after a brief historical review, I will discuss the main features of regular black holes and their implications for fundamental physics. Finally, I will...
Black holes in general relativity possess curvature singularities. These are not the only type of singularities that a spacetime can possess though. The Kerr solution, for example, also exhibits a thermodynamic singularity at a specific, non-extremal value of the spin parameter where the heat capacity diverges ("Davies' point"). Given the deep connections between black holes and...
In this talk, we will discuss the formation of black hole shadows and how some cases of non-Kerr compact objects can produce distinctive non-Kerr shadows with fractal structures. We will also discuss how these shadows can be illuminated by accretion and explore the possibilities for measuring the properties of the spacetime.
Horizonless compact objects may produce phenomenological features which distinguish them observationally from black holes. In particular, the images of the accretion disks around them may possess a characteristic morphology including a series of central bright rings instead of a shadow. We demonstrate how the central ring structure arises relating it to the behavior of the deflection angle on...
Tidal interactions play a fundamental role in shaping binary systems and affect their gravitational wave (GW) signals, which is crucial for future detectors such as LISA and ET. While tidal effects on binaries are often studied in a weak-field approximation, their role in the strong-gravity regime remains largely uncharted.
We present two frameworks to analyze strong-gravity tidal effects...
Binary neutron star mergers (BNSM) are associated with powerful gravitational and electromagnetic astronomical transients. Multimessenger observations of BNSMs promise to deliver unprecedented insights on fundamental physics questions, including constraints on dense matter models and the production of heavy elements. Detailed theoretical predictions of the merger dynamics are crucial for...
One of the most significant open questions in the theoretical understanding of neutron stars and black hole–neutron star binaries is the amplification of magnetic fields in the remnant. This amplification initially occurs on small scales via a turbulent dynamo, primarily driven by the Kelvin–Helmholtz instability. This turbulent phase is followed by a large-scale reorganization of the field,...
Neutron stars are useful laboratories for many areas of physics, including subatomic physics and the coupling of nonlinear gravity with matter. To gain further insights into the fundamental information contained in observable neutron star properties and potential degeneracies between modifications to different sectors of physics, it is useful to consider theories of gravity beyond General...
The correct interpretation of multimessenger data obtained from binary neutron star mergers, including gravitational waves and electromagnetic signals, requires accurate theoretical predictions that can be cross-correlated with observations. These models can be constructed by combining ab initio numerical-relativity simulations with derived analytical knowledge. In addition, an efficient...
Binary neutron star mergers provide insights into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data....
One of the most promising and challenging future gravitational wave (GW) sources are core-collapse supernovae. The oscillation modes of the newly born proto-neutron star (PNS) and the stalled accretion shock will be excited triggering the GW emission. Due to the stochastic nature of these signals, it is not possible to use template matching techniques. An alternative way to analyse the signal...
We present a full 3D numerical evolution code to study rotating neutron stars in massive-scalar-tensor (MST) theories.
The implementation consists in a modified version of the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formalism such that the simulations are performed in the physical Jordan frame, where the scalar field is directly coupled with the spacetime evolution. This approach allows to...
Having well posed time evolution is an important property we seek to have in our theories of nature. In this talk, I will present how some simple classical field theories can be ruled out due to a breakdown of time evolution in their dynamics. I will also speculate on using similar analyses to constrain the alternative theory landscape of gravity, helping the observational and experimental tests.
The magneto-rotational instability (MRI) is a cornerstone of accretion disk theory, and it is often invoked to explain the generation of large-scale, poloidal magnetic fields in binary neutron star mergers. However, simulations that begin with weak seed fields and follow their amplification to saturation lack convincing evidence of MRI activity, casting doubts on its role in this setting. In...
We investigate the imprint of magnetic fields on gravitational waves from the inspiral phase of eccentric binary neutron star binaries (BNS). While neutron stars are typically observed to exhibit strong magnetic fields ranging from $10^{14}$ to $10^{15}$ G, theoretical models predict strengths up to $10^{18}$ G. BNS systems formed through dynamical capture may retain substantial eccentricity...
The orbital evolution of binary black hole (BBH) systems is determined by the component masses and spins of the black holes and the governing gravity theory. General relativity (GR) is the simplest theory of gravity that lays the foundation for successfully explaining the current gravitational wave (GW) observations. We present a method of stacking up the time-frequency pixel energies through...
Black holes represent an ideal laboratory to test Einstein’s theory of general relativity and alternative theories of gravity. Among the latter, Einstein-Scalar-Gauss-Bonnet Theories have received much attention in recent years. In this talk some properties of their black holes are recalled, which depend significantly on the coupling function of the scalar field. Linear mode stability of the...
According to General Relativity, astrophysical black holes are remarkably simple and their properties are determined by just two quantities, their mass and their angular moment. Gravitational waves and other strong gravity observations promise to probe the nature of black holes more precisely that ever before. Any observed deviation from the simple description General Relativity provides can...
In this talk I will discuss in which cases black holes carry a scalar charge, and the implications when the latter scales with the black hole mass. I will talk about the phenomenological consequences of these insights for the physics of compact binaries, and how asymmetric systems evolving in the LISA band are ideal sources for searches of new fundamental fields coupled to gravity. I will lay...
We will explore the long path from Einstein's equations to computational simulations. I will discuss how Numerical Relativity can serve as a tool to study nonlinear dynamics in alternative theories of gravity. In this talk, I will consider two subclasses of the broader Horndeski theory: those that have a screening mechanism and those involving scalar-Gauss-Bonnet gravity. In particular, I will...
In this talk I will discuss our recent progress in performing numerical simulations in the Einstein-scalar-Gauss-Bonnet theory of gravity, both for equal and unequal mass binary black hole systems, which result in waveforms with dephasings from GR consistent with PN calculations.
I will discuss the Cauchy problem for self-interacting massive vector fields, often facing instabilities and apparent pathologies when performing numerical simulations. After showing that these issues are due to the breakdown of the well-posedness of the initial-value problem, I will show how the pathologies can be classified, building on previous work done for 𝑘-essence, and how these issues...
The Cosmological Principle is one of the pillars of the standard model of cosmology and it is commonly realised in a trivial way with homogeneous SO(3)-scalars. I will discuss several scenarios where the matter sector realises the Cosmological Principle in a non-trivial manner by resorting to combinations of spacetime and internal symmetries. A natural consequence of some of these scenarios is...
Gravitational wave observations of compact objects have provided new opportunities to test our understanding of gravity in the strong-field, highly dynamical regime. To perform model-dependent tests of General Relativity with these observations, as well as to guide theory-agnostic tests, it is crucial to develop inspiral-merger-ringdown waveforms in alternative theories of gravity. In this...
We study binary neutron stars in the framework of Damour-Esposito-Farese-type (DEF) scalar-tensor theory of gravity with a massive scalar field. In this talk, I will start from the quasiequilibrium sequences of binary neutron stars, paying particular attention to the case where neutron stars are already spontaneously scalarized at distant orbits, i.e., in the high-coupling constant case. In...
Extensions of General Relativity can give rise to black holes with nontrivial scalar structure, commonly called "hairy" black holes. This work investigates the dynamics of spherically symmetric black holes in scalar–Gauss–Bonnet gravity through fully nonlinear simulations incorporating excision techniques. These theories often suffer from an ill-posed initial value formulation, which limits...
A black hole quantum state (Hartle-Hawking, Boulware or Unruh) is usually defined in a fixed background of a classical black hole. In my talk I will discuss the corresponding space-time geometry when the back-reaction is taken into account. The important questions include: does the back-reacted geometry always contain a horizon?; how it depends on the choice of the quantum state? and what is...
Next generation of gravitational wave detectors will have the sensibility to detect potential deviations in gravitational waveforms with respect to general relativity. However, current agnostic tests of gravity with gravitational waves are plagued by a lack of realistic deviations, making it difficult to interpret such detections with respect to specific theories. In this talk, I present a...
This talk will address the study of stationary and axisymmetric exact solutions in scalar-tensor theories. We will review de Weyl problem in GR and its stationary extension, to present then a generalisation valid in the presence of scalar fields. We will show new rotating spacetimes in four and five dimensions and a systematic procedure to construct them. Extensions to generalised...
The most well-studied class of theories of gravity beyond General Relativity is represented by scalar-tensor gravity, in which at least one fundamental scalar degree of freedom is included in the gravitational sector. This family is theoretically appealing due to its simplicity and due to its capability to describe cosmic dynamics at large scales.
Within this class, a particularly relevant...
The direct observation of gravitational waves gives us the opportunity to test gravity in the highly dynamical, strong curvature regime probed by coalescing black hole binaries. In particular, gravitational wave detectors are sensitive to large-curvature corrections of general relativity, mostly unconstrained by other astrophysical observations. However, theoretical consistency imposes...
We construct static and axially symmetric magnetically charged hairy black holes in the gravity-coupled Weinberg-Salam theory. Large black holes merge with the Reissner-Nordstr\"om (RN) family, while the small ones are extremal and support a hair in the form of a ring-shaped electroweak condensate carrying superconducting W-currents and up to $22\%$ of the total magnetic charge. The extremal...
I will discuss properties of general stationary and axisymmetric spacetimes, with a particular focus on circularity - an accidental symmetry enjoyed by the Kerr metric, and therefore widely assumed when searching for rotating black hole solutions in alternative theories of gravity as well as when constructing models of Kerr mimickers. It can be shown the local existence of a Kerr-like gauge,...
Gravitational waves offer the promising prospect of testing one of the main predictions of general relativity, namely the presence of black holes beyond which nothing can escape.
The ringdown is the final stage of a compact binary coalescence when the remnant settles down to a stationary configuration. It is modelled as a superposition of exponentially damped sinusoids whose frequencies and...
In this talk we will discuss the quasinormal mode spectrum of rapidly rotating black holes in Einstein-Gauss-Bonnet-dilaton theory, which is crucial for understanding the ringdown phase that follows from a black hole merger. Unlike previous studies that relied on pproximations, we compute the QNM spectrum non-perturbatively, providing robust results even for large coupling constants. Using a...
Black hole spectroscopy, applied to the ringdown phase of compact binary mergers, is one of the most promising tools to test the nature of black holes. It allows us to quantify whether black holes and their perturbative dynamics are well described by general relativity and, thus, serve as a magnifier to explore fundamental physics. The parametrized quasi-normal mode framework has been...
Nonlinear effects in black hole perturbation theory may be important for describing a black hole ringdown, as suggested by recent works. I will describe a new class of "quadratic" quasi-normal modes at second order in perturbation theory. Remarkably, not only their frequency but also their amplitude is completely determined by the linear modes themselves. I will present how one can compute...
In this talk we discuss the quasinormal mode (QNM) spectrum of charged and static wormholes, which is interesting for understanding gravitational-wave phenomena in this kind of objects. We compute the QNMs employing a spectral method, which allows us to study in a systematic way the properties of the modes as we vary the charges of the wormholes. We discuss several properties of the QNM...
We present a model of black hole scalarization where a scalar field couples simultaneously to the Gauss–Bonnet invariant and the electromagnetic Maxwell field. This combined interaction broadens the conditions for spontaneous scalarization. We track how the electric charge and the two coupling constants control the onset of the scalar field and uncover new solution branches with non-trivial...
The aim of the current investigation is to determine the evolution characteristics of anisotropic and homogeneous universe within the context of fractal theory of gravitation. The Bianchi type-V space-time has been employed to derive the field equations of fractal theory. Based on the signature flip property of the deceleration parameter, we have derived the scale factor and the Hubble...
This paper presents a comprehensive study of binary black hole systems using general relativistic magnetohydrodynamics (GRMHD) simulations within the Einstein Toolkit framework. We investigate the complex interactions and accretion dynamics of merging black holes, highlighting the role of magnetic fields in shaping the gravitational wave signals and electromagnetic counterparts associated with...
Testing the strong-field regime of gravity has become of great interest in the scientific community following the first gravitational wave detections and the growing theoretical and experimental indications that General Relativity (GR) may require modifications in extreme conditions.
In this context Scalar-Gauss-Bonnet (sGB) not only provides a natural framework for such deviations but is...
In this study, we used the ( f(T) ) gravity framework with the energy-momentum tensor for a perfect fluid to derive key cosmological parameters, including the Hubble parameter ( H ), deceleration parameter ( q ) and Statefinder diagnostics. Model parameters were optimized using an ( R^2 ) test, resulting in ( \beta = 1.312^{+0.013}{-0.014} ), ( \xi = 1.273^{+0.0065}{-0.0071} ), and...
A proto-neutron star forms after a successful supernova when the stellar remnant decouples from the ejecta. we explore a relativistic framework for the finite-temperature $\beta$-equilibrium limit of equation of state, constrained via a Bayesian inference methodology, subject to minimal constraints on a few nuclear saturation properties, low-density pure neutron matter constraints from chiral...
In this paper, we construct exact rotating wormholes using the Ehlers solution-generating technique. This is based on the Ernst description of four-dimensional, stationary, and axially symmetric solutions of the Einstein-Maxwell theory. We adopt the static Barceló-Visser wormhole derived from the Einstein-Maxwell-conformal-scalar theory as a seed and demonstrate, through the Ernst approach,...
We investigate gravitational perturbations of a rotating (Kerr) black hole within a noncommutative geometry framework for quantum gravity. Using a Drinfeld twist that deforms the spacetime symmetries (a semi-Killing twist), we formulate a noncommutative extension of Einstein field equations and derive the effective potential for axial (odd-parity) gravitational perturbations of a...
The neutron star observables, such as mass, radius, and tidal deformability, non-radial oscillations are directly related to the equation of state (EOS). The exact nature of dense matter in neutron stars(NS) remains unknown. Several efforts have been made to constrain the EOS through theoretical modelling, incorporating inputs from nuclear physics experiments and astrophysical...
Gravitational wave data from BBH coalescence have recebtly been analyzed to validate Hawking's Area Theorem. We discuss how this validation constrains theoretical calculations of black hole entropy, including corrections to quantal or classical modifications of classical general relativity. We show how observational data discriminates between quantum gravity corrections to the...
In this study, we explore cosmological models within the framework of ( f(T) ) gravity by utilizing the energy-momentum tensor for a perfect fluid to solve the corresponding field equations. We derive key cosmological parameters, including the Hubble parameter ( H ). Parameter constraints were applied using the ( R^2 ) test, resulting in best-fit values of ( \beta =...
By restoring the symmetry between time and space in a matter field, we reconciled the properties of a zero-spin quantum field from a system that has vibrations of matter in time. This quantized real scalar field obeys the Klein-Gordon equation and Schrodinger equation. The particles observed are oscillators in proper time. In motion, the proper time oscillation translates to the oscillations...
This work investigates Buchdahl transformations within the framework of Einstein and Einstein-Scalar theories. Specifically, we establish that the recently proposed Schwarzschild–Levi-Civita spacetime can be obtained by means of a Buchdahl transformation of the Schwarschild metric along the spacelike Killing vector. The study combines Buchdahl’s original theorem with the Kerr–Schild...
The intrinsic spin of fermions can generate torsion in spacetime. This gives rise to an effective four-fermion interaction that fermions experience within a fermionic distribution. This interaction is expected to become significant when densities become large, such as in early universe cosmology or in compact astrophysical objects like neutron stars. In this contribution, I will discuss the...
We propose a three-flavor nonlocal Nambu–Jona-Lasinio model of quark matter with attractive scalar and diquark, and repulsive vector interaction channels to study the question whether an approximately conformal behavior of the strongly interacting quark matter in neutron star interiors is possible. The model qualitatively agrees with the perturbative quantum chromodynamics (pQCD), which...
Mrk 841 is known for its high variability, complex iron line component and strong soft excess. We use the most recent and longest XMM-Newton observations, with durations exceeding 100 ks. We explore several theoretical models to explain the puzzling behaviors of the reflecting component, considering parameters such as disk ionisation and relativistic effects. This offers a rare glimpse into...
