I will review how non-linearities can allow for screening solar-system scales from non-tensorial gravitational polarizations, focusing on the case of scalar-tensor theories with derivative self-interactions (K-essence). I will then present fully relativistic simulations in these theories in 1+1 dimensions (stellar oscillations and collapse) and 3+1 dimensions (binary neutron stars), showing...

We discuss a new approach to the thermodynamics of scalar-tensor gravity and to its possible `diffusion'' toward general relativity, seen as an`

equilibrium state'' in a space of theories. This new approach echoes ideas from the thermodynamics of spacetime, but it is different. The main idea consists of describing scalar-tensor gravity as an effective dissipative fluid and applying...

The non-linear and Lorentz invariant theory of a massive spin--2 field, proposed by de Rham, Gabadadze and Tolley (dRGT), has attracted considerable attention in the last decade, thanks to its potential to provide an alternative to dark energy. However, due to the pathologies of the cosmological solutions, the community has moved on to extensions with additional degrees of freedom and broken...

We present a generalized piecewise polytropic parametrization for the neutron-star equation of state using an ansatz that imposes continuity in not only pressure and energy density, but also in the speed of sound. The universe of candidate equations of state is shown to admit preferred dividing densities, determined by minimizing an error norm consisting of integral astrophysical observables....

The classical singularity theorems of General Relativity rely on energy conditions that are easily violated by quantum fields. In this talk I will provide motivation for an energy condition obeyed by semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. I will then then present the...

The C-metric in 4D is an interesting exact solution of Einstein's equations, representing a black hole being pulled by a "cosmic string". In 3D, we can construct similar metrics, but these turn out to have sometimes quite different properties. I will give a complete classification of "3DC" metrics, and show how their global structure varies from their 4D cousins, and the BTZ solution.

We consider theories of gravity that include many coupled scalar fields with arbitrary couplings, in the geometric framework of wave maps. The possibility of obtaining acceptable cosmological solutions without the inclusion of a potential term to the scalar fields is examined. We also address the issue of the conditions that must be satisfied by the wave maps for an accelerated phase of the Universe.

The Standard Model of Cosmology, namely Λ-Cold Dark Matter (ΛCDM) plus inflation in the framework of general relativity, proves to be very efficient in describing the universe evolution, both at the background and perturbation levels. However, theoretical issues such as the cosmological constant problem and the non-renormalizability of general relativity, as well as the possibility of a...

Recently, the 4D Einstein-Gauss-Bonnet gravity has received a lot of attention. Remarkably, it possesses an exact vacuum solution that deviates from general relativity. I will discuss the important features of black holes and neutron stars in this theory. In particular, for very compact objects, the sequence of neutron stars matches asymptotically to the black hole limit, closing the mass gap...

Testing different beyond-Kerr alternatives has been amongst the major goals of astrophysics and especially gravitational wave physics. In the present talk, we will focus more specifically on black holes endowed with scalar hair. We will discuss the peculiar properties these objects can possess and their astrophysical manifestations both in the electromagnetic and gravitational wave channels....

We investigate the use of neural networks for surrogate modeling of non-spinning EOB BBH waveforms. Specifically, we use autoencoders to first uncover any underlying structure in the empirical interpolation coefficients and discover a spiral pattern wherein the spiral angle is linearly related to the mass ratio q of the waveforms. We then design a neural spiral module with learnable...

The automorphisms of the various Bianchi-type Lie algebras are seen to arise from particular g.c.t.'s of the base manifold. They can be used as Lie-point symmetries of the corresponding Einstein field equations, entailing a reduction of their order and ultimately leading to the entire solution space. The example of the Kasner-like (spatially flat) 4+1 geometry is presented.

In general relativity black holes are fully characterised by their mass, spin, and electromagnetic charge. No-hair theorems indicate that scalar fields cannot affect black hole spacetimes. However, the devil is on the details and, in practice, no-hair theorems allow us to identify a list of interesting exceptions in which scalar field leave their imprint on black holes. Such scenarios are of...

Entangled relativity is a new general theory of relativity that is free of any new parameter at the classical level. It is based on the same principles as general relativity, has the same fields and number of dimensions, but it changes the way spacetime and matter interact with each other, in a way that avoids the possibility of defining the theory of relativity without defining matter and the...

We examine parity violation in gravity during a transitory non-attractor phase of inflation, which amplifies the would be decay tensor mode and enhances tensor fluctuations at super horizon scales. This is realised in a kinetically driven scenario of inflation which is extended to include higher order corrections to gravity.

The Mathisson-Papapetrou-Dixon (MPD) equations describe the motion of an extended test body in general relativity. This system of equations, though, is underdetermined and has to be accompanied by constraining supplementary conditions, even in its simplest version, which is the pole-dipole approximation corresponding to a spinning test body. In particular, imposing a spin supplementary...

We compute the extreme mass ratio inspiral in a system of a Schwarzschild black hole perturbed by an additional matter located far in the equatorial plane. First the geodesic equation is solved using an approximate transformation of our hamiltonian to the action-angle coordinates. The approximate solution is then expressed as a Fourier-like expansion which is subsequently inserted to the...

It has been shown that massive geodesics may admit certain nonlocal integrals of motion associated with conformal Killing vectors. In the exceptional case of pp-wave space-times these charges reduce to local expressions generated by a mass dependent distortion of the conformal Killing algebra. We demonstrate under which modification of the Noether symmetry procedure these vectors can be...

Bimetric gravity is a ghost-free extension of general relativity, exhibiting both a massless and a massive graviton. We show how the theory can be parameterized with five observables with specific physical interpretations and then constrain the parameter space by requiring: (i) observationally viable cosmology, (ii) a working screening mechanism that restores general relativity locally, and...

In 1970, Taub sought to construct a particular two-parameter family of spherically symmetric self-similar shock-wave solutions to the Einstein field equations with a perfect fluid source. This family would consist of a Friedmann-like interior spacetime expanding into a static exterior spacetime, the physical realisation of which would be a general relativistic explosion. Taub was not...

We construct, for the first time, the time-domain gravitational wave strain waveform from the collapse of a strongly gravitating Abelian Higgs cosmic string loop in full general relativity. We show that the strain exhibits a large memory effect during merger, ending with a burst and the characteristic ringdown as a black hole is formed. Furthermore, we investigate the waveform and energy...

We have studied a non-ntegrable analogue of a perturbed Kerr metric and found that the passage of an orbit through a resonance is further prolonged when the self-force itself is used to evolve the orbit, instead of the average losses of energy and angular momentum caused by the same self-force. The enhancement is of the order of (but less than) 10. This result renders the revealing of...

The ESA Laser Interferometer Space Antenna (LISA) is a space born Gravitational-Wave (GW) observatory scheduled to be launched in the early 2030s. LISA will be comprised by a constellation of three satellites forming a triangle with sides of 2.5 million kilometres, following a heliocentric orbit. In this talk I will present the measuring principle of LISA, as well as the different GW sources...

The open-source Python-based computer algebra system SageMath [[1]] has some differential geometry and tensor calculus capabilities, which have been implemented through a community effort --- the SageManifolds project [[2]]. I shall briefly present the project and illustrate it by various examples relevant to relativistic gravity, among which the demonstration that the Poincaré horizon of AdS...

I will talk about disformal versions of the Kerr spacetime in higher order scalar tensor theories. Properties of the constricted solutions are rather non-trivial and in many aspects differ from those of the Kerr solution. Although the disformed metric has only a ring singularity and asymptotically is quite similar to Kerr, it is found to be neither Ricci flat nor circular. Non-circularity has...

Gravitational wave observations are crucial in the effort to determine the high-density equation of state. Fluid modes in neutron stars can lead to the emission of gravitational waves. Various empirical relations have been proposed between the frequencies of such modes and stellar properties of the system. In this talk we focus on two distinct systems and their oscillation modes. On the one...

I will develop briefly the existing plans of ugrade of the current

detectors, their projected physics horizons, as well as the horizons of 3rd generation detectors, on earth (ET and CE) and space (LISA) including new proposals for the Moon. I will also briefly visit, the technological fronts, the multimessenger and more generally the interdisciplinary context, including climate change...

The observation of compact binary mergers by the LIGO/Virgo collaboration marked the dawn of a new era in astronomy. LISA will expand this vision by opening a new observational window at low frequencies. The gravitational radiation emitted by compact binary systems in these two frequency windows encodes important information on their astrophysical formation mechanism. Furthermore, compact...

I will first define the gravitational-wave background (GWB) and highlight the method we are using to detect it in the presence of correlated magnetic noise. I will then discuss astrophysical (compact binary coalescences) and cosmological (cosmic strings, first-order phase transitions) sources and report on the current constraints imposed from a non-detection during the last observing run of...

Extreme Mass Ratio Inspirals (EMRIs) are one of the prominent sources for gravitational wave detection by the Laser Interferometer Space Antenna (LISA). EMRIs consist of a stellar compact object ispiralling into a supermassive black hole due to gravitational radiation reaction. During this process the stellar object traces the background and the gravitational waves it emits carry away the...

During the late stages of a neutron star binary inspiral finite-size effects come into play, with the tidal deformability of the supranuclear density matter leaving an imprint on the gravitational-wave signal. As demonstrated in the case of GW170817—the first direct detection of gravitational waves from a neutron star binary—this can lead to strong constraints on the neutron star equation of...

Transient compact remnants briefly supported by differential rotation and thermal pressure are a possible outcome of binary neutron star (BNS) mergers, with the post-merger phase expected to yield pivotal constraints for the equation of state of high density matter. Modelling remnants as equilibrium configurations can aid in interpreting the post-merger gravitational wave (GW) signal, deducing...

Causal sets are a theory that encodes space-time through the causal relations between events. This leads to a fundamentally Lorentzian, discrete, formulation, in which space-time is reduced to partial orders. One possible way to quantize causal sets, is to calculate the path integral over these partial orders. This can either be attempted analytically or explored through Monte Carlo...

In this work we consider black holes surrounded by anisotropic fluids in four dimensions. We first study the causal structure of these solutions showing some similarities and differences with Reissner-Nordström-de Sitter black holes. In addition, we consider scalar perturbations on this background geometry and compute the corresponding quasinormal modes. Moreover, we discuss the late-time...

In the context of a five-dimensional braneworld model with a warped extra dimension, we construct novel localized, analytic black-hole solutions. The geometry of the bulk spacetime possesses a higher-dimensional spherical symmetry, while on the brane the geometry is of a Schwarzschild-like form. The singularity of these solutions occupies a single point in the higher-dimensional space, which...

Recently proposed statistical mechanics arguments [1] and hydrodynamical presentation of quantum wave equations [2] have revealed that the quantum liquids with logarithmic nonlinearity, often referred as “logarithmic fluids”, are very instrumental in describing generic condensate-like matter, including strongly-interacting quantum liquids, one example being He II, a superfluid component of...

In this talk two cosmological studies will be presented and their results discussed. In these studies we have tried to remain model agnostic as much as possible. In particular, our first study ([arxiv;1905.08512][1]) performs dynamical analysis of a broad class of non-minimally coupled real scalar fields in spatially curved Friedmann-Robertson-Walker (FRW) spacetimes with unspecified positive...

We apply the gravity-thermodynamics conjecture, namely the first law of thermodynamics on the Universe horizon, but using the generalized Kaniadakis entropy instead of the standard Bekenstein-Hawking one. The former is a one-parameter generalization of the classical Boltzmann-Gibbs-Shannon entropy, arising from a coherent and self-consistent relativistic statistical theory. We obtain new...

In this review talk we'll discuss the main properties of a large family of modified gravity models, aiming to explain the cosmic acceleration. In the second part of the presentation we'll test their performance against the recent cosmological data.

Scalar fields in cosmology have been intensively studied during the last decades due to their potential application in the physics of the early and late universe cosmology. In this talk, we will explore the high-energy motivation and the theoretical implications of considering more than one scalar fields in the early universe and then discuss the observational viability of these models. We...

The detection of gravitational waves 6 years ago and the first detection of binary neutron stars mergers two years later signalled the beginning of a new era for Gravitation and Astrophysics. Neutron stars is a prominent source of gravitational waves and the first observations from them already provided unique information for their structure but also for the associated physics. We will review...

We will discuss the existence of a new fully nonlinear dynamical mechanism for the formation of scalarized black holes which is different from the spontaneous scalarization. We consider a class of scalar-Gauss-Bonnet gravity theories within which no tachyonic instability can occur. Although the Schwarzschild black holes are linearly stable against scalar perturbations, we show dynamically that...

The original Bondi–Metzner–Sachs (BMS) group B is the common asymptotic symmetry group of all asymptotically ﬂat Lorentzian radiating 4-dim space–times. As such, B is the best candidate for the universal symmetry group of General Relativity (G.R.). In 1973, with this motivation, McCarthy classiﬁed all relativistic B-invariant systems in terms of strongly continuous irreducible unitary...

ABSTRACT:

Spherical energy shells in General Relativity tend to collapse due to gravitational effects and/or due to tension effects. Shell stabilization may be achieved by modifying the gravitational properties of the background spacetime. Thus, gravastars consist of stiff matter shells with an interior de Sitter space with repulsive gravitational properties and an exterior Schwarzschild...

We will discuss theoretical and experimental results studying the wave-vortex interaction arising from rotating fluid and superfluid flows. The dynamical equation describing the wave-vortex interaction can be mapped to scalar fields exhibiting an effective rotating black hole. This opens the possibility of studying a variety of rotating black hole processes in hydrodynamic systems. The focus...

We formulate Positivity Bounds for scattering amplitudes including exchange of gravitons in four dimensions. We generalize the standard construction through dispersion relations to include the presence of a branch cut along the real axis in the complex plane for the Maldestam variable s. In general, validity of these bounds require the cancellation of divergences in the forward limit of the...

We investigate the modification of the gravitational field equations for the Szekeres spacetimes in the content of de Broglie–Bohm theory for quantum cosmology. We determine a nonzero contribution of quantum potential. Finally, the physical properties provided by the quantum terms in the semiclassical limit are discussed.

In the quest for exact solutions of the Einstein-Maxwell (EM) equations a considerable amount of research has been devoted to the study of aligned EM fields, in which at least one of the principal null directions (PND) of the electromagnetic field $\mathbf{F}$ is parallel to a PND of the Weyl tensor, a so called Debever-Penrose (DP) direction. One of the main triumphs of this effort - spread...

In this talk, I will discuss how a newly proposed gravitational theory (arXiv: 2007:00082, PRL in press) can solve the dark matter problem by reducing to Milgrom’s Modified Newtonian Dynamics at the scale of galaxies and to the LambdaCDM model on cosmological scales. I will show that the theory (i) leads to correct gravitational lensing on galactic scales, (ii) propagates tensor modes at the...

Spontaneous scalarization of compact objects provides one of the most interesting manifestations of new strong gravity physics while remaining undetected in the weak field regime. We demonstrate that there are theories that exhibit spontaneous scalarization while having General Relativity as a cosmological attractor. For that to happen, we assume a scalar-Ricci coupling in addition to the...

We consider compact objects solutions of a Horndeski subclass

which includes a massless scalar field non-minimally coupled to the Einstein tensor. We study the stability of such solutions under scalar and axial perturbations and we find that they are gravitationally stable at the linear level.

The imaging of black-hole shadows with the Event Horizon Telescope has opened a new window into the strong-field spacetimes of these extreme astrophysical objects. I will first discuss the technological and theoretical advances that led to the first image of the black hole in the M87 galaxy. I will describe how this observation allows us to perform new tests of General Relativity. I will...

The first observations of gravitational waves (GWs) from the coalescence of a black-hole binary in 2015 and a neutron-star binary in 2017 inaugurated a new era in experimental gravity. In less than 5 years and with the continuous upgrades of our GW observatories, LIGO, Virgo and now KAGRA, the detection of GWs evolved from non-existent to a weekly business and has led to a plethora of results...

The $R+R^2$ (Starobinsky), where $R$ is the Ricci scalar, and the Higgs inflationary models represent the simplest phenomenological inflationary models which are internally

consistent, have only one free dimensionless parameter taken from observations, produce a smooth exit from inflation to the subsequent hot radiation-dominated stage through an intermediate matter-dominated one, and which...

I will discuss the mathematical status of the problem of the nonlinear stability of black holes in classical general relativity.

We reconsider the thermodynamics of AdS black holes in the context

of gauge-gravity duality. In this new setting where both the cosmological

constant $\Lambda$ and the gravitational Newton constant $G$ are varied

in the bulk, we rewrite the first law in a new form containing both $\Lambda$

(associated with thermodynamic pressure) and the central charge $C$ of

the dual CFT theory and their...

The scheduled launch of the LISA Mission has called attention to the gravitational self-force problem. Accurate long-time numerical computations of gravitational waves from extreme-mass-ratio-inspirals (EMRI) remain challenging. First, we discuss a class of evolution schemes suitable to this problem based on Hermite integration. Their time-reversal symmetry and unconditional stability allows...

A particularly interesting property emerging in Horndeski (and beyond) solutions is the presence of regions with negative effective energy density – this is due to the presence of the higher-curvature gravitational terms in the action and is therefore of purely gravitational nature. This negative effective energy density leads to the violation of both the Weak and the Null Energy Conditions in...