Since 2015’s celebrated first detection of gravitational waves from merging black holes, detectors have measured waves from nearly a hundred different events. Many current detectors are being upgraded, improving their sensitivity and broadening their sensitive wave bands; future detectors are planned which will likewise have wider bands and deeper sensitivity. These future instruments will be...

According to General Relativity, an isolated black hole in vacuum shall be described by the Kerr metric, whose geodesic equations are integrable. The violation of integrability leads to chaos for particles moving around the black hole. This chaotic dynamics could leave imprints on the associated gravitational waveform and could be tested with upcoming observations. In this talk, we discuss the...

It has been thirty years since the breakthrough paper of M. Choptuik on critical phenomena in the gravitational collapse of a real massless scalar field in spherical symmetry. This celebrated paper led to a rich exploration of different extreme spacetimes in numerical relativity in the years following its publication. Those numerical studies persist in questioning the weak cosmic censorship...

The historic detection of gravitational waves paved the way for precision gravitational-wave astrophysics to blossom at unprecedented proportions. In this talk, I will focus on extreme-mass-ratio inspirals (EMRIs) that consist of a primary supermassive compact object, and a stellar-mass secondary companion. Even though we are currently performing pertinent tests on order to characterize the...

Spontaneous scalarization of neutron stars has been extensively studied in the Damour and Esposito-Farèse model, in which a scalar field couples to the Ricci scalar or, equivalently, to the trace of the energy-momentum tensor. However, scalarization of both black holes and neutron stars may also be triggered by a coupling of the scalar field to the Gauss-Bonnet invariant. The case of the...

Black-hole binary spin precession admits equilibrium solutions corresponding to systems with (anti-) aligned spins. Among these, binaries in the up–down configuration, where the spin of the heavier (lighter) black hole is co- (counter-) aligned with the orbital angular momentum, might be unstable to small perturbations of the spin directions. The occurrence of the up–down instability leads to...

Gravitational self-force theory is the primary way of modelling extreme-mass-ratio inspirals (EMRIs). One difficulty that appears in second-order self-force calculations is the strong divergence at the worldline of the small object, which causes both numerical and analytical issues. Previous work [Phys. Rev. D 95, 104056 (2017); ibid. 103, 124016 (2021)] demonstrated that this could be...

Action-angle (AA) coordinates are commonly used in theoretical description of extreme mass ratio inspirals (EMRI), as they are naturally associated with the two dynamical timescales present in EMRI. However the AA coordinates are rarely used in practical calculations. In our work we implemented analytic methods of canonical perturbation theory to transform geodesic Hamiltonian into AA...

We will present results regarding the instability of extremal black holes. We will show how these results can be used to derive observational signatures for extremal black holes.

Working within the space of smooth solutions of GR and tuning to the threshold of black hole formation, we arrive at extreme spacetimes which are generally expected to contain naked singularities. In the spherical setting such configurations have been accurately studied by numerical work for three decades. This resulted in a beautiful understanding of the threshold of collapse through a...

We investigate the robustness of inflation models to inhomogeneous initial conditions. In the simplest case inflation is described by a single scalar field $\phi$ minimally coupled to gravity with homogeneous initial value $\phi_0 and canonical momentum $\Pi_0 = 0$. However, there is no fundamental reason to reckon that inflation started with such homogeneous initial conditions. In our recent...

We explore the potential for detecting low-$|T/W|$ rotational instabilities (known also as dynamical shear instabilities) with future gravitational wave observatories. Our study employs numerically generated post-merger waveforms, which reveal the re-excitation of the l=m=2 f-mode. We evaluate the detectability of these signals by injecting them into colored Gaussian noise. The signals are...

We provide accurate universal relations that allow to estimate the moment of inertia $I$ and the ratio of kinetic to gravitational binding energy $T/W$ of uniformly rotating neutron stars from the knowledge of mass, radius, and moment of inertia of some related non-rotating neutron star. Based on these, several other fluid quantities can be estimated as well. Astrophysical neutron stars rotate...

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 constraints on the neutron star equation of state....

LISA (Laser Interferometer Space Antenna) is the future large mission by the European Space Agency aimed at observing the Universe with gravitational waves. It was approved in 2017 following the success of the technological demonstrator LISAPathfinder and the detection of gravitational waves by ground-based observatories LIGO-Virgo. LISA has nearly completed its detailed definition phase...

I will discuss some explicit solutions of higher order scalar tensor theories. We will start by reviewing classical GR solutions and some of their key properties such as integrability of geodesics in Kerr spacetime. We will then construct stealth solutions, i.e. solutions that are still Einstein metrics but with a non trivial scalar field. We will construct solutions which are distinct from...

I will describe recent developments related to the search for low-frequency gravitational waves with pulsar timing arrays

In this work we explore the motion of massive particles in curved backgrounds. We demonstrate how new symmetries emerge by distorting the conformal vectors of the spacetime metric and how they lead to additional conserved quantities. Beside (pseudo-)Riemannian geometry, we also apply this scheme to certain Finslerian extensions. Finally, we explain how the emergence of these symmetries is...

This paper investigates the integrability properties of Einstein's theory of gravity in the context of accelerating NUT spacetimes by utilizing Ernst's description of stationary and axially symmetric electro-vacuum solutions. We employ Ehlers transformations, Lie point symmetries of the Einstein field equations, to efficiently endorse accelerating metrics with a nontrivial NUT charge. Under...

We investigate some aspects of GB-BTZ black holes. Perturbations associated to scalar and massless spinorial fields are studied suggesting the dynamical stability of the geometry. Moreover, the quasinormal modes are found for different parameters, mainly exploring the influence of the coupling constant of the theory. The hydrodynamical modes are also obtained in the small coupling limit....

In this talk we discuss the issue whether the center of the mass of a spinning extended test body moving in a black hole background is just a gauge. The discussion is in the framework of the Mathisson-Papapetrou-Dixon equations, for which a spin supplementary condition is used in order to fix the center of the mass. To get the answer we employ novel and in principle analytical methods of...

In this work we are revisiting the well studied Ellis wormhole solution in a generalized Horndeski theory motivated from the Kaluza-Klein compactification procedure of the more fundamental higher dimensional Lovelock gravity. We show that the Ellis wormhole is analytically supported by a gravitational theory with a non-trivial coupling to the Gauss-Bonnet term and we expand upon this notion by...

Primordial black holes (PBH) can account for a wide variety of cosmic conundra, among which the origin of the primordial magnetic fields threading the intergalactic medium. In this talk, by considering PBHs naturally furnished with a disk due to the vortexlike motion of the primordial plasma around them, we will propose a novel natural ab initio mechanism for the generation a battery induced...

We develop a general algorithm that enables the consistent embedding of any four-dimensional static and spherically symmetric geometry into any five-dimensional single-brane braneworld model, characterized by an injective and nonsingular warp factor. Furthermore, we supplement the algorithm by introducing a method that allows one to, in principle, reconstruct 5D field theories that support the...

We discuss exploration for isotropic gravitational wave backgrounds around 1 mHz by correlation analysis, targeting both parity odd and even polarization modes. Even though the space interferometer LISA alone cannot probe the two modes due to cancellations, the outlook is being changed drastically by the strong development of other space detectors such as Taiji. In fact, a heliocentric...

There are various ways to deploy symmetries in order to extract exact solutions of Einstein’s Field Equations. The Study of the Canonical forms of Killing Tensor in the frame of General Theory of Relativity proved fruitful in the past providing new solutions or general families of already known spacetimes. Regarding the latter the most general versions of Killing Tensor, its four Canonical...

In this talk, I will introduce a novel formalism for any field theory and apply it to the effective field theories of large-scale structure. The new formalism is based on functors of actions composing those theories. This new formalism predicts the actionic fields. Furthermore, I will discuss a generalised manifold with N-correlators of Nt- objects with or without contaminants. I will discuss...

I will review recently proposed probes of gravity and dark matter based on the oscillations of stars. In this regard, I explain how current and future solar/stellar oscillations can place tight constraints on the properties of dark energy and dark matter.

We consider some cosmological models where soft future singularities arise and discuss how the crossing of these singularities can change the properties of matter fields. We also compare different approaches to the possibility of the crossing of the singularities of the Big Bang – Big Crunch type. We discuss what happens with the singularities in quantum cosmology and study what happens with...

The relevant time-depended automorphisms are used to reduce the form of

the line-element. There are, thus revealed, enough first integrals of motion

which enable the presentation of the entire solution space for the 5-dim

Kasner (4 A_1) and/or Type V (A^{1,1}_{4,5}) models.

Nonparametric reconstructions of cosmological parameters from observational data sets are usually associated with Gaussian processes (GP). It is known though, that GPs are plagued with overfitting issues and they introduce some statistical bias through the selection of the kernel. The last few years, with the advent of Machine Learning, artifical neural networks are being used in cosmology for...

We investigate the alleviation of both H0 and σ8 tensions simultaneously within the framework of Tsallis cosmology. Such a modified cosmological scenario is obtained by the application of the gravity-thermodynamics conjecture, but using the non-additive Tsallis entropy, instead of the standard Bekenstein-Hawking one. Hence, one obtains modified Friedmann equations, with extra terms that depend...

In this talk I am going to present our recent work where we derive for the first time a three-peaked GW signal associated to no-scale Supergravity within the frequency ranges of nHz, Hz and kHz, with the former being in excellent agreement with NANOGrav/PTA GW data. We concentrate on the primordial gravitational wave (GW) spectrum induced due to second-order gravitational interactions by...

Various studies, such as references 2207.00878 [gr-qc], 2304.14465 [gr-qc], and 2304.14465 [gr-qc], have explored the potential of Fractional Cosmology to address the H0 tension. They have analyzed the Equation of State’s value attained from the Supernova H0 and Planck’s value for z<1.5 and have reported a trend of H0 that aligns with these values. However, there is still a discrepancy between...

We summarize the famous tensions between various observational

datasets and theoretical predictions of the Standard Model of Cosmology,

such as the H0 and S8 tensions, that could be a sign that we are

approaching New Physics. Then we provide possible solutions, arising

from modifications /extensions of the standard lore.

In recent years, the field of Gravitational Wave Astronomy has flourished. With the advent of more sophisticated ground-based detectors and space-based observatories, it is anticipated that Gravitational Wave events will be detected at a much higher rate in the near future. One of the future data analysis challenges is performing robust statistical inference in the presence of detector noise...