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...
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...
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....
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 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...
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...
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...
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...
Entropy is a physical shock detector. Entropy is only produced during irreversible processes, like shocks, therefore it can be used to flag and track highly non-smooth features present in the solution space of the problem we are investigating. In this talk, I will demonstrate how entropy can be employed in the design of a flux-limiter for a number of different problems that can be written in...
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.
