In a series of previous works, several initial boundary value problems in numerical relativity were demonstrated as numerically stable even with boundaries placed in the strongly gravitating region of a dynamical spacetime. Now, care is being taken to define boundary conditions that can be placed on the radiating degrees of freedom based on quasi-local conservation laws. Incoming gravitational...
Numerical relativity (NR) has revolutionized our understanding of strong-field gravity, enabling high-fidelity simulations of compact binary systems. This paper reviews recent breakthroughs in NR methodologies, with emphasis on binary neutron star (BNS) and black hole–neutron star (BHNS) mergers. We discuss advancements in adaptive mesh refinement, constraint-damping formulations, and...
Functions of bound Kerr geodesic motion play a central role in many calculations in relativistic astrophysics, ranging from gravitational-wave generation to self-force and radiation-reaction modeling. Although these functions can be expressed as a Fourier series using the geodesic fundamental frequencies, reconstructing them in coordinate time is challenging due to the coupling of the radial...
In the realm of general relativity, given two metric solutions in different gauges, determining whether they describe the same physical scenario poses significant challenges. This study proposes a novel application of machine learning techniques to address this issue within the context of numerical relativity. We introduce the first implementation of neural networks to learn the mapping...
In this work, we perform the first fully three-dimensional simulations of the ekpyrotic cosmological scenario. Our numerical relativity code is based on previous work that assumed some symmetry. In particular, we use a tetrad formulation of the Einstein-scalar field equations, but in this case using the parallel infrastructure PAMR. Our goal is to test the robustness of the ekpyrotic...
Many binary population-synthesis (BPS) codes have been developed over the last few decades to evolve binary stars throughout their entire evolutionary history. BPS codes are extremely useful for investigating astrophysical processes we observe in the universe such as supernovae, x-ray binaries, generation of r-process elements, and gravitational-wave populations. However, many current BPS...
The most general bound binary black hole (BBH) system has an eccentric orbit and precessing spins. Detecting a BBH with significant eccentricity near merger would be a clear signature of dynamical formation, while the spin–orbit misalignment angles (spin tilts) provide a powerful probe of different astrophysical formation channels. We have developed publicly available codes that evolve...
The Kiselev metric has gained increasing relevance in recent years due to its ability to model a variety of physical systems. It describes a black hole influenced by the presence of an anisotropic fluid—composed of the superposition of a perfect fluid, an electromagnetic field, and a scalar field. Depending on the equation of state parameter $\omega$, this metric can model dispersed gases in...
Current waveform models of binary black hole mergers incorporate a large amount of analytical information during the inspiral phase. On the other hand, post-merger templates use a phenomenological ansatz informed by numerical simulations, with the quasinormal frequencies as the only analytical input.
Working in the perturbative limit, we develop the first analytical model predicting the...
Black Hole Perturbation Theory (BHPT) has been shown to be very successful in describing physical processes around BHs. In particular, the scattering of electromagnetic and gravitational waves (and other physical fields) and the so-called quasinormal mode (QNM) oscillations, the damped BH oscillations that dominate the ringdown phase of binary BH (BBH) coalescences.
A key feature of BHPT is...
We evolve for the first time in full GR a small, collisional N-body black hole cluster of arbitrary total mass M. The bound cluster is initially compact (radius R/M≈10), stable, and consists of 25 equal-mass, nonspinning black holes. The dynamical interactions of compact objects in N-body clusters is of great interest for the formation of black holes in the upper mass gap as well as...
Recent millimeter-wavelength observations of the near-horizon regions of
M87 and Sgr A by the Event Horizon Telescope have significantly advanced our understanding of gravity and astrophysics under extreme conditions. A key goal for future observations, with improved angular resolution and sensitivity, is the detection of the photon ring in these black holes. For Kerr black holes, photon...
The application of $V/V_{max}$ statistics to gravitational-wave cosmology aids in our understanding of compact object parameter evolution, particularly the redshift and mass distributions of binary systems. Transforming $V/V_{max}$ from a distance-dependent to a population-dependent diagnostic provides a direct pipeline for testing various gravitational-wave source distributions. Through this...
Multi-messenger astronomy is a new and promising area in astrophysics where the same phenomenon is seen using multiple methods. In this project we examine binary systems of dense stellar core remnants known as white dwarfs (WDs) which may be seen both in the electromagnetic spectrum as light and in gravitation as gravitational waves (GWs). Gravitational waves were first predicted by Albert...
Gravitational waves from compact binaries provide excellent opportunities for testing general relativity (GR) in the strong- and dynamical-field regime. So far, tests using inspiral signals have employed certain variants of the parametrized post-Einsteinian (ppE) framework to model deviations from GR. However, this approach has several limitations: the parameters introduced do not fully...
As detections of gravitational waves are increasing, they offer a testing ground for General Relativity. One test for deviations from General Relativity and the Standard Model is the inspiral-merger-ringdown test. In this project, we investigate the applicability and reliability of this test based on simulated observations of injected boson star waveforms. We find that the test in its most...
Horava Gravity is a field-theoretic model that shares many properties of GR. Namely it has metric as a basic degree of freedom and possesses spatial diffeomorphisms, as well as global time reparameterizations, as its symmetries. The feature of the model is the potentially much improved high energy behavior at the expense of forbidding the symmetry of local time reparameterizations and...
Abstract
Introduction of stochastic definitions of universal Planck and Boltzmann constants (h, k) reveals hydrodynamic origins of both quantum mechanics and quantum field theory. Also, new perspectives regarding the quantum nature of internal atomic (spacetime, mass) versus external (space, time, mass), black hole, white hole, and Dirac anti-matter are discussed. The temperature limits...
In this presentation, we investigate traversable wormhole spacetimes within the context of $f(R,T_{ab}T^{ab})$ modified-gravity theory. By considering a linear form of the function, we demonstrate the existence of numerous wormhole solutions where the matter fields satisfy all energy conditions. However, due to the lack of natural localization of these solutions, it is necessary to match them...
We explore the possibility that quadratic gravity, as a renormalizable theory, describes the interior of quantum black holes. We find new exact power-law solutions to pure quadratic gravity under spherical symmetry, which are complex valued. The resulting solutions, dubbed powerballs, are horizonless compact objects that become Schwarzschild-like a small distance (of the order of the Planck...
We use holography as a tool to explore all-order in velocity gradients relativistic hydrodynamics. We demonstrate the link between causality and instability of the theory.
The (degenerate) geometry of event horizons is linked to Carrollian fluids. We investigate the behaviour of event horizons via a perturbative coupling to a massless scalar field, making connections to Carrollian hydrodynamics with a driving source, and discuss the fluid equilibration in tandem with the horizon’s relaxation to equilibrium. We observe that after the perturbation dies off, the...
The nature of the compact objects within the supposed 3-5M⊙ “lower mass gap” remains uncertain. Observations of GW190814 and GW230529 highlight the challenges gravitational waves face in distinguishing neutron stars from black holes. Interpreting these systems is especially difficult because classifications depend simultaneously on measurement noise, compact binary population models, and...
In this era of gravitational waves from blackhole and neutron star mergers, GW data is being collected at an amazing pace. There is a possibility of GWs of BH mergers scattering from wormholes and/or other exotic compact objects (ECOs), if they exist. Specialized groups are searching for wormhole signatures. We share the results of our study of scalar field perturbations and quasi-normal modes...
Formulating precise definitions of conserved charges like energy-momentum and angular momentum in General Relativity is extremely challenging. Instead of entering directly into the stress-energy tensor, gravitational energy manifests as an obstruction to integrating the local stress-energy conservation law to yield globally conserved charges. A very nice quasi-local definition for mass was...
Traversable wormholes have captivated the interest of the public and of theoretical physicists since their inception, with a recent resurgence of interest in the context of holography. While wormhole geometries are permissible within the framework of general relativity, they require matter with "negative energy" to sustain them; specifically, the matter must violate the null energy condition...
Classical singularity theorems and topological‑censorship arguments seem to forbid the formation of a wormhole in 4‑dimensional general relativity, even at the kinematical level, through the emergence of naked singularities. I will present a concrete example, developed in [1], in which the would‑be singular set is excised and replaced by a controlled 0‑surgery on a compact spacetime...
Observers have always played a central role in quantum mechanics, yet their consistent incorporation into quantum gravity remains poorly understood. In asymptotically flat spacetimes, this issue becomes especially acute due to the infrared structure of gravity and the infinite-dimensional BMS symmetry group, which render the definition of observables highly nontrivial. In standard quantum...
We study propagation of high-frequency electromagnetic and gravitational waves in the gravitational field of a rotating black hole. Due to the interaction of the spin of the field with the spacetime curvature, the standard geometric optics approximation that is used for obtaining the approximate high-frequency solutions of the wave equation should be modified. The corresponding modified...
The usual definition of asymptotic flatness at spatial infinity requires that the flat metric be approached at a particular rate. However, slower rates of fall off are compatible with well-defined evolution under the Einstein vacuum equations. Since initial data must satisfy the constraint equations, we want to know if we can specify the fall off rate that we want and still have...
I will discuss ongoing work on the construction of asymptotically flat vacuum initial data sets in General Relativity via the conformal method. My collaborators and I have demonstrated that certain asymptotic structures may be prescribed a priori through the method's seed data, including the ADM momentum components, the leading- and next-to-leading-order decay rates, and anisotropy in the...
GR, while one of the most successful and well-tested theories to date, is expected to receive corrections at high energies—through higher-curvature terms, additional degrees of freedom, or both. Given the vast landscape of possible extensions, how can we test them in a systematic way?
In this talk, I will present a general framework for interpreting deviations in gravitational wave data,...
Binary black holes can interact with surrounding matter, producing unique electromagnetic signatures and influencing their long-term evolution. Numerical simulations are crucial to understand the nonlinear behavior of gas and particles moving on this dynamical spacetime. We present a general binary black hole metric approximation valid at all binary separations for all practical purposes. We...
I present a new formulation of General Relativity. The action is quadratic in the curvature and the equations of motion involve the divergence of the Riemann tensor. I show that this formulation is well posed and is equivalent to the Einstein equations. Overall, this formulation provides a surprising and peculiar new point of view on the Einstein equations.
I will present a detailed introduction to my Python package, OGRePy: An Object-Oriented General Relativity Package for Python, which will be of great interest to anyone doing research in general relativity. I will demonstrate the package's usage and features, including its ability to calculate arbitrary tensor formulas involving any combination of...
The horizon of a black hole, the "surface of no return," is characterized by its rotation frequency $\Omega_H$ and surface gravity $\kappa$. A striking signature is that any infalling object appears to orbit at $\Omega_H$ due to frame dragging, while its emitted signals decay exponentially at a rate set by $\kappa$ as a consequence of gravitational redshift. Recent theoretical work predicts...
This paper argues for a Relativistic light speed limit to escape velocity and a process that will transform higher elements into Hydrogen-1. It proposes a Relativistic process that will free energy to decay all aspects of the Periodic Table into a single Proton and Electron. They could then attract and form into Hydrogen-1. The relativistic slowdown will mean that Structural energies that...
The real working process of universal phenomenon is described in this paper and unraveling the misconceptions of the theory of relativity.Some experimental evidence and universal phenomenon indicates that the speed of light (c) is not constant in space.Speed of light varies one to another place in space and show relative speed in space.As which this effect predicts to existence of light wave...
Einstein worked on General Relativity for a decade before releasing it in 1916. For several of those years he struggled to include gravity’s own energy into his equation. He couldn’t get it right, so he just dropped it. How do we consistently put gravitational energy back into the equations? In this essay, Einstein’s own solution to this problem - the energy pseudo tensor, along with variants...
I began graduate studies in Fall 1993, arriving at Caltech vaguely interested in work being done in Kip Thorne’s group, but without a clear sense of what I really wanted to do. Kip’s advice to me was “talk to Eric.” Eric not only got me started thinking about subjects that continue to influence the calculations my group and I do today, but provided the standard that I have attempted to meet...
Neutron stars are the densest known gravitationally-stable objects in the Universe. Their strong gravitational fields, rapid rotation rates, and supra-nuclear central densities allow for a fascinating interplay between general relativistic effects and nuclear physics theory. Pulse-profile modeling is a technique that uses the gravitationally-lensed X-ray flux emitted from hot spots on the...
