Gravitational wave radiation, our window for probing the strong field and dynamical regime of gravity, is unambiguously defined only at future null infinity - the location in spacetime where light rays arrive and thus where signals and global properties of spacetimes can be measured. A convenient way to reach it within numerical relativity simulations is by means of hyperboloidal foliations,...

One of the challenges in numerical relativity is to include future null infinity in the computational domain with a well-posed formulation. Success will not only enable us to evolve any system of astrophysical interest, e.g. binary black holes and extracting the gravitational wave signal at future null infinity, with any desired accuracy, but also help in studying various phenomena of...

The Einstein and Maxwell equations are both systems of hyperbolic equations which need to satisfy a set of elliptic constraints throughout evolution. However, while electrodynamics and magnetohydrodynamics have benefited from a large number of evolution schemes that are able to enforce these constraints and are easily applicable to curvilinear coordinates, unstructured meshes, or N-body...

In this talk I will present the equations of the Einstein-scalar-Gauss-Bonnet theory, a modified theory of gravity, in the 3+1 formulation with a modified gauge that proves to be strongly hyperbolic. Then I'll show some of the numerical results that we have obtained from the implementation of these equations with GRChombo, a numerical general relativity code with fully adaptive mesh refinement.

The study of vacuum critical collapse has acquired an aura of unapproachability in its 30 years of existence. It is often thought that only highly sophisticated numerical methods together with immense supercomputer clusters will suffice for treating this problem. In our recent work we refute this folk lore by using only open-source code running on single machines to discover universal echoing...

We use our adapted pseudo-spectral code bamps, with its new hp adaptive mesh refinement, to tune close to the barrier between gravitational collapse and dispersed fields, in order to study the critical phenomena that emerges near that threshold. In spherical symmetry, we observe critical phenomena by evolving massless scalar fields. This has allowed us to assess the adequate choice of gauge...

We studied the asymptotic behavior of perturbed Kerr initial data by solving the evolutionary form of the vacuum Einstein constraint equations. Unlike in the elliptic formulation, solving the constraints in their evolutionary form, we have direct control over the constrained data only on a single 2-surface. This immediately raises the question of whether it can guarantee asymptotic flatness of...

Detection and parameter inference of gravitational-wave signals relies on the comparison of the incoming detector strain data to waveform templates for the gravitational-wave strain $h(t)$ which ultimately rely on the resolution of Einstein's equations via numerical relativity simulations. These, however, commonly output a quantity known as the Newman-Penrose scalar $\psi_4(t)$, related to the...

Compact binary coalescence waveforms generated by numerical relativity codes and associated phenomenological models are a critical part of gravitational-wave searches in detector data. However, the computational cost of the conventional matched-filtering searches is steeply rising as increased sensitivity of detectors expands the waveform parameter space. Lately, machine-learning based search...

The formation of Primordial black holes is naturally enhanced during the QCD phase transition, because of the softening of the equation of state: at a scale between 1 and 3 solar masses, the threshold is reduced of about 10% with a corresponding abundance of primordial black holes increased by more than 100 times. Such black holes could be an interesting source of gravitational waves emitted...

GW190425 was the second gravitational wave (GW) signal compatible with a binary neutron star (BNS) merger detected by the Advanced LIGO and Advanced Virgo detectors. Despite intense follow-up campaigns, no electromagnetic counterpart was identified. Whether the associated kilonova was too dim or the localisation area too broad is still an open question. We simulate 28 BNS mergers with the...

The impact of nuclear reactions on observed signals from neutron star mergers is uncertain. We make a first attempt at quantifying that uncertainty by studying two cases that, intuitively, are the most extreme. In one case we assume that the reactions happen instantaneously (on timescales much faster than can be resolved in simulations). In the other we assume reactions do not happen (or occur...

Modelling the transport of neutrinos is crucial for core-collapse supernavoae and neutron star mergers. Evolving the complete Boltzmann equations for radiative transport is extremely expensive, so several more efficient approximation formalisms have been developed to face such difficulty. In this talk, I will focus on the truncated-moment formalism, considering only the first two moments (M1)...

Authors: Nigel T. Bishop, Monos Naidoo, and Petrus J. van der Walt. Using linearized perturbations within the Bondi-Sachs formalism, we have considered the problem of a gravitational wave (GW) source surrounded by a spherical dust shell. It was shown that the shell causes the GWs to be modified both in magnitude and phase; and that if the shell is viscous, then the shear induced in the...

In my talk, I will present the latest results of general relativistic hydrodynamic simulations of binary neutron star mergers with the recently introduced finite-temperature extension of the V-QCD equation of state. The V-QCD model is based on the gauge/gravity duality and provides a consistent description of nuclear and quark matter at densities beyond nuclear saturation which are realized in...

The detection of a binary neutron star merger in 2017 through both gravitational waves and electromagnetic emission opened a new era of multimessenger astronomy. During the merger, several mechanisms like the Kelvin-Helmholtz instability, the winding up effect and the MRI, can amplify the initial magnetic field in the remnant to be powerful enoguh for launching a jet, with an associated short...

Binary neutron star mergers provide the unique opportunity to study matter at densities and temperatures unreachable in laboratories on earth. Its properties are encoded in the equation of state whose influence on the graviational wave signal of merging neutron stars can be used to constrain the physics of the strong interaction. But it is not only the equation of state which carries...

The construction of constraint-satisfying initial data is an essential element for the numerical exploration of the dynamics of compact-object binaries. While several codes have been developed over the years to compute generic quasi-equilibrium configurations of binaries comprising either two black holes, or two neutron stars, or a black hole and a neutron star, these codes are often not...

The solution of elliptic equations is an integral part of computational physics. This talk presents a new elliptic solver which is based on a discontinuous Galerkin scheme applicable to a wide class of elliptic partial differential equations. The solver is employing task-based parallelism and scales to a few thousand of compute cores. We show applications to the construction of initial data...

Binary black hole simulations become increasingly computationally expensive with smaller mass ratios because the Courant-Friedrich-Lewy condition imposes smaller time steps by the need to resolve the small black hole. Here we propose and explore a method for alleviating the scale disparity in simulations with mass ratios in the intermediate astrophysical range, where purely perturbative...

Numerical simulations are the only way to calculate exact gravitational waveforms from binary neutron star mergers and to design templates for gravitational-wave astronomy. Our knowledge about the physical properties of the inspiraling neutron stars, the mechanisms inherent in the inspiral and the massive object produced after the merger depends crucially on the accuracy of these numerical...

`GR-Athena++`

is a general-relativistic, high-order, vertex-centered solver that extends the oct-tree, adaptive mesh refinement capabilities of the astrophysical (radiation) magnetohydrodynamics code `Athena++`

. To simulate dynamical spacetimes `GR-Athena++`

uses the Z4c evolution scheme of numerical relativity coupled to the moving puncture gauge. Stable and accurate binary black hole merger...

We will quickly review the last 30 years of open general codes, platforms, and frameworks for Numerical Relativity, with special emphasis on the lessons that we have learnt and the challenges that lie ahead. We will also describe results and features of the last versions of different codes, with special emphasis on Simflowny: an open platform which automatically generates efficient parallel...

The numerical solution of a system of hyperbolic PDEs all the way to future null infinity requires the knowledge of asymptotics. The Good-Bad-Ugly-F model is known to mimic the asymptotic properties of Einstein equations in generalized harmonic gauge. In this talk I will present the results of numerical evolution of this system, both in spherical symmetry and full 3D, with the scope of using...

Since the first general-relativistic simulation of a binary neutron-star (BNS) merger less than a decade ago, the numerical relativity community has embraced increasingly sophisticated and accurate descriptions of the physics of these systems, most notably, the gravitational-wave signal. On the other hand, the electromagnetic signals, which are crucial to understand in the novel context of...

We discuss the construction of quasi-equilibrium configurations of compact binaries, in which each component of the binary is modeled as a mixture of two ideal fluids. For the first fluid we use an ordinary baryonic-matter equation-of-state and for the second fluid, describing dark matter, we trial different dark-matter equation-of-state. We use the obtained quasi-equilibrium configurations as...

Fuzzy dark matter (FDM) is an exciting alternative to the standard cold dark matter (CDM) paradigm, reproducing its many successful large scale predictions, but solving most of the existing tension with small scale (galactic) observations. FDM models postulate that dark matter (DM) is constituted by light bosons of mass~$m_\psi\sim 10^{-22}\,\text{eV}$, which behave like CDM on scales larger...

A promising way to study the properties and behaviour of dark matter on small scales is through the effects it may have on binary mergers. In particular, its presence around black holes may lead to a distinctive dephasing of the signal due to dynamical friction. In this talk I will present how we calculate this force numerically and discuss how our findings extend some already existing...

The multi-messenger detection of GW170817, GRB170817A, and AT2017gfo originating from the merger of two neutron stars have been a scientific breakthrough. Under the assumption that dark matter accumulates in and around neutron stars, multi-messenger observations of compact binary mergers will provide a new way to search for and constrain the nature of dark matter. In this context, we extended...

Scalar fields around compact objects are of interest for scalar-tensor theories of gravity and dark matter models consisting of a massive scalar, e.g. axions. These fields can form long-lived clouds around black holes via superradiance or simple gravitational accretion. Black hole binary mergers occuring inside such clouds, or in dense scalar dark matter environments, may modify the...

Recently, a new class of fully nonlinear numerical solutions to the static, spherically symmetric Einstein-Klein-Gordon system for a collection of an arbitrary odd number of complex scalar fields with an internal U(N) symmetry was found. These solutions are parametrized by an angular momentum number l, an excitation number n, and a continuous parameter representing the amplitude of the fields....

Initial data play a vital role in numerical simulations -- the ability to construct constraint satisfying initial data closely representing the physical system determines the quality of the subsequent numerical evolution. It has been shown by Helfer et. al. (arxiv:1802.06733) that plainly superposed initial data can lead to significant constraint violations that affect the physics of the...

New light fundamental fields are natural candidates for all or a fraction of dark matter. Self-gravitating structures of such fields might be common objects in the universe, and could comprise even galactic haloes. These structures would interact gravitationally with black holes, process of the utmost importance, since it dictates their lifetime, the black hole motion and possible...

Particle physics models of dark matter, and extensions to the Standard Model, predict the existence of a large abundance of light scalar degrees of freedom in the universe. From a diffuse cloud, these can form into clumps of energy - boson stars. Additionally, due to their high compactness, close to that of black holes, these solutions serve as test beds to study the non-linear dynamics of a...

Einstein-dilaton-Gauss-Bonnet is a theory of modified gravity in which a scalar field, called dilaton, is nonminimally coupled to the metric via an exponential function. Black holes (BHs) in this theory are particularly interesting since they possess a critical configuration with minimum mass and finite Hawking temperature. This means that a critical BH loses mass due to Hawking's radiation,...

It was recently shown that a broad class of gravity theories that couples a dynamical scalar field to the Gauss-Bonnet invariant can lead to spontaneous scalarization of black holes, allowing these objects to grow "scalar hair" once certain conditions are met and to remain "bald" otherwise. While most works on the topic have focused on isolated black holes, progress has recently been made in...

In recent years, gravitational wave observation of black holes have furnished new opportunities to test our understanding of gravity in the strong field, highly dynamical regime. However, in order to perform model dependent tests of General Relativity with these observations one needs accurate waveforms for modified gravity theories which is still an outstanding theoretical problem. In this...

The black hole merger in scalar-Gauss-Bonnet gravity can lead to dynamical descalarization this is a spontaneous release of the scalar hair of the newly formed black hole. Depending on the exact form of the Gauss-Bonnet coupling function, the stable scalarized solutions can be either continuously connected to the Schwarzschild black hole, or the transitions between the two can happen with a...

Though General Relativity has been successfully tested so far, concepts such as dark energy and string theory suggest the need of modifying it. Scalar-tensor theory is one of the most popular alternatives discussed. We produce studies of stellar core collapse in spherical symmetry that were performed by adapting the numerical code GR1D to the case of massive scalar-tensor gravity. We...

I present the first numerical-relativity simulations for black holes in Quadratic Gravity, i.e., including the leading-order (quadratic) curvature corrections to General Relativity. I review the nonlinear degrees of freedom and discuss a well-posed initial value formulation for Quadratic Gravity, both in spherical symmetry and in (3+1) dimensions. In spherical symmetry, self-convergence tests...

Using the standard methods of numerical relativity we study axisymmetric gravitational waves undergoing gravitational collapse. Because it is known that the 1+log lapse choice breaks down in this situation, we propose a computationally effective alternative to maximal slicing and show that it allows the simulation to proceed until either the gravitational waves disperse or an apparent horizon...