### Conveners

#### Numerical Relativity and Gravitational Wave Observations: Block 1

- Nigel Bishop (Rhodes University)

#### Numerical Relativity and Gravitational Wave Observations: Block 2

- Nigel Bishop (Rhodes University)

### Description

The session is open for talks on all aspects of computation related to the calculation of gravitational waves that are potentially observable. This includes the development and applications of codes, as well as the development of relevant mathematical theory or computational methods.

Characteristic formulations of General Relativity are based on a null folliation of the spacetime. When combined with the standard Cauchy evolution they can in principle provide highly accurate waveform modelling. During this modelling process it is typical that the full non-linear Einstein field equations are solved numerically. A numerical solution to a PDE problem can converge to the...

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...

We develop new strategies to build numerical relativity surrogate models for eccentric binary black hole systems, which are expected to play an increasingly important role in current and future gravitational-wave detectors. We introduce a new surrogate waveform model, NRSur2dq1Ecc, using 47 non-spinning, equal-mass waveforms with eccentricities up to 0.2 when measured at a reference time of...

As detections of mergers of compact bodies begin to flow in, and as we enter an era of precision GW measurements, our understanding of compact bodies, their physics and that of the surrounding astrophysical environment, will continue to grow and at times even be challenged. The need to revise the mass bounds of compact bodies such as BHs and NSs and the possibility of the existence of GW...

The long-awaited detection of a gravitational wave from the merger of a binary neutron star in August 2017 (GW170817) marked the beginning of the new field of multi-messenger gravitational wave astronomy. Reaching densities a few times that of nuclear matter and temperatures up to 100 MeV, such mergers also represent potential sites for a phase transition from confined hadronic matter to...

We present the first numerically stable nonlinear evolution for the leading-order gravitational effective field theory (Quadratic Gravity) in the spherically-symmetric sector. The formulation relies on (i) harmonic gauge to cast the evolution system into quasi-linear form (ii) the Cartoon method to reduce to spherical symmetry in keeping with harmonic gauge, and (iii) order-reduction to...

A burst of gravitational waves creates a permanent change in separation between two initially comoving test particles; this is known as the gravitational wave memory effect. Near null infinity, two contributions to the memory effect arise: linear memory, which appears in linearized gravity and is due to changes in conserved quantities, and nonlinear memory, which arises due to the nonlinear...

Current PyCBC-based searches detect gravitational-wave (GW) transients by matched-filtering the advanced LIGO-Virgo detector data with model waveforms. They have, to date, detected or confirmed more than 50 compact binary merger signals. But these searches perform poorly when it comes to identifying short-duration compact binary signals in Advanced LIGO-Virgo data. In this talk, we will...

Spinning neutron stars are sources of long-duration continuous waves that may be detected by interferometric detectors. We focus on long, but not infinite duration signals and derive the precise signal-to-noise ratio (SNR) when the duration is not a priori known. We illustrate the effect of gaps in the data on the SNR.

We propose a method for localising a long-duration signal (longer than a few hours) when the start time and duration of the signal are unknown. We show how the uncertainties in the time-localization of the signal reflect on signal-to-noise ratio (SNR) of the recovered signal.

Intermediate-mass black holes (IMBHs) span the approximate mass range $100$--$10^5$ $M_{\odot}$, between black holes (BHs) formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of massive BHs in a binary system are the most energetic gravitational-wave sources accessible by the ground-based gravitational-wave detector network, so is IMBH binary. The third...

The Coherent WaveBurst (cWB) search algorithm identifies generic gravitational wave (GW) signals in the LIGO-Virgo data by looking for excess power events in the time-frequency domain, with minimal assumptions on the signal model. The standard cWB pipeline improves signal significance by removing excess noise through the application of a set of a priori defined vetoes on summary statistics...

We introduce a gravitational waveform inversion strategy that discovers mechanical models of binary black hole (BBH) systems. We show that only a single time series of (possibly noisy) waveform data is necessary to construct the equations of motion for a BBH system. Starting with a class of universal differential equations parameterized by feed-forward neural networks, our strategy involves...

If the number of extra dimensions is odd, the full spacetime becomes odd-dimensional and the formation of gravitational radiation is obscured by violation of the Huygens principle. Gravitational waves must travel with the speed of light, while the full retarded gravitational field of a localized source propagates with all velocities lower or equal to the speed of light. To calculate...