**Numerical relativity simulations of binary black holes on hyperbolic and highly eccentric orbits**

# Parallel Session: GRAVITY - NUMERICAL RELATIVITY (D)

## Location: Park 2.07

#### D1 - Monday 14:00-15:40 (Helvi Witek)

**14:00-14:25**

**Harald Pfeiffer (Max Planck Institute for Gravitational Physics)**

Eccentric binaries or hyperbolic encounters of two black holes can occur in dense stellar environments like globular or nuclear clusters. Precise knowledge about the dynamics and emitted gravitational waves of such encounters is needed for analyzing gravitational wave events, like measurement of the eccentricity of compact binary inspirals, or applications of post-Minkowskian theory to waveform modelling. This talk reports on the efforts of the Numerical Relativity group at the Albert-Einstein-Institute to simulate highly binary black holes on highly eccentric or hyperbolic orbits. We will also discuss applications to gravitational wave astrophysics, most notably the relevance for tests of general relativity.

**IMRPhenomX: A Phenomenological Waveform Model for the Advanced Detector Era**

**14:25-14:50**

**Geraint Pratten (Physics and Astronomy, University of Birmingham)**

The gravitational-wave signal emitted by coalescing compact binaries carries a wealth of information on the intrinsic properties of the binary. By using accurate, high-fidelity waveform models we can accurately recover information regarding the masses and spins of the observed binary. In this talk I will introduce IMRPhenomX, a state-of-the art phenomenological waveform model that acts as the foundation to numerous improvements: higher modes, precession and tidal effects. I will highlight some of the key innovations and show preliminary parameter estimation results, demonstrating how the model can help improve constraints on masses, spins and tidal information in the advanced detector era.

**Gravitational wave propagation through a cosmological medium using the Bondi-Sachs formalism.**

**14:50-15:15**

**Nigel Bishop (Mathematics, Rhodes University)**

Gravitational waves (GWs) propagating over cosmological distances do so in a medium whose properties (the dark matter) are largely unknown. Depending on the physical properties of the dark matter, the GWs may experience changes such as dispersion, phase shift and attenuation. The approach adopted here is that the nature of dark matter is unknown, and the coefficients in the equations of state are arbitrary. The only property that is fairly well-constrained is the density. We wish to determine formulas for any deviation from vacuum propagation, and whether the magnitudes of any effects are such that, in principle, the signature could be observable.

This problem has received attention in previous works. Here, we use the Bondi-Sachs formalism, which has given additional insights in other cases. We construct exact solutions to the linearized Einstein equations. We are able to do so not for the general case, but when the background density is spherically symmetric with specific functional forms for the radial dependence. However, that is sufficient to be able to obtain expressions for the effect of a thin shell, and then a general density distribution can be modelled by integration over a sequence of thin shells.

**Optical Properties in Black Hole Universe Under Continuum Limit **

**15:15-15:40**

**Chi Tian (Physics, Case Western Reverse University)**

The distance redshift relation encodes important information about matter content in our Universe. In this talk, I will present our recent progress analyzing the distance redshift relations in a black-hole-lattice universe using numerical relativity tools. We will especially focus on the continuum limit, where the black hole mass tends to zero but density stays constant. Under such a limit, the black hole size is negligible compared to the Hubble length, and the universe tends to be matter dominated. However, complications arise when considering optical properties. By exploring those properties, we will discuss some new ways to constrain properties of black holes or dark matter.

#### D2 - Monday 16:10-17:50 (Helvi Witek)

**New approaches to the self-force problem for Extreme-Mass-Ratio Inspirals**

**16:10-16:35**

**Carlos Sopuerta (Institute of Space Sciences, National Spanish Research Council (CSIC))**

In this talk I will describe recent analytical and numerical work on the formulation and computation of the self-force that describes the inspiral of a stellar-mass compact object into a (super)massive black hole, one of the main sources of gravitational waves for the future space-based gravitational-wave observatory LISA, known as EMRIs (Extreme-Mass-Ratio Inspirals). The analytical work consists in the use of quasilocal conservation laws for the formulation of the equation of motion and the introduction of the self-force. The work on numerical techniques for EMRIs refers to new advances in the Particle-without-Particle method for the computation of the self-force, in particular its formulation in the frequency domain.

**Gravitational wave signal from tidal disruption events: analytical and numerical approach**

**16:35-17:00**

**Martina Toscani (Physics Department, University of Milan)**

In this talk I will describe the gravitational wave (GW) signal from Tidal Disruption Events (TDEs). First of all, I will determine the GW emission from a hot rotating accretion torus, formed after a TDE, that is subject to a specific hydrodynamical instability, called the Papaloizou-Pringle instability (Toscani et al, 2019). This study is performed both through an analytical and a numerical study, using the Smoothed Particle Hydrodynamics (SPH) code PHANTOM (Price et al, 2018). Then, I will talk about the new feature for the calculation of GWs that I have implemented in GR PHANTOM, the new version of the PHANTOM code with general relativity implemented (Price & Liptai, 2019). Finally, I will talk about our new investigation of the GW background associated to TDEs.

**Uncertainty quantification for astrophysical simulations**

**17:00-17:25**

**Ian Hawke (STAG, University of Southampton)**

Numerical simulations are required for astrophysical problems where energy cascades to ever-smaller scales, such as neutron star mergers and core-collapse supernovae. Recent results from the field of uncertainty quantification suggest that these simulations may converge *only* in a statistical sense. We will show examples where, when using standard models, astrophysically useful observables can only be predicted pointwise in a statistical sense, and discuss the restrictions this places on numerical simulation for parameter estimation in neutron star mergers.

**Hypermassive hybrid stars within the phase diagram of Quantum chromodynamics**

**17:25-17:50**

**Matthias Hanauske (Relativistic Astrophysics, Institute of Theoretical Physics/Goethe University Frankfurt)**

Hypermassive hybrid stars (HMHS) are extreme astrophysical objects. In contrast to hypermassive neutron stars (HMNS), these highly differentially rotating objects contain deconfiend strange quark matter in their slowly rotating inner region. HMHS and HMNS are formed in a binary neutron star merger event and can survive only a view seconds. During the inspiral, merger and post-merger evolution of the system, gravitational waves (GW) are emitted and the measured GW GW170817 has verified this picture impressively for the late inspiral phase. GWs, emitted from merging neutron star binaries, will be observed frequently within the coming years and numerical simulations are needed to understand the main characteristics of the underlying merging system in order to predict the expected GW signals. The appearance of a QCD-phase transition in the interior region of the HMHS and its conjunction with the spectral properties of the emitted GW will be addressed during this talk.