The gravitational wave (GW) group at the Chinese University of Hong Kong (CUHK) is pursuing various lines of research, all threading with gravitational waves. One of its main focus, and area of expertise, is GW lensing. Hereafter, you can find some description of different research directions the group has been delving into, as well as some of our most recent publications.
Gravitational Waves Lensing
Ania Liu, Hemantakumar Phurailatpam, Jason Poon, Laura Uronen, Paul Martens
Almost since the inception of general relativity, it has been known that massive objects bend spacetime, and, with it, the path of light. This effect is gravitational lensing! And the same is expected to be true for gravitational waves. Consequently, we are constantly searching to find real lensed gravitational wave events. Currently, no such event has been confirmed, but the forecasts are very optimistic for upcoming detector runs.
We are part of the LVK lensing group, with collaborators mainly in India and Europe, which conducts searches for strongly-lensed (multiple images), millilensed (frequency-independent amplitude modulation, from overlapping signals) and microlensed (frequency-dependent modulation) signals during LVK detector runs [1-2]. Our group members are involved in multiple lensing search pipelines such as Gravelamps [3] and GOLUM [4], which run lensed gravitational-wave parameter estimations, as well as the lensing forecast and rate estimator ler [5].
We also study the potential applications of gravitational-wave lensing. Laura is working on combining GW and EM data in the dark siren event regime, and probing whether we can confidently identify the lensed host galaxy of a GW event without a direct EM counterpart [6], based on work done in [7-8]. The group works on various aspects of lensing, including data searches, statistics, lens reconstruction, and applications of GW lensing.
[1] Search for gravitational lensing signatures in LIGO-Virgo binary black hole events
[2] Follow-up Analyses to the O3 LIGO-Virgo-KAGRA Lensing Searches
[3] Gravelamps: Gravitational Wave Lensing Mass Profile Model Selection
[4] GOLUM: A fast and precise methodology to search for, and analyze, strongly lensed gravitational-wave events
[5] hemantaph/ler: Gravitational waves lensing rate calculator
[6] Finding Black Holes: an Unconventional Multi-messenger
[7] Localizing merging black holes with sub-arcsecond precision using gravitational-wave lensing
[8] On the detection and precise localisation of merging black holes events through strong gravitational lensing
Tests of GR & Modified Gravity
Ania Liu, Samson Leong, Thomas Ng, Paul Martens
While general relativity (GR) has certainly proven to be an incredibly successful theory for gravity so far, there are good reasons to expect it to break down at some scales: either in the low energy regime, or at very high energies and large curvatures. The binary systems the LVK collaboration observes are precisely evolving in this latter regime. Therefore, the gravitational wave signals coming from them represent a fantastic testbed to constrain deviations from GR, and/or test alternative descriptions of gravity. In particular, Thomas had looked into the constraints GWTC-3 can provide on amplitude birefringence [1].
[1] Constraining gravitational wave amplitude birefringence with GWTC-3
Numerical Relativity
Anson K. L. Yip, Liiyung Yeow
Aside from observations and data analysis, we also take advantage of numerical relativity simulations to solve and analyze problems in relativity. Numerical relativity (NR) is a numerical method developed within the framework of Einstein’s general relativity. Since conducting controlled experiments on astrophysical objects is not feasible, NR is employed to investigate high-energy astrophysical phenomena of interest. Various formulations of Einstein’s field equations and matter field equations are used to model the dynamics of compact objects and the spacetime surrounding them. Typical applications of NR include simulating supernovae, neutron stars, black holes, and the corresponding gravitational waves.
Anson and Liiyung are members of the Gmunu collaboration, which involves participants from Hong Kong, Europe, and the United States. Our collaboration has developed a novel numerical infrastructure for generic astrophysical simulations called Gmunu (General-relativistic multigrid numerical solver) [1 – 4]. Gmunu is an open-source code that is parallelized, block-grid adaptive, and capable of multi-dimensional general-relativistic resistive radiation magnetohydrodynamics (GRRRMHD) simulations in curvilinear geometries within dynamical spacetimes. The Einstein equations in Gmunu are solved using a multigrid method within the constrained-evolution formulation under the conformally flat condition (CFC) approximation. Gmunu is the first and currently the only code that employs a multigrid metric solver in dynamical simulations. Furthermore, Gmunu incorporates a state-of-the-art neutrino microphysics library called Weakhub [5]. This library encompasses advanced techniques for multi-species, multi-energies neutrino transport, coupled with novel neutrino microphysics. By integrating these capabilities, Gmunu stands at the forefront of simulating various astrophysical scenarios.
Presently, Anson has used Gmunu to investigate the dynamics and oscillations of strongly magnetized neutron stars [6 – 8] as well as differentially rotating neutron stars [9]. Meanwhile, Liiyung is in the process of implementing higher-order Riemann solvers for neutron star and core-collapse simulations into Gmunu.
[1] Gmunu: toward multigrid based Einstein field equations solver for general-relativistic hydrodynamics simulations
[2] Gmunu: paralleled, grid-adaptive, general-relativistic magnetohydrodynamics in curvilinear geometries in dynamical space-times
[3] An Extension of Gmunu: General-relativistic Resistive Magnetohydrodynamics Based on Staggered-meshed Constrained Transport with Elliptic Cleaning
[4] General-relativistic Radiation Transport Scheme in Gmunu. I. Implementation of Two-moment-based Multifrequency Radiative Transfer and Code Tests
[5] General-relativistic radiation transport scheme in Gmunu II: Implementation of novel microphysical library for neutrino radiation — Weakhub
[6] Oscillations of highly magnetized non-rotating neutron stars
[7] General-relativistic simulations of the formation of a magnetized hybrid star
[8] Gravitational wave signatures from the phase-transition-induced collapse of a magnetized neutron star
[9] Universal relations for fundamental modes of rotating neutron stars with differential rotations
Cosmology & Black Hole Population
Thomas Ng, Paul Martens
Similarly to light sources, gravitational waves can be also be exploited to infer the properties of the underlying black hole population, and, in fine, characterize the cosmological model of our Universe. In particular, Thomas has been looking into using non-parametric methods to infer Hubble constant with population information. Other works include investigations into the Universe’s anisotropy.
And more…
Naturally, all the above topics are broad research directions undertaken by the group member, and our interests are broader and evolve with time. The group remains fully open to new perspectives and ideas to better understand the Universe we all live in!