arXiv subjects

Hwei-Jang Yo

Publications and source records attributed to Hwei-Jang Yo.

16 recordsLinked to original sources

arXiv2025

Denoising gravitational wave with deep learning in the time-frequency domain

Gravitational wave denoising is an ongoing task for revealing the events of compact binary objects in the universe. Recently, with the aid of deep learning, gravitational waves have been efficiently and delicately extracted from the noisy data compared with the traditional match-filtering. While most of the relevant studies adopt the data in the time series only, the time-frequency data processing is also in progress due to its several advantages for the waveform denoising. Here, we target the gravitational waves events emitted by binary black hole (BBH) mergers, with their total mass larger than 30 solar masses. For denoising, we propose a deep learning model utilizing the Griffin-Lim algorithm, an existing numerical approach to restore the phase information from the related amplitude spectrogram. This design allows extra attention on the phase recovery by using a priorly denoised amplitude spectrogram. The denoising results fit well in both the amplitude and the phase alignments of the mock injected waveforms. We also apply our model to the real detected events and discover a nice consistency with the simulated template waveforms, especially the high accuracy around the merger stage. Our work suggests the possibility of a better methodological design for gravitational wave data analysis.

gr-qc↗

arXiv2019

Accuracy of source localization for eccentric inspiraling binary mergers using a ground-based detector network

The problem of gravitational wave parameter estimation and source localization is crucial in gravitational wave astronomy. Gravitational waves emitted by compact binary coalescences in the sensitivity band of second-generation ground-based detectors could have non-negligible eccentricities. Thus it is an interesting topic to study how the eccentricity of a binary source affects and improves the accuracy of its localization (and the signal-to-noise ratio). In this work we continue to investigate this effect with the enhanced postcircular waveform model. Using the Fisher information matrix method, we determine the accuracy of source localization with three ground-based detector networks. As expected, the accuracy of source localization is improved considerably with more detectors in a network. We find that the accuracy also increases significantly by increasing the eccentricity for the large total mass ($M\ge 40M_\odot$) binaries with all three networks. For the small total mass ($M<40 M_\odot$) binaries, this effect is negligible. For the smaller total mass ($M<5 M_\odot$) binaries, the accuracy could be even worse at some orientations with increasing eccentricity. This phenomenon comes mainly from how well the frequency of the higher harmonic modes induced by increasing eccentricity coincides with the sensitive bandwidth of the detectors. For the case of the $100 M_\odot$ black hole binary, the improvement factor is about $2$ in general when the eccentricity grows from $0.0$ to $0.4$. For the cases of the $22 M_\odot$ black hole binary and the $2.74 M_\odot$ neutron star binary, the improvement factor is less than $1.1$, and it may be less than 1 at some orientations.

gr-qc↗

arXiv2017

Gravitational wave source localization for eccentric binary coalesce with a ground-based detector network

Gravitational wave source localization problem is important in gravitational wave astronomy. Regarding ground-based detector, almost all of the previous investigations only considered the difference of arrival time among the detector network for source localization. Within the matched filtering framework, the information beside the arrival time difference can possibly also do some help on source localization. Especially when an eccentric binary is considered, the character involved in the gravitational waveform may improve the source localization. We investigate this effect systematically in the current paper. During the investigation, the enhanced post-circular (EPC) waveform model is used to describe the eccentric binary coalesce. We find that the source localization accuracy does increase along with the eccentricity increases. But such improvement depends on the total mass of the binary. For total mass 100M${}_\odot$ binary, the source localization accuracy may be improved about 2 times in general when the eccentricity increases from 0 to 0.4. For total mass 65M${}_\odot$ binary (GW150914-like binary), the improvement factor is about 1.3 when the eccentricity increases from 0 to 0.4. For total mass 22M${}_\odot$ binary (GW151226-like binary), such improvement is ignorable.

gr-qc↗

arXiv2015

General Poincaré Gauge Theory Cosmology

For the quadratic Poincaré gauge theory of gravity (PG) we consider the FLRW cosmologies using an isotropic Bianchi representation. Here the considered cosmologies are for the general case: all the even and odd parity terms of the quadratic PG with their respective scalar and pseudoscalar parameters are allowed with no \emph{a priori} restrictions on their values. With the aid of a manifestly homogeneous and isotropic representation, an effective Lagrangian gives the second order dynamical equations for the gauge potentials. An equivalent set of first order equations for the observables is presented. The generic behavior of physical solutions is discussed and illustrated using numerical simulations.

gr-qc↗

arXiv2015

Pseudotensor applied to Numerical Relativity in Calculating Global Quantities

In this work we apply the Landau-Lifshitz pseudotensor flux formalism to the calculation of the total mass and the total angular momentum during the evolution of a binary black hole system. We also compare its performance with the traditional integrations for the global quantities. It shows that the advantage of the pseudotensor flux formalism is the smoothness of the numerical value of the global quantities, especially of the total angular momentum. Although the convergence behavior of the global quantities with the pseudotensor flux method is only comparable with the ones with the traditional method, the smoothness of its numerical value allows using a larger radius for surface integration to obtain more accurate result.

gr-qc↗

arXiv2015

Improved fast-rotating black hole evolution simulations with modified Baumgarte-Shapiro-Shibata-Nakamura formulation

Different formulations of Einstein's equations used in numerical relativity can affect not only the stability but also the accuracy of numerical simulations. In the original Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formulation, the loss of the angular momentum, $J$, is non-negligible in highly spinning single black hole evolutions. This loss also appears, usually right after the merger, in highly spinning binary black hole simulations, The loss of $J$ may be attributed to some unclear numerical dissipation. Reducing unphysical dissipation is expected to result in more stable and accurate evolutions. In the previous work \cite{yhlc12} we proposed several modifications which are able to prevent black hole evolutions from the unphysical dissipation, and the resulting simulations are more stable than in the traditional BSSN formulation. Specifically, these three modifications (M1, M2, and M3) enhance the effects of stability, hyperbolicity, and dissipation of the formulation. We experiment further in this work with these modifications, and demonstrate that these modifications improve the accuracy and also effectively suppress the loss of $J$, particularly in the black hole simulations with the initially large ratio of $J$ and the square of the ADM mass.

gr-qc↗

arXiv2012

Modifications for numerical stability of black hole evolution

We experiment with several new modifications for the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formulation of the Einstein field equations and demonstrate how these modifications affect the stability of numerical black hole evolution calculations. With these modifications, we evolve both single non-rotating excised Kerr-Schild black holes and punctured binary black holes, and obtain accurate and stable simulations.

gr-qc↗

arXiv2012

Perturbational Treatment of the Gravitational Potential Effect on Binary Black Hole Evolution

Binary black hole (BBH) systems are usually located in the gravitational potential well formed by a massive black hole (BH), which is mostly located in the center of a galaxy. In most existing studies, the BBH systems are treated as isolated systems, while the effect of the background is ignored. The validity of the approximation is based on the belief that the background gravitational field from other sources is extremely weak compared with the strong gravitational field produced by the BBH itself during the evolution, and can be neglected in gravitational wave detection. However, it is still interesting to check how valid this approximation is. In this work, instead of simulating the three-BH problem with a fully relativistic treatment, we use a perturbational scheme to investigate the effect of the background gravitational potential on the evolution of a BBH, especially on the waveform of its gravitational radiation. Four scenarios are considered including the head-on collision and the inspiral-to-merger process of a BBH which is either freefalling towards or circularly orbiting around a third large BH. The head-on collision and the circular inspiral are two limits of all possible configurations. The existence of the background gravitational potential changes the arrival time of the gravitational wavefront of a BH, prolongs the wavelength, and increases the gravitational radiation energy. And most interestingly, the background gravitational potential induces the higher-order modes of the gravitational wave of a BBH. These interesting phenomena can be explained by the gravitational redshift effect and the change of eccentricity of a BBH's orbit from the background gravitational potential.

gr-qc↗

arXiv2009

Cosmological dynamics with propagating Lorentz connection modes of spin zero

The Poincare gauge theory of gravity has a Lorentz connection with both torsion and curvature. For this theory two good propagating connection modes, carrying spin-$0^+$ and spin-$0^-$, have been found. The possible effects of the spin-$0^+$ mode in cosmology were investigated in a previous work by our group; there it was found that the $0^+$ mode could account for the presently accelerating universe. Here, we extend the analysis to also include the spin-$0^-$ mode. The resulting cosmological model has three degrees of freedom. We present both the Lagrangian and Hamiltonian form of the dynamic equations for this model, find the late-time normal modes, and present some numerical evolution cases. In the late time asymptotic regime the two dynamic modes decouple, and the acceleration of the Universe oscillates due to the spin-$0^+$ mode.

gr-qc↗

arXiv2008

A Reinvestigation of Moving Punctured Black Holes with a New Code

We report on our code, in which the moving puncture method is applied and an adaptive/fixed mesh refinement is implemented, and on its preliminary performance on black hole simulations. Based on the BSSN formulation, up-to-date gauge conditions and the modifications of the formulation are also implemented and tested. In this work we present our primary results about the simulation of a single static black hole, of a moving single black hole, and of the head-on collision of a binary black hole system. For the static punctured black hole simulations, different modifications of the BSSN formulation are applied. It is demonstrated that both the currently used sets of modifications lead to a stable evolution. For cases of a moving punctured black hole with or without spin, we search for viable gauge conditions and study the effect of spin on the black hole evolution. Our results confirm previous results obtained by other research groups. In addition, we find a new gauge condition, which has not yet been adopted by any other researchers, which can also give stable and accurate black hole evolution calculations. We examine the performance of the code for the head-on collision of a binary black hole system, and the agreement of the gravitational waveform it produces with that obtained in other works. In order to understand qualitatively the influence of matter on the binary black hole collisions, we also investigate the same head-on collision scenarios but perturbed by a scalar field. The numerical simulations performed with this code not only give stable and accurate results that are consistent with the works by other numerical relativity groups, but also lead to the discovery of a new viable gauge condition, as well as clarify some ambiguities in the modification of the BSSN formulation.

gr-qc↗

arXiv2008

Torsion Cosmology and the Accelerating Universe

Investigations of the dynamic modes of the Poincare gauge theory of gravity found only two good propagating torsion modes; they are effectively a scalar and a pseudoscalar. Cosmology affords a natural situation where one might see observational effects of these modes. Here we consider only the ``scalar torsion'' mode. This mode has certain distinctive and interesting qualities. In particular this type of torsion does not interact directly with any known matter and it allows a critical non-zero value for the affine scalar curvature. Via numerical evolution of the coupled nonlinear equations we show that this mode can contribute an oscillating aspect to the expansion rate of the Universe. From the examination of specific cases of the parameters and initial conditions we show that for suitable ranges of the parameters the dynamic ``scalar torsion'' model can display features similar to those of the presently observed accelerating universe.

gr-qc↗

arXiv2006

Dynamic Scalar Torsion and an Oscillating Universe

For the Poincare gauge theory of gravity we consider the dynamical scalar torsion mode in a cosmological context. We explore in particular the possibility of using dynamical torsion to explain the current state of the accelerating Universe. With certain suitable sets of chosen parameters, this model can give a (qualitatively) proper description of the current universe without a cosmological constant, and the universe described is oscillating with a period of the Hubble time.

astro-ph↗

arXiv2004

Quasi-equilibrium Binary Black Hole Initial Data for Dynamical Evolutions

We present a formalism for constructing quasi-equilibrium binary black hole initial data suitable for numerical evolution. We construct quasi-equilibrium models by imposing an approximate helical Killing symmetry appropriate for quasi-circular orbits. We use the sum of two Kerr-Schild metrics as our background metric, thereby improving on conformally flat backgrounds that do not accommodate rotating black holes and providing a horizon-penetrating lapse, convenient for implementing black hole excision. We set inner boundary conditions at an excision radius well inside the apparent horizon and construct these boundary conditions to incorporate the quasi-equilibrium condition and recover the solution for isolated black holes in the limit of large separation. We use our formalism both to generate initial data for binary black hole evolutions and to construct a crude quasi-equilibrium, inspiral sequence for binary black holes of fixed irreducible mass.

gr-qc↗

arXiv2004

Relativistic Hydrodynamic Evolutions with Black Hole Excision

We present a numerical code designed to study astrophysical phenomena involving dynamical spacetimes containing black holes in the presence of relativistic hydrodynamic matter. We present evolutions of the collapse of a fluid star from the onset of collapse to the settling of the resulting black hole to a final stationary state. In order to evolve stably after the black hole forms, we excise a region inside the hole before a singularity is encountered. This excision region is introduced after the appearance of an apparent horizon, but while a significant amount of matter remains outside the hole. We test our code by evolving accurately a vacuum Schwarzschild black hole, a relativistic Bondi accretion flow onto a black hole, Oppenheimer-Snyder dust collapse, and the collapse of nonrotating and rotating stars. These systems are tracked reliably for hundreds of M following excision, where M is the mass of the black hole. We perform these tests both in axisymmetry and in full 3+1 dimensions. We then apply our code to study the effect of the stellar spin parameter J/M^2 on the final outcome of gravitational collapse of rapidly rotating n = 1 polytropes. We find that a black hole forms only if J/M^2<1, in agreement with previous simulations. When J/M^2>1, the collapsing star forms a torus which fragments into nonaxisymmetric clumps, capable of generating appreciable ``splash'' gravitational radiation.

gr-qc↗

arXiv2003

Improved numerical stability of stationary black hole evolution calculations

We experiment with modifications of the BSSN form of the Einstein field equations (a reformulation of the ADM equations) and demonstrate how these modifications affect the stability of numerical black hole evolution calculations. We use excision to evolve both non-rotating and rotating Kerr-Schild black holes in octant and equatorial symmetry, and without any symmetry assumptions, and obtain accurate and stable simulations for specific angular momenta J/M of up to about 0.9M.

gr-qc↗

arXiv2000

Gravitational Wavetrains in the Quasi-Equilibrium Approximation: A Model Problem in Scalar Gravitation

A quasi-equilibrium (QE) computational scheme was recently developed in general relativity to calculate the complete gravitational wavetrain emitted during the inspiral phase of compact binaries. The QE method exploits the fact that the the gravitational radiation inspiral timescale is much longer than the orbital period everywhere outside the ISCO. Here we demonstrate the validity and advantages of the QE scheme by solving a model problem in relativistic scalar gravitation theory. By adopting scalar gravitation, we are able to numerically track without approximation the damping of a simple, quasi-periodic radiating system (an oscillating spherical matter shell) to final equilibrium, and then use the exact numerical results to calibrate the QE approximation method. In particular, we calculate the emitted gravitational wavetrain three different ways: by integrating the exact coupled dynamical field and matter equations, by using the scalar-wave monopole approximation formula (corresponding to the quadrupole formula in general relativity), and by adopting the QE scheme. We find that the monopole formula works well for weak field cases, but fails when the fields become even moderately strong. By contrast, the QE scheme remains quite reliable for moderately strong fields, and begins to breakdown only for ultra-strong fields. The QE scheme thus provides a promising technique to construct the complete wavetrain from binary inspiral outside the ISCO, where the gravitational fields are strong, but where the computational resources required to follow the system for more than a few orbits by direct numerical integration of the exact equations are prohibitive.

gr-qc↗