Critical behavior in 3D gravitational collapse of massless scalar fields

We present results from a study of critical behavior in 3D gravitational collapse with no symmetry assumptions. The source of the gravitational field is a massless scalar field. This is a well-studied case for spherically symmetric gravitational collapse, allowing us to understand the reliability and accuracy of the simulations. We study both supercritical and subcritical evolutions to see if one provides more accurate results than the other. We find that even for nonspherical initial data with 35% of the power in the ℓ=2 spherical harmonic, the critical solution is the same as in spherical symmetry

Collective filters: a new approach to analyze the gravitational-wave ringdown of binary black-hole mergers

We propose two frequency-domain filters to analyze ringdown signals of binary black hole mergers. The first rational filter is constructed based on a set of (arbitrary) quasi-normal modes (QNMs) of the remnant black holes, whereas the second full filter comes from the transmissivity of the remnant black holes. The two filters can remove corresponding QNMs from original time-domain ringdowns, while changing early inspiral signals in a trivial way - merely a time and phase shift. After filtering out dominant QNMs, we can visualize the existence of various subdominant effects. For example, by applying our filters to a GW150914-like numerical relativity (NR) waveform, we find second-order effects in the (l = 4, m = 4), (l = 5, m = 4) and (l = 5, m = 5) harmonics; the spherical-spheroidal mixing mode in the (l = 2,m = 2) harmonic; and a mixing mode in the (l = 2,m = 1) harmonic due to a gravitational recoil. In another NR simulation where two component spins are anti-aligned with the orbital angular momentum, we also find retrograde modes. Additionally, we propose to use the rational filter to estimate the start time of a QNM. The filters are sensitive to the remnant properties (i.e., mass and spin) and thus have a potential application to future data analyses and parameter estimations. We also investigate the stability of the full filter. Its connection to the instability of QNM spectra is discussed

Simulating neutron stars with a flexible enthalpy-based equation of state parametrization in SpECTRE

Numerical simulations of neutron star mergers represent an essential step toward interpreting the full complexity of multimessenger observations and constraining the properties of supranuclear matter. Currently, simulations are limited by an array of factors, including computational performance and input physics uncertainties, such as the neutron star equation of state. In this work, we expand the range of nuclear phenomenology efficiently available to simulations by introducing a new analytic parametrization of cold, beta-equilibrated matter that is based on the relativistic enthalpy. We show that the new $\textit{enthalpy parametrization}$ can capture a range of nuclear behavior, including strong phase transitions. We implement the enthalpy parametrization in the $\texttt{SpECTRE}$, code, simulate isolated neutron stars, and compare performance to the commonly used spectral and polytropic parametrizations. We find comparable computational performance for nuclear models that are well represented by either parametrization, such as simple hadronic EoSs. We show that the enthalpy parametrization further allows us to simulate more complicated hadronic models or models with phase transitions that are inaccessible to current parametrizations.Comment: 20 pages, 14 figures, submitted to PRD, additional information on software including input files available at https://github.com/sxs-collaboration/paper-2023-spectre-enthalpy-eo

Characterizing the Directionality of Gravitational Wave Emission from Matter Motions within Core-collapse Supernovae

We analyze the directional dependence of the gravitational wave (GW) emission from 15 3D neutrino radiation hydrodynamic simulations of core-collapse supernovae. We develop a new analytic technique to characterize the distribution of GW emission over all angles. We use physics-informed toy models to provide closed form expressions for the distribution of GW emission for different CCSN phases. Using these toy models, we approximate the PNS dynamics during multiple CCSN stages and obtain similar GW distributions to simulation outputs. By applying this new technique throughout the supernova duration, we construct a distribution of preferred directions of GW emission. Our findings indicate CCSNe do not have a single `optimal' viewing angle along which the strongest GWs can be detected. For nonrotating cases, this dominant viewing angle drifts isotropically throughout the supernova, set by the dynamical timescale of the protoneutron star. For rotating cases, during core bounce and the following tens of ms, the strongest GW signal is observed along the equator. During the accretion phase, comparable -- if not stronger -- GW amplitudes are generated along the axis of rotation, which can be enhanced by the low T/|W| instability. We show two dominant factors influencing the directionality of GW emission are the degree of initial rotation and explosion morphology. Lastly, looking forward, we note the sensitive interplay between GW detector site and supernova orientation, along with its effect on detecting individual polarization modes.Comment: 32 pages, 17 Figures, submitted to Ap

High-accuracy numerical models of Brownian thermal noise in thin mirror coatings

Brownian coating thermal noise in detector test masses is limiting the sensitivity of current gravitational-wave detectors on Earth. Therefore, accurate numerical models can inform the ongoing effort to minimize Brownian coating thermal noise in current and future gravitational-wave detectors. Such numerical models typically require significant computational resources and time, and often involve closed-source commercial codes. In contrast, open-source codes give complete visibility and control of the simulated physics and enable direct assessment of the numerical accuracy. In this article, we use the open-source SpECTRE numerical-relativity code and adopt a novel discontinuous Galerkin numerical method to model Brownian coating thermal noise. We demonstrate that SpECTRE achieves significantly higher accuracy than a previous approach at a fraction of the computational cost. Furthermore, we numerically model Brownian coating thermal noise in multiple sub-wavelength crystalline coating layers for the first time. Our new numerical method has the potential to enable fast exploration of realistic mirror configurations, and hence to guide the search for optimal mirror geometries, beam shapes and coating materials for gravitational-wave detectors.Comment: 9 pages, 5 figures. Results are reproducible with the ancillary input file