arXiv subjects

Justin L. Ripley

Publications and source records attributed to Justin L. Ripley.

At least 19 recordsLinked to original sources

arXiv2025

Dissipative tidal effects to next-to-leading order and constraints on the dissipative tidal deformability using gravitational wave data

Dissipative tidal interactions can be used to probe the out-of-equilibrium physics of neutron stars using gravitational wave observations. In this paper, we present the first post-Newtonian (PN) corrections to the orbital dynamics of a binary system containing objects whose tidal interactions have a dissipative contribution. We derive the 1PN-accurate equations of motion in the center-of-mass frame and a generalized energy-balance law that is valid for dissipative tidal interactions. We show how mass and energy loss due to the absorption of orbital energy change the orbital dynamics and derive the next-to-leading order correction to the gravitational wave phase of a binary system in a quasi-circular orbit containing initially non-spinning components. We then use this waveform model to constrain, for the first time, the individual dissipative tidal deformabilities of each of the binary components that generated the GW170817 event using real data. We find that the GW170817 data requires $Ξ_{1} \lesssim 1121$ and $Ξ_{2} \lesssim 1692$ at 90\% confidence, where $Ξ_{1,2}$ are the individual tidal deformabilities of the primary and secondary binary components that produced the GW170817 event.

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arXiv2025

Probing internal dissipative processes of neutron stars with gravitational waves during the inspiral of neutron star binaries

We study the impact of out-of-equilibrium, dissipative effects on the dynamics of inspiraling neutron stars. We find that modeling dissipative processes (such as those from the stars internal effective fluid viscosity) requires that one introduce a new tidal deformability parameter--the dissipative tidal deformability--which modifies the phase of gravitational waves emitted during the inspiral phase of a neutron star binary. We show that the dissipative tidal deformability corrects the gravitational-wave phase at 4 post-Newtonian order for quasi-circular binaries. This correction receives a large finite-size enhancement by the stellar compactness, analogous to the case of the tidal deformability. Moreover, the correction is not degenerate with the time of coalescence, which also enters at 4PN order, because it contains a logarithmic frequency-dependent contribution. Using a simple Fisher analysis, we show that physically allowed values for the dissipative tidal deformability may be constrained by measurements of the phase of emitted gravitational waves to roughly the same extent as the (electric-type, quadrupolar) tidal deformability. Finally, we show that there are no out-of-equilibrium, dissipative corrections to the tidal deformability itself. We conclude that there are at least two relevant tidal deformability parameters that can be constrained with gravitational-wave phase measurements during the late inspiral of a neutron star binary: one which characterizes the adiabatic tidal response of the star, and another which characterizes the leading-order out-of-equilibrium, dissipative tidal response. These findings open a window to probe dissipative processes in the interior of neutron stars with gravitational waves.

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arXiv2024

Radial Mode Stability of Two-Fluid Neutron Stars

Radial mode stability is a necessary condition for the astrophysical viability of compact objects. In recent years, astrophysical models with two fluids have gain popularity, especially in their ability to model dark matter admixed neutron stars. Just as is the case of single-fluid stars, a stability criterion based on the background equations has been developed -- the critical curve for the particle numbers of the two fluids in the two-dimensional configuration space determines a one-dimensional sequence that labels the marginally stable configurations -- but its validity depends on the linear stability of radial perturbations which remains unstudied. In this paper, we establish a set of stability criteria for two perfect-fluid relativistic stars by carefully studying the radial mode perturbation equations. We prove that modes are complete, have real eigenvalues with a minimum eigenvalue (i.e. a fundamental mode), thus a configuration is stable if and only if the fundamental mode is positive. As a consequence, our work formally and rigorously proves these necessary conditions for the stability criterion based on the background equations.

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arXiv2024

Nonlinear effect of absorption on the ringdown of a spinning black hole

The ringdown gravitational wave signal arising e.g., in the final stage of a black hole binary merger, contains important information about the properties of the remnant, and can potentially be used to perform clean tests of general relativity. However, interpreting the ringdown signal, in particular when it is the loudest, requires understanding the role of nonlinearities and their potential impact on modelling this phase using quasinormal modes. Here, we focus on a particular nonlinear effect arising from the change in the black hole's mass and spin due to the partial absorption of a quasinormal perturbation. We isolate and systematically study this third-order, secular effect by evolving the equations governing linear metric perturbations on the background of a spinning black hole, but allowing the properties of the background to evolve in a prescribed way. We find that this leads to the excitation of quasinormal modes with higher polar angular number, retrograde modes (counter-rotating with respect to the black hole), and overtones, as well as giving rise to a component of the signal at early times that cannot be fully described using quasinormal modes. Quantifying these effects, we find that they may be relevant in analyzing the ringdown in black hole mergers.

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arXiv2024

First constraint on the dissipative tidal deformability of neutron stars

The gravitational waves (GWs) emitted by neutron star binaries probe the physics of matter at supra nuclear densities. During the late inspiral, tidal deformations raised on each star by the gravitational field of its companion depend crucially on the star's internal properties. The misalignment of a star's tidal bulge with its companion's gravitational field encodes the strength of internal dissipative processes, which imprint onto the phase of the gravitational waves emitted. We here analyze GW data from the GW170817 (binary neutron star) event detected by LIGO and Virgo and find the first constraint on the dissipative tidal deformability of a neutron star. From this constraint, \emph{assuming} a temperature profile for each star in the binary, we obtain an order of magnitude bound on the averaged bulk ($ζ$) and shear ($η$) viscosity of each star during the inspiral.: $ζ\lesssim 10^{31} \mathrm{g}\;\mathrm{cm}^{-1}\mathrm{s}^{-1}$ and $η\lesssim 10^{28} \mathrm{g}\;\mathrm{cm}^{-1}\mathrm{s}^{-1} $. We forecast that these bounds could be improved by two orders of magnitude with third-generation detectors, like Cosmic Explorer, using inspiral data. These constraints already inform nuclear physics models and motivate further theoretical work to better understand the interplay between viscosity and temperature in the late inspiral of neutron stars.

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arXiv2024

Spin dependence of black hole ringdown nonlinearities

The nonlinear character of general relativity leaves its imprint in the coalescence of two black holes, from the inspiral to the final ringdown stage. To quantify the impact of nonlinearities, we work at second order in black hole perturbation theory and we study the excitation of second-order modes relative to the first-order modes that drive them as we vary the black hole spin and the initial data for the perturbations. The relative amplitude of second-order modes is only mildly dependent on the initial data that we consider, but it strongly decreases for large black hole spins. This implies that the extrapolation of calculations based on the Kerr-CFT correspondence to subextremal Kerr black holes should be viewed with caution.

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arXiv2024

Dynamical tidal response of non-rotating relativistic stars

Accurately modeling the tidal response of neutron stars is crucial to connecting gravitational wave observations of binaries to ultra-dense nuclear physics. Most current models of the tidal response of relativistic stars either assume a static response model, or use phenomenological models inspired by Newtonian gravity. In this work, we present a general formalism for computing the linear dynamical tidal response function of relativistic, spherically symmetric stars. Our formalism incorporates stratification due to thermal and chemical imbalances, allowing one to study the effects of g modes on the tidal response function. We also describe how to incorporate sources of dissipation due to shear and bulk viscosity. To showcase the utility of our approach, we present several applications for polytropic stars in general relativity. We show how our formalism can capture the dynamical tidal resonance due to the f and g modes of inviscid stars and explore the sensitivity of the dynamical tidal response to the compactness of the star. We also compute the dissipative tidal deformability due to bulk and shear viscous dissipation assuming a simple viscous profile for the bulk and shear viscosity.

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arXiv2024

Challenges in Quasinormal Mode Extraction: Perspectives from Numerical solutions to the Teukolsky Equation

The intricacies of black hole ringdown analysis are amplified by the absence of a complete set of orthogonal basis functions for quasinormal modes. Although damped sinusoids effectively fit the ringdown signals from binary black hole mergers, the risk of overfitting remains, due to initial transients and nonlinear effects. In light of this challenge, we introduce two methods for extracting quasinormal modes in numerical simulations and qualitatively study how the transient might affect quasinormal mode fitting. In one method, we accurately fit quasinormal modes by using their spatial functional form at constant time hypersurfaces, while in the other method, we exploit both spatial and temporal aspects of the quasinormal modes. Both fitting methods leverage the spatial behavior of quasinormal eigenfunctions to enhance accuracy, outperforming conventional time-only fitting techniques at null infinity. We also show that we can construct an inner product for which the quasinormal eigenfunctions form an orthonormal (but not complete) set. We then conduct numerical experiments involving linearly perturbed Kerr black holes in horizon penetrating, hyperboloidally compactified coordinates, as this setup enables a more precise isolation and examination of the ringdown phenomenon. From solutions to the Teukolsky equation, describing scattering of an ingoing gravitational wave pulse, we find that the contributions from early-time transients can lead to large uncertainties in the fit to the amplitudes of higher overtones ($n\geq 3$). While the methods we discuss here cannot be applied directly to data from merger observations, our findings underscore the persistence of ambiguities in interpreting ringdown signals, even with access to both temporal and spatial information.

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arXiv2024

Nonlinear Effects In Black Hole Ringdown From Scattering Experiments I: spin and initial data dependence of quadratic mode coupling

We investigate quadratic quasinormal mode coupling in black hole spacetime through numerical simulations of single perturbed black holes using both numerical relativity and second-order black hole perturbation theory. Focusing on the dominant $\ell=|m|=2$ quadrupolar modes, we find good agreement (within $\sim10\%$) between these approaches, with discrepancies attributed to truncation error and uncertainties from mode fitting. Our results align with earlier studies extracting the coupling coefficients from select binary black hole merger simulations, showing consistency for the same remnant spins. Notably, the coupling coefficient is insensitive to a diverse range of initial data, including configurations that led to a significant (up to $5\%$) increase in the remnant black hole mass. These findings present opportunities for testing the nonlinear dynamics of general relativity with ground-based gravitational wave observatories. Lastly, we provide evidence of a bifurcation in coupling coefficients between counter-rotating and co-rotating quasinormal modes as black hole spin increases.

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arXiv2023

The non-relativistic limit of first-order relativistic viscous fluids

Out-of-equilibrium effects may play an important role in the dynamics of neutron star mergers and in heavy-ion collisions. Bemfica, Disconzi, Noronha and Kovtun (BDNK) recently derived a causal, locally well-posed, and modally stable relativistic fluid model that incorporates the effects of viscosity and heat diffusion. We study the non-relativistic limit of this fluid model and show that causality for relativistic motion restricts the transport coefficients in the non-relativistic limit. This restriction provides an upper bound on the ratio of the shear viscosity to the entropy density for fluids that can be described as relativistic within the BDNK model and can be exactly modeled using the Navier-Stokes equation in the non-relativistic limit. Furthermore, we show that the Fourier law of heat conduction must be modified by higher gradient corrections for such fluids. We also show that the non-relativistic limit of the BDNK equations of motion are never hyperbolic, in contrast to the non-relativistic limit of extended variable models, whose non-relativistic equations of motion can be hyperbolic or not depending on the scaling of the transport coefficients present in the auxiliary equations for viscous degrees of freedom.

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arXiv2023

Measuring the ringdown scalar polarization of gravitational waves in Einstein scalar Gauss-Bonnet gravity

We model the scalar waves produced during the ringdown stage of binary black hole coalescence in Einstein scalar Gauss-Bonnet (EsGB) gravity, using numerical relativity simulations of the theory in the decoupling limit. Through a conformal coupling of the scalar field to the metric in the matter-field action, we show that the gravitational waves in this theory can have a scalar polarization. We model the scalar quasi-normal modes of the ringdown signal in EsGB gravity, and quantify the extent to which current and future gravitational wave detectors could observe the spectrum of scalar radiation emitted during the ringdown phase of binary black hole coalescence. We find that within the limits of the theory's coupling parameters set by current theoretical and observational constraints, the scalar ringdown signal from black hole remnants in the $10^1 - 10^3 \, M_{\odot}$ mass range is expected to be well below the detectability threshold with the current network of gravitational-wave detectors (LIGO-Virgo-KAGRA), but is potentially measurable with next-generation detectors such as the Einstein Telescope.

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arXiv2023

Where and why does Einstein-Scalar-Gauss-Bonnet theory break down?

We present a systematic exploration of the loss of predictivity in Einstein-scalar-Gauss-Bonnet (ESGB) gravity. We first formulate a gauge covariant method of characterizing the breakdown of the hyperbolicity of the equations of motion in the theory. With this formalism, we show that strong geodesic focusing leads to the breakdown of hyperbolicity, and the latter is unrelated to the violation of the null convergence condition. We then numerically study the hyperbolicity of the equations during gravitational collapse for two specific ESGB gravity theories: "shift symmetric Gauss-Bonnet gravity" and a version of the theory that admits "spontaneously scalarized" black holes. We devise a "phase space" model to describe the end states for a given class of initial data. Using our phase space picture, we demonstrate that the two theories we consider remain predictive (hyperbolic) for a range of GB couplings. The range of couplings, however, is small, and thus, the presence of "spontaneously scalarized" solutions requires fine-tuning of initial data. Our results, therefore, cast doubt as to whether scalarized black hole solutions can be realistically realized in Nature even if ESGB gravity happened to be the correct gravitational description.

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arXiv2023

Nonlinear studies of binary black hole mergers in Einstein-scalar-Gauss-Bonnet gravity

We study the nonlinear dynamics of binary black hole systems with scalar charge by numerically evolving the full equations of motion for shift-symmetric Einstein scalar Gauss-Bonnet gravity. We consider quasi-circular binaries with different mass-ratios, varying the Gauss-Bonnet coupling and quantifying its impact on the emitted scalar and gravitational waves. We compare our numerical results to post-Newtonian calculations of the radiation emitted during the inspiral. We demonstrate the accuracy of the leading-order terms in post-Newtonian theory in modeling the amplitude of the scalar waveform, but find that, at least for the last few orbits before merger, the currently available post-Newtonian theory is not sufficient to model the dephasing of the gravitational wave signal in this theory. We further find that there is non-negligible nonlinear enhancement in the scalar field at merger, but that the effect on the peak gravitational wave emission is small.

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arXiv2022

Dynamics of a nonminimally coupled scalar field in asymptotically $AdS_4$ spacetime

We numerically investigate the stability of four-dimensional asymptotically anti-de Sitter ($AdS_4$) spacetime for a class of nonminimally coupled scalar field theories. In particular, we study how the coupling affects the formation of black holes and transfer of energy to different spatial/temporal scales. We conclude by detailing the well-known analogy between the nonminimally coupled scalar-field stress-energy tensor and that of a viscous relativistic fluid, and discuss the limitations of that analogy when it is applied to anisotropic scalar field configurations in asymptotically $AdS$ spacetimes.

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arXiv2022

Evolution of black holes through a nonsingular cosmological bounce

We study the classical dynamics of black holes during a nonsingular cosmological bounce. Taking a simple model of a nonsingular bouncing cosmology driven by the combination of a ghost and ordinary scalar field, we use nonlinear evolutions of the Einstein equations to follow rotating and non-rotating black holes of different sizes through the bounce. The violation of the null energy condition allows for a shrinking black hole event horizon and we find that for sufficiently large black holes (relative to the minimum Hubble radius) the black hole apparent horizon can disappear during the contraction phase. Despite this, we show that most of the local cosmological evolution remains largely unaffected by the presence of the black hole. We find that, independently of the black hole's initial mass, the black hole's event horizon persists throughout the bounce, and the late time dynamics consists of an expanding universe with a black hole of mass comparable to its initial value.

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arXiv2022

Numerical relativity for Horndeski gravity

We present an overview of recent developments in the numerical solution of Horndeski gravity theories, which are the class of all scalar-tensor theories of gravity that have second order equations of motion. We review several methods that have been used to establish well-posed initial value problems for these theories, and discuss well-posed formulations of the constraint equations. We also discuss global aspects of exact, strongly coupled solutions to some of Horndeski gravity theories: the formation of shocks, the loss of hyperbolicity, and the formation of naked curvature singularities. Finally we discuss numerical solutions to binary black hole and neutron star systems for several Horndeski theories.

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arXiv2022

Computing the quasinormal modes and eigenfunctions for the Teukolsky equation using horizon penetrating, hyperboloidally compactified coordinates

We study the quasinormal mode eigenvalues and eigenfunctions for the Teukolsky equation in a horizon penetrating, hyperboloidally compactified (HPHC) coordinate system. Following earlier work by Zenginoğlu (arXiv:1102.2451), we show that the quasinormal eigenfunctions for the Teukolsky equation are regular from the black hole horizon to future null infinity in these coordinates. We then present several example quasinormal eigenfunction solutions, and study some of their properties in the near-extremal Kerr limit.

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arXiv2022

Lessons for adaptive mesh refinement in numerical relativity

We demonstrate the flexibility and utility of the Berger-Rigoutsos Adaptive Mesh Refinement (AMR) algorithm used in the open-source numerical relativity code GRChombo for generating gravitational waveforms from binary black-hole inspirals, and for studying other problems involving non-trivial matter configurations. We show that GRChombo can produce high quality binary black-hole waveforms through a code comparison with the established numerical relativity code Lean. We also discuss some of the technical challenges involved in making use of full AMR (as opposed to, e.g. moving box mesh refinement), including the numerical effects caused by using various refinement criteria when regridding. We suggest several "rules of thumb" for when to use different tagging criteria for simulating a variety of physical phenomena. We demonstrate the use of these different criteria through example evolutions of a scalar field theory. Finally, we also review the current status and general capabilities of GRChombo.

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