arXiv subjects

Lam Hui

Publications and source records attributed to Lam Hui.

At least 19 recordsLinked to original sources

arXiv2026

A Collapsar-Disk Origin for GW190814

GW190814 was a remarkable gravitational-wave (GW) event: a merger between a $23 M_\odot$ black hole (BH) and a $2.6\,M_\odot$ compact object, with an extreme mass ratio that is difficult to reproduce through standard isolated-binary or dynamical formation channels. Recent work has shown that neutrino-cooled collapsar disks can become gravitationally unstable and fragment, producing neutron stars (NSs) or low-mass BHs in orbit around the newly formed central BH. These fragments may subsequently interact, scatter, merge with one another, or inspiral into the central remnant. We explore the possibility that GW190814 originated from such a collapsar-disk fragment merging with the central BH. A key prediction of this scenario is a temporal association with a stripped-envelope supernova preceding the GW event, and we identify the Type Ib supernova candidate SN2019npv, which occurred inside the GW190814 credible volume approximately 60 days before coalescence, as a possible electromagnetic precursor. Although this delay is too long for a conventional kilonova counterpart, we show that three-body interactions among disk fragments can excite some compact objects to wide orbits and produce merger delays of weeks to months. While GW190814 itself was not expected to produce detectable tidal-disruption-powered emission, future delayed mergers in this channel could generate luminous transients through either reprocessed kilonova heating or shocks driven as merger ejecta collide with the preceding supernova ejecta. Finally, treating SN2019npv as the electromagnetic counterpart makes GW190814 a bright standard siren and yields $H_{0} = 71.1^{+9.3}_{-7.5} km s^{-1}Mpc^{-1}$.

astro-ph.HE↗

arXiv2026

Gravitational wave signatures of subsolar neutron star formation in collapsar disks

Standard models for stellar core collapse predict the birth of neutron stars with masses exceeding a solar mass. In the collapsar model, a rapidly rotating massive star collapses to a black hole surrounded by a dense, neutrino-cooled accretion disk, whose outer regions may in some cases fragment into neutron-rich clumps that collapse to form subsolar-mass neutron stars. These disk-formed objects can then merge either with one another or, following possibly hierarchical evolution, with the central black hole. In this work, we characterize the gravitational-wave signatures of these binaries, focusing on distinctive features of the collapsar environment: Doppler and other phase modulations from orbital motion about the central black hole, gas-driven migration in the disk, and tidal interactions during inspiral. We show that the combination of anomalously low component masses with these environmental imprints produces unique waveform signatures that can distinguish the collapsar channel from other subsolar merger scenarios. Current and next-generation detectors have good prospects for identifying the intrinsic properties of such objects and, if subsolar mergers are discovered, for testing and constraining the collapsar formation pathway.

gr-qc↗

arXiv2026

An ultra-broadband axion dark matter experiment

We propose a novel broadband strategy to search for axions by leveraging observables controlled by the axion field squared. We present a practical implementation of this concept for probing the axion--photon coupling. This is done by operating a dc SQUID at the flux sweet spot, where the voltage depends quadratically on the magnetic flux, and using lock-in modulation to evade low-frequency noise. The proposed setup is ultra-broadband, spanning over 15 orders of magnitude in axion mass, with further expansion of the mass range possible. The projected sensitivity is $|g_{aγγ}| \gtrsim 10^{-16} \text{ GeV}^{-1}$, orders of magnitude better than current bounds, and largely independent of axion mass. We discuss the sources of systematic background and a nulling technique to reduce them to an acceptable level. We also discuss how our strategy could be adapted to probe the axion-fermion coupling, as well as to detect other dark matter candidates such as dark photons.

hep-ph↗

arXiv2026

Ladder Symmetries of Higher Dimensional Black Holes

We compute the ladder operators for static tidal perturbations to higher-dimensional black holes. These operators map between solutions of the relevant equation of motion at different multipole orders. We focus on spin 0, 1 and 2 perturbations to the Schwarzschild-Tangherlini black hole and on spin 0 perturbations to the 5D Myers-Perry black hole. The ladder structure, used in conjunction with the existence of special ground state solutions, explains why the Love numbers of these higher-dimensional black holes vanish for specific combinations of the multipole moment and number of spacetime dimensions. This generalizes previous work on a ladder symmetry explanation for the vanishing of 4D black hole static Love numbers to higher dimensions.

hep-th↗

arXiv2025

Caustic fringes for wave dark matter

Wave dark matter is composed of particles sufficiently light that their de Broglie wavelength exceeds the average inter-particle separation. A typical wave dark matter halo exhibits granular substructures due to wave interference. In this paper, we explore the wave interference effects around caustics. These are locations of formally divergent density in cold collisionless systems. Examples include splashback in galaxy clusters, and tidal shells in merging galaxies, where the pile-up of dark matter close to apogee gives rise to caustics. We show that wave interference modifies the density profile in the vicinity of the caustics, giving rise to a fringe pattern well-described by the Airy function. This follows from approximating the gravitational potential as linear close to apogee. This prediction is verified in a series of numerical simulations in which the gravitational potential is computed exactly. We provide a formula expressing the fringe separation in terms of the wave dark matter mass and halo parameters, which is useful for interpreting and stacking data. The fringe separation near caustics can be significantly larger than the naive de Broglie scale (the latter set by the system's velocity dispersion). This opens up the possibility of detecting caustic fringes for a wide range of wave dark matter masses.

astro-ph.CO↗

arXiv2025

Geometric Symmetries for the Vanishing of the Black Hole Tidal Love Numbers

We present a unified geometric perspective on the symmetries underlying the spin 0, 1 and 2 static perturbations around a Schwarzschild black hole. In all cases, the symmetries are exact, each forming an SO(3,1) group. They can be formulated at the level of the action, provided the appropriate field variables are chosen. For spin 1 and 2, the convenient variables are certain combinations of the gauge fields for even perturbations, and dual scalars for odd perturbations. The even and odd sector each has its own SO(3,1) symmetry. In addition, there is an SO(2) symmetry connecting them, furnishing an economical description of Chandrasekhar's duality. When decomposed in spherical harmonics, the perturbations form a non-trivial representation of SO(3,1), giving rise to ladder symmetries which explain the vanishing of the tidal Love numbers. Our work builds on earlier discussions of ladder symmetries, which were formulated in terms of the Newman-Penrose scalar at the level of the Teukolsky equation. Our formulation makes it possible to state the symmetries responsible for the vanishing of the Wilson coefficients characterizing the spin 0, 1 and 2 static tidal response, in the effective point particle description of a black hole.

hep-th↗

arXiv2025

Heating and scattering of stellar distributions by ultralight dark matter

Due to wave interference, an ultralight light dark matter halo has stochastic, granular substructures which can scatter stars, leading to the heating of stellar distributions. Studies of this phenomenon have placed lower bounds on the ultralight dark matter mass. In this paper we investigate a number of relevant systematic effects, including: (1) the heating by the central soliton, (2) the self-gravity of the stars, (3) the suppression of heating in a tidally stripped halo, and (4) the tidal field suppression of heating when the stellar cluster is much smaller than the de Broglie wavelength. The first three effects are quantified by studying the dynamics of stellar particles in Schrodinger-Poisson simulations of ultralight dark matter halos, while the last effect is studied using analytic approximations.

astro-ph.CO↗

arXiv2025

Microcausality without Lorentz invariance

Microcausality -- the vanishing of commutators outside the lightcone -- is a fundamental property of relativistic quantum field theories. We derive its implications for two-point functions of scalar operators on {\it Lorentz-breaking} states. We restrict to spatially homogeneous and isotropic states, at zero and finite temperature, such as finite-density states of matter and primordial inflationary states. In a mixed $(t, \vec k \, )$ representation, we find certain analyticity and exponential boundedness conditions, which we verify in a variety of examples. Crucially, we discuss how our conditions can be tested within the regime of validity of Lorentz-breaking low-energy effective field theories, clarifying the role of the group velocity of low-energy excitations. In the cosmological case, we derive a positivity condition on an EFT coefficient in an inflationary background. Lastly, we comment on how microcausality can be used to constrain higher-point correlation functions, via suitable nested commutators.

hep-th↗

arXiv2025

Symmetries of Vanishing Nonlinear Love Numbers of Schwarzschild Black Holes

The tidal Love numbers parametrize the conservative induced tidal response of self-gravitating objects. It is well established that asymptotically-flat black holes in four-dimensional general relativity have vanishing Love numbers. In linear perturbation theory, this result was shown to be a consequence of ladder symmetries acting on black hole perturbations. In this work, we show that a black hole's tidal response induced by a static, parity-even tidal field vanishes for all multipoles to all orders in perturbation theory. Our strategy is to focus on static and axisymmetric spacetimes for which the dimensional reduction to the fully nonlinear Weyl solution is well-known. We define the nonlinear Love numbers using the point-particle effective field theory, matching with the Weyl solution to show that an infinite subset of the static, parity-even Love number couplings vanish, to all orders in perturbation theory. This conclusion holds even if the tidal field deviates from axisymmetry. Lastly, we discuss the symmetries underlying the vanishing of the nonlinear Love numbers. An $\mathfrak{sl}(2,\mathbb R)$ algebra acting on a covariantly-defined potential furnishes ladder symmetries analogous to those in linear theory. This is because the dynamics of the potential are isomorphic to those of a static, massless scalar on a Schwarzschild background. We comment on the connection between the ladder symmetries and the Geroch group that is well-known to arise from dimensional reduction.

gr-qc↗

arXiv2024

Black hole spectroscopy with nonlinear quasi-normal modes

The future detection of quasi-normal modes (QNMs) from black hole ringdown will allow for consistency and independent tests of general relativity (GR) in the strong-field regime. In this paper, we perform a ringdown Fisher forecast when including the dominant quadratic QNM (QQNM) expected in nearly equal-mass quasi-circular binary black holes (BBHs) observed by next-generation ground-based detectors, Einstein Telescope (ET) and Cosmic Explorer (CE). We consider a ringdown model with a total of four modes: three linear QNMs labeled by $(\ell m n)=(220), (330), (440)$ and one QQNM coming from the self-interaction of the dominant linear (220) QNM. We perform a forecast in two scenarios, when the QQNM parameters are considered to be: (a) independent of the linear QNMs; (b) dependent on the (220) QNM parameters, according to GR. In Scenario (a) we find the QQNM to generally be measured with better precision than the (440) mode but worse than the (330) mode. As shown in the past, high-spin BBHs tend to have higher relative QQNMs. Even in such cases, we only expect to confidently detect and resolve these four independent QNMs for nearby events, below redshift $z\sim 0.5$ in ET and CE for intermediate-mass BHs. In Scenario (b) we find the QQNM to be extremely useful for improving the precision on the (440) parameters, with negligible improvements on the (220) parameters. In this case, the (440) parameters are expected to be measured even better than those of the (330) QNM. As a result, we expect to confidently detect and resolve the three independent linear QNMs for events even at high redshifts, up to $z\sim 35$ in ET and CE for intermediate-mass BHs. Therefore, thanks to the inclusion of the QQNM, virtually all second-generation BBH events will provide excellent consistency tests of GR.

gr-qc↗

arXiv2024

The impact of initial conditions on quasi-normal modes

This study investigates the influence of initial conditions on the evolution and properties of linear quasi-normal modes (QNMs). Using a toy model in which the quasi-normal mode can be unambiguously identified, we highlight an aspect of QNMs that is long known yet often ignored: the amplitude of a QNM (after factoring out the corresponding exponential with a complex frequency) is not constant but instead varies with time. We stress that this is true even within the regime of validity of linear perturbation theory. The precise time variation depends on the initial conditions. In particular, it is possible to find initial conditions for which the QNM fails to materialize; it is also possible to find those for which the QNM amplitude grows indefinitely. Focusing on cases where the QNM amplitude does stabilize at late times, we explore how the timescale for amplitude stabilization depends on the shape and location of the initial perturbation profile. Our findings underscore the need for care in fitting linear QNMs to ringdown data. They also suggest recent computations of quadratic QNMs, sourced purely by {\it stabilized} linear QNMs, do not fully capture what determines the amplitude of the quadratic QNMs, even at late times. Our results motivate a detailed investigation of the initial perturbations generated in the aftermath of a binary merger.

gr-qc↗

arXiv2024

Fragmentation in Gravitationally-Unstable Collapsar Disks and Sub-Solar Neutron Star Mergers

Although stable neutron stars (NS) can in principle exist down to masses Mns ~ 0.1Msun, standard models of stellar core-collapse predict a robust lower limit Mns >~ 1.2Msun, roughly commensurate with the Chandrasekhar mass Mch of the progenitor's iron core (electron fraction Ye ~ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Ye << 0.5) for which Mch ~ Ye^2 is reduced to < Msun. Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly-rotating stars ("collapsars"), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form sub-solar mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNS, their subsequent coalescence offers a channel for precipitating sub-solar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ~Hz frequency Doppler modulation of the ssNS-merger gravitational wave signals due to the binary's orbital motion in the disk; (2) at least one additional gravitational wave event (coincident within <~ hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched with r-process elements from the NS merger(s) embedded within the exploding stellar envelope ("kilonovae inside a supernova").

astro-ph.HE↗

arXiv2024

Massive-ish Particles from Small-ish Scales: Non-Perturbative Techniques for Cosmological Collider Physics from Large-Scale Structure Surveys

Massive particles produced during inflation impact soft limits of primordial correlators. Searching for these signatures presents an exciting opportunity to uncover the particle spectrum in the inflationary epoch. We present non-perturbative methods to constrain intermediate-mass scalars ($0\leq m/H<3/2$, where $H$ is the inflationary Hubble scale) produced during inflation, which give rise to a power-law scaling in the squeezed primordial bispectrum. Exploiting the large-scale structure consistency relations and the separate universe approach, we derive models for the late-time squeezed matter bispectrum and collapsed matter trispectrum sourced by these fields. To validate our models, we run $N$-body simulations with the "Cosmological Collider" squeezed bispectrum for two different particle masses. Our models yield unbiased constraints on the amplitude of non-Gaussianity, $f_{\rm NL}^Δ$, from the squeezed bispectrum and collapsed trispectrum deep into the non-linear regime ($k_{\rm max}\approx 2~h/{\rm Mpc}$ at $z=0$). We assess the information content of these summary statistics, emphasizing the importance of sample variance cancellation in the matter sector. We also study the scale-dependent halo bias in our simulations. For mass-selected halos, the non-Gaussian bias estimated from our simulations agrees with predictions based on (i) separate universe simulations and (ii) universal mass functions. With further work, these results can be used to search for inflationary massive particle production with upcoming galaxy surveys.

astro-ph.CO↗

arXiv2024

Relativistic drag forces on black holes from scalar dark matter clouds of all sizes

We use numerical simulations of scalar field dark matter evolving on a moving black hole background to confirm the regime of validity of (semi-)analytic expressions derived from first principles for both dynamical friction and momentum accretion in the relativistic regime. We cover both small and large clouds (relative to the de Broglie wavelength of the scalars), and light and heavy particle masses (relative to the BH size). In the case of a small dark matter cloud, the effect of accretion is a non-negligible contribution to the total force on the black hole, even for small scalar masses. We confirm that this momentum accretion transitions between two regimes (wave- and particle-like) and we identify the mass of the scalar at which the transition between regimes occurs.

gr-qc↗

arXiv2024

Static response and Love numbers of Schwarzschild black holes

We derive the quadratic action for the physical degrees of freedom of massless spin-0, spin-1, and spin-2 perturbations on a Schwarzschild--(A)dS background in arbitrary dimensions. We then use these results to compute the static response of asymptotically flat Schwarzschild black holes to external fields. Our analysis reproduces known facts about black hole Love numbers, in particular that they vanish for all types of perturbation in four spacetime dimensions, but also leads to new results. For instance, we find that neutral Schwarzschild black holes polarize in the presence of an electromagnetic background in any number of spacetime dimensions except four. Moreover, we calculate for the first time black hole Love numbers for vector-type gravitational perturbations in higher dimensions and find that they generically do not vanish. Along the way, we shed some light on an apparent discrepancy between previous results in the literature, and clarify some aspects of the matching between perturbative calculations of static response on a Schwarzschild background and the point-particle effective theory

hep-th↗

arXiv2023

$S$-matrix positivity without Lorentz invariance: a case study

We investigate the analytic structure of scattering amplitudes in theories in which Lorentz invariance is spontaneously broken. We do so by computing and studying the S-matrix for a simple example: a superfluid described by a complex scalar with quartic interactions. The computation is confined to tree-level, for there are no absolutely stable single-particle states, though the lifetime can be made long by lowering the chemical potential. For the $2 \to 2$ amplitude in center-of-mass configurations, not only is crossing symmetry violated, there appears a {\it tree level} branch cut for unphysical kinematics. Its appearance is a consequence of non-analyticity in the dispersion relation. The branch point defines a new scale in the problem, which scales inversely with the chemical potential. In this example, even derivatives of the forward amplitude are positive while odd derivatives are negative. This pattern can be understood in a general way in the limit of a small chemical potential, or weak Lorentz breaking.

hep-th↗

arXiv2023

Consistently constraining $f_{\rm NL}$ with the squeezed lensing bispectrum using consistency relations

We introduce a non-perturbative method to constrain the amplitude of local-type primordial non-Gaussianity ($f_{\rm NL}$) using squeezed configurations of the CMB lensing convergence and cosmic shear bispectra. First, we use cosmological consistency relations to derive a model for the squeezed limit of angular auto- and cross-bispectra of lensing convergence fields in the presence of $f_{\rm NL}$. Using this model, we perform a Fisher forecast with specifications expected for upcoming CMB lensing measurements from the Simons Observatory and CMB-S4, as well as cosmic shear measurements from a Rubin LSST/Euclid-like experiment. Assuming a minimum multipole $\ell_{\rm min}=10$ and maximum multipole $\ell_{\rm max}=1400$, we forecast $σ_{f_{\rm NL}}=175$ ($95$) for Simons Observatory (CMB-S4). Our forecasts improve considerably for an LSST/Euclid-like cosmic shear experiment with three tomographic bins and $\ell_{\rm min}=10$ and $\ell_{\rm max}=1400$ ($5000$) with $σ_{f_{\rm NL}}=31$ ($16$). A joint analysis of CMB-S4 lensing and LSST/Euclid-like shear yields little gain over the shear-only forecasts; however, we show that a joint analysis could be useful if the CMB lensing convergence can be reliably reconstructed at larger angular scales than the shear field. The method presented in this work is a novel and robust technique to constrain local primordial non-Gaussianity from upcoming large-scale structure surveys that is completely independent of the galaxy field (and therefore any nuisance parameters such as $b_ϕ$), thus complementing existing techniques to constrain $f_{\rm NL}$ using the scale-dependent halo bias.

astro-ph.CO↗

arXiv2023

An analytic approach to quasinormal modes for coupled linear systems

Quasinormal modes describe the ringdown of compact objects deformed by small perturbations. In generic theories of gravity that extend General Relativity, the linearized dynamics of these perturbations is described by a system of coupled linear differential equations of second order. We first show, under general assumptions, that such a system can be brought to a Schrödinger-like form. We then devise an analytic approximation scheme to compute the spectrum of quasinormal modes. We validate our approach using a toy model with a controllable mixing parameter $\varepsilon$ and showing that the analytic approximation for the fundamental mode agrees with the numerical computation when the approximation is justified. The accuracy of the analytic approximation is at the (sub-) percent level for the real part and at the level of a few percent for the imaginary part, even when $\varepsilon$ is of order one. Our approximation scheme can be seen as an extension of the approach of Schutz and Will to the case of coupled systems of equations, although our approach is not phrased in terms of a WKB analysis, and offers a new viewpoint even in the case of a single equation.

gr-qc↗