arXiv subjects

Asad Hussain

Publications and source records attributed to Asad Hussain.

9 recordsLinked to original sources

arXiv2026

Binary-black hole spin population results may be driven by prior degeneracies

Gravitational waves carry information on the spins of merging binary-black holes. The orientations of their spins relative to their orbits ("tilts")---while challenging to measure---differentiate the astrophysical formation mechanisms by which merging black hole binaries may form. Multiple analyses have reported tentative evidence that black hole spins are preferentially oriented in the plane of the binary orbit. We show that these results are likely extrapolated from more easily measurable "effective spin" parameters which characterize the gravitational-wave inspiral of black hole binaries but may not uniquely constrain their astrophysical formation. In particular, we reproduce a peak in the spin tilt distribution with information about the marginal effective spin distributions alone. We propose a geometric picture to compare constraints on effective spins to standard population modeling assumptions, suggesting that current inferences of a preferred spin orientation are spurious.

gr-qc↗

arXiv2026

Evidence for mass-dependent spin subpopulations in GWTC-4

While the origin of merging black-hole binaries observed in gravitational waves remain uncertain, different formation channels are expected to leave distinct imprints on their observed mass and spin distributions. In this work, we focus on the mass dependence of the spin magnitudes $(\chi_1,\chi_2)$, allowing for multiple spin subpopulations whose relative fraction varies with mass. Using the binaries from the fourth LIGO--Virgo--KAGRA gravitational wave transient catalog (GWTC-4), we find strong evidence for two subpopulations, with a log Bayes factor of $\log_{10} \mathcal B = 4^{+6}_{-2}$. The data support a picture in which low-mass systems are composed almost entirely of slowly spinning black holes ($\chi \lesssim 0.2$), while high-mass systems are dominated by a broader component with moderate to rapid spins, peaking at $(\chi_1,\chi_2)\approx(0.7,0.7)$ and retaining a non-negligible tail toward asymmetric spins $(\chi_1,\chi_2)\approx(0.7,0)$. The transition between these regimes spans a broad range from $m_1 \approx 35^{+20}_{-20} \,M_\odot$ to $m_1 \approx 70^{+90}_{-25}\,M_\odot$. For the low-mass, slowly-spinning regime, a flexible model of spin orientations does not allow us to exclude identically vanishing spins, $\chi_1 = \chi_2 = 0$. Meanwhile, the high-mass, rapidly-spinning population does not neatly display the spin-magnitude asymmetry expected from hierarchical mergers, although the data do not exclude that. This kind of localized structure is generally difficult to access with standard Monte-Carlo likelihood estimators, which we replace with a regularized representation in terms of truncated Gaussian mixtures. Taken together, our results provide a new target for formation models, which should reproduce not only the mass and effective spin distributions, but also the mass-dependent structure of the individual component-spin magnitudes.

astro-ph.HE↗

arXiv2025

Computing spectral shifts for Johannsen-Psaltis black holes

The growing number of gravitational wave (GW) detections and the increasing sensitivity of GW detectors have enabled precision tests of General Relativity (GR) in the strong-field regime. The recent observation of multiple quasinormal modes (QNMs) in GW250114 marks a major advance for observational black hole spectroscopy. This clear signal, together with the growing number of GW detections, highlights the need for accurate predictions of QNM spectra in beyond-GR theories in order to carry out precision searches for new physics. In this work, we continue to lay the foundation for such predictions using a modified Teukolsky formalism in conjunction with the eigenvalue perturbation method. We compute the spectral shifts of slowly rotating Johannsen-Psaltis black holes for $2 \leq \ell \leq 10$, all $m$, and overtones $n = 0, 1, 2$, and confirm the large-$\ell$ behavior of the modes by comparing with the WKB approximation. We find that these black holes admit definite-parity modes but break the isospectrality between even- and odd-parity QNMs at all spins, and that the shifts depend linearly on $m$ for slow spins. We further derive a general parity condition that any beyond-GR modification to the metric must satisfy to support definite-parity modes, providing new insights into isospectrality breaking and parity structure in gravitational perturbations.

gr-qc↗

arXiv2025

Living on the edge: Testing for compact population features at the edges of parameter space

Many astrophysical population studies involve parameters that exist on a bounded domain, such as the dimensionless spins of black holes or the eccentricities of planetary orbits, both of which are confined to $[0, 1]$. In such scenarios, we often wish to test for distributions clustered near a boundary, e.g., vanishing spin or orbital eccentricity. Conventional approaches -- whether based on Monte Carlo, kernel density estimators, or machine-learning techniques -- often suffer biases at the boundaries. These biases stem from sparse sampling near the edge, kernel-related smoothing, or artifacts introduced by domain transformations. We introduce a truncated Gaussian mixture model framework that substantially mitigates these issues, enabling accurate inference of narrow, edge-dominated population features. While our method has broad applications to many astronomical domains, we consider gravitational wave catalogs as a concrete example to demonstrate its power. In particular, we maintain agreement with published constraints on the fraction of zero-spin binary black hole systems in the GWTC-3 catalog -- results originally derived at much higher computational cost through dedicated reanalysis of individual events in the catalog. Our method can achieve similarly reliable results with a much lower computational cost. The method is publicly available in the open-source packages gravpop and truncatedgaussianmixtures.

astro-ph.IM↗

arXiv2024

Hints of spin-magnitude correlations and a rapidly spinning subpopulation of binary black holes

The complex astrophysical processes leading to the formation of binary black holes and their eventual merger are imprinted on the spins of the individual black holes. We revisit the astrophysical distribution of those spins based on gravitational waves from the third gravitational wave transient catalog GWTC-3, (Abbott et al. 2023a), looking for structure in the two-dimensional space defined by the dimensionless spin magnitudes of the heavier ($χ_1$) and lighter ($χ_2$) component black holes. We find support for two distinct subpopulations with greater than $95\%$ credibility. The dominant population is made up of black holes with small spins, preferring $χ_1 \approx 0.2$ for the primary and $χ_2 \approx 0$ for the secondary; we report signs of an anticorrelation between $χ_1$ and $χ_2$, as well as as evidence against a subpopulation of binaries in which both components are nonspinning. The subdominant population consists of systems in which both black holes have relatively high spins and contains $20^{+18}_{-18}\%$ of the binaries. The binaries that are most likely to belong in this subpopulation are massive and slightly more likely to have spin-orientations aligned with the orbital angular momentum--potentially consistent with isolated binary formation channels capable of producing large spins, like chemically homogeneous evolution. This hint of a rapidly spinning subpopulation hinges on GW190517, a binary with large and well-measured spins. Our results, which are enabled by novel hierarchical inference methods, represent a first step towards more descriptive population models for black hole spins, and will be strengthened or refuted by the large number of gravitational wave detections expected in the next several years.

astro-ph.HE↗

arXiv2023

Isospectrality breaking in the Teukolsky formalism

General relativity, though the most successful theory of gravity, has been continuously modified to resolve its incompatibility with quantum mechanics and explain the origin of dark energy or dark matter. One way to test these modified gravity theories is to study the gravitational waves emitted during the ringdown of binary mergers, which consist of quasinormal modes. In several modified gravity theories, the even- and odd-parity gravitational perturbations of non-rotating and slowly rotating black holes have different quasinormal mode frequencies, breaking the isospectrality of general relativity. For black holes with arbitrary spin in modified gravity, there were no avenues to compute quasinormal modes except numerical relativity, until recent extensions of the Teukolsky formalism. In this work, we describe how to use the modified Teukolsky formalism to study isospectrality breaking in modified gravity. We first introduce how definite-parity modes are defined through combinations of Weyl scalars in general relativity, and then, we extend this definition to modified gravity. We then use the eigenvalue perturbation method to show how the degeneracy in quasinormal mode frequencies of different parity is broken in modified gravity. To demonstrate our analysis, we also apply it to some specific modified gravity theories. Our work lays the foundation for studying isospectrality breaking of quasinormal modes in modified gravity for black holes with arbitrary spin.

gr-qc↗

arXiv2022

Thermalization of isolated Bose-Einstein condensate under a PT-symmetric environment

The postulates of the eigenstate thermalization hypothesis (ETH) express that thermalization occurs due to the individual eigenstate of the system's Hamiltonian. But the ETH put no light on the dynamics that lead toward thermalization. In this paper, we observe the thermalization of a Bose-Einstein Condensate (BEC) confined in an optical lattice potential that is embedded in the harmonic trap. Such optical lattice potential offers local friction to the oscillating BEC. The spread in the temporal density plot of BEC shows the thermalization of the BEC. Moreover, we observe that the presence of a PT-symmetric potential greatly influences the BEC dynamics and the thermalization of the system. The presence of a PT-symmetric potential offers a way to manipulate the mean position of the BEC to the desired location and for a desired length of time.

cond-mat.quant-gas↗

arXiv2022

An approach to computing spectral shifts for black holes beyond Kerr

Recent measurements of gravitational-wave ringdown following the merger of binary black holes raise the prospect of precision black hole spectroscopy in the near future. To perform the most sensitive tests of the nature of black holes using ringdown measurements, it is critical to compute the deviations to the spectrum of black holes in particular extensions of relativity. These spectral shifts are also needed to interpret any violations of the predictions of relativity that may be detected during ringdown. Here we present a first step towards computing the shifts to the spectrum of Kerr black holes with arbitrary spins, by deriving a modified Teukolsky equation governing the perturbations of black holes in theories beyond GR. Our approach applies to a class of theories which includes dynamical Chern-Simons gravity and shift-symmetric scalar Gauss-Bonnet gravity, in the case where the deviations from relativity are small. This allows for a perturbative approach to solving the equations of motion. Further, we show how to use the modified equation to compute the leading-order spectral shifts of Kerr black holes, using eigenvalue perturbation methods. Our formalism provides a practical approach to predicting ringdown for black holes in a range of promising extensions to relativity, enabling future precision searches for their signatures in black hole ringdown.

gr-qc↗

arXiv2018

Decay of Qubits under arbitrary space-time trajectories: The Zeno & Anti-Zeno Effects

Modeling an arbitrarily accelerating qubit as an open quantum system, we derive an exact solution for the pure-dephasing model ($σ_z$ coupling) under arbitrary qubit space-time trajectories, as well as general expressions for the survival probabilities of finite-length qubits interacting with a massless scalar field under $σ_x$ coupling (an Unruh-DeWitt detector) to second order. We follow the regularization scheme presented in a previous paper by S. Schlicht, to allow a finite length quantum detector to couple to the massless scalar field. We compute the decay rate of the qubit in different coupling regimes, (pure dephasing, Unruh-DeWitt) and explore the Quantum Zeno (QZE) and Quantum Anti Zeno Effect (QAZE) as the qubit decoheres under it's interaction with the environment. We compute results for four example trajectories: stationary, uniform acceleration, oscillation and circular orbits.

quant-ph↗