arXiv subjects

Hui-Yu Zhu

Publications and source records attributed to Hui-Yu Zhu.

11 recordsLinked to original sources

arXiv2026

Real-time Gravitational Wave Response in Thermal Spinning fields

We study how the spin content of the thermal plasmas affects the propagation of gravitational waves in a radiation-dominated universe. As a simple but representative setup, we consider conformal scalar, Weyl fermion, and Maxwell fields that provide the background radiation, and we ask whether the resulting damping and phase shift of gravitational waves retain any memory of their spins. We revisit this question in a real-time quantum-field-theoretic framework, where the stress tensor splits into a background part, a dynamical (history-dependent) response, and local contact terms, with an additional on-shell projection fixed by the Friedmann equation. We find that the dynamical spin-dependent response arises on a short time scale characterized by the radiation temperature, which is exactly canceled by the local responses. As a result, the remaining long-time response is universal and consistent with kinetic theory in the hard thermal limit. Although the underlying mechanism exhibits strong spin dependence, it leaves no observable imprint on the large-scale effective dynamics of gravitational waves in this setup.

hep-th↗

arXiv2025

Quantum Treatment of Black Hole Superradiance

Rotating black holes can form dense boson clouds through superradiant instability, making Kerr black holes a powerful probe of ultralight massive bosons. Previous studies of black hole superradiance have often treated bosonic fields classically, leaving open questions about how particles are produced and how the clouds grow over time. In this work, we canonically quantize a massive scalar field around a Kerr black hole, providing a fully quantum description of black hole superradiance. We show that the evolution of the particle number in the cloud, as well as the energy and angular momentum of the scalar field, can be consistently explained within the standard framework of quantum field theory in curved spacetime. Furthermore, we prove that the growth of the cloud occurs independently of the choice of initial state. We also explore several phenomena related to a massive scalar field in a rotating black hole spacetime, including Hawking radiation, adiabatic backreaction on the black hole spin, and the direction of level transitions in the presence of self-interactions of the field. Our analysis provides a consistent quantum-mechanical perspective that includes all these phenomena.

gr-qc↗

arXiv2025

Extracting Properties of Dark Dense Environments around Black Holes from Gravitational Waves

Dark matter (DM) can form dense condensates around black holes (BHs), such as superradiant clouds and ultracompact mini halos, which can significantly affect the orbital evolution of their companion objects through dynamical friction (DF). In this work, we define a novel quantity to quantify such effects in the emitted gravitational waves (GWs) in terms of GW amplitude, frequency, and their time derivatives. The information about the density profile can be extracted from this quantity, which characterizes the type of condensate and, therefore, the corresponding DM property. This quantity allows us to probe the dark dense environment by multi-wavelength GW observation with existing ground-based and future space-based GW detectors, potentially revealing the properties of the dark sector and shedding light on the primordial origin of the stellar mass BHs. A null detection can place strong constraints on the relevant DM parameters.

gr-qc↗

arXiv2025

Dark Matter-Independent Orbital Decay Bounds on Ultralight Bosons from OJ287

Ultralight bosons, predicted in scenarios beyond the Standard Model and viable dark matter (DM) candidates, can form superradiant clouds around spinning black holes influencing their dynamics. Using century-long monitored OJ287 supermassive black hole binary we set first DM-independent, dynamical constraints on their masses $\mu = (8.5-22) \times 10^{-22}$ eV. These dynamical constraints, driven by boson cloud friction, are robust against DM-model uncertainties and offer a novel ultralight boson probe. We show that analogous superradiant dynamics across the cosmic population of supermassive black hole systems could help resolve final-parsec evolution stalling problem and imprint a detectable suppression and break in the gravitational wave background.

hep-ph↗

arXiv2024

Survival of the Fittest: Testing Superradiance Termination with Simulated Binary Black Hole Statistics

The superradiance instability of rotating black holes leads to the formation of an ultralight boson cloud with distinctive observational signatures, making black holes an effective probe of ultralight bosons. However, around black holes in a binary system, the superradiance effect of such clouds can be terminated by tidal perturbations from the companion, leading to cloud depletion. In this study, we focus on the superradiance of a scalar boson, and perform the first analysis of the impact of this termination effect on superradiant black hole binaries which are realistically modeled after their statistics in our Galaxy. Working with a dataset of approximately $10^7$ black hole binaries simulated using the Stellar EVolution for N-body (SEVN) population synthesis code, we identify the superradiant candidates and those that manage to survive the termination effect. We then calculate the cloud survival rate for various boson masses and black hole spin models. Our findings reveal that the $l=m=1$ cloud modes are generally stable against termination, whereas the $l=m=2$ modes can be significantly affected, with survival rates dropping below $10\%$ for boson masses below approximately $0.5\times 10^{-12}$ eV. In addition, our analysis indicates that clouds that overcome termination typically exhibit a higher superradiant growth rate and therefore a higher detectability.

gr-qc↗

arXiv2024

Modulating binary dynamics via the termination of black hole superradiance

A superradiant cloud of ultralight bosons near a rotating black hole provides a smoking gun for particle physics in the infrared. However, tidal perturbations from a nearby binary companion can destabilise the boson cloud and even terminate superradiance. In this work, we consider the backreaction of superradiance termination to the dynamics of general binary orbits parametrised by their semi-latus rectum, eccentricity and inclination angle. Our analysis focuses on Extreme Mass Ratio Inspiral (EMRI) systems and employs the period-average approximation to derive evolution equations of these binary parameters in the Newtonian limit. We find that the binary evolution history can be significantly modulated by the backreaction towards large circular equatorial orbits with reduced termination rate. This process can generically happen even away from the resonance bands. Our work therefore serves as a first step towards probing ultralight bosons through the statistics of EMRI binary parameters in the future.

gr-qc↗

arXiv2022

Missing one-loop contributions in secondary gravitational waves

We find several missing one-loop-order contributions in previous considerations about secondary gravitational waves induced at nonlinear order in cosmological perturbations. We consider a consistent perturbative expansion to third-order in cosmological perturbations, including higher-order interactions and iterative solutions ignored in the previous literature. Tensor fluctuations induced by the source with two scalar and one tensor perturbations are correlated with the first-order tensor fluctuation and thus give a one-loop-order correction to the tensor power spectrum. The missing loop correction is \textit{scale-invariant} and \textit{negative} in the superhorion region, which secondarily reduces the initial primordial tensor power spectrum prior to the horizon re-entry. Such an IR behavior is very different from the auto-spectrum of second-order induced tensor modes discussed in the previous literature and can be important for the actual gravitational wave measurements. For a sharp peak of scalar fluctuations with $A_ζ=10^{-2}$ at $k_*=10^{5}h/{\rm Mpc}$ motivated by the LIGO/Virgo events, we show that the tensor power spectrum at the cosmic microwave background scale reduces by at most 35%. Hence, the polarization B-mode might not be seen because of the reduction of the original tensor spectrum due to the secondary effect of primordial black hole formation.

astro-ph.CO↗

arXiv2022

Termination of Superradiance from a Binary Companion

We study the impact of a binary companion on black hole superradiance at orbital frequencies away from the gravitational-collider-physics (GCP) resonance bands. A superradiant state can couple to a strongly absorptive state via the tidal perturbation of the companion, thereby acquiring a suppressed superradiance rate. Below a critical binary separation, this superradiance rate becomes negative, and the boson cloud gets absorbed by the black hole. This critical binary separation leads to tight constraints on GCP. Especially, a companion with mass ratio $q>10^{-3}$ invalidates all GCP fine structure transitions, as well as almost all Bohr transitions except those from the $|ψ_{211}\rangle$ state. Meanwhile, the backreaction on the companion manifests itself as a torque acting on the binary, producing floating/sinking orbits that can be verified via pulsar timing. In addition, the possible termination of cloud growth may help to alleviate the current bounds on the ultralight boson mass from various null detections.

gr-qc↗

arXiv2022

Probing The Neutrino Sector via A Statistical Approach

We apply the idea of landscape (motivated by string theory) to study the statistical nature of parameters/couplings in the standard model of strong and electroweak interactions. Following the success of this approach on the fermion masses, we discuss the divergent behavior of the probability distributions of other physical parameters/couplings to obtain some insights on the quantities that cannot be measured by current experiments but can be relevant in cosmology, in particular those in the neutrino sector. From a relatively strongly mixed PMNS matrix, we argue that the probability distribution of heavy neutrino mass $P(M)$ does not diverge at $M=0$. This analysis favors the degenerate heavy neutrino scenarios.

hep-ph↗

arXiv2021

Gravitational Collider Physics via Pulsar-Black Hole Binaries II: Fine and Hyperfine Structures are Favored

A rotating black hole can be clouded by light bosons via superradiance, and thus acquire an atom-like structure. If such a gravitational atom system is companioned with a pulsar, the pulsar can trigger transitions between energy levels of the gravitational atom, and these transitions can be detected by pulsar timing. We show that in such pulsar-black hole systems, fine and hyperfine structure transitions are more likely to be probed than the Bohr transition. Also, the calculation of these fine and hyperfine structure transitions are under better analytic control. Thus, these fine and hyperfine structure transitions are more ideal probes in the search for gravitational collider signals in pulsar-black hole systems.

astro-ph.HE↗

arXiv2019

Approaching a spacetime singularity in conformal gravity

We hereby show that the Kasner spacetime turns out to be singularity-free in Einstein's conformal gravity in vacuum or in presence of matter. Such a statement is based on the regularity of the curvature invariants and on the geodesic completion of the spacetime when it is probed by massive, massless, and conformally coupled particles. As a universal feature of the regular metric, nothing can reach the singularity located in t=0 in a finite amount of proper time (for massive particles and conformally coupled particles) or for finite values of the affine parameter (for massless particles).

gr-qc↗