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Abstract We determine the metric of a Kerr black hole subject to external tidal fields using metric reconstruction techniques. Working within the Newman–Penrose formalism, we solve the Teukolsky master equation for static, quadrupolar modes associated with a slowly varying tidal environment, and reconstruct the corresponding metric perturbation in the outgoing radiation gauge. As an application, we derive the secular Hamiltonian governing the motion of a test particle in the tidally deformed Kerr spacetime and investigate long-term tidal effects relevant to extreme-mass-ratio inspirals. In particular, we compute tidal-induced shifts of the innermost stable circular orbit and the light ring. We find that these tidal corrections are strongly spin dependent, with significantly larger effects for retrograde orbits around rapidly rotating black holes. Our results provide a fully analytic framework for studying tidal interactions and secular dynamics in rotating black-hole spacetimes, with direct applications to gravitational-wave modeling and tests of gravity in the strong-field regime. 

Abstract The ‘ringdown’ radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave (GW) data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and dynamical gravitational fields, and search for physics beyond the Standard Model or new gravitational degrees of freedom. We summarize the state of the art in our understanding of black hole quasinormal modes in general relativity and modified gravity, their excitation, and the modeling of ringdown waveforms. We also review the status of LIGO-Virgo-KAGRA ringdown observations, data analysis techniques, and the bright prospects of the field in the era of LISA and next-generation ground-based GW detectors. 

A<sc>bstract</sc> We study second order gravitational perturbations on plane wave spacetimes from both the metric and curvature perturbation points of view. For the former, we explicitly use the isometries of the background to introduce tensor oscillator harmonics, which render Einstein’s equations algebraic around symmetric plane waves. For the latter, we formulate the first and second order Teukolsky equations in a Geroch-Held-Penrose covariant way. Both approaches are useful in their own right, and together with our discussion on gauge freedom, they provide a foundation for the study of higher-order gravitational dynamics around plane wave spacetimes. Taking the perspective that these plane wave spacetimes arise from Penrose limits, we subsequently use these results to explore the nonlinear gravitational dynamics close to black hole lightrings. Specifically, we define and discuss quadratic quasinormal mode ratios, observe that they satisfy emergent selection rules, and make publicly available a code to compute them. 

Abstract Supermassive black holes are prevalent at the centers of massive galaxies, and their masses scale with galaxy properties, increasing evidence suggesting that these trends continue to low stellar masses. Seeds are needed for supermassive black holes, especially at the highest redshifts explored by the James Webb Space Telescope. We study the hierarchical merging of galaxies via cosmological merger trees and argue that the seeds of supermassive black holes formed in nuclear star clusters via stellar black hole mergers at early epochs. Observable tracers include intermediate-mass black holes, nuclear star clusters, and early gas accretion in host dwarf galaxies, along with a potentially detectable stochastic gravitational-wave background, ejection of intermediate and supermassive black holes, and consequences of a significant population of early tidal disruption events and extreme mass ratio inspirals. 

ABSTRACT Rapid formation of supermassive black holes occurs in dense nuclear star clusters that are initially gas-dominated. Stellar-mass black hole remnants of the most massive cluster stars sink into the core, where a massive runaway black hole forms as a consequence of combined effects of repeated mergers and Eddington-limited gas accretion. The associated gravitational wave signals of high-redshift extreme mass-ratio inspirals are a unique signature of the nuclear star cluster scenario. 

Abstract The intergalactic helium became fully ionized by the end of cosmic noon (z∼ 2).Similarly to the reionization of hydrogen, helium reionization is expected to be patchy, driven by luminous quasars that ionize the intergalactic gas in their surrounding environment.Probing the morphology of ionized electrons during this epoch can provide crucial information about early structure formation, including the clustering and luminosities of quasars, the accretion rates, variability, and lifetimes of active galactic nuclei, as well as the growth and evolution of supermassive black holes.In this study, we present how measurements of the cosmic microwave background (CMB) can be used to reconstruct the optical-depth fluctuations resulting from patchy helium reionization.As helium reionization occurred at lower redshifts, upcoming probes of large-scale structure surveys will present a significant opportunity to enhance the prospects of probing this epoch by their combined analysis with the CMB.Using a joint information-matrix analysis of hydrogen and helium reionization, we show that near-future galaxy and CMB surveys will have enough statistical power to detect optical-depth fluctuations due to doubly-ionized helium, providing a way of measuring the redshift and duration of helium reionization to high significance.We also show that modeling uncertainties in helium reionization can impact the measurement precision of parameters characterizing hydrogen reionization. 

Abstract Astrometry, the precise measurement of star motions, offers an alternative avenue to investigate low-frequency gravitational waves through the spatial deflection of photons, complementing pulsar timing arrays reliant on timing residuals. Upcoming data from Gaia, Theia, and Roman can not only cross-check pulsar timing array findings but also explore the uncharted frequency range bridging pulsar timing arrays and LISA. We present an analytical framework to evaluate the feasibility of detecting a gravitational wave background, considering measurement noise and the intrinsic variability of the stochastic background. Furthermore, we highlight astrometry's crucial role in uncovering key properties of the gravitational wave background, such as spectral index and chirality, employing information-matrix analysis. Finally, we simulate the emergence of quadrupolar correlations, commonly referred to as the generalized Hellings-Downs curves. 

Abstract Black hole (BH) demographics in different environments is critical in view of recent results on massive star binarity, and of the multimessenger detectability of compact object mergers. But the identification and characterization of noninteracting BHs are elusive, especially in the sparse field stellar population. A candidate noninteractive BH + red giant (RG) binary system, 2MASS J05215658+4359220, was identified by T. A. Thompson et al. We obtained Astrosat/UVIT far-ultraviolet (FUV) imaging and Hubble Space Telescope (HST) UV−optical imaging and spectroscopy of the source to test possible scenarios for the optically elusive companion. HST/STIS spectra from ≈1600 to 10230 Å are best fit by the combination of two stellar sources, a RG withT_eff= 4250 ± 150 K, logg= 2.0,R_RG∼ 27.8R_⊙(assuming a single-temperature atmosphere), and a subgiant companion withT_eff= 6000 K,R_comp= 2.7R_⊙, orT_eff= 5270 K,R_comp= 4.2R_⊙using models with one-tenth or one-third solar metallicity, respectively, logg= 3.0, extinctionE_B−V= 0.50 ± 0.2, adopting the Data Release 3 Gaia distanceD= 2463 ± 120 pc. No FUV data existed prior to our programs. STIS spectra give an upper limit of 10⁻¹⁷erg cm⁻²s⁻¹Å⁻¹shortwards of 2300 Å; an upper limit of ≳25.7 ABmag was obtained in two UVIT FUV broad bands. The nondetection of FUV flux rules out a compact companion such as a hot white dwarf. The STIS spectrum shows strong Mgiiλ2800 Å emission, typical of chromospherically active RGs. The masses inferred by comparison with evolutionary tracks, ∼1M_⊙for the RG and between 1.1 and 1.6M_⊙for the subgiant companion, suggest past mass transfer, although the RG currently does not fill its Roche lobe. WFC3 imaging in F218W, F275W, F336W, F475W, and F606W shows an unresolved source in all filters. 

We present reying, a new model for the postmerger signal in black-hole binary coalescences based on the graybody factor of the remnant. The model accurately reproduces the full frequency-domain ringdown signal for a large set of comparable-mass, aligned-spin numerical relativity waveforms, including the $(\ell ,m)=(2,2)$ , (3,3), and (4,4) modes. It achieves mismatches of order $\mathcal{O}\left({10}^{-6}\right)$ for the dominant (2,2) mode, demonstrating that the graybody-factor parametrization provides a very accurate phenomenological description of the postmerger signal. Building on this model, we introduce a novel consistency test of strong gravity based on the graybody factor: the remnant mass and spin inferred from reying can be compared with those obtained through standard black-hole spectroscopy. This agnostic test relies exclusively on the postmerger signal and does not require the inclusion of overtones or the choice of very early ringdown starting times, combining the advantages of inspiral-merger-ringdown consistency tests and traditional black-hole spectroscopy. We apply the test to GW250114 and find that the remnant mass and spin inferred from reying are consistent with those measured from the full signal. Remarkably, the inferred parameters can be measured with a precision comparable to, or slightly better than, that achieved with standard black-hole spectroscopy. Our graybody-factor waveform model allows for new precision tests of strong gravity using the ringdown signal. 

The remnant of a black hole binary merger settles into a stationary configuration by “ringing down” through the emission of gravitational waves that consist of a superposition of damped exponentials with discrete complex frequencies—the remnant black hole’s quasinormal modes. While the frequencies themselves depend solely on the mass and spin of the remnant, the mode amplitudes depend on the merger dynamics. We investigate quasinormal mode excitation by a point particle plunging from the innermost stable circular orbit of a Kerr black hole. Our formalism is general, but we focus on computing the quasinormal mode excitation coefficients in the frequency domain for equatorial orbits, and we analyze their dependence on the remnant black hole spin. We find that higher overtones and subdominant multipoles of the radiation become increasingly significant for rapidly rotating black holes. This suggests that the prospects for detecting overtones and higher-order modes are considerably enhanced for highly spinning merger remnants.