The pulsar timing array (PTA) is a powerful technique for detecting nanohertz gravitational wave backgrounds (GWBs). However, conventional PTAs lack sensitivity to parity violation in the GWB. In this work, we propose a dipole pulsar timing array system (dPTA). By deriving the overlap reduction functions (ORFs) from the cross-correlation of timing signals, we find that this system exhibits sensitivity to chiral GWBs in the nanohertz regime. Furthermore, through numerical calculations of its sensitivity curves, we demonstrate that the dPTA extends the detectable frequency range of PTAs for GWBs from the nanohertz to the microhertz regime.

We investigate black hole accretion in self-interacting dark matter (SIDM) halos using a self-gravitating fluid model with thermal conduction. We develop a robust one-dimensional spherically symmetric hydrodynamic code based on an operator-splitting finite-volume method. Simulating both Singular Isothermal Sphere (SIS) and Navarro-Frenk-White (NFW) profiles, we find that black hole growth is regulated by the competition between gravity-driven inflow and SIDM heat transport. Our results demonstrate that an SIS-like environment facilitates rapid accretion, allowing a 100 M_⊙ seed to grow to 10^4 M_⊙ within 2 Myr. Furthermore, we show that larger initial black hole masses, steeper density profiles, and higher scattering cross sections significantly enhance the accretion rate. This study provides a comprehensive fluid-dynamical picture of black hole growth in SIDM halos.

We study images of spacetimes containing continuous photon spheres (CPS). For a self-gravitating, isotropic, spherically symmetric spacetime with CPS, we find that a thin accretion disk produces images that closely resemble those of a Schwarzschild black hole, despite significant differences in photon dynamics. More generally, for any static, spherically symmetric spacetime with a luminous CPS core, the image profile is universal: members of this class produce identical image shapes, differing only by an overall normalization factor. This universality is, however, sensitive to the nature of the accretion flow and breaks down for spherically symmetric infalling accretion, where Doppler shifts and non-static emission introduce image features that depend on the flow dynamics and the metric. Finally, we investigate photon regions in a rotating CPS spacetime and find that unlike in Kerr spacetime, the photon region appears as one or two angular sectors in a constant-phi cross section. These distinctive photon region properties could produce observable signatures that distinguish rotating CPS spacetimes from the Kerr one.

We present a numerical investigation of primordial black hole (PBH) formation from superhorizon curvature perturbations and the subsequent generation and propagation of sound waves, which can serve as a new source of stochastic gravitational-wave backgrounds presented in a companion letter. Using the Misner-Sharp formalism with an excision technique, our simulations extend to significantly later times than previous work and indicate that the near-critical perturbations produce a distinct compression wave featuring both over-and underdense shells, while significantly supercritical perturbations yield only an underdense shell. We also show that a softer equation of state suppresses the formation of compression waves. Furthermore, the comoving thickness of sound shells remains nearly constant during propagation and scales with the Hubble radius at horizon reentry, thereby serving as a key link between the gravitational-wave peak frequency and PBH mass in the companion letter. These results offer new insights into the dynamics of PBH formation and suggest potential observational signatures of PBHs in the gravitational-wave spectrum from associated sound waves.

The surface r=∞ of the Kerr–Bertotti–Robinson (KBR) spacetime is not the collection of the endpoints of infinitely extended light rays, and the Coulomb type component of gravitational field strength represented by Ψ_2 remains nonvanishing there, indicating that this surface is not a real boundary. We construct a natural extension across this surface, which connects the exterior of one KBR region to the interior of a neighboring one and, upon iteration, produces an infinite chain of regions connected by wormhole-like bridges. The extension also exposes the neighboring ring singularity without an intervening horizon, challenging the weak cosmic censorship conjecture and raising the question of whether the extended geometry is stable under perturbations. We therefore study the quasinormal modes (QNMs) of a test massless scalar field on a two-universe scattering segment. For axisymmetric perturbations, the existence of purely imaginary unstable QNMs is analytically proved for every ℓ, with their origin tied to the chronology-violating region. In the (ℓ,m)=(2,2) sector, unstable QNM branches driven by a black-hole-bomb mechanism are found. Finally, the wormhole geometry produces a double-barrier cavity and families of weakly damped QNMs, suggesting echo-like responses.

We propose a novel mechanism where a first-order phase transition modulates the decay rate of a massive field. This modulation, even if the scalar field has negligible energy density, subsequently generates an observable stochastic gravitational-wave (GW) background. The stochastic nature of bubble nucleation leads to the asynchrony of phase transitions, generating superhorizon-scale density perturbations through spatial variations in the decay rate Gamma. These perturbations subsequently source second-order GWs with peak amplitudes governed by the phase transition parameter beta/H* and decay rate Gamma. We apply this mechanism in the reheating scenario where the decay rate of the inflaton is modulated by the scalar field that undergoes a first-order phase transition. Numerical calculations reveal that the GW energy spectrum typically reaches Omega GW similar to 10-10, demonstrating prospects for detection by space-based interferometers like Laser Interferometer Space Antenna, TianQin, and Taiji. This work establishes a new approach to probe phase transition processes in the early Universe without requiring significant vacuum energy release.

Early detection and localization of gravitational waves (GWs) are essential for identifying electromagnetic (EM) counterparts, playing a key role in multi-messenger astronomy. However, second-generation (2G) ground-based detectors are most sensitive to frequencies of tens to hundreds of hertz, limiting the in-band duration of GW signals to O(0.1) to several tens of seconds. This constraint hinders early-warning capabilities and early localization. We present the first theoretical study on how eccentricity-induced higher harmonic modes, which enter the detector band significantly earlier than the dominant mode, enhance early detection and localization in a 2G detector network. By decomposing each harmonic mode in the frequency domain and tracking their sequential entry into the detector band, we analyze the evolution of the average signal-to-noise ratios (SNRs) and localization accuracy as functions of time-to-merger. For a GW170817-like BNS, an eccentricity of e0 = 0.4 at 10 Hz allows the signal to reach SNR 4 and the detection threshold of SNR 8 approximately 12 and 5 min before merger, respectively—gains of 4.5 and 1.5 min over the circular case. Localization within 1000(100)deg2 is achievable 5 (1) min before merger, improving by 2 min (15 s). Our results highlight the potential of eccentricity-induced higher harmonics in improving early warnings and localization, particularly for BNS mergers, enhancing the prospects for multi-messenger astronomy.

Long gamma-ray bursts (GRBs) frequently exhibit complex prompt emission structures with multiple temporally distinct episodes, such as a main emission (ME) phase followed by a weak extended emission (EE) tail. Whether these subcomponents from a common physical origin with similar classification properties, or instead represent fundamentally different emission mechanisms within a single event, remains an open question. Here, we present a systematic, pulse-resolved analysis of 22 Swift/BAT long-duration GRBs, each exhibiting a well-separated, bright ME (G_1) followed by a fainter EE (G_2) after a background-consistent quiescent gap. For each component, we independently measure standard classification diagnostics, including duration (T_90), spectral hardness ratio (HR), minimum variability timescale (MVT), and spectral lag. We then compare these properties between the ME and EE within individual bursts. We find that the EE is systematically softer (lower HR in 19 of 22 events), smoother (longer MVT in 17 of 22 events), and more diverse in spectral lag than the ME. However, both components still occupy the long-GRB track in the traditional duration-hardness and duration-MVT planes, indicating a common Type II (collapsar) origin. These results suggest that the EE in long GRBs represents a physically distinct regime of the central engine, characterized by a lower luminosity, longer emission timescales, and evolved spectral properties, rather than a simple continuation of the main burst. This picture is consistent with late-time fallback accretion onto a black hole or proto-magnetar spin-down.

Abstract We investigate vacuum decays in the early Universe in the presence of curvature perturbations. For sufficiently large perturbations associated with over-densities, we find that the bounce solution develops an oscillating middle stage near the bubble wall. For small perturbations, we analytically show, within the thin-wall approximation, that an over-(under-) density would enhance (suppress) the vacuum decay rate for a smaller (larger) initial bubble radius. By numerically solving for the bounce solutions and evaluating the corresponding Euclidean action, we further confirm this behavior in thick-wall cases. Our results indicate that overdensities can, in general, trigger vacuum decay earlier.

Boson clouds formed via superradiance around spinning black holes offer a novel gravitational-wave probe for weakly interacting ultralight particles. We show that such gravitational atoms can undergo a self-stimulated avalanche: a coherent quadrupolar transition is seeded and then amplified by gravitational radiation feedback. We formulate an effective two-level description, validated by numerical simulations, that captures the logistic population transfer and the resulting delayed gravitational-wave pulse with a characteristic envelope, and assess its detectability with future detectors. As a gravitational analogue of superfluorescence, this cooperative emission mechanism opens a new observational avenue into the ultralight dark sector.

Thermodynamics and transport are investigated in a holographic QCD model that extends the Einstein–Maxwell–Dilaton framework by incorporating Gauss–Bonnet corrections. Model parameters are fixed using state-of-the-art lattice QCD thermodynamics. The analysis then examines the equation of state at zero and finite baryon chemical potential, the phase structure in the temperature and chemical potential plane, as well as the shear and bulk viscosity to entropy ratios, η/s and ζ/s, via the corresponding fluctuation equations. For a constant Gauss–Bonnet coupling, the model preserves a reasonable description of the equation of state and generates a temperature-dependent η/s, although the resulting profile remains monotonic near the crossover region, which does not satisfy the phenomenological expectation. When the Gauss–Bonnet coupling is allowed to depend on the dilaton, a non-monotonic η/s and a peaked ζ/s are obtained while maintaining agreement with thermodynamic constraints. The resulting phase diagram contains a critical end point in a phenomenologically relevant region.

We show that oscillating (real-scalar) boson stars generically host an oscillating radial caustic. Sources near this caustic cross it every half period, thereby producing periodic caustic-crossing lensing. The resulting observables are phase locked to the lens oscillation: image-pair creation or annihilation, changing image morphology, achromatic photometric spikes, and astrometric motion. This signal provides a distinctive target for time-domain astronomy, and its detection would reveal an intrinsically time-dependent compact dark-sector object. Event-number estimates indicate a measurable discovery space with current astrometric and high-cadence photometric surveys, while null searches would constrain the abundance of such objects as dark matter. The predictions rely only on the dynamics of real-scalar condensates and extend naturally to self-interacting real scalars, including axionlike particles, and to ultralight vector bosons.

The Planck measurement of the cosmic microwave background (CMB) has established the A cold dark matter (ACDM) model as the concordant model along with other observations. However, recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) have renewed the matter fraction S2m tension between Planck-ACDM and DESI-ACDM. Directly reconciling this CMB-BAO tension with a dynamical dark energy in Chevallier-Polarski-Linder parametrization seems to imply a crossing of the equation of state (EOS) through w = -1 at low redshifts. In this paper, we resolve this S2m tension by allowing for the dark matter nonminimally coupled to gravity via a quintessence field. This nonminimal coupling is preferred over 36 confidence level. Consequently, even though the usual effective EOS of the coupled quintessence apart from the standard CDM part never crosses but always is above w = -1, a misidentification with the w0waCDM model would exactly fake such a crossing behavior, and the tensions on neutrino mass and growth rate in the ACDM model are also relieved in our model as a result of the resolved S2m tension.

We investigate primordial black hole (PBH) formation in a cosmological background with bulk viscosity. Using numerical simulations, we determine the collapse threshold and the resulting PBH mass. We find that the critical threshold μ_c retains a dependence on the equation-of-state parameter w similar to that in the inviscid case, but is enhanced by an amount comparable to the bulk-viscosity strength ε. For fixed w, the increase in μ_c is approximately linear in ε. By fitting the standard critical-scaling law for near-threshold collapse, we find that the bulk viscosity leads to an enhancement in the resulting PBH mass. These results indicate that bulk viscosity can systematically modify both the PBH threshold and PBH mass scaling law in the early universe.

The quantum chromodynamics (QCD) phase diagram is crucial for understanding strongly interacting matter under extreme conditions, with major implications for cosmology, neutron stars, and heavy-ion collisions. We present a novel holographic QCD model utilizing neural ordinary differential equations (ODEs) to map the QCD phase diagram under magnetic field B, baryon chemical potential μB, and temperature T. By solving the inverse problem of constructing a gravitational theory from Lattice QCD data, we reveal an unprecedentedly rich phase structure at finite B, including multiple critical endpoints (CEPs) in strong magnetic fields. Specifically, for B = 1.618 GeV2 = 2.592 × 1019 Gauss, we identify two distinct CEPs at (TC = 87.3 MeV, μC = 115.9 MeV) and (TC = 78.9 MeV, μC = 244.0 MeV). Notably, the critical exponents vary depending on the CEP’s location, and the conventional scaling relations can be violated in the presence of strong magnetic fields. These findings significantly advance our understanding of the QCD phase structure and provide concrete predictions for experimental validation at upcoming facilities such as FAIR, JPARC-HI, and NICA.

We investigate the thermodynamic properties of the Kerr-Bertotti-Robinson black hole, an exact Petrov type D solution of Einstein-Maxwell theory describing a rotating black hole immersed in an external electromagnetic field. While the conserved angular momentum and electric charge can be computed straightforwardly, the conserved mass cannot be obtained through standard integrability methods due to the nontrivial asymptotically uniform external electromagnetic field. To overcome this difficulty, we adopt the Christodoulou-Ruffini mass relation as a thermodynamic definition of the conserved mass, and identify the associated generator, thereby fixing the ambiguity in defining this conserved mass and constructing the thermodynamic potentials. These thermodynamic quantities naturally satisfy the first law of black-hole thermodynamics as well as the Smarr formula.

We develop an efficient three-dimensional numerical-relativity framework for primordial black-hole (PBH) formation from superhorizon curvature perturbations in a radiation-dominated Universe. We implement flux-conservative relativistic hydrodynamics in the adaptive-mesh-refinement code GRChombo and introduce a cosmologically scaled Gamma-driver that allows the cosmic-time step to grow in proportion to the scale factor. For a representative long-term simulation, the scaled driver preserves the apparent-horizon mass evolution and constraint behavior while reducing the number of coarse-level advances by a factor of approximately 94 relative to the standard driver. We also construct a conformal-time version of the moving-puncture gauge as an independent check. Applying the framework to a spherical Gaussian curvature profile, we find a collapse threshold 0.79578 < μ_c < 0.79580 and a critical exponent γ≃ 0.3559, consistent with previous spherically symmetric results. We further fit the late-time PBH mass growth to the Zel'dovich–Novikov accretion law, demonstrating that the code can follow both near-critical collapse and long-term post-formation evolution in three dimensions. The framework provides a foundation for future studies of PBH formation beyond spherical symmetry.

The temporal and spatial coincidence between the gravitational wave (GW) event GW190425 and the fast radio burst (FRB) event FRB 20190425A raises the intriguing possibility of a physical connection between the two. The widely discussed possibility invoking the collapse of a supermassive neutron star as the merger product suffers the inconsistency between the model prediction and the measured inclination angle of the system. Here, we propose a novel physical mechanism to account for the association. We envisage a magnetar located at about 2.5 light hours away from the binary neutron star merger site. The kiloherz GWs generated by the merger are converted into kiloherz electromagnetic (EM) radiation via the Gertsenshtein-Zeldovich (GZ) effect near the magnetar. Subsequent inverse Compton scattering off the kilohertz EM waves by relativistic particles generates the observed gigahertz FRB emission. Our calculation reveals that, with appropriate parameter choices, the properties of FRB 20190425A can be reproduced.

The recent preference for a dynamical dark energy (DE) from the dark energy spectroscopic instrument seems to call for interactions between DE and dark matter (DM), either from direct DE-DM interaction or indirect interaction induced by modified gravity. Therefore, an independent probe for these kinds of DE-DM interactions would be appealing from observational aspects. In this paper, we propose the black hole superradiance as a novel astrophysical probe for field-theoretic interacting DE-DM models, providing complementary constraints independent of large-scale cosmological observations. The core principle is that the DE-DM interaction can alter the effective mass of the superradiant ultralight boson, thereby modifying its superradiant instability rate around spinning black holes. We explore this connection through two distinct scenarios: a model where the DE field mediates a dark fifth force within the DM sector, affecting the superradiance from DM particles and a novel mechanism where the DE field itself becomes superradiant due to the effective mass enhancement induced by dense DM spikes around supermassive black holes. By applying a statistical framework to black hole observations in both scenarios, we derive constraints on the fundamental DE-DM coupling strength. Although the current constraints are rather loose due to small samples and inaccurate measurements, our work provides new astrophysical constraints on these interacting DE-DM scenarios and establishes a new synergy between black hole physics and cosmology for probing the fundamental nature of the dark sector.

We perform fully nonlinear, spherically symmetric numerical simulations of superhorizon false-vacuum-domain (FVD) collapse in a coupled gravity-scalar-fluid system to study primordial black hole (PBH) formation during delayed first-order phase transitions (FOPTs). Using adaptive mesh refinement to resolve the bubble wall, we identify three dynamical outcomes: type B (supercritical) PBHs with an interior baby universe and a bifurcating trapping horizon, type A (subcritical) PBHs with an apparent horizon formed by direct wall collapse, and dispersal with no PBH formation. To separate these three cases, we evaluate two commonly used PBH-formation criteria: the time scale ratio $t_\mathrm{H}/t_\mathrm{V}$ (horizon crossing time versus vacuum-energy domination time) and the local density contrast $δ(t_\mathrm{H})$ at horizon crossing. For the parameter space explored, we find that $t_\mathrm{H}/t_\mathrm{V}$ is a more robust predictor of outcome: type B PBHs form when $t_\mathrm{H}/t_\mathrm{V} \gtrsim 1$ (critical range $\sim 1.1 - 1.6$ in our survey), type A PBHs arise when $t_\mathrm{H}/t_\mathrm{V}$ is below this threshold but remains above a lower bound (typical range $\sim 0.35 - 0.7$), and no-PBH dispersal occurs when $t_\mathrm{H}/t_\mathrm{V}$ falls below this lower bound. When a clear thin-wall FVD boundary exists, $δ(t_\mathrm{H})$ can correspondingly distinguish different outcomes (roughly $δ_c(t_\mathrm{H}) \sim 1 - 1.7$ for type B and $δ_c(t_\mathrm{H}) \sim 0.35 - 0.5$ for type A), but is highly sensitive to wall structure and model details and thus less universal. These results offer new insights into the dynamics of FVD collapse, quantify practical PBH-formation thresholds, and pave the way for precise predictions of PBH abundance from delayed FOPTs.