高能暂现源 · 每日文献综述

Recent JWST observations have revealed a population of compact, optically red sources known as Little Red Dots (LRDs). A popular interpretation is that LRDs host massive black holes embedded in dense gaseous envelopes, yet the physical origin of such envelopes remains unclear. We propose that the optically thick outflow driven by supercritical accretion onto black holes may explain the envelope. Based on an analytic radiative hydrodynamic outflow model, we relate the outflow properties to the black hole mass $M$ and dimensionless accretion rate $\dot{m}$. With $M\sim10^5-10^7\,M_\odot$ and $\dot m \sim 1500-5000$, the outflow photosphere reaches luminosities of $10^{43}-10^{45}\,\rm erg\,s^{-1}$ and temperatures of $\sim 3000-6000$ K, effectively matching the red optical continua observed in LRDs. The presence of a thick scattering region beyond the photosphere is central to deciphering the distinctive properties of LRDs. For each object, if one derives $M$ and $\dot{m}$ from the observed luminosity and temperature, while accounting for both kinematic broadening and scattering effects, the model predicts an emission line FWHM consistent with observations within a factor of 1.5 for more than 80\% objects. Furthermore, with Cloudy simulations, we find that the partially ionized gas beyond the photosphere produces Balmer breaks broadly consistent with measurements.

We report a $γ$-ray excess toward the radio-quiet narrow-line Seyfert 1 galaxy 1H 1934-063 using data collected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope and taking into account the LAT 16-year Source List (FL16Y). During the flare interval, the excess is detected in the 1--500 GeV band at a significance of $\sim 5.2σ$ (TS = 27.12), with a photon flux of $(4.94\pm2.33)\times10^{-10}$ ph cm$^{-2}$ s$^{-1}$ and a hard photon index of $Γ=1.50\pm0.25$. The best-fit $γ$-ray position is consistent with the radio position of 1H 1934-063, while the nearby source FL16Y J1936.9-0552 is not significantly detected during the same interval. In the absence of contemporaneous multiwavelength data, the broadband interpretation cannot be tightly constrained. A compact nonthermal component can produce the hard GeV emission, but the present data do not allow a unique physical interpretation. The excess therefore provides an interesting case for probing high-energy activity in radio-quiet NLS1 galaxies, although the underlying physical mechanism remains uncertain.

In 1971 a group from Columbia University detected polarized X rays from the Crab Nebula at about 3 sigma level during a short sounding rocket flight (Novick et al. (1972)). This measurement, later confirmed to high precision (greater than 20 sigma) by the OSO-8 satellite in 1976/77 (Weisskopf et al. (1978)) marks the real beginning of astrophysical X-ray polarimetry. Since then and before the launch of IXPE, surprisingly few additional measurements had been made. X-ray polarimetry is a demanding field of study. To achieve per-cent-level polarization sensitivity, the collection of more than 106 photons in a polarization-sensitive detector is required. As most cosmic X-ray sources are much fainter than the Crab, to do a systematic study of the X-ray sky demands a highly-sensitive payload and a dedicated mission. Further, as the expected polarization will vary with location in extended sources, such as supernovae and pulsar wind nebula, it is vital that the polarimeter should be imaging. First proposed in its current form in 2007, and selected for funding in 2017, the Imaging X-ray Polarimetry Explorer (IXPE) fits this bill. In this chapter we describe: the design and construction of the IXPE payload and spacecraft; its testing and launch; commissioning activities; science operations and; a high-level overview of IXPE science to date. Finally, we conclude with the current status of the Observatory (as of 2025) and prospects for continued operations.

Solar wind alpha particles exhibit preferential heating and acceleration relative to protons; however, their behavior in the vicinity of turbulent coherent structures remains less understood. We report the first evidence of localized alpha particle and proton heating within coherent structures identified using the Partial Variance of Increments (PVI) method, based on Parker Solar Probe (PSP) observations. Our results show that high-PVI events are associated with significant, species-dependent temperature enhancements: protons undergo a relative larger temperature increase than alpha particles. This preferential proton heating produces a localized decrease in the alpha-to-proton temperature ratio, indicating that the plasma is driven toward thermal equilibration between species. The heating is also anisotropic, being dominated by enhancements in the perpendicular temperature. These temperature-signatures coincide with a pronounced reduction in the normalized alpha-proton differential flow speed and a localized minimum in the Coulomb collision age, suggesting that the relaxation is affected primarily by collisionless kinetic effects. These findings provide new insight into the intermittent energy conversion and ion thermodynamics in the solar wind.

Recent high-resolution observations from the Solar Orbiter mission can help detect the indirect signatures of heating at the smallest scales. In this work we measure the properties and investigate the physical origin of propagating disturbances (PDs) in the intensity at the smallest resolvable scales with Solar Orbiter/EUI along coronal loops. We used two sequences of EUI/HRIEUV at high spatial (down to 125 km per pixel) and temporal resolutions (5 s of cadence). We placed slits along 13 active region (AR) coronal loops. We measured the plane-of-sky (PoS) velocities and the intensity perturbation damping along the slits of PDs. We also measured the periodicity of PDs in one slit by using a Fourier analysis. We report the detection of PDs that have high PoS velocities ranging between 500 and 2000 km/s (which we call "fast" PDs). They are only visible in the upper part of the coronal loops. The intensity increase associated with these fast PDs is on the order of 4\% to 8\% of the HRIEUV intensity, and we measured little to no damping of their intensity with distance. We also measured a peak in the Fourier power spectra at 2 min above the 95\% confidence limit that is associated with fast PDs. The fast PDs are detected in the same coronal loops as PDs with a lower PoS velocity (70 to 90 kms/s), which we refer to as "slow" PDs. Unlike the fast PDs, these slow PDs are only visible in the lower part of the coronal loop, and they show clear intensity damping. Slow PDs show properties consistent with slow magneto-acoustic modes or upflows. On the other hand, fast PDs cannot be explained by slow magneto-acoustic modes. Instead, they show properties consistent with fast flows induced by magnetic reconnection, current sheet generated by propagating transverse oscillations, and fast magnetohydrodynamics modes or Alfvén waves.

We study accretion and neutrino-sensitive thermal signatures of a static black hole reconstructed from a three-parameter generalized entropy. The construction is thermodynamic by design: the entropy deformation is not mapped to a radial-coordinate redefinition or to a prescribed Reissner--Nordström-like correction. Instead, the generalized entropy fixes the horizon response factor $Ξ_h=dS_G/dS|_{S_h}$. The effective exterior geometry is then reconstructed by requiring its surface-gravity temperature to reproduce the generalized thermodynamic temperature. As a result, the metric preserves the horizon area and the Schwarzschild asymptotics, and reduces smoothly to Schwarzschild when $Ξ_h\to1$. The deformation is controlled by $λ_G=1/Ξ_h-1$, while the integer $p\geq2$ determines the radial localization of the near-horizon correction. We compute the photon sphere, critical shadow scale, circular geodesics, ISCO, Novikov--Thorne flux, disk temperature, radiative efficiency, energy-at-infinity luminosity, a redshifted spectral proxy, and neutrino-sensitive temperature moments. In particular, positive $λ_G$ moves the photon sphere and ISCO inward, decreases the shadow scale, raises the radiative efficiency, and concentrates the energy release toward the inner disk. By contrast, negative $λ_G$ produces the opposite trend. Finally, the neutrino sector is modeled conservatively using dimensionless temperature moments instead of a full neutrino-dominated accretion flow or annihilation-deposition calculation. The hierarchy among the $ n=4$, $n=6$, and $n=9$ moments reveals that entropy-induced disk deformation becomes increasingly apparent with higher temperature exponents. Thus, observable changes in compact orbits and thin-disk emission can be directly linked to generalized entropy via the horizon response.

AGN emission is intrinsically variable across the electromagnetic spectrum. Mapping the coupling between the accretion disk, the X-ray corona, and ejection flows is key to understanding the energy flow within the central engine. We characterize the multi-wavelength variability of 14 hard X-ray selected AGN from the INTEGRAL/IBIS catalog in the radio, optical, and X-ray bands, to determine the coupling between these frequencies and how variability relates to the physical properties of the central engine, with emphasis on the radio band. We analyzed multi-epoch observations from AMI-LA at 15 GHz, ZTF in the g and r bands, and Swift/XRT over 2018--2020. Variability was quantified using the normalized excess variance, the fractional variability amplitude, and the Mexican Hat filter at 70- and 200-day timescales. We also characterized the radio-loudness of the sample and evaluated the impact of variability on the Fundamental Plane of black hole activity by comparing time-averaged with strictly simultaneous data. Significant variability is detected in 86% of the sample, with a clear amplitude stratification: the fractional rms amplitude is highest in X-rays, with a median of 30% (11-67%), followed by the optical g and r bands at 19% (2-33%) and 8.5% (0.2-24%), and the radio band at 10% (4-23%). The Mexican Hat analysis reveals a red-noise power spectrum dominated by long-term fluctuations. The sample follows the expected Fundamental Plane scaling; although individual sources shift within the relation due to stochastic fluctuations, this dispersion accounts for only ~3% of the total scatter. Our findings support a core-dominated origin for the 15 GHz emission, likely a compact jet base or a magnetized corona, while differences in variability patterns, radio-loudness, and Fundamental Plane location point toward distinct accretion/ejection processes and degrees of corona-jet coupling.

We investigate gravitational lensing in Kerr spacetime in the presence of a modified photon propagation law arising from higher-curvature effective field theory corrections. Adopting a deformed dispersion relation, we analytically derive the deflection angle and the propagation time delay for null trajectories in a rotating background. We show that the modified propagation leads to explicit and calculable deviations from the standard Kerr predictions, affecting both the bending angle and the time delay. These corrections exhibit a nontrivial dependence on the black hole spin and are expected to become more relevant in the strong-field regime. In particular, near the photon region-where the spin-dependent geometry influences photon trajectories-such effects may play a role in shaping observable propagation features. Our results establish a concrete and systematic framework to quantify deviations from standard photon propagation in gravitational lensing. They further indicate that observables such as relativistic image separations and time delays provide a potential avenue to probe ultraviolet corrections to gravity in strong-field environments.

We describe a classical (non-quantum) radiation process: additional (to Hawking) emission by a black hole evaporating in an external magnetic field in vacuum. The electromagnetic radiation process is completely electric charge-free and bears some resemblance to the Gertsenshtein-Zel'dovich effect. The time evolution of the spacetime metric perturbs a static background magnetic field, inducing a radiative field that acts as an effective electromagnetic source even in the absence of physical charges or currents. To isolate the dynamic effects of the time-dependent spacetime on the external magnetic field, we approximate Hawking radiation as a spherically symmetric outflow of null fluid governed by the prescribed time-dependence of the central mass $M(t)$. We employ Laplace transform, which selects the retarded outgoing branch of the electromagnetic response, producing fields proportional to $Θ(t-r)$ and thereby fixing a causal radiative arrow of emission. The emitted spectral energy is red-dominated, scaling as $d\mathcal{E}_B/dω\propto B_0^2 m_0^2 τ_H^{-2/3} ω^{-8/3}$, where $τ_H$ is the Hawking evaporation timescale, and corresponds to a pure Transverse Electric (TE) mode. There is no final bright burst at the end of the evaporation. We offer classical analogues for this mechanism as transmission-line emission and, separately, as the displacement current emission from a medium with time-varying dielectric permittivity.

In this article, We investigate the cosmological consequences of a recently developed $f(R,G,\mathcal{T})$ gravitational framework, in which the action is formulated as a general function of the Ricci scalar $R$, the Gauss-Bonnet invariant $G$, and the trace of the energy-momentum tensor $\mathcal{T}$. As one of the most reliable probes of the physical conditions in the early universe, Big Bang nucleosynthesis offers a stringent framework for testing deviations from standard cosmology. We consider four representative models that are analyzed and constrained using observational limits on $\left|ΔT_f/T_f\right|$ and the primordial helium mass fraction $Y_p$. The bounds obtained identify the allowed parameter regions for each model and demonstrate that significant departures from standard cosmology are compatible with nucleosynthesis observations. Our analysis shows that broad regions of the parameter space satisfy existing nucleosynthesis constraints, indicating the consistency of $f(R,G,\mathcal{T})$ gravity with the observed primordial light-element abundances and the established picture of the early universe preserving the observed abundances of light nuclei.

We present optical observations on the young type Ia supernova (SN Ia) SN 2021fxy obtained within a few days after the explosion, with a focus on its prominent high-velocity features (HVFs). It reached a $B$-band maximum of $M_{\rm max}(B) = -19.36\pm0.31$ mag, corresponding to a bolometric luminosity of $\sim 1.3\times10^{43}~\rm{erg~s^{-1}}$ with a synthesized $^{56}$Ni mass of $0.58\pm0.14$ M$_{\odot}$. The early spectra exhibit strong HVFs of intermediate-mass elements that are significantly detached from the photospheric components. In particular, the velocity of the Si II $\lambda6355$ HVFs follows a power-law evolution ($β\approx 0.1$), shallower than the expected photospheric velocity evolution expected for an assumed $n=10$ density profile ($β\approx 0.22$) under homologous expansion. This behavior is consistent with the HVFs forming in intrinsic ejecta structures at least partially decoupled from the bulk outer ejecta, providing a possible constraint on the explosion physics of SNe Ia.

We present a comprehensive multiband photometric and spectroscopic study of the G-type binary CW Cas whose parameters have not been well determined. Our double-lined spectroscopic radial velocity curve of this system yields a reliable mass ratio of $q = 1.88(9)$. By combining $BVR_{c}I_{c}$ bands, TESS light curves and radial velocity curves, we found that CW Cas is a W-subtype shallow contact binary with a fill-out factor of 15\%. The components have masses of $0.98(6)M_{\odot}$ and $0.52(4)M_{\odot}$, separated by $2.25(5)R_{\odot}$. A notable asymmetry in the maxima of the light curves was detected and explained by a dark spot located on the surface of at least one component. Comparison of light curves from different years revealed that these dark spot activities exhibit cyclic variations with an approximate period of 1250 days. Orbital period analysis via O-C diagram spanning 125 years shows a long-term decrease superimposed with periodic oscillation caused by the light-travel-time effect (LTTE) due to a third body. This tertiary component has an orbital period of $P_{3} = 99.4(6)$ years and a minimal mass of $M_{3}=0.91(1)M_{\odot}$. The absence of detectable signatures for this massive object in either spectroscopic or photometric datasets implies it must be a compact object such as a white dwarf or neutron star. Furthermore, a visual companion was identified based on Gaia DR3 astrometric data, suggesting that CW Cas is part of a hierarchical quadruple system. As such, CW Cas represents a valuable laboratory for probing 2+1+1 hierarchical multiple system hosting compact object.

The growth of galaxies in the early Universe is thought to be dominated by compact, intensely star-forming systems, yet the corresponding growth of supermassive black holes (SMBHs) within such environments remains poorly constrained. Green Pea galaxies are nearby analogs of rapidly assembling galaxies in the early Universe owing to their compact morphologies, intense star formation, low metallicities, and extreme ionization conditions. Although galaxy interactions are thought to trigger episodes of rapid SMBH growth, direct observations of simultaneous accretion onto multiple SMBHs in compact, intensely star-forming galaxies are lacking. Here we report the discovery of the first confirmed dual active galactic nucleus (AGN) in a Green Pea system, SDSS J162209.41+352107.5. Chandra imaging resolves two luminous hard X-ray sources separated by 8.4 kpc in projection, demonstrating simultaneous accretion onto two SMBHs. Follow-up Keck spectroscopy confirms that the two optical nuclei share a common redshift and independently exhibit broad Balmer emission and high-ionization AGN emission lines. Unlike most known dual AGN, which are typically found in massive mergers, J162209.41+352107.5 is a compact low-mass system analogous to galaxies thought to dominate early phases of galaxy assembly. These findings demonstrate that efficient growth of multiple SMBHs can occur in such environments and establish Green Pea galaxies as nearby laboratories for investigating the interplay between galaxy interactions, star formation, and black-hole growth under conditions analogous to those prevalent in the young Universe.

Gravitational waves (GWs) have proven to be a powerful probe of compact binary populations. In the millihertz frequency range accessible to Laser Interferometer Space Antenna LISA, binary white dwarfs (BWDs) are expected to constitute a dominant source, forming both individually resolvable signals and an unresolved Galactic background. In this work, we construct a Milky Way like population model and calculate the GW background from unresolved Galactic BWD in the LISA sensitivity range, with particular emphasis on exploring and constraining uncertainties in binary stellar evolution. We employ COMPAS binary population synthesis framework to generate synthetic populations of BWD in the Milky Way. Various physically motivated evolution prescriptions and initial model parameters are used to study diverse population of BWDs. From these populations, we construct the GW background and investigate the dependence of the background spectrum on the assumptions on binary analysis. We discuss the possibility of constraints on binary evolution that LISA GW observations may yield. We find that the shape and amplitude of the background are sensitive to key binary evolution parameters like common envelope evolution and mass-transfer efficiencies. Variations in these assumptions lead to measurable differences in the predicted background spectrum. Our results demonstrate that LISA observations of the unresolved BWD background have the potential to constrain binary evolution models. This highlights the importance of GW background modelling as a complementary tool for studying the formation and evolution of compact binaries in the Milky Way.

Simulation-based inference (SBI) has become a major tool for extracting cosmological information from weak-lensing (WL) surveys, particularly from non-Gaussian observables. We compare its two main paradigms: explicit likelihood inference (ELI), based on a Gaussian likelihood built from an emulator and covariance matrix, and likelihood-free inference (LFI), which learns the likelihood directly from simulations using neural density estimators. Using Gaussian random field mocks representative of the non-tomographic final Euclid data release, we analyse shear two-point correlation functions (shear-2PCFs), compressed with linear or non-linear methods, together with a fundamentally different map-level convolutional neural network (CNN) statistic, focusing on $Ω_{\rm m}$ and $S_8$. We deploy posterior calibration diagnostics developed for LFI, including the test of accuracy with random points (TARP), showing that ELI becomes strongly miscalibrated under emulation inaccuracies or likelihood non-Gaussianity, whereas LFI remains well calibrated. These effects drive substantial disagreement between ELI and LFI, which largely vanishes once addressed. We further show that the compression scheme can significantly degrade ELI while leaving LFI largely unaffected. Although shear-2PCFs should capture all the information in Gaussian fields, finite compression and non-Gaussian likelihoods cause ELI constraints to differ by up to a factor of two from those inferred with the CNN, while the discrepancy drops to $\approx 30\%$ for LFI, underscoring the robustness of the deep-learning probe. Overall, our results indicate that in our simple setup, which neglects systematic biases, LFI provides a more robust and better-calibrated framework, while highlighting accurate non-Gaussian likelihood modelling and posterior calibration diagnostics as essential for future ELI analyses.

Cosmic Domain Wall networks are among the most interesting sources of a stochastic Gravitational Wave (GW) background from the early Universe. We present a thorough analysis of their annihilation, with a focus on scenarios where the collapse is induced by a population bias, whereby one of two degenerate vacua is initially preferred over the other. Our state-of-the-art $3+1$ lattice field theory simulations in the expanding Universe reveal that the network decays around the temperature $T_\text{ann}\sim T_s\,\mathcal{B}_s^{0.8}$, where $\mathcal{B}_s$ quantifies the preference for one vacuum over the other at the onset of the scaling regime at the temperature $T_s$. Furthermore, we obtain the spectrum of GWs from such networks, and provide a detailed comparison with the alternative potential bias annihilation mechanism that relies on a small explicit symmetry breaking in the potential. En passant, we update results on the evolution of these networks and on their GWs, and clarify existing disagreements in the recent literature. Our results sharpen the phenomenological viability of spontaneously broken discrete symmetries, and provide GW spectra that Pulsar Timing Arrays (PTAs) and ground-based interferometers (LIGO-Virgo-KAGRA) can readily use in their searches for a cosmological GW background.

The fourth observing run (O4) of Advanced LIGO, Virgo, and KAGRA presents unparalleled sensitivity, rendering unsupervised pipelines highly vulnerable to the non-stationary domain shift of the detectors' noise manifolds. We present DANTE V3, concluding a longitudinal investigation into unmodeled anomalies during early O4a. By expanding to a multi-scale geometric framework (0.5 s to 4.0 s), we amplify morphological sensitivity, extracting 10,372 unique candidates. To mitigate domain-shift artifacts, we introduce a block-bootstrap Domain Shift Defense (DSD) against a vector-quantized native background index. While 28.3% of candidates survive recalibration, global topological analysis reveals they lack discrete morphological cohesion. The survivors coalesce into a single macro-cluster without compact substructure; we show explicitly that morphological "diffusivity" comparisons used previously are confounded and retire them. Pristine background is even more monolithic (100% in one cluster vs 99.77% for survivors): this topology is a property of the embedding geometry, not of anomaly status. Executing a definitive physical environment monitoring (PEM) cross-correlation defense using an empirically calibrated null, one singleton is vetoed by a control-line coupling, while another survives as an uncatalogued instrumental outlier. Replacing the embedding-similarity cross-detector coincidence test with a physical normalized cross-correlation test, we find no coincident events among 8,749 candidates. We quote 90% frequentist Poisson upper limits on the rate of novel uncatalogued instrumental morphologies -- $R_{90} \le 5.83 \mathrm{yr}^{-1}$ (H1) and $R_{90} \le 5.63 \mathrm{yr}^{-1}$ (L1). This underscores the absolute necessity of native background recalibration and physical auxiliary vetoes in unsupervised gravitational-wave astronomy.

The parameterized post-Newtonian (PPN) approach is the state of the art formalism for performing theory independent tests of weak-field gravity, and for constraining possible deviations from Einstein's theory. Within this framework, global conservation laws are useful for the calculation of dynamics and for giving meaning to parameters. In this paper we extend the concept of semi-conservative and fully-conservative theories of gravity to include situations in which compact astrophysical bodies are modeled as masses that are sensitive to their local environment, as relevant for theories that violate the strong equivalence principle. We find that globally conserved quantities can still exist in the presence of such sensitivities, and find their explicit forms when they do. We identify new ways of writing the coefficients that enter into the PPN metric when a theory of gravity admits conserved quantities in the presence of a sensitive body, and demonstrate the applicability of our approach by comparing it to known results in scalar-tensor theories of gravity.

We perform a search for continuous gravitational waves from J0435+3233 using LIGO O4a public data. J0435+3233 is unique among millisecond pulsars as it exhibits an exceptionally large spin-down and marks the first pulsar observed to date with a spin-down larger than $10^{-12}$ Hz/s in the sub $10$ ms spin period range, making it a potentially strong source of continuous gravitational waves. We target signals at exactly twice the rotation frequency, a narrow band around this frequency, and also signals corresponding to r-modes. Our results are consistent with a non-detection. Our most stringent upper limit on the intrinsic gravitational wave amplitude is $h_0=\targetedULninetyfive$, at the 95\% confidence level, which is a factor of $\approx14$ times lower than the spin-down limit. With an estimated source distance of $1.2$ kpc our upper limit on the gravitational wave amplitude excludes source ellipticities greater than $\targetedellipULninetyfive$, making this the first source for which the spin-down upper limit is beaten by over an order of magnitude {\it{and}} the ellipticity is constrained to a physically interesting range, in the low $10^{-8}$ region.

Ultralight bosonic dark matter (UBDM) that couples to electromagnetism can generate an oscillating magnetic-field signal at the Earth's surface. This is referred to as the "Earth transducer" effect, as the Earth converts UBDM into a detectable magnetic field. Similar DM-induced fields in laboratory experiments typically scale with the size $L$ of the experiment. Because the Earth transducer signal instead scales with the large radius of the Earth, $R$, it is one of the most powerful direct probes of UBDM with masses $m_\mathrm{DM}\lesssim1/R\sim3\times10^{-14}\,\mathrm{eV}$. It has many other favorable properties, such as high spatial and temporal coherence and robustness to atmospheric modeling. In this review, we derive the Earth transducer effect and its properties for multiple UBDM models, and discuss current and future prospects to detect it.

We investigate the production of particles much heavier than the characteristic scale of a cosmological first-order phase transition through collisions of highly boosted bubble walls. Using the scalar order-parameter field, we derive the ultraviolet behavior of its Fourier-space profile for both elastic and inelastic collisions. In the regime $χ\equivω^2-\mathbf{k}^2\gg M_h^2$, we find the universal result $\tildeφ(χ) = -2V^\prime(2v_φ)χ^{-2}+O(χ^{-3}),$ implying that the spectral density scales as $F(χ)\propto [V^\prime(2v_φ)]^2χ^{-4}$. Thus, heavy-particle production is localized near the instant of collision and, at leading order, depends on the scalar potential only through $V^\prime(2v_φ)$. We verify this behavior using high-precision numerical solutions of the trapping equation, carefully suppressing spectral leakage from the finite integration domain, and obtain agreement over a broad ultraviolet range. We then derive analytical production rates for general heavy-particle thresholds and for fermion pairs, together with their cosmological number density and yield. Finally, we extend the analysis to $(3+1)$ dimensions and incorporate the finite bubble radius, finding an order-one suppression relative to the parallel-wall approximation. Our results revise the ultraviolet scaling used in previous treatments and have direct implications for superheavy dark-matter production and baryogenesis from bubble collisions.

In Jordan-frame scalar--tensor gravity, a timelike scalar gradient defines an effective thermodynamic medium. We show that a transverse--traceless perturbation of a homogeneous shear-free background activates the local shear-work channel $\mathcal W_φ=π_{ab}^{(φ)}σ^{ab}$, with leading pure-TT contribution $\mathcal P_{\rm TT}^{[1,1]} =\overline{\mathcal K\mathcal T} \dotχ_{ij}^{\rm TT}\dotχ_{\rm TT}^{ij}/4$. This stress power follows from local traction mechanics, enters the scalar-sector energy balance, and is gauge invariant at second order. The same scalar--tensor coupling controls non-GR tensor damping and appears with the opposite sign in a GR-normalized amplitude balance, although its interpretation as a separate tensor-energy source is normalization dependent. Scalar--tensor gravity thus provides a controlled example in which a propagating spin-2 mode acquires an intrinsically gravitational thermodynamic interpretation.

We present effective temperature (Teff) estimates of low-mass stars and T Tauri stars (TTS) candidate detections derived from automated spectroscopic measurements and low-complexity machine-learning models applied to the LAMOST DR10 V2 survey. Equivalent widths of key diagnostic spectral features, including the atomic lines Halpha, LiI 6708 AA and TiO/VO molecular bands, are automatically measured for all stars within 1 kpc observed by LAMOST. Using nine spectral features as inputs, we train a Gradient Boosting Machine tree-based regression model, calibrated with synthetic spectra from the PHOENIX library, to predict Teff over the range 2,500 - 5,100 K. We apply a logistic regression model to the principal components derived from the measured spectral features, enabling efficient identification of TTS candidates. Both models exhibit strong performance in validation tests. Finally, a Monte Carlo framework is employed to propagate input uncertainties and estimate Teff and its associated uncertainties for low-mass stars from 1,733,802 spectra and to identify 2,534 candidate TTS from 3,121 spectra.

This study investigates the dynamical behavior of particle's test around a spinning charged Einstein-Maxwell-dilaton (EMd) quantum corrected black hole (QCBH) by examining the effects of spinning parameter $a$, quantum correction parameter $b$, and charge parameter $Q$. Using EMd spacetime geometry, we analyze the effective potential and effective force to determine the stability and structure of the circular orbits in a strong gravitational field. Furthermore, we calculate the oscillation frequencies radial $(Ω_r)$, vertical $(Ω_θ)$, and angular $(Ω_φ)$, as well as the corresponding Periapsis and Lense-Thirring precession frequencies. The spin parameter primarily determines the spinning properties of spacetime, while the quantum correction and charge particle introduce additional biases to Kerr-Newman geometry, particularly near the event horizon. These corrections alter the position and stability of the circular orbits, and the spectrum effectively associates with particle's motion. The results provide a comprehensive description of the orbital dynamics of spinning EMd QCBH through future high-precision astrophysical observations.

Recent DESI results exhibit preference for a Null Energy Condition violating dynamical dark energy, with early phantom behaviour. We explore an alternative interpretation in which this preference arises from unconventional dark matter dynamics rather than from dynamical dark energy. We propose a dynamical dark matter (DDM) model, with a non-zero equation of state (EoS) that smoothly interpolates between early time and late time asymptotes across a transition scale factor $a_t$, and study its consequences against cosmological datasets including CMB, DESI DR2 BAO, SNeIa (PantheonPlus, Union3, and DESY5) and growth rate data. We find the early time EoS to be consistent with zero, while the present day value is negative at a significance ranging from $0.42σ$ to $3.02σ$ depending on the dataset combination. The strongest preference occurs from the combination of CMB, DESI, and DESY5 giving the present day EoS to be $-0.060^{+0.013}_{-0.028}$ and $a_t = 0.41^{+0.088}_{-0.13}$ at 68\% CL. This preference for a non-zero, late time DM EoS persists when growth rate data are included and across all three SNeIa compilations considered, while the matter density $Ω_m$ mildly shifts to higher values relative to $Λ$CDM. The model also predicts a lower $σ_8$ and $S_8$ than $Λ$CDM, in better agreement with weak-lensing data, while $H_0$ remains unchanged and in tension with local distance-ladder measurements. The DDM model is preferred over $Λ$CDM ($Δχ^2_{\rm MAP} = -14.093$, $Δ{\rm DIC} = -7.838$ for Planck+DESI+DESY5) but disfavored relative to the CPL parameterization of DE ($Δχ^2_{\rm MAP} = 6.755$, $Δ{\rm DIC} = 7.966$). This preference is consistent among other combination of datasets as well.

We construct an $R^2$ cosmology based on scale invariance and unimodular gravity in which the Goldstone boson of dilatations establishes a predictive connection between inflation and dark energy. The conservation of the corresponding Noether current confines the inflationary trajectory to a one-dimensional orbit in field space, freezing the Goldstone direction and rendering the inflationary dynamics effectively single field. After inflation, the system settles on a Minkowski vacuum manifold. The unimodular integration constant, already present in the full theory but negligible during inflation, lifts the flat Goldstone direction, generating an exponential potential for the canonically normalised field. Unlike in phenomenological quintessence models, its slope is not a free DE parameter, but is instead fixed by the field-space geometry inherited from the inflationary attractor. This geometry excludes matter-era tracking and, throughout the inflationary viability region, places the field in the thawing regime compatible with accelerated expansion. For a representative inflationary benchmark, the now pseudo-Goldstone field remains frozen until close to the present epoch, yielding DE equation-of-state parameters $(w_0,w_a)\simeq(-0.992,-0.011)$ in the CPL parametrisation $w(a)=w_0+w_a(1-a)$. More generally, the same parameter controlling the inflationary spectral tilt also fixes the slope of the DE potential, leading to a tight correlation between early- and late-Universe observables: increasing the thawing signal lowers $n_s$, while values of $n_s$ closer to unity drive the model towards $Λ$CDM.

Dynamical friction prescriptions used for common-envelope and planetary engulfment inspirals often assume a homogeneous medium and/or rectilinear perturber motion. A gravitating object embedded in a giant-star envelope instead excites an orbit-scale wake while moving on a curved orbit through a finite, radially stratified medium. We formulate the linear barotropic acoustic response of a weak point perturber on a circular orbit in a hydrostatic, spherically stratified gaseous medium. We apply the formulation to single perturbers in power-law density profiles and giant-star envelope models, and to double perturbers in power-law density backgrounds. We find that stratification affects dynamical friction through the global structure of the wake. The radial component is set by the low-order, orbit-scale wake and can strongly differ in amplitude and sign from the homogeneous-medium result. The azimuthal component is also modified by stratification, but in the supersonic regime it retains the Coulomb-logarithmic sensitivity of the homogeneous problem. In double-perturber systems, the companion wake can substantially change the radial force and reduce the azimuthal drag on a given component, but, unlike the perturber's own wake, it has no local Coulomb-logarithmic contribution. For the adopted giant-star envelope profiles, the azimuthal drag exerted by the stratified wake gives shorter inspiral times than uniform-medium prescriptions evaluated with the same local background quantities. The formulation provides a flexible tool for computing embedded-perturber wakes in prescribed radial stratifications and is a first step toward computationally efficient, self-consistent models of common-envelope and planetary-engulfment inspirals.

The fragmentation of massive molecular clumps into dense cores sets the initial conditions for stellar clusters, yet how core mass distributions evolve remains unclear. Using the ALMAGAL survey (1,013 clumps and 6,348 cores), we analyze the Gini coefficient of core mass distributions for the 514 clumps containing at least four cores ($N_{\rm frag} \ge 4$), which host 5,728 cores in total. We find that the most massive core consistently accounts for $\sim 38\%$ of the total core mass ($f_{\rm core,max} \approx 0.38$), with no systematic trend with the total mass of the core per clump $M_{\rm core,tot}$. The Gini coefficient increases with total core mass ($M_{\rm core,tot}$, $r=0.55$) and evolutionary stage ($L/M$, $r=0.38$). Clumps with $L/M > 6.0$ have significantly higher Gini values (median $0.62$) than those with $L/M \le 6.0$ (median $0.49$; $p = 4.23\times10^{-15}$). These results demonstrate that while the mass fraction of the most massive core is constant, overall mass inequality among cores grows systematically as clumps evolve.

A direct separation of the linearized Einstein equations in Kerr is presented. The trace of the perturbed metric is found to obey the spin-0 perturbation equation and so is irrelevant for the spin-2 perturbations. By combining the traceless condition, the Killing-Yano symmetry, a parity requirement, and the de Donder gauge condition, it has been found possible to reduce the linearized Einstein equations in Kerr to a manageable level, thereby enabling the direct separation of the equations and the derivation of a unique set of decoupled mode functions for the spin-2 perturbation of the Kerr black hole.

Three-body encounters are frequent events in stellar systems, intrinsically chaotic, and computationally costly to model with direct N-body integration. Predicting whether such encounters lead to binary formation is therefore challenging, particularly in large-scale simulations. We aim to develop an accurate and physically interpretable machine-learning model that predicts binary formation from the initial conditions of three-body encounters, to assess its reliability, and to identify the physical parameters that most strongly determine the outcome. We trained an XGBoost binary classifier on a balanced dataset of three-body scattering experiments computed with the REBOUND code and the IAS15 integrator. The input to the model consists of 30 physically motivated features describing the masses, energies, and kinematics of the initial configuration. The classifier achieves excellent performance on a balanced test set, with accuracy, precision, recall, and F1-score all above 0.94, and with ROC-AUC and PR-AUC values of 0.99. Feature-importance analysis shows that the outcome is governed primarily by the mass hierarchy and hardness ratio of the encounter, followed by velocity fraction and mass entropy. The predicted probabilities are well calibrated, with an expected calibration error (ECE) of 0.02. Inference is approximately 400 times faster than direct N-body integration. The model also generalizes well across encounter radii, although its performance decreases in the weak-interaction regime where binary formation becomes intrinsically rare. These results show that machine learning can provide fast, accurate, and physically interpretable predictions of binary formation in three-body encounters. Such models offer a practical complement to direct N-body simulations and may enable efficient probability estimates in large-scale simulations of stellar systems.

We investigate potential singularities in isotropic cosmological models featuring a time-varying speed of light and the gravitational constant. The governing field equations generally simplify into two-dimensional systems, subject to analysis through dynamical systems techniques within phase space and the application of asymptotic splitting methods. In the broader context, our findings reveal the existence of initially expanding closed models that undergo subsequent recollapse towards a future singularity, and open universes exhibiting perpetual expansion into the future. The specific characteristics of these singularities are also expounded upon.

Observations reveal a pronounced deficit of white dwarfs (WDs) in open clusters (OCs) relative to theoretical expectations, suggesting that a significant fraction of WDs may have escaped from their parent clusters after formation. In this work, we perform a systematic search for escaped WD candidates from OCs by back-tracing the motions of WDs and star clusters from Gaia DR3 catalogs. We identify 476 candidate WDs with kinematics consistent with having escaped from one of 175 OCs. A control-field Monte Carlo (MC) test yields a contamination rate of 87.6% +/- 4.3%. Given this high contamination rate, we filter the candidates by ensuring each WD's total age is consistent with its host cluster age, thereby establishing a more reliable follow-up sample of 109 stars. The excluded candidates with anomalous ages are more likely field interlopers or products of accelerated binary evolution. Among these, the low-mass regime exhibits a clear excess over expected field binary merger rates, whereas the high-mass regime remains broadly consistent with binary population synthesis predictions. Finally, comparing escaped WDs with cluster properties, absolute escape counts appear limited by Gaia's distance-dependent incompleteness. The normalized escape fraction shows little dependence on cluster age, possibly favouring WD loss near formation over gradual evaporation, but is strongly anticorrelated with cluster mass, plausibly because deeper potential wells of massive clusters retain more of their WDs.

Based on eigendecomposition and matrix perturbation theory, we propose an alternative method for analyzing perturbations in massive gravity that is applicable to both the base theory and its various modifications. We demonstrate the validity of this approach even in seemingly singular regimes. Applying this framework to mass-varying massive gravity, we compute perturbations for all constant-scalar Schwarzschild-(A)dS black hole solutions. We show that, under the condition $β= α^2$, the tensor sector of these black holes reduces to that of general relativity, while the scalar sector remains free from tachyonic instabilities given appropriate parameter choices. A deeper relationship is also revealed between the solutions with generic parameters and those with $β= α^2$: their solution manifolds intersect at a specific locus where the perturbation vanishes.

The redshifted 21 cm line from the hyperfine structure of neutral hydrogen atoms is a promising tracer for the three-dimensional evolution of our universe. Its broad spatial and temporal coverage is crucial for understanding the complex nature of dark matter and dark energy. However, it is very challenging to directly detect the 21 cm signal due to the existence of radio foreground contaminants that are orders of magnitude brighter. Therefore, mitigating the foreground contamination becomes an indispensable task for detecting the 21 cm signal, which is also expected to be correlated with the dark-matter-dominated large-scale structure. The cross-correlations between the neutral hydrogen intensity mapping and galaxy surveys in the future can not only confirm a detection of the 21 cm signal but can also be a complementary probe for 21 cm cosmology. To meet the precision requirements for cosmological studies, it is important to investigate the complex features of the estimated cross-correlations using numerical simulations. In this work, we simulate the low-redshift HI observations and narrow-band optical surveys in the future and obtain the foreground- and bias-mitigated HI-galaxy cross-power spectra, with which we perform a Bayesian analysis to infer cosmological parameters of dynamical dark energy model. The method developed in this work will be important for cosmological studies with the 21 cm intensity mapping.

Microlensing, the influence of stars within a galactic gravitational lens, has emerged as a powerful probe of compact mass and, through differential magnification, sub-parsec scale sources at cosmological distances. The recent discovery of a double-source-plane gravitational lens system in which the most distant source is a quasar offers the prospect of compound microlensing, in which quasar light rays are influenced by compact masses within the two foreground lensing galaxies. Here, we present the first numerical simulations of this "microlensing of microlensing". We consider the recently discovered "Einstein zig-zag" lens, J1721+8842, as a fiducial case, and construct microlensing magnification maps for each of the six quasar images in this system. Due to the secondary microlensing effects of the myriad of initial microimages, the resulting maps contain more complex caustic features than seen in the case of single plane microlensing. This is reflected in the expected lightcurves seen for each of the images.

Context. Accretion streamers connected to protostellar disks and/or envelopes are thought to transport material across several thousand of au. Whether the motions of the gas comprising these streamers are dominated by gravity, large scale external turbulence or the action of magnetic fields is still under scrutiny. Aims. The aim of this work is to understand the role of the magnetic fields in the star-formation processes, in particular the role that magnetic forces have in potentially leading flows of gas and the accretion onto the envelopes and disks orbiting protostars. Methods. First, we try to identify the large-scale accretion streamers toward the high-mass Young Stellar Object GGD 27-MM1 and fit their trajectories using the so-called Mendoza's model, a modification of the classical model of pure gravitational infalling motion of fluid particles in a potential well. Second, we estimate the strength of the magnetic field associated with the streamers. Then, we determine if the streamers are dominated by magnetic or centrifugal forces. Results. Inspecting the Atacama Large Millimeter/submillimeter Array (ALMA) H$_2$CO cube we were able to identify four accretion streamers spreading up to $\sim$7,000 au and fit their trajectories in the position-position-velocity space. The polarized continuum emission reveals a good alignment of the magnetic field and the trajectory of the streamers. Using the Davis-Chandrasekhar-Fermi method, we derive estimates for the magnetic field strength, find that the streamers are sub-alfvénic, and discuss (after estimating energy terms for turbulence, ordered motions, magnetic forces and gravity) a possible qualitative scenario in which, the gravitational well of the GGD 27-MM1 protostar dominates streamer gas motions over turbulence and magnetic forces at distances of $\sim 3,000$ au.

Protostellar core formation and growth in high-mass star-forming regions remain key to understanding massive star birth. We analyze the masses of 839 cores (resolved at scales of a few thousand au) from the ASHES project targeting 39 massive infrared dark cloud clumps. The masses of the three most massive cores scale linearly with the total core mass. They maintain a constant mass fraction of ~25%, 16%, and 10% along the mass growth sequence. These fractions reveal that the progenitor seeds destined to become high-mass cores establish their mass dominance very early. Additionally, the Gini coefficient (a statistical measure of inequality) of the core mass distributions increases along the mass growth sequence, confirming that the relative population of low-mass cores builds up toward later stages. This points to an environment-dependent fragmentation picture: central prestellar seeds rapidly evolve into high-mass cores via transport-driven super-Jeans fragmentation under rapid, non-stationary mass accumulation in high-density, turbulent hubs, subsequently maintain their dominance through supply-limited synchronized growth (at R<1 pc, n_H2>10^5 cm^-3), while the formation of the surrounding low-mass cores is relatively delayed due to their lower gas densities and the lack of non-stationary inflow acceleration effect, resulting in their continuous emergence through Jeans-like fragmentation in lower-density envelopes. This picture is consistent with a gravity-driven scenario where the local free-fall time is the controlling factor. Our analysis suggests that non-stationary density-regulated fragmentation and supply-limited accretion jointly drive the synchronized co-evolution of the core cluster, seamlessly linking small-scale core growth with large-scale reservoir regulation.

The importance of massive stars cannot be overstated: they are powerful probes of the early universe, play a vital role in the chemical and mechanical evolution of their host environments and their end products allow us to study the most extreme physics in the universe. Obtaining accurate stellar and surface parameters for large samples of massive stars is vital to our understanding of how they evolve, and how their births, lives and deaths affect their surroundings. With the large volume of data expected from upcoming spectroscopic surveys, computational limitations will likely be the most important bottleneck impeding our progress. To address this and dramatically decrease computing times, we aim to develop a robust emulator for the FASTWIND radiative transfer and spectral synthesis code. Additionally, we aim to explore alternative fitting methods that have not been feasible until now due to computational costs. We calculate a set of FASTWIND synthetic spectra of OB-type stars, and we train a collection of neural networks to emulate these models. We also develop the open-source python package SpecFANN, which provides users with a suite of fitting methods that can be used with these or other user-generated neural networks. The majority of the trained neural networks reach average accuracies of better than ~0.01-0.1% for photospheric lines and better than ~0.1-1% for wind lines. SpecFANN is able to obtain robust and accurate stellar parameters that are consistent with the literature for a sample of 52 early-type stars. Using SpecFANN we find that we can achieve the same fit in ~1/360,000 of the time when compared to alternative techniques that rely on on-the-fly FASTWIND computations. We have demonstrated that neural networks offer a viable path forward to address the computational limitations of our current atmosphere analysis and stellar parameter determination methods for hot stars.

We demonstrate that in a scalar-tensor theory where dark matter is conformally coupled to a scalar field, a black hole surrounded by a Hernquist dark matter halo can undergo spontaneous scalarization, acquiring a nontrivial scalar field configuration. The astrophysical properties of the dark matter halo directly dictate the quantum-like spectrum of scalar field configurations around the central black hole. Remarkably, the conformal coupling constant becomes discretized and is determined solely by the halo's compactness parameter.

A growing number of phenomena and theoretical problems indicate that quantum gravity theory is necessary. In this paper, we investigate the evaporation of covariant quantum BHs for particles with different spins and compare the results with the Schwarzschild case. Our results show that the Hawking radiation and mass loss rate of covariant quantum BHs differ from those of Schwarzschild BHs and they depend on the spins of the emitted particles. Therefore, these results suggest that it may be insufficient to consider only BH evaporation in the massless scalar field case and may provide a possible way to test loop quantum gravity in the future.

Double red giant stars are very important for studies of the stability of mass transfer, common-envelope evolution, and the formation of double white dwarfs with short orbital periods. However, no double red giant system undergoing mass transfer has yet been found. We present the discovery of a close Algol-type binary system composed of two red giant stars. This is the first known semi-detached system observed during the very short phase when the accretor has expanded into a red giant just before entering the common envelope phase. The $H_α$ line suggests that the system has recently lost some material, which is now moving toward us. We present a consistent analysis of all the available spectroscopic and photometric observations of this system, constraining its orbital parameters and the fundamental properties of the components. Our findings are supported by a binary evolution model that successfully reproduces the currently observed parameters. The model suggests that the system will eventually merge into a single star.

Primordial magnetic fields (PMFs) provide a well-motivated source of cosmological perturbations through their anisotropic stress and may leave observable imprints in both the scalar and tensor sectors. In this work, we study scalar metric perturbations sourced by a PMF evolving through a finite post-inflationary epoch characterized by a constant equation-of-state (EoS) parameter $w$. Working in a gauge-invariant framework, we derive the sourced evolution equation for the Bardeen gravitational potential in a general constant-$w$ background. Our analysis indicates faster (slower) growth of PMF-sourced scalar perturbations on superhorizon scales for a stiffer $w>1/3$ (softer $w<1/3$) background compared to the marginal radiation-dominated scenario with $w=1/3$. We then derive the scalar-induced gravitational wave (SIGW) background induced at second order by the magnetically generated scalar perturbations. Our analysis indicates a narrow viable parametric region for a kination-like post-inflationary era and a nearly scale-invariant PMF that may give rise to a PMF-sourced SIGW signal dominating over the direct PMF-generated tensor background. Simultaneously, the induced tensor modes are found to be perturbatively small compared to the magnetic sector, which, in turn, exerts negligible backreaction on the background. Interestingly enough, for an inflationary scale $H_{\rm inf}\sim10^3-10^9$ GeV and duration of reheating in $e$-folds $N_{\rm reh}\sim8-10$, the dominant PMF-sourced SIGW signal may be detectable by next-generation terrestrial and space-based interferometric GW detectors spanning the mHz$-$kHz frequency range.

We compute the instantaneous angular momentum flux from nonspinning black hole binaries in Einstein-Maxwell-dilaton theory for generic noncircular orbits up to relative first post-Newtonian order. Working in the Einstein frame and using the direct integration of the relaxed field equations approach, we construct the scalar, electromagnetic, and tensor gravitational fields in the wave zone and express the required source multipole moments in the center-of-mass frame. In addition to the leading $1/R$ radiative fields, we retain the next-to-leading $1/R^2$ terms in the wave-zone fields, which describe finite-distance corrections and do not contribute to the flux at null infinity. We obtain separately the scalar, electromagnetic, and tensor contributions to the angular momentum flux. The scalar and electromagnetic channels begin with dipole radiation, while the tensor channel begins at quadrupole order. Our results recover the quasicircular balance relation, the known general-relativistic limit, and the expected scalar- and electromagnetic-dipole suppression limits. Together with the previously known energy flux, these results provide the dissipative information required for future studies of orbit-averaged eccentric evolution and waveform phasing in Einstein-Maxwell-dilaton theory.

High-performance computing is increasingly dominated by hardware acceleration using Graphics Processing Units (GPUs). To take advantage of this long-term trend, we implement a major overhaul of the parallelisation approach in the relativistic particle-mesh N-body code gevolution. The new version 2.0 of gevolution employs three layers of parallelisation: MPI for scalability on a distributed memory system, shared memory parallelisation on each MPI rank using OpenMP, and offloading all compute intensive tasks to GPUs using CUDA. The code also includes many new features that have been developed over the past years. We provide an overview of the code structure and show key performance benchmarks. The public release of gevolution 2.0 can be found at https://github.com/gevolution-code/gevolution-2.0. We also release a GPU-ready version of the LATfield2 library which provides the parallelisation backend, available at https://github.com/gevolution-code/LATfield2.

We study the cosmological core-halo relation in vector dark matter using three-component Schrödinger-Poisson simulations. Starting from cosmological vector-field initial conditions, which due to the evolution of the vector field during inflation are enhanced on small scales, we find that nonlinear evolution begins almost immediately following matter-radiation equality and produces compact self-gravitating Proca-star condensates at the centers of halos. After confirming the central condensates through their radial density profiles, we find the empirical relation \(\widetilde M_\star\propto \widetilde M_{\rm h}^{0.6403}\) between the Proca star mass and halo mass, although interestingly we find that we are only able to confirm Proca stars in $\mathcal{O}(10\%)$ of halos. This serves as important input for future studies of the abundance and merger rates of Proca stars in models of vector dark matter. We also examine the vector-field structure of the objects through global longitudinal and transverse polarization fractions and local spin density inside halos, which increases over cosmic time.

Exploring how matter cycles through the Galaxy, from the birth of stars in dense interstellar clouds to the ejection of matter and energy during a star's final stages-requires a multi-faceted approach. Radio observations are essential to reveal the intricate interactions at play. By studying our Galaxy in detail, we can use it as a model to better understand these processes in galaxies as well. The Square Kilometre Array offers unique capabilities in wide-field, high-sensitivity, high-resolution spectroscopy and precise astrometry to revolutionise Galactic astrophysics. This chapter presents an overview of the science addressed in detail in chapters pertinent to the SKAO Science Working Group \textit{Our Galaxy}.

Wide stellar binaries are sensitive dynamical probes of dark matter substructure. We generalize existing point-mass disruption constraints to spherical extended dark objects and apply the resulting framework to the population of wide-binary candidates identified in the ultra-faint dwarf galaxy Bootes I. We derive 95% confidence limits on the dark matter fraction as a function of the perturber mass, the radius enclosing 90% of the mass, and the density profile, considering Navarro-Frenk-White, r^-3/2, and r^-9/4 models. Sufficiently compact objects converge to the point-mass limit, whereas finite size suppresses binary disruption when the perturber becomes comparable to the relevant encounter and binary scales. The constraints remain sensitive to dark matter fractions well below unity over a broad region of the mass-radius plane and extend to objects substantially larger than those accessible to conventional microlensing searches. As an application, we map the r^-3/2 results onto ultracompact minihalos and derive an illustrative constraint on the primordial curvature power spectrum. Ultra-faint-dwarf wide binaries therefore offer a purely gravitational probe of the abundance of extended dark matter objects and of the small-scale primordial curvature power spectrum.

A reliable computation of the bubble wall velocity during a cosmological phase transition requires an adequate modeling of the non-equilibrium dynamics in the vicinity of this expanding bubble. This task can be made computationally faster by imposing an \emph{Ansatz} on the shape of the non-equilibrium particle distribution function, thus simplifying the collision terms and making the Boltzmann equation solvable in terms of some out-of-equilibrium fluctuations. Two different \emph{Ansätze} have prevailed in the recent literature: the so-called fluid \emph{Ansatz} and an expansion in a basis of Chebyshev polynomials, consolidated in the public code \texttt{WallGo}. In this work we show that the two approaches yield essentially the same wall velocity in the regime of reasonably mild phase transitions, $α\lesssim 0.01$. Interestingly, the agreement is excellent when only top-quark annihilation is considered, but a noticeable discrepancy appears once scattering processes are included. We also investigate the limitations of linearizing the Boltzmann equation when the fluid \emph{Ansatz} is applied to stronger phase transitions, showing that non-linear contributions induce significant shifts in the predicted terminal velocity as $α\to 1$, even though the non-linear contribution to the wall pressure remain quantitatively small compared to the equilibrium and linearized non-equilibrium parts. We discuss possible consequences of this result for both \emph{Ansätze}, while also highlighting the possible limitations of the WKB approach itself when applied to the regime of strong transitions. Since strong phase transitions are precisely the primary targets for future gravitational waves observatories, our study emphasizes that not only higher precision computations of $v_w$ in the semi-classical approach are required, but a treatment beyond the WKB approximation may be needed.

Massive cosmological correlators exhibit a rich hypergeometric structure already at tree level, reflecting the distorted propagation of particles in de Sitter spacetime. In this paper, we reveal that this apparent complexity conceals a remarkably simple underlying mathematical structure. Using the spectral representation, we compute arbitrary tree-level correlators of scalar fields with generic masses and show that they are constructed from fundamental building blocks belonging to the family of Lauricella generalised hypergeometric functions, glued together by spectral integrals. We develop a spectral gluing algorithm that evaluates these integrals through elementary graph combinatorics, yielding explicit series representations that resum the dependence on internal energies away from soft limits. This algorithm naturally generates solutions to the differential equations satisfied by massive correlators as expansions in the corresponding eigenfunctions. Acting with a set of graph annihilators, we uncover a new class of magical identities among generalised hypergeometric functions, revealing an unexpected simplification: once the dynamical propagators are stripped away, the remaining hypergeometric kinematic dependence collapses to rational functions. Our results expose a hidden simplicity in the rigid hypergeometric analytic structure dictated by graph combinatorics, and hint at an intrinsic geometric principle from which properties of massive correlators naturally emerge.

Constraining local primordial non-Gaussianity through scale-dependent bias requires an accurate determination of the PNG bias parameter $b_φ$, which is completely degenerate with $f_\mathrm{NL}$ in the galaxy power spectrum. The standard universality relation predicts $b_φ$ from the linear bias, but breaks down for realistic galaxy populations due to PNG assembly bias. The time evolution estimator offers a promising alternative, inferring $b_φ$ from the redshift evolution of tracer number counts. We test it against the separate universe estimator using the IllustrisTNG and SIMBA suites of CAMELS. Our main contribution is to demonstrate that the estimator can be extended to realistic observer-frame photometric selections, provided correction factors derivable from the galaxy SEDs are applied to account for luminosity-distance dimming and $K$-corrections. We further show that physical evolution not directly tracing the growth of matter fluctuations can also be corrected under assumptions about the star formation history: magnitude cuts require correcting for passive stellar fading, after which agreement with the separate universe estimator is restored. This establishes the robustness of the time evolution estimator of PNG bias with realistic survey selections, paving the way to future applications with real data.

Future interferometers may detect a gravitational-wave (GW) signal from a cosmological first-order phase transition. Reconstructing the underlying particle-physics model from such a signal requires theoretical control over the map from microphysics to the spectrum. For classically scale-invariant extensions of the Standard Model, which generically predict strongly supercooled transitions and strong GW signals, this map depends sensitively on the treatment of quantum and thermal corrections to the nucleation rate. Taking the classically conformal ${\rm U}(1)_X$ model as representative of this class, we scan its parameter space and compare two resummation schemes. The first is a high-temperature effective field theory, matched at two-loop level and including next-to-leading-order corrections to the bounce action, with the nucleation-rate prefactor given by the full one-loop functional determinants. The second is a commonly employed daisy-resummed effective potential, with the prefactor estimated on dimensional grounds. Reconstructing the fundamental model parameters through a Fisher-matrix analysis of injected GW signals at LISA, we find that the daisy-resummation scheme is strongly disfavored, as its theoretical error dominates over the reconstruction uncertainty.

In Friedmann cosmology with Einstein--Cartan torsion, the homogeneous torsion mode compatible with a separately conserved matter sector scales as $Φ\propto a^{-3}$ and enters the Friedmann constraint as a stiff component of negative energy density, $-3Φ^{2}\propto a^{-6}$. It has recently been claimed that this mode rescues the Hubble radius as an infrared cutoff for holographic dark energy, producing late-time acceleration and a phantom-divide crossing of possible relevance to DESI. We show that it cannot. With the Hubble cutoff the holographic density drops out of the deceleration parameter, and the only accelerating regime is the transient window $\bar{a}\leq a<4^{1/3}\bar{a}$ around the torsion bounce at $H(\bar{a})=0$; placing that window at observable redshifts would force the Hubble rate to vanish in our recent past, against the measured expansion history. Requiring a viable history yields nested upper bounds on $Ω_Φ\equiv(Φ_{0}/H_{0})^{2}$: fitting the official DESI DR2 BAO likelihood gives $Ω_Φ<8.7\times10^{-4}$ ($95\%$ CL), the existence of a spectroscopically confirmed galaxy at $z=14.32$ gives $8.4\times10^{-5}$, the CMB gives $3.1\times10^{-10}$, and Big Bang nucleosynthesis gives $5\times10^{-24}$. Today's imprint on the dark energy equation of state, $|1+ω_{0}|\leq2Ω_Φ/Ω_Λ$, falls short of the DESI preference by two orders of magnitude at best and twenty-two at worst, and on the phantom side. In the Granda--Oliveros cutoff, torsion deepens rather than prevents the big-rip singularity. An appendix derives the Friedmann pair from the Einstein--Cartan field equations and shows that the $a^{-3}$ scaling is the kinematics of a diluting spin fluid; escaping the no-go requires breaking exactly that.

Motivated by growing observational indications that dark energy may be dynamical, we introduce Rolling Galileon gravity: a minimal shift-symmetry-breaking extension of the cubic Galileon in which the coupling coefficients are allowed to vary, giving rise to an evolving braiding strength. The full theory space, shown to be closed under field redefinitions, is characterised by two functions. We derive analytical conditions to satisfy three phenomenological requirements: i) a phantom-crossing equation of state at late times, ii) a positive integrated Sachs-Wolfe signature, and iii) absence of pathologies in the screened scalar force in cosmic voids. We show that these conditions are collectively satisfied by an increasing braiding strength relative to the kinetic sector. A Bayesian analysis of minimal Rolling Galileon models finds that they can satisfy the viability requirements i)-iii) whilst providing an acceptable fit to expansion-history data.

The Local Volume Complete Cluster Survey (LoVoCCS) is a volume-complete survey of over one-hundred nearby ($0.03 < z < 0.12$), X-ray luminous ($L_{500} > 10^{44} \text{ erg s}^{-1}$) galaxy clusters in the southern sky. Observations for the survey concluded in December 2025, reaching Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) Year 1-2 depth in each field and providing observations with $\lesssim 1"$ seeing for weak lensing science. In this paper, we present the latest pipeline for reducing observations using the third-generation of the LSST Science Pipelines. We use recent observations of the Hercules Supercluster to validate the pipeline's data-products and conduct an extensive multi-plane weak-lensing analysis of a $\sim 16 \text{ deg}^2$ complex covering Abell 2147, 2151, 2152, and several additional structures. We confirm that the dynamical mass of the complex is biased due to the dynamical state of Abell 2147, which is consistent with being $\sim 0.2-0.4 \text{ Gyr}$ out-of periapsis, and estimate that the total mass of the supercluster is $8.9^{+1.7}_{-1.4} \times 10^{14}~M_{\odot}$.

The 21 cm line of atomic hydrogen (HI) is a sensitive tracer of the outer disk of galaxies, where environmental signatures are most apparent. The relative extent of HI disks compared to optical disks ($D_{\rm HI}$ vs $D_{25}$) is thought to provide a quantitative measure of such imprint, yet systematic comparisons between extreme environments remain scarce. We quantify the relative extent of HI disks in Hickson Compact Groups (HCGs) and in the Analysis of the interstellar Medium in Isolated GAlaxies (AMIGA) sample, using AMIGA as a control sample that captures secular evolution with minimal external influence. We calculate HI diameters by directly fitting an ellipse to the $1\,M_{\odot}\,{\rm pc}^{-2}$ iso-density contour. Because $D_{\rm HI}$ and $D_{25}$ are nonlinearly related, we avoid the traditional $D_{\rm HI}/D_{25}$ ratio, which carries a size-dependent bias, and instead quantify truncation as the residual from the isolated-galaxy $D_{\rm HI}$-$D_{25}$ baseline, which we establish for AMIGA via Bayesian analysis. The full analysis is provided as a reproducible Python package and Snakemake workflow. HCG galaxies lie systematically below the isolated-galaxy baseline in the $D_{\rm HI}$-$D_{25}$ plane. When members with HI nondetections are included as upper limits, HCGs have HI disks at least ~71% smaller than expected for isolated galaxies of the same optical diameter. The truncation increases monotonically along the HCG evolutionary sequence, from Phase 1 to Phase 3. A comparison with literature samples places HCGs at the most-truncated end, statistically indistinguishable from the Virgo cluster sample (VIVA). Compared to AMIGA, HI disks are typically smaller relative to the optical disk in loose groups, compact groups, and cluster infall/field environments, and are most strongly truncated in HCGs and in the Virgo cluster sample.

We present a detailed thermodynamic analysis of a sloshing spiral discovered in the intracluster medium (ICM) of the galaxy cluster Abell 2107, based on deep Chandra X-ray Observatory observations. Spectral analysis reveals that the spiral arm is significantly cooler and exhibits lower entropy and higher metallicity compared to the surrounding ICM, consistent with an origin in the cool, dense cluster core. We detect three cold fronts at the edges of the spiral, roughly aligned along an east-west axis. This alignment is consistent with predictions from numerical simulations of off-axis merger events. Subregion analysis of the spiral shows that the coldest and lowest-entropy gas is concentrated in the central part of the arm, while the side regions show evidence of mixing and interaction with the ambient medium. Among the examined thermodynamic quantities, entropy provides the most robust contrast between the sloshing gas and its surroundings. Notably, no optical or X-ray counterpart to a potential perturber is detected, suggesting that the responsible perturber may currently be below the detection limit or otherwise difficult to identify, while a dark matter-dominated or dark matter-only subhalo remains one possible scenario. Our findings provide new insights into the dynamical history of A2107 and demonstrate the importance of deep X-ray exposures for revealing subtle ICM structures in apparently relaxed clusters.

Fuzzy Dark Matter (FDM) predicts the existence of virialized halos with interference-driven granular structure generated by the wave nature of ultralight bosons. Since these fluctuations produce time-dependent gravitational fields, they can be proposed as a potential source of observable dynamical effects on baryonic matter. In this work we test whether such fluctuations generate measurable temporal correlations between FDM and a coupled gas component. We evolve coupled FDM-gas configurations with the Schrödinger-Poisson-Euler system in three dimensions. The FDM component is initialized through multimerger configurations that relax toward core-halo structures, while the baryonic component is modeled as an ideal gas. After virialization, we analyze the temporal fluctuations of both components through correlation functions and spectral diagnostics. The FDM density field shows fluctuations with significant high-frequency content, whereas the gas develops smoother and longer-lived coherent structures. Although the gas dynamically responds to the gravitational potential generated by the FDM halo, we find no strong temporal correlation between the interference-driven fluctuations of FDM and the gas dynamics. This behavior persists across the explored range of gas temperatures. Our results indicate that baryonic matter reacts to the global gravitational potential of the virialized halo rather than to the detailed phase-dependent granular structure of FDM. This weak temporal coupling may limit the possibility of directly detecting FDM granularity through local baryonic temporal fluctuations.

The polar magnetic field is the most reliable precursor for predicting the amplitude of the solar cycle, and the 2D surface flux transport (SFT) model is widely used to reconstruct its evolution. Traditional 2D SFT models can not capture the surface-interior coupling of the surface field, causing delays in polar field reversals. This deficiency is conventionally corrected by adding a decay term $-B_r/τ$ with a poorly constrained radial decay timescale $τ$. Here, we present a self-consistent estimate of $τ$ through a comparative study of radial flux transport in the 2D SFT model and the 3D kinematic dynamo model, STABLE. By keeping the same transport parameters for both models and assuming surface-interior coupling is diffusive, the poloidal field evolution equation reduces to an eigenvalue problem, which yields a spectrum of $τ$ that decrease with increasing angular modes $l$. To capture realistic surface-interior coupling, we further perform data-assimilated 2D SFT simulations with real magnetograms and compare those results with that of the data-assimilated 3D STABLE model to constrain $τ$ and effective decay modes. With our choice of transport parameters, a value of $τ=~2~\text{yr}$ keeps the surface dynamics of the two models consistent, and this timescale corresponds to the angular mode $l=8$ from our self-consistent estimate. We also perform 2D SFT simulations with only large-scale active regions, as the source. We find that $τ=7~\text{yr}$ accurately captures the radial decay of the dipole mode ($l=1$) and removes the secular drift in the polar fields.

In this paper, we investigate the shadow of the generalized Vaidya black hole described by the Husain solution. Assuming a barotropic equation of state for the additional matter sector, we first identify the self-similar class admitting a homothetic Killing vector and transform the geometry to a conformally static form. This allows us to derive the effective potential for null geodesics in the conformally static representative, determine the corresponding homothetic photon surface, and then transform the result back to the original generalized Vaidya coordinates. We show that, within the weak-energy-condition branches, the parameter $α$ controlling the equation of state determines whether the shadow is enlarged or reduced relative to the Vaidya case: the branch $0\leqα<1/2$ increases the photon-sphere and shadow radii, whereas the branch $1/2<α\leq 1$ decreases them. We then turn to the genuinely time-dependent Husain metric and derive quasistatic slow-evolution equations for the instantaneous photon surface and critical impact parameter. In particular, we show that the growth or contraction of the shadow is governed by the local effective influx in the photon region rather than by the signs of $\dot M$ and $\dot N$ separately. For the charge-like branch, this yields a transparent criterion for when accretion enlarges the shadow and when rapid charge growth instead causes it to shrink.

We build an ultralight dark matter model with repulsive self-interactions, starting from a 5D action that only includes a $U(1)$ gauge field and several massive and charged free fermions. The dark matter scalar field corresponds to the fifth component of the gauge field, after compactification to 4D. Although the single-fermion case only leads to attractive self-interactions, two fermions can already give rise to repulsive self-interactions, thanks to a Scherk-Schwarz twist around the fifth dimension. We check the observational and theoretical self-consistency of this scenario, from the inflation stage to the current time. We find that large ranges of model parameters are allowed. For scalar masses below $10^{-12}$ eV the self-interactions are negligible and the model behaves as fuzzy dark matter. For higher masses the repulsive self-interactions govern the formation of solitons of astrophysical size. Larger sizes require a large initial misalignment or small masses in the fuzzy dark matter regime.

The intracluster light (ICL) has been proposed as a luminous tracer of the dark matter (DM) halo shape in galaxy clusters, with recent observational studies and $Λ$CDM simulations suggesting close alignment between the two components. We test whether this correspondence holds in a self-interacting dark matter (SIDM) cosmology using matched CDM and SIDM ($σ_\mathrm{DM}/m = 0.1$ and $0.5\;\mathrm{cm^{2}\,g^{-1}}$) hydrodynamical cluster simulations from TheThreeHundred project. We measure 3D axis ratios and major-axis orientations as a function of radius for both the DM halo and the stellar (BCG+ICL) distribution, and quantify the spatial correspondence using the Weighted Overlap Coefficient. In the SIDM simulations, DM haloes become significantly rounder in their inner regions with increasing DM cross-section, while the stellar distribution retains the same elongated morphology in both CDM and SIDM. This decoupling is robust across two baryonic physics models and two resolution levels, and is detected at $>5σ$ for $σ_\mathrm{DM}/m = 0.5\;\mathrm{cm^{2}\,g^{-1}}$. It arises because the stellar shape is governed by the accretion geometry of tidally stripped satellites along the large-scale structure, which is identical in CDM and SIDM, whereas, in SIDM, halo shapes respond additionally to local self-interactions. The ICL-DM shape correspondence demonstrated in CDM therefore breaks down in an SIDM universe.

The Solar, Heliospheric and Ionospheric (SHI) Physics Science Working Group of the Square Kilometre Array Observatory (SKAO) addresses the full chain of plasma processes linking the solar corona to the terrestrial environment. This overview chapter synthesises 16 topical contributions to Advancing Astrophysics with the SKA-II}, spanning the quiet and active solar atmosphere, eruptive phenomena, heliospheric turbulence and solar-wind diagnostics, ionospheric science, stellar-solar connections, and the observational frameworks required to deliver these science goals. The primary focus is on the capabilities of Array Assembly 4 (AA4), the design baseline for both SKA-LOW (50-350,MHz) and SKA-MID (0.35-15.4,GHz), which together provide continuous spectral coverage, sub-arcsecond angular resolution, full-Stokes polarimetry, and sensitivity gains of an order of magnitude over existing facilities. From resolving fine-scale coronal heating events to mapping coronal mass ejection magnetic fields and characterising multi-scale heliospheric turbulence, SKAO will deliver transformative advances in solar and space-weather science. We frame these contributions as a single end-to-end Sun-to-Earth system. We identify cross-cutting themes and gaps not fully addressed by individual chapters and outline the staged roadmap from early operations through to the full AA4 capability.

In general relativity, cosmic acceleration is timelike defocusing of the comoving congruence and requires a negative total active gravitational mass density, $\mathcal{M}_{\rm tot}=ρ_{\rm tot}+3p_{\rm tot}<0$. The criterion $w\equiv p/ρ<-1/3$ diagnoses sector repulsion only for $ρ>0$: the inequality reverses for $ρ<0$, and ratio variables are ill-defined at $ρ=0$ even when the stress-energy tensor is finite. For sign-changing effective dark energy (DE), as in $Λ_{\rm s}$CDM-type histories, we instead use the signed density $ρ_{\rm de}$ and two branch-independent combinations. The regular null energy condition (NEC) boundary $\mathcal{I}_{\rm de}=ρ_{\rm de}+p_{\rm de}=0$ replaces the phantom divide $w_{\rm de}=-1$, while $\mathcal{M}_{\rm de}=ρ_{\rm de}+3p_{\rm de}<0$ governs sector-level Raychaudhuri repulsion. For a separately conserved DE sector with a smooth negative-to-positive density crossing of finite odd order $n$ at $z_\dagger$, we prove that $\mathcal{I}_{\rm de}$ and $\mathcal{M}_{\rm de}$ are negative in a punctured neighborhood and non-positive at the crossing, while $w_{\rm de}$ develops a kinematic pole with universal residue $n(1+z_\dagger)/3$. If $\mathcal{M}_{\rm de}>0$ at some sufficiently high redshift, continuity requires at least one repulsion boundary $z_{\rm rep}>z_\dagger$: the sector is already repulsive while $ρ_{\rm de}<0$. We derive the exact range of $ρ_{\rm de}'(z_\dagger)$ for acceleration at the crossing with the total NEC satisfied. Under the stated single-impulse and stationary-point assumptions, the deceleration parameter has one or three sign-changing zeros. A smooth $Λ_{\rm s}$CDM profile, an exponential infrared $f(T)$ model, and the minimal phantom brane illustrate the results. These results motivate organizing late-time inference around $(ρ,p,\mathcal{I},\mathcal{M})$ rather than around $w$ alone.

We develop a deterministic, scale-resolved formulation of energy transfer in the three-dimensional incompressible Navier-Stokes equations based on an explicit triadic decomposition of the nonlinear term in Fourier space. Using a systematic dyadic localization of the velocity field, we derive an exact representation of the nonlinear energy flux across scales and organize it in terms of interactions between well-defined scale components. Under suitable smoothness assumptions, we obtain an absolutely convergent triadic expansion and quantitative bounds that distinguish local and nonlocal contributions in scale space. This framework provides a transparent and fully explicit description of how energy transfer is mediated by triadic interactions and how scale locality emerges as a structural property of the nonlinearity. Building on this formulation, we revisit the classical inertial-range picture of turbulence from a deterministic perspective. We show that, under a scale-invariant flux assumption, the Kolmogorov $-5/3$ scaling is formally consistent with the triadic energy-transfer mechanism at a structural level. The result does not rely on statistical assumptions, but instead follows from the structural properties of the Navier-Stokes equations combined with a scale-resolved representation of the energy flux. The present work thus provides a coherent synthesis of triadic interaction analysis, dyadic scale decomposition, and classical turbulence phenomenology, offering a deterministic framework that clarifies how Kolmogorov-type scaling constraints arise in the scale-resolved structure of the underlying equations.

The process of Hawking evaporation, shadows and thermal fluctuations are investigated within the fabric of f(R) AdS Black Hole (BH). Specifically, the Hawking evaporation process is analyzed numerically using the Stefan-Boltzmann law. The results indicate that the BH lifetime is always infinite, which means the BH becomes a remnant in the late time. Additionally, the evaporation rate depends on the AdS radius and coupling parameters. We further examined the visual properties of BH shadows observed for various values of the two parameters. The results reveal that the BH shadow radius decreases with $ψ_{0}$, while it increases with $λ$. Consequently, we further investigate the infalling accretion matter in the vicinity of BHs. The results depict that while variations in relevant parameters do influence the central region, the important factor is the change in optical appearance of the bright photon ring, which is exhibited at the position of the photon sphere. Next, we discuss many thermodynamical quantities, such as temperature, entropy, Helmholtz free energy, internal energy, corrected pressure, enthalpy, Gibbs free energy and specific heat and interpret how the variations in $λ$ and $ψ_{0}$ impact on the stability and phase transitions of the AdS BHs.

Non-Lorentzian theories of gravity, most common of which are Galilean and Carrollian gravity, arise from General relativity under suitable scalings. General relativity can be obtained by gauging the Poincaré algebra. A convenient formulation of non-Lorentzian gravity follows the contraction of the Poincaré algebra in the tangent space to its non-Lorentzian counterparts, e.g. Galilean and Carrollian algebras. In existing literature, different non-Lorentzian theories of gravity have been addressed separately. In this paper, we introduce a single unified framework of expansion to address all these different theories. We show that different scalings can be unified into a single covariant form parametrized by $(s,n)$, alongside the contraction parameter $ε$. Keeping these parameters unfixed in the limit $ε\to 0$ defines a $\textit{unified flat geometry}$ and its $\textit{unified algebra}$, which reduces to a specific non-Lorentzian geometry for a particular choice of $s$ and $n$. Using this setup in the tangent space, we systematically expand the Einstein-Hilbert action in even powers of $ε$, which we call a $\textit{unified expansion}$ of Einstein's gravity, whose leading-order theory is fixed by $(s,n)$. This reproduces various classes of gravitational theories, including Einstein gravity (the trivial case), Galilean gravity, Carroll gravity, all of which can be extracted from this expansion. Using the expansion, we then formulate String Carroll (SC) gravity, where the local metric has two vanishing eigenvalues. The near-horizon region of generic non-extremal black holes has been recently shown to be a SC geometry. By considering explicit examples, we confirm that these near-horizon geometries constitute solutions of SC gravity, paving the way of understanding physics near the horizon of generic black holes in terms of SC gravity.

We investigate the geometric quantum discord of bipartite Werner states influenced by Hawking radiation in the quantum atmosphere of a Schwarzschild black hole. We find that the geometric quantum discord in the physically accessible region exhibits a nonmonotonic behavior: it first decreases and then increases as the normalized radial distance increases, while the discord in the physically inaccessible region shows the exact opposite trend. A pronounced extremum of geometric quantum discord occurs precisely at the position corresponding to the peak intensity of Hawking radiation. Furthermore, we find that the Hartle Hawking constant and the event horizon radius exert opposite effects on the strength of quantum correlation redistribution: a larger Hartle Hawking constant enhances the redistribution effect, whereas a larger event horizon radius suppresses it.

We present a uniform IRAM-30\,m survey analysis of four oxygen-bearing complex organic molecules (COMs), methanol (CH$_3$OH), acetaldehyde (CH$_3$CHO), methyl formate (CH$_3$OCHO), and dimethyl ether (CH$_3$OCH$_3$), toward 50 high-mass star-forming regions (HMSFRs) associated with 6.7\,GHz methanol masers. Column densities were derived through a homogeneous rotation-diagram approach, with CH$_3$CN used as a proxy excitation-temperature reference when needed. In CH$_3$OH-normalized abundance-ratio space, CH$_3$OCHO/CH$_3$OH and CH$_3$OCH$_3$/CH$_3$OH show the strongest pairwise correlation, whereas the correlations involving CH$_3$CHO are weaker. No clear monotonic trends are found with Galactocentric distance or beam-averaged H$_2$ column density. Comparison with previous observations places the CH$_3$OCHO--CH$_3$OCH$_3$ behavior within the range of earlier abundance-ratio measurements, while CH$_3$CHO shows larger inter-study variation. A representative warm-up chemical model is used only for qualitative comparison with the observed abundance ranges, which are most closely matched during the decline from the post-desorption abundance peaks in the model. These results provide homogeneous beam-averaged abundance-ratio constraints for common O-bearing COMs in high-mass star-forming regions and show that their source-to-source behavior is molecule-dependent rather than fully described by a single common abundance pattern.

Binary--single encounters occur frequently in stellar systems, constitute a non-integrable chaotic three-body problem, and are computationally expensive when treated with N-body codes. We aim to construct an accurate and physically interpretable surrogate model for binary--single scattering final states (ionization, flyby, exchange, or hierarchical), evaluate the reliability of its probabilistic predictions, and trace its predictive failures to the underlying chaotic nature of the scattering problem. We trained an XGBoost multiclass classifier on a class-balanced dataset of numerically integrated binary--single scattering experiments, using physically motivated features. The model achieves a test accuracy of 88.32 percent and a top-2 accuracy of 98.77 percent. Feature importance indicates binary hardness as the dominant predictor. Misclassifications concentrate near the chaotic boundary separating exchange and hierarchical outcomes. Basin-entropy analysis reveals that such misclassifications are due to intrinsically ambiguous regions of parameter space. Therefore, a fast machine-learning surrogate can predict binary--single scattering final states with high accuracy, and residual errors arise from chaotic regions rather than model inadequacy.

Estimating the iron abundance ([Fe/H]) of Classical Cepheids (CCs) is crucial both for validating the cosmic distance scale through the Period-Luminosity relation and for tracing the chemical evolution of young stellar populations. However, standard spectroscopic determinations are highly time-consuming, limiting their application to large or distant samples. This work delivers a practical, multi-band alternative by investigating whether light-curve Fourier parameters derived from Gaia mission photometry can serve as reliable proxies for [Fe/H]. Our reference sample comprises 398 fundamental-mode CCs from Gaia DR3 with high-resolution spectroscopic metallicities available in the literature, spanning a wide range of pulsation periods. Light curves in the G, G_BP, and G_RP bands were modeled with a truncated Fourier series to extract characteristic amplitudes, amplitude ratios, and phase differences. We implemented a replicated Random Forest algorithm as a feature selection tool to isolate the most predictive parameters based on their total increase in node purity. Using the selected features, we calibrated an empirical, multi-variable relation via linear regression, incorporating higher-order polynomial terms to capture complex morphological dependencies. The primary unweighted OLS model yields a residual rms of 0.213 dex and a Pearson correlation of 0.61. As an external validity check, we applied this calibration to large target samples of CCs in the Milky Way, LMC, and SMC. The resulting photometric metallicity distributions correctly reproduce the expected chemical enrichment history and peak values of the three galaxies, proving the reliability of this relation for population studies in the Gaia DR4 era.

Conical geometries provide the standard description of disclinations, but they concentrate the curvature at a singular apex. We introduce a Yukawa-type regularization that replaces this singularity by a smooth curvature distribution while preserving the asymptotic topology of the defect. Exact expressions are obtained for the conformal factor, curvature, and enclosed topological charge. The resulting geometry exhibits a scale-dependent topological charge and a corresponding radius-dependent holonomy, establishing a direct connection between distributed curvature and geometric phases. We further investigate the dynamics of massless Dirac quasiparticles in this background and show that the regularized core modifies the spin connection while preserving the asymptotic topological signature of the defect. These results provide a finite-core extension of the conventional conical description and offer a natural framework for studying geometric and topological effects in graphene-like systems.

We investigate standard quantum teleportation in a relativistic setting where one participant, Rob, undergoes uniform acceleration while Alice remains inertial. Rob interacts locally with a massive scalar field modeled by an Unruh-DeWitt detector. We show that for small detector energy gaps, the entanglement shared between Alice and Rob increases monotonically with acceleration. For larger gaps, entanglement initially decreases at low accelerations due to the Unruh effect, but is subsequently restored and enhanced at higher accelerations as a consequence of the anti-Unruh effect. This behavior is associated with a reduction of the effective detector temperature, leading to a recovery of quantum coherence previously lost to the field. As a result, the teleportation fidelity increases with acceleration and approaches unity in the high-acceleration regime. Our results demonstrate that the anti-Unruh effect can protect and recover quantum information, providing a potential mechanism to mitigate relativistic degradation of quantum correlations.

We identify two general obstructions to a regular rotating constant-norm vector vacua. Firstly, at the poles of a nondegenerate bifurcation surface, horizon and axial invariance force every smooth symmetry-inheriting one-form to vanish, contradicting a strictly nonzero constant norm. Secondly, we show that a conicity varying along a rotation axis generates a singular curvature that diverges as the inverse proper distance from the axis. We apply these results to a general three parameter family Kerr-disformal solution in Einstein--bumblebee gravity. Consequently, its vector vacuum is not smooth on the axes because of a nonzero constant norm, and the position-dependent conicity leads to genuine curvature singularities on the polar axes. A distinguished nonextremal branch nevertheless admits a regular outer Killing horizon away from the axes and defines an exact local rotating solution in the nonpolar region.

The QCD axion with charm-only Peccei-Quinn coupling evades the isospin-violation problem of light-quark models in $χ$PT. Since $m_c>Λ_{\rm QCD}$, we employ heavy meson chiral perturbation theory (HMChPT) to constrain this GeV-scale axion, including its CP-even radial partner $σ$ and the axion $a$. In HMChPT, $σ$ enters at leading order with $O(1)$ coupling, while $a$ couples only at $O(1/m_c)$ and manifests via axion-pion mixing with $θ_{aπ}\simeq f_π/f_a\sim0.04$. We analyze $D$-meson masses, $D$-$D^*$ splitting, charmonium, $D$-$D$ scattering, $B_s$ mixing, rare $D$ decays, and the channels $K^+\toπ^+a$, $B\to K a$. No fatal HMChPT constraint emerges, unlike the $χ$PT isospin issue. The strongest bound is from $σ$-mediated $B_s$-$\bar B_s$ mixing; axion-pion mixing opens testable rare decays, though rates are slightly below current sensitivity.

We have determined the initial mass function (IMF) and structural properties of the open clusters Alpha Persei, Pleiades and Praesepe using Gaia DR3 astrometry and photometry. Cluster members were identified using primarily Gaia astrometry, supplemented with near-infrared UKIDSS and optical HIPPARCOS survey data with stellar masses down to $0.10-0.17$ MSun. We measure and correct for unresolved binaries in each cluster using photometry and Monte Carlo simulations in a Bayesian framework, finding present-day fractions between ($20.0\pm0.8$)% and ($23.8\pm1.2$)%. Through a novel approach that combines N-body simulations with machine learning emulators and MCMC algorithms, we have also determined the most probable initial number of stars, binary fraction, half-mass radius and mass function under the assumption that early gas removal does not significantly influence the subsequent cluster evolution. We find a best-fitting initial mass function described by a three-stage broken power-law distribution with break masses in the ranges $0.24-0.50$ MSun and $0.91-1.20$ MSun and average slopes of $α_{\rm med}=1.72\pm0.09$ and $α_{\rm high}=2.98\pm0.22$. While the low-mass slope remains sensitive to the adopted mass-luminosity relation, this IMF reproduces the present-day clusters well once their dynamical evolution is taken into account. We find evidence of scatter in the high mass IMF between the individual clusters, as described by the measured dispersions in mass function slopes of $σ_{\rm high}=0.29\pm0.16$, whereas the intermediate mass slope shows no significant variation with $σ_{\rm med}=0.00\pm0.14$. Our findings provide additional evidence for a (compared to a Salpeter MF) top-light IMF and a cluster-to-cluster variation of the IMF. They therefore provide constraints on the universality of the IMF and its dependence on environmental conditions.

The ultrahigh-sensitivity polarization imaging of the cosmic microwave background (CMB) is a treasure trove for new physics. Searching for a predicted polarization angle rotation known as the cosmic birefringence effect is a pursuit of the next-generation CMB experiments, and this new polarization signal would be important for testing fundamental physics theories. However, such a delicate rotation effect can be confused by different mechanisms including polarized Galactic foregrounds and instrumental calibration effects. Also, the cosmological rotation effects may arise from both the background evolution and spatial fluctuations of axion-like particles, leaving complicated imprints on CMB polarization fluctuations. In this work, we establish a comprehensive modeling of cosmological rotation effects, the instrumental miscalibration of polarization angles and complex foreground polarization with higher-order fluctuations, and perform a Bayesian analysis to infer both isotropic and anisotropic rotation angles simultaneously for foreground-mitigated polarization datasets from submillimeter to millimeter wavelengths. We find that such a Bayesian formalism can effectively extract the rotation effects for the next-generation CMB experiments and investigate the impact of the known sources including Galactic foregrounds and lensing on the inferred cosmological signals. The method in this work is well suited for future polarization data analyses, and the inferred rotation effects may shed light on the nature of the axion-like particles.

Planet formation begins with dust growth and planetesimal formation within protoplanetary disks surrounding young stars. To understand these processes, it is essential to estimate dust grain sizes from disk observations. In this study, we develop a new method to constrain grain size based on the estimation of cooling timescales. Our approach applies to transitional disks that possess an inclined inner disk casting shadows on the outer disk, whose temperature variations serve as a tracer of dust properties. By constructing a three-dimensional model of the disk surface using near-infrared scattering light images and comparing it with submillimeter dust continuum maps, we estimate the spatial offset between the irradiated and shadowed regions to derive the cooling timescale. We then build an analytic model that calculates the cooling timescale at the dust thermal emission height with an assumed turbulent diffusion intensity to infer the dust surface density and dust grain size. Applying this method to the protoplanetary disk around HD~142527, we find that the disk's northern shadowed region cools on a timescale of a few percent of the orbital period and that the maximum grain size consistent with the observations is approximately 0.1-1 mm. We also find that the conditions required for the vertical shear instability, which needs a short cooling timescale, are satisfied, allowing turbulence with an intensity consistent with near-infrared observations. This study demonstrates that estimating cooling timescales is an effective tool for constraining dust grain size. Our approach can be generally applied to other transition disks with inner-disk-induced shadows.

Stellar age is a fundamental quantity for Galactic archaeology, but reliable age estimation for large stellar samples remains challenging. In this work, we develop an uncertainty aware NGBoost framework for stellar age estimation using Gaia XP-derived atmospheric parameters and chemical abundances. Different from the standard NGBoost model, we modify the loss function by incorporating the uncertainties of the training age labels. We further use a Monte Carlo strategy to quantify the influence of input-feature uncertainties on the predicted ages. The resulting model provides age estimates together with uncertainty estimates. Applying this framework to Gaia XP stars, we construct a stellar age catalog containing 15,175,107 stars.

AI Phe is an eclipsing binary star with an orbital period of 24.6 days for which the surface gravity and effective temperature are known from direct measurements to very high precision and accuracy. We have obtained high-quality spectroscopy of the K0IV star during the total eclipse of the F7V companion, and also obtained spectra with a very high signal-to-noise ratio for this star and its F7V companion using the spectral disentangling technique. We have used these spectra to measure the abundances of iron and magnesium for both stars. We compare the values of [Fe/H] and [Mg/H] for the F7V star and the K0IV star to stars in M67, an open cluster of similar age and metallicity to AI Phe. We find that our [Fe/H] and [Mg/H] measurements clearly show the signature of elemental diffusion in the F7V star. This suggests that AI Phe can be used to test models of single stars that include diffusion and mixing of elements.

A compact form of the stress tensor $\langle T^{ab}T^{cd}\rangle$ correlation function of a quantum scalar field of arbitrary mass $m$ and curvature coupling $ξ$ is presented, with particular emphasis on decoupling in the $m\to\infty$ limit and infrared origin of the conformal anomaly as $m \to 0$. The Ward Identity (WI) of covariant conservation is verified for the full second order metric variation of the one-loop effective action, of which $\langle T^{ab}T^{cd}\rangle$ is part, including local contact terms. This WI is satisfied by two tensors, one spin-2 (traceless) and the second spin-0 (traceful), each multiplied by a Lorentz invariant form factor computed in $n$-dimensional regularization. Minimal subtraction of pole terms at $n =4$ produces form factors that fail to satisfy decoupling for general $ξ\neq 1/6$, but can be simply amended to do so. Particular interest attaches to the $ξ=1/6$ case, in which the spin-0 form factor at $n =4$ is completely finite, requires no UV regularization or subtractions, and satisfies decoupling directly. Its imaginary part defines a spectral function that obeys a UV finite sum rule for any $m$, while its real part unambiguously determines the $\square R$ in the massless limit. Its $m^2$ dependence describes a Wilsonian renormalization group flow to an effective field theory limit at large distances. At $m= 0$ the spin-0 spectral function becomes a Dirac $δ$-function at zero energy, demonstrating the existence of a massless scalar Goldstone collective excitation due to the conformal anomaly in the low energy effective theory of gravity.

We re-investigate the open cluster UBC 63 using the Gaia DR3 data and show that, rather than being a single cluster as previously classified, it is a compelling candidate for a double cluster undergoing an unbound flyby interaction. A GMM decomposition performed in the 5D astrometric space reveals the two statistically distinct components, namely UBC 63A (98 members, Age = 21 $\pm$ 4 Myr) and UBC 63B (148 members, Age = 562 $\pm$ 43 Myr). A significant age difference of $Δ\mathrm{Age} = 541 \pm 43$ Myr between the clusters, rules out coeval formation. Their 3D separation of $60 \pm 29$ pc at the birth-epoch of the younger cluster, indicates that the clusters might have originated from the same molecular cloud complex. At present, the system exhibits a 3D separation of $26 \pm 8$ pc, with a relative velocity of $3.60 \pm 1.80$ km s$^{-1}$. Orbital integrations and \textit{N}-body simulations of the pair suggest that the systems had a close encounter, reaching a separation of $7 \pm 2$ pc only $\sim$~6 Myr ago and predict a rapid divergence to a separation of $491 \pm 213$ pc within the next $\sim$100 Myr. The low escape velocity ($V_{\rm esc} = 0.51 \pm 0.12$ km s$^{-1}$) of the system compared to the relative 3D velocity indicates that they are gravitationally unbound. Their low tidal factors, elongated structures and populations extending beyond the Jacobi radii may reflect a strong transient tidal interaction between the clusters.