高能暂现源 · 每日文献综述

We investigate the spherical accretion onto a static Konoplya Zhidenko black hole and examine how the deformation of the spacetime affects the dynamics of the accreting matter. By applying the conservation laws of particle number and energy-momentum, we formulate the accretion problem and derive the corresponding Hamiltonian describing the fluid motion. The critical points and sonic transitions are then analyzed for ultra stiff, ultra relativistic, radiation, and sub relativistic fluids. We study the dependence of the critical point structure and flow trajectories on the KZ deformation and compare the results with those obtained for asymptotically safe, Schwarzschild MOG, and Reissner Nordström black holes. The comparison shows that the location and behavior of the critical points vary significantly with the underlying BH geometry and the equation of state of the fluid. In particular, the KZ deformation produces noticeable changes in the radial structure of the accretion flow, with different effects observed for the considered fluid models. These results provide a comparative picture of spherical accretion in different black hole spacetimes and help clarify how modifications of the background geometry influence matter dynamics in the strong field region.

Astrophysical black holes exist within non-vacuum environments, and their motion through these environments generically result in a force on these black holes through processes such as dynamical friction and Bondi accretion. We explore these forces numerically for black holes moving through a complex scalar field of mass $m$ with constant acceleration $a$, while also revisiting the constant velocity case considered in the existing literature. The former is modeled by the C-metric, while we use Painlevé-Gullstrand metric with an additional divergence and vorticity free velocity field to mimic the constant velocity motion. Our simulations reveal several novel and interesting aspects of the drag force in both cases. For the constant velocity case, the force saturates at late times, with a value that depends on velocity in a manner distinct from known dependence. For the constant acceleration case, the force increases to a maximum value $F_\star$ and then reduces drastically, with $F_\star \approx 0.3~ m a$. Moreover, the density wake in this case shows revivals separated by decreasing time intervals.

Constraining the location and geometry of ultra-fast outflows (UFOs) is essential for identifying where they are launched and how they are accelerated. We investigate the soft X-ray variability of the luminous quasar PDS 456 using simultaneous March 2024 observations with XRISM/Xtend and NuSTAR. A model-independent comparison between the flare and quiescent phases reveals spectral variability around 1 keV in the rest frame, while the hard X-ray spectral shape remains nearly unchanged. Broadband spectral fitting shows that the soft X-ray structure is well described by a partial-covering low-ionization UFO with $\log (ξ/(\mathrm{erg~cm~s^{-1}})) \simeq 3.1$ and $v_{\mathrm{out}} \simeq 0.30c$. Time-sliced spectral analysis further reveals significant variability in the covering fraction of this absorber. Interpreting this variability as transverse motion across the X-ray source, we constrain the crossing velocity to be $v_{\mathrm{cross}} \lesssim 5 \times 10^{-3}c$ and derive a lower limit on the absorber distance of $r \gtrsim 4 \times 10^3 R_{\mathrm{g}}$. This location is substantially farther out than the high-ionization UFO previously inferred at ~200-600 $R_{\mathrm{g}}$, while the two phases have comparable outflow velocities. The resulting velocity-distance structure disfavors a self-similar magnetocentrifugal wind and instead suggests either radiation-pressure acceleration following a Castor-Abbott-Klein-like velocity law or compact magnetic acceleration through magnetic reconnection. These results demonstrate that soft X-ray partial-covering variability can provide a geometrical probe of UFOs and directly connect spectral variability to wind acceleration.

Pulsar glitches display diverse behaviors. They are commonly labeled as "Vela-like" or "Crab-like," yet these qualitative categories do not provide reproducible quantitative boundaries. We constructed an objective classification scheme based on the joint distribution of backward waiting time $Δt_{-}$ and fractional glitch amplitude $Δν/ν$. A sample of 349 glitches from 22 pulsars was compiled by cross-matching the Jodrell Bank and ATNF glitch databases. For each source, we used two-dimensional kernel density estimation (KDE) together with highest-density regions (HDRs) to derive geometric descriptors, including the HDR area, anisotropy ratio, and out-of-HDR fraction. Hierarchical clustering of the KDE--HDR descriptors revealed two morphology classes independent of visual inspection. In this convention, the Compact class is associated with small HDR areas, elevated anisotropy, and limited peripheral occupancy. The Extended class is associated with broader HDR occupancy and stronger low-density extensions. We found that bootstrap resampling and hyperparameter sensitivity tests confirmed this partition for most sources. Nevertheless, two prolific pulsars (PSR J0537-6910 and PSR J1341-6220) were found to occupy intermediate positions. Logistic regression identified the mean backward waiting time $\langleΔt_{-}\rangle$ as the strongest class predictor (in-sample $\mathrm{AUC}=0.88$; leave-one-out $\mathrm{AUC}_{\mathrm{CV}}=0.59$, reflecting the limited sample size). Additionally, a parallel analysis using $Δt_{+}$ recovered the same two-class structure but with reduced stability. The data suggest that the Compact class is consistent with near-complete reservoir depletion, whereas the Extended class reflects partial, avalanche-like releases. As a result, short-term measurements of $G$ in Extended-class pulsars may underestimate the true crustal superfluid fraction.

We report a multi-wavelength study of two flaring episodes of the blazar BL Lacertae during MJD 60500-60800 (9 July 2024 - 5 May 2025). The source reached a daily-averaged $γ$-ray flux of $(1.03 \pm 0.05) \times 10^{-5} \, \mathrm{ph \, cm^{-2} \, s^{-1}}$ ($E > 100$ MeV) on MJD 60588 (5 October 2024). Using orbit-binned data from the Large Area Telescope (LAT) onboard the \textit{Fermi Gamma-ray Space Telescope}, we identify a minimum flux halving timescale of $τ= 1.33 \pm 0.29$ hr. This constrains the upper limit on the $γ$-ray emitting region size to $R \le 2.0 \times 10^{15}$ cm, as well as its distance from the central supermassive black hole to $R_\mathrm{H} \le 5.9 \times 10^{16}$ cm, assuming a Doppler factor of $δ= 14.8$ derived from the spectral energy distribution (SED) modeling. We find tentative evidence for sub-minute $γ$-ray variability with a minimum doubling time of $0.7 \pm 0.2$ min ($p$-value = 0.03). This may originate from an extremely compact region with a size of $R \le 1.8 \times 10^{13}$ cm, suggesting that the emission arises from magnetohydrodynamic substructures, such as plasmoids within a magnetic reconnection zone. Spectral analysis reveals a significant ``softer-when-brighter'' trend ($r = 0.96, p = 4.5 \times 10^{-4}$) during the minute-scale flare peaks, indicating a complex interplay between particle acceleration and radiative cooling. The SED is reproduced using a one-zone leptonic model, in which synchrotron self-Compton (SSC) and external Compton (EC) scattering effectively account for the high-energy emissions. The reduced magnetic field strengths and hard electron injection spectral indices observed during the flaring states suggest enhanced particle acceleration efficiency, possibly associated with relativistic magnetic reconnection.

Binary black holes (BBHs) in gaseous medium, such as active galactic nucleus (AGN) disks, are important gravitational-wave sources, yet the gas-driven torque that governs their orbital evolution remains to be fully understood. Most existing studies of BBHs in gas often approximate the net torque as the sum of independent dynamical friction (DF) forces exerted on each black hole by its own wake, neglecting the mutual gravitational coupling between the two wakes. We perform three-dimensional hydrodynamic simulations of circular BBHs in a uniform flow and find the torque is determined by the wake-wake interactions. The net torque is controlled by a single parameter $η\equiv v_g/v_o$, the ratio of the gas flow velocity to the binary orbital velocity. At small $η$, the wakes merge into a single overdense envelope and the time-averaged torque is negative; as $η$ increases, the wakes separate and the torque becomes positive. In AGN disks, capture-channel binaries in a disk model naturally produce $η$ in the positive-torque regime; the expansion timescale is comparable to or shorter than the disk lifetime, suggesting that gas-driven expansion can compete with gravitational-wave inspiral and suppress the capture-channel merger rate.

We investigate the origin of long-term spectroscopic and photometric variability in the blue supergiant rho Leo, with particular emphasis on distinguishing between intrinsic pulsations and variability induced by a possible companion. Our analysis is based on an 11.5-year spectroscopic time series obtained at Tartu Observatory, complemented by high-cadence, high-resolution spectroscopy from the Hertzsprung SONG telescope and space-based photometry from TESS. We studied line-profile variability using normalised moments of the HeI 6678, HeI 5875, and SiIII 4552 lines. Periodic signals were identified using the generalised Lomb-Scargle periodogram with iterative pre-whitening, and their temporal stability was examined with the weighted wavelet Z-transform. We detect a persistent periodic signal at P = 16.46 d in the first and third moments, present throughout the full observing interval, with a radial-velocity amplitude of 3.9 km/s. This signal is also present in the SONG data and is visible in multiple spectral lines, indicating a global origin. Photometric observations reveal a dominant variability timescale near 33 d, approximately twice the spectroscopic period. The stable 16.46-day period, present throughout the entire observing interval, most likely results from non-radial pulsations of the supergiant. A binary origin of the signal is not excluded, but distinguishing between these scenarios is complicated by the supergiant's complex variability pattern.

We present space-based follow-up observations of the superluminous Type I supernova (SLSN-I) SN 2025wny at redshift $z_{SN}=2.0151$, gravitationally lensed by two galaxies at redshifts $z_{G1}=0.3755$ and $z_{G2}=0.3766$ into five resolved images. SN 2025wny is the first strongly lensed SLSN discovered and the first galaxy-scale lensed supernova for which both photometric and spectroscopic time-delay measurements are feasible. As such, it opens a new observational window for precision cosmology and the study of stellar explosions near the epoch of peak cosmic star formation. Our follow-up observations comprise two epochs of Hubble Space Telescope (HST) imaging, together with near-infrared imaging and spectroscopy obtained with the James Webb Space Telescope (JWST). From these data, we measure precise astrometry and multi-band photometry for the five resolved supernova images, the host galaxy, and the two deflecting galaxies. HST provides accurate relative image positions and rest-frame ultraviolet photometry, while JWST delivers complementary near-infrared imaging and spectroscopy probing the rest-frame optical at high signal-to-noise ratio. Together they yield a detailed characterization of both the lensing configuration and the supernova spectral energy distribution over a broad wavelength range. The data presented here provide the observational foundation for the accompanying analyses of the supernova properties, lens modeling, and time-delay cosmography, including the astrometric, photometric, and spectroscopic information required to measure $H_0$.

In-ice radio detectors have been developed primarily for the detection of high-energy neutrinos via the Askaryan effect, but have recently been shown to also be sensitive to cosmic ray air showers impacting the ice sheet. Using CORSIKA 8 to simulate impacting air showers, we find that the lateral width of the in-ice cascade is sensitive to the atomic mass of the primary cosmic ray. The width of the shower core is accessible through the shape of the Askaryan radio spectrum, while the amplitude encodes the total in-ice charge excess. Using these two observables, we show that in-ice radio detectors can be used to measure cosmic ray mass composition in the region above $10^{17}$eV, if the in-ice receivers are placed to adequately sample the Askaryan emission pattern. We also show that the in-ice core width is highly, but not exactly, correlated with the depth of maximum particle count ($\mathrm{X}_{\mathrm{max}}$), the most commonly-used observable for reconstructing cosmic ray mass composition.

We construct regular, spherically symmetric, compact energy-cutoff equilibria for gravitational Vlasov--Poisson systems sourced by positive and negative mass distributions in the context of stellar dynamics. Motivated by Bondi's notion of negative mass and its relation to the weak field limit of Einstein gravity with a signed mass--energy source, we show that the internal structure of the steady states depends qualitatively on the type of negative mass present. In the Bondi convention, the signs of inertial, passive gravitational, and active gravitational mass are reversed together, preserving the equivalence principle. The resulting equilibria consist of a central overlap core containing both mass species, surrounded by a finite positive mass shell and an exterior vacuum. Thus, although a pure Bondi negative mass gas cannot form a compact steady state, Bondi negative mass can be spatially confined by a suitable positive mass distribution. In the gravitational-charge convention, only the passive and active gravitational masses change sign, so that the two species obey opposite free-fall laws and unlike masses repel. The resulting equilibria are spatially segregated into a negative mass core, a vacuum gap, a positive mass shell, and an exterior vacuum. For cutoff exponent $n=-1/2$, the radial matching is explicit, while for the regular cutoff $n=1/2$ the matter regions satisfy Lane--Emden-type equations. These results provide a kinetic framework for investigating the distribution of negative mass in astrophysical and cosmological settings.

In this work, we investigate whether machine learning can be used to infer averaged cosmological quantities directly from the present day matter density distribution. Using an implementation of the simplified silent universe framework, we generate a dataset consisting of 92610 independent relativistic, simplified cosmological simulations spanning a range of initial conditions and average cosmological parameters. We train a convolutional neural network to take the present-time matter density field as input and predict the averaged matter, cosmological constant, curvature, and kinematical backreaction density parameters as well as the Hubble parameter, at both initial and present time. The network achieves coefficients of determination exceeding 0.9 for all predicted quantities, with error distributions showing that only a small fraction of predictions reach percent-level errors or above. Although our use of the simplified silent universe approximation precludes direct application of the trained model to observational data, the results provide a proof-of-principle that neural networks can successfully recover averaged cosmological properties, including backreaction, at different epochs, simply from the late-time matter distribution. More broadly, our findings demonstrate that information about the averaged cosmological history of a universe is encoded in its present-time density field and can be extracted using machine learning techniques. This opens the possibility of applying similar approaches to more realistic cosmological simulations and observational probes such as N-body simulations and weak-lensing maps, ultimately providing a new avenue for constraining the large-scale evolution of the Universe.

Ultralight scalar fields nonminimally coupled to curvature can endow rotating black holes with stationary hair and alter nearby orbits. Using recently constructed hairy black-hole spacetimes, we model the transition to plunge of an extreme-mass-ratio inspiral. For scalar masses $μM=0.01$ and $0.1$ and dimensionless curvature coupling $ζ=10^{-3},10^{-4}$, we find $\mathcal{O}(10)$ rad of gravitational-wave dephasing relative to general relativity over $\mathcal{O}(10^3)$ orbits. A multimode Fisher forecast at $μM=0.2$, where neighbouring hair solutions permit a numerical $μ$-derivative, suggests that $ζ$ and $μ$ could be measured to precisions of $\simeq0.2\%$ and $0.6$--$1.1\%$, respectively, for a source with signal-to-noise ratio of about 80 in LISA.

Contact binaries (CBs) are key systems for investigating the late evolutionary stages that may lead to stellar coalescence. Systems with an extremely low mass ratio and high fill-out factor are of particular interest as potential merger candidates and possible progenitors of luminous red novae. A combined photometric and spectroscopic study of the CB systems AL Lep, ASAS J082243+1927.0, and ZZ PsA is presented. The aim is to derive self-consistent orbital and physical parameters, assess their evolutionary state, and examine whether they have reached the terminal and unstable phase of their evolution. High-resolution VLT/UVES spectra were analysed to derive radial velocity values, spectroscopic mass ratio, and metallicity. The results were combined with multi-band light-curve modelling within a Monte Carlo framework to determine the system geometries and absolute parameters. Orbital period variations were investigated through O-C analysis, while the evolutionary status of the systems was assessed using the Darwin instability criterion, critical mass ratio estimates, fill-out factor, and orbital period variation. Mass ratio values of q = 0.1332, 0.1041, and 0.0789 were derived for AL Lep, ASAS J082243+1927.0, and ZZ PsA, respectively. The corresponding primary mass values are M1 = 1.656 +/- 0.081 Msun, 1.275 +/- 0.078 Msun, and 1.795 +/- 0.111 Msun. The locations of their components show significant deviations from single-star ZAMS and TAMS reference curves, indicating strong binary interaction and energy redistribution through the common envelope. The spin-to-orbital angular momentum ratio for AL Lep and ASAS J082243+1927.0 remains below the standard Darwin instability threshold, while ZZ PsA lies close to the instability regime and may be consistent with an unstable configuration within the derived uncertainties.

Recent space missions such as Kepler have provided large-scale observations of the γ-Doradus instability strips (IS), which can be used to constrain models and explore their limitations. One persistent limitation is the prediction of the blue edge of the γ- Doradus IS, where a significant number of γ-Doradus stars are observed. Despite these observational advances, no systematic study has been undertaken to explore the effects of different physical processes on the γ-Doradus. Our aim is to systematically explore the theoretical γ-Doradus IS with modern tools, accounting for the effects of rotation and metallicity, providing a large grid of models and their oscillation parameters for the scientific community. We investigated the non-adiabatic pulsation properties of stars in a grid of stellar models with masses between 1.35 M{_\odot} and 2.5 M{_\odot}, metallicities between Z= 0.01 and Z= 0.025, and solid-body rotation rates ranging from Ω = 0 to 0.5 {Ω_{crit}}, where {Ω_{crit}} denotes the critical rotation rate. We find that, across all computations, the theoretical γ-Doradus IS agrees well with the observed IS, except in the extended blue region. In terms of radial orders, our models consistently reproduce the excited modes in broad agreement with observations for l= 1, l= 2, and Rossby modes. We also show that the range of excited radial orders is strongly dependent on the effective temperature. The range of excited radial orders can be used to constrain the position of a star within the γ-Doradus IS and may provide insights into the physical mechanisms responsible for the discrepancies observed in the blue region. All computed grids are made available and include mode damping and growth rates for γ-Doradus stars, obtained with a time-dependent treatment of the convection-oscillation interaction.

We investigate black hole shadows and photon sphere properties for two families of covariant quantum-corrected black-hole metrics (hereafter called BH-I and BH-II) formulated within a general $μ$-scheme, parameterised by a power-law exponent $s$ and an amplitude $ξ$. The extension to general (including non-integer) $s$ is a phenomenological interpolation between the $μ_0$-scheme ($s=0$) and $\barμ$-scheme ($s=1$) and reveals a rich phenomenology masked when $s$ is usually fixed to 1 in previous literature. For BH-I, the photon sphere exists for all parameter values and exhibits an exact cancellation at $s=1$ where the coordinate location $r_{\rm ph}=3M$ is restored for any $ξ$. For BH-II, both the horizon and the photon sphere exhibit critical curves in the $(ξ,s)$ parameter plane; the photon sphere disappears when $ξ$ exceeds a closed-form critical value $ξ_c^{\rm PS}(s)$. We prove analytically that the photon sphere is always unstable ($λ_{\rm ph}>0$) throughout the physical parameter space of both metrics, and derive closed-form expressions for the Lyapunov exponent. Systematic parameter scans reveal that for fixed $ξ$ the shadow radius decreases monotonically with $s$ for BH-II and non-monotonically for BH-I. We derive small-parameter analytic expansions for the photon-sphere radius, shadow radius, and Lyapunov exponent, and demonstrate that a single shadow measurement suffers an observational degeneracy in the two-dimensional $(ξ,s)$ space; the degeneracy can be broken by a simultaneous measurement of the Lyapunov exponent. Using Event Horizon Telescope (EHT) measurements, we derive constraints on the $(ξ,s)$ parameter space and find that BH-II is constrained roughly $2$--$4$ times more tightly than BH-I.

In cosmology, the Hubble tension has become one of the most serious challenges in the last decade. Assuming that all the observational data are right, one of the ways out is to modify the standard $Λ$CDM cosmology. In the literature, the theoretical modifications are mainly made in the early and the late/local universes. But on both sides, some no-go arguments have been proposed recently. In particular, the general no-go arguments for the late-time modifications are mainly based on the PAge parameterization. In the present work, we note that there are some flaws in the (original) PAge parameterization, and then propose various generalized PAge (GPAge) parameterizations, which can be extended to $z>z_{\rm CMB}$ and hence the cosmic microwave background (CMB) can be taken into account, while they are also more accurate at low redshifts. With these GPAge parameterizations, we revisit the Hubble tension by using not only the late-time observations but also the observational data of CMB. We find that the Hubble tension could be alleviated (or even resolved) in GPAges significantly different from the standard $Λ$CDM cosmology, while they are overwhelmingly preferred over $Λ$CDM by the observational data in terms of all the information criteria. Surprisingly, we find that the new physics in the mid-time might be the key to resolve the Hubble tension.

Earth is surrounded by two highly dynamic concentric belts of particle radiation. The outer radiation belt exhibits coherent variability at solar-cycle, seasonal, and 27-day (Carrington) timescales. While the solar-cycle and Carrington variations have been attributed to the recurrence of coronal hole solar wind, the seasonal variation has long been explained by local geometric effects that modulate the solar wind-magnetosphere coupling. Here, we challenge this paradigm by showing that the periodic recurrence of coronal hole solar wind likewise drives the seasonal variation. We further demonstrate that the Alfvénic nature of this solar wind is responsible for the observed electron flux enhancement in the outer radiation belt. These findings provide a unifying framework linking solar magnetic topology, solar wind properties, and magnetospheric dynamics across multiple timescales at Earth and beyond.

Extragalactic foregrounds contaminate cosmic microwave background (CMB) temperature maps at small angular scales and limit their utility for precision cosmology. The internal linear combination (ILC) is a well-known technique for suppressing these contaminants, but residual foreground power remains a limiting factor. Kusiak et al. (2023) proposed adding galaxy number-density maps as additional ILC channels, exploiting their correlation with the large-scale structure sourcing these foregrounds to suppress contamination. Here we apply this framework to forecast the gains in CMB-based cosmological parameter constraints from near- and next-generation experiments. Using a halo-model foreground pipeline and a Fisher forecast from joint TT+TE+EE power spectra, we quantify the improvement from galaxy-tracer-assisted ILC cleaning across three configurations: enhanced Simons Observatory (SO) with unWISE or Rubin-like galaxy catalogs, and a futuristic CMB-HD configuration with a hypothetical deep galaxy survey. We find that adding galaxy tracers reduces the residual foreground power in the cleaned temperature map by $\sim4\%$, $\sim22\%$, and $\sim32\%$ at $\ell\sim10,000$ for the unWISE, Rubin-like, and futuristic samples, respectively. For the overall variance of the cleaned map at $\ell\sim10,000$, it provides $8\%$, $24\%$, and $17\%$ improvements for each combination. The resulting reduction in marginalized parameter error bars is modest for the base six-parameter $Λ$CDM model: sub-percent for SO+unWISE, rising to $\sim2\%$ for SO+Rubin-like tracer. Including the effective number of relativistic species $N_{\rm eff}$, we find at most $2.2\%$ improvements for both SO+Rubin-like and CMB-HD+Futuristic tracer. These results establish the expected gains from combining near-term CMB experiments with current and forthcoming large-scale-structure surveys.

We improve upon previous efforts to optimally combine overlapping galaxy samples in the DESI baryon acoustic oscillation analysis. By weighting each galaxy by its linear bias, overlapping galaxies are combined into a single, unified catalog, naturally avoiding double counting of cosmic volume and including all auto- and cross- information at the catalog level. Improvements over the previous effort include the addition of QSO out to $z=1.6$ to account for all overlapping DR2 tracers and redshift-dependent bias treatment to improve reconstruction. We report distance measurements using this unified tracer, and find them to be highly consistent with the baseline DR2 BAO analysis. We also test for tracer-dependent systematics within the DESI data, and find no evidence of tracer-dependent systematics within $0.8<z<1.6$. Finally, we take advantage of the unified tracer to rebin the analysis in redshift in order to more finely resolve the redshift-to-distance relation. Dynamical dark energy results on this finer redshift binning indicate that there is no missed feature in the expansion history in the redshifts $0.8<z<1.6$, and reproduces DESI's preference for an evolving dark energy equation of state.

We present photometric and spectroscopic investigations of 11 totally eclipsing contact binaries with mass ratios below 0.15, classifying them as extremely low mass ratio contact binaries. The ground-based multiband light curves were modeled with PHysics Of Eclipsing BinariEs to derive the photometric parameters of these systems. The TESS light curves of J011323 and J002747 show pronounced asymmetries and continuous variations. Markov Chain Monte Carlo modeling suggests that these variations are related to longitudinal migration of spots on the primary components. Spectral subtraction of the LAMOST spectra reveals excess chromospheric emission in six systems, while no detectable excess emission is found in the other five. The O$-$C analysis indicates secular period increases in six systems and secular decreases in five systems. We estimated their orbital angular momenta and performed a dynamical analysis. {Within our adopted semi-analytical framework without energy transfer, the formation of these observed systems requires additional angular momentum loss beyond saturated magnetic braking and gravitational radiation, but this requirement could be modified by different assumptions on mass transfer or radius evolution.

We conduct an investigation of the global structure of Kerr-Newman-BR spacetime and its various subcases such as Schwarzschild-BR spacetime. In this work, we extend the result of our previous studies, where we found that $r\to \infty$ is not the proper conformal infinity. By using the reciprocal coordinate $y=1/r$, we show that the whole family of Kerr-Newman-BR spacetimes can be continuously extended through the surface $r\to \infty$. However, the conformal structure after passing $r\to\infty$ strongly depends on the type of spacetime one is considering (either rotation is present, or not) and on the polar direction one chooses to be fixed (namely, whether we consider generic polar angle $θ$, or we focus on the behaviour near the equatorial plane $θ=π/2$, or near the poles $θ=0,π$). Finally, we discuss the universality of such a continuous extension.

General relativity coupled to multiple scalar fields is a diffeomorphism-invariant constrained system. Consequently, a naive counting of the perturbative degrees of freedom unavoidably overestimates the true number of physical modes propagating in the theory, as gauge redundancies and constraint equations remove non-dynamical ones. While this problem has been solved for linear fluctuations, this work presents the first explicit calculation of all large-scale gauge-invariant phase-space variables in multifield inflation and at second order in perturbation theory, in a Hamiltonian language. Building upon the well-known Sasaki-Mukhanov variables, we show how to construct a finite-dimensional basis of quadratic corrections which are invariant under gauge transformations. Although our procedure is generic to any number of fields and at any scale, we restrict to super-Hubble scales for their explicit solution, which we deliver. Henceforth, we prove that it is possible to recover the usual flat-gauge and comoving-gauge fluctuations as large-scale gauge-invariant combinations, making for a robust consistency check of the gauge-fixed procedure to connect theoretical predictions above the horizon to observations. We derive the quadratic and cubic Hamiltonian of multifield inflation in a gauge independent manner, then we gauge fix our theory by going into the flat gauge, and we show perfect agreement with the literature on this topic, usually based on a Lagrangian approach. After these concrete steps, we propose a more formal proof of the existence of gauge-invariant variables at quadratic order, and we provide a sketch of the procedure that should allow to go to higher orders in perturbation theory.

Red clump (RC) stars are excellent standard candles for mapping the three-dimensional structure of the Milky Way. We construct an all-sky catalog of 6.5 million primary RC stars using spectroscopic estimates of asteroseismic parameters inferred from Gaia DR3 low-resolution XP spectra. We train a mixture density network (MDN) on a cross-matched sample. The network maps each 343-dimensional corrected XP spectrum to estimates of $ΔΠ_{1}$, the asymptotic period spacing of dipole gravity modes, and $Δν$, the large frequency separation. We select primary RC stars using these two estimates. We release two complementary catalogs defined by different parameter-space cuts and thresholds: a high-purity Tier 1 sample of 532,189 stars with 97% purity and a high-completeness Tier 2 sample of 6,534,931 stars with 86% completeness. Both catalogs cover the full celestial sphere. We provide distances and extinction values for all stars. We obtain a median photometric distance precision of 6% for Tier 1 RC stars. Using the resultant distances, we independently calibrate the Gaia DR3 parallax zero-point offset. The three-dimensional density distribution traced by the Tier 2 sample extends continuously to $R\approx25$ kpc. Both catalogs are publicly available. These catalogs provide a valuable resource for Galactic archaeology, delivering a homogeneous dataset to trace the chemo-dynamical evolution of the Milky Way and to calibrate models of stellar populations.