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In the 170 years since its discovery, U Gem has been intensively studied as the prototypical cataclysmic binary star. Its massive white dwarf (WD) (~ 1.2 Msun) is cannibalizing its ~ 0.42 Msun red dwarf companion. The WD's resulting accreted hydrogen-rich envelope has previously undergone thermonuclear runaways seen as nova eruptions. These weekslong transient events brighten U Gem to m ~ -3 and eject the accreted envelope (~ 0.00001 Msun) at thousands of km/s. Ultraviolet observations show that the accreted envelope of U Gem's WD is greatly enhanced in nitrogen and depleted in carbon relative to solar composition, and that the WD's effective temperature is much hotter than that of most cataclysmic binary WDs. These suggest that U Gem underwent a classical nova eruption quite recently and should therefore still be surrounded by the ejecta of that event. Modeling of U Gem predicts a nova event ~ 1000 years ago, and the existence of two concentric shells, each of order 1 degree in size, surrounding U Gem. We obtained deep narrowband H-alpha imaging of U Gem with the Condor Array Telescope. The two concentric H-alpha-bright shells that we find, centered on U Gem, may be the first-ever predicted old nova ejecta. A transient "guest star" in the asterism Shuiwei, recorded by Chinese imperial astrologers in November 829 CE is consistent with the timing and location of U Gem's last predicted nova eruption, but the transient's absence in Japanese and Korean records weighs against this suggestion.

We report the discovery of new and candidate Galactic supernova remnants made through over 1500 hours of H$α$ and [O III] imaging using amateur class instrumentation. These nebulae range from 1 to over 8 degrees in size. Five of these, namely G27.8-17.1, G115.6+9.2, G190.5+25.3, G195.9+2.2 and G239.9+7.0 exhibit low-dispersion optical spectra indicating shock emission by the presence of strong [S II] $λλ$6716,6731 and [O I] $λ$6300 line emissions relative to H$α$, or exhibit a nonradiative shock generated Balmer dominated spectrum. Their filamentary emission morphology also supports the presence of interstellar shock fronts. Two other optical nebulae, G191.3+11.1 and G205.7-1.7, appear likely to be remnants and are proposed as new SNR candidates. We also present [O III] and H$α$ images of optical filaments associated with the 25 degree diameter Monogem Ring SNR which reveal extensive emission-line filaments around most of the remnant's X-ray boundaries. Deep images of the large and [O III] emission dominated SNR G206.6+6.1 seen toward Monogem SNR's center are also included. Key findings of this work include: 1) optical surveys can sometimes detect SNRs especially at high Galactic latitudes and toward the Galactic anti-center direction better than radio and X-ray SNR searches, 2) Balmer nonradiative shocks appear not uncommon in high Galactic latitude SNRs, and 3) faint [O III] emission dominated SNRs may have been missed in H$α$ emission surveys.

Modern solar observatories produce vast amounts of detailed spectral data. To infer atmospheric parameters from these data, in particular for the solar chromosphere, requires immense computational resources. This problem is becoming more acute as spatial resolution improves, and faster methods are much needed to make sense of the data. Archive-based inversions provide a cost-effective way to infer atmospheric parameters from solar spectra, and involve building databases of synthetic spectra and atmospheric models. We extend this idea to a $k$-nearest neighbour inversion with non-LTE chromospheric spectra from a three-dimensional radiative-magnetohydrodynamic simulation. Our database consists of 150 million \theline\ line profiles from a magnetically quiet simulation. We validate our method on a different simulation, and then apply the database inversion on high-cadence observations taken with the Swedish 1-m Solar Telescope and find excellent agreement between the observed and fitted spectra for almost the entire field of view. We recover the chromospheric temperature and line-of-sight velocity, and calculate the uncertainty in their inversions. The gas temperature has small uncertainties over a broad range of optical depths, and we recover a physically consistent solution over a broad range of optical depths, even beyond the line sensitivity region. Compared with the traditional inversion method, our approach is four orders of magnitude faster. Limitations and future improvements of the method are discussed.

We study collective slow flavor conversion (SFC) of supernova neutrinos with multi-energy, multi-angle simulations for three representative neutrino spectra in the early accretion, late accretion, and cooling phases, in which multiple crossings between the initial electron- and heavy-lepton-flavor spectra are present. By numerically solving the neutrino quantum kinetic equations in a local periodic box, we find that SFC triggered predominantly by the spatially inhomogeneous instabilities drives the system toward a spatially coarse-grained, quasi-stationary state, whose flavor conversion probability depends strongly on energy, angle, and the neutrino mass ordering. While we find that not all of the initial spectral crossings are completely erased in the final state, a simple, box-like analytical prescription inspired by studies of fast flavor conversions, which eliminates the spectral crossings, can reasonably approximate the post-SFC spectra. Using the initial and post-SFC spectra, we also evaluate the changes of the corresponding $ν_e$ and $\barν_e$ heating rates as well as the absorption equilibrium electron fraction ($Y_e$). Within the considered scenarios, we find that the heating rates are generally enhanced by up to $\sim 80\%$ due to the net conversion of $ν_x$ to $ν_e$ (and $\barν_x$ to $\barν_e$) above their crossing energy, provided that the energy spectra above the crossing energy differ substantially. For the absorption equilibrium $Y_e$, spectra changes due to SFC increase it by $\sim 0.03$ due to the relatively more enhanced $ν_e$ absorption rate than $\barν_e$, which potentially drives supernova materials to be more proton-rich. These results highlight the importance of energy-dependent treatments of SFC for supernova neutrinos.

The spatial distribution of the cosmic-ray (CR) flux above $8\,\mathrm{EeV}$ exhibits a significant dipole in agreement with expectations from extragalactic sources following the large-scale structure. At the ankle, the direction changes suddenly and is compatible with the Galactic center for energies $\lesssim4\,\mathrm{EeV}$. We leverage the direction and strength of the dipole anisotropy as diagnostics of the possible sources of the intermediate-mass nuclei below the ankle that cannot be associated with the continuation of the Peters cycle of lower-energy Galactic CRs. Using 3D-simulations of CR propagation in up-to-date Galactic magnetic field models, we show that the dipole anisotropy of both continuous and transient individual Galactic sources and source distributions exceeds the measurements by $>3σ$. An extragalactic origin of the intermediate nuclei below the ankle instead agrees with all data, where the observed change in dipole direction is guided by the fading heavy Galactic component dominant at the second knee.

Characterizing the high-energy radiation environments of stars plays a critical role in determining which systems' planets are capable of retaining atmospheres and sustaining habitable conditions. X-ray through ultraviolet (UV) radiation drives atmospheric photochemistry, heating, and escape, making accurate characterization of stellar high-energy emission essential for both interpreting future exoplanet observations and identifying the most promising targets for life detection. We construct panchromatic spectral energy distributions (SEDs) spanning the X-ray through radio for 12 nearby low-mass and Sun-like stars with directly imageable habitable zones that are prioritized targets for the Habitable Worlds Observatory (HWO) and Extremely Large Telescopes (ELTs). These SEDs are generated using forward stellar atmosphere models guided and constrained by available archival X-ray and UV observations. We find that many stars in this sample exhibit elevated high-energy radiation environments relative to the modern Sun, with habitable zone X-ray and extreme UV (XUV) fluxes frequently exceeding solar values by 1-2 orders of magnitude. The elevated emission likely reflects a combination of sample selection effects, differences in stellar age and rotation, and intrinsic magnetic variability, with multi-epoch observations demonstrating that variability alone can significantly alter inferred radiation environments. These results highlight high-energy radiation as an important discriminator in identifying the most promising habitable planet hosts and demonstrate the need for expanded X-ray and UV observations to complete the stellar characterization necessary for HWO target prioritization.

We propose a new early dark energy with a potential that has an inverse power-law asymptotic tail to alleviate the Hubble tension. Fitting this model to the datasets of CMB+BAO+SN+H0DN, one obtains a 68$\%$ confidence-level value of $H_0\sim 70.20^{+0.60}_{-0.47}$ km s$^{-1}$Mpc$^{-1}$ and a best-fit value of $H_0\sim 70.56$ km s$^{-1}$Mpc$^{-1}$ for index $n=10$. We show that this model can be tested by future data of CMB and matter power spectra. Finally, we briefly discuss fine-tuning problems related to this model.

Dark matter halos, which host galaxies and galaxy clusters, have peculiar velocities far from thermal equilibrium. Characterizing the nonlinear and non-Gaussian features of this velocity distribution improves our understanding of the gravitational evolution of the cosmic web and supports related cosmological applications. We endeavor to establish a connection between the peculiar velocity distribution of halos and nonequilibrium statistical mechanics, with the objective of obtaining a model that is both concise and accurate for practical applications. We extracted halo samples from large N-body simulations and performed maximum-likelihood fits to the peculiar velocity distributions using a two-parameter Tsallis model, derived from non-extensive statistical mechanics. On the theoretical side, we reformulated the halo distribution in the superstatistics framework by means of a generalized Gram-Charlier expansion based on the gamma distribution. For halo peculiar velocities below 1000 km/s, the Tsallis model achieves 5 percent accuracy over z=0-2, with performance improving toward lower redshifts. Our results show that the halo velocity distribution becomes increasingly non-Gaussian and departs further from equilibrium over time. The best-fit parameters depend only weakly on mass, though low-mass halos exhibit slightly stronger non-Gaussianity. The two parameters, especially the velocity dispersion, offer promising probes of cosmological parameters. Theoretically, we find that in general the halo peculiar velocity distribution function is expressible as a superposition of a series of Tsallis distribution functions, while simulation results demonstrate that the zeroth-order approximation, namely a single Tsallis function, already achieves sufficient accuracy.

We introduce a novel method for reconstructing the cosmic mass, tidal, and velocity (MTV) fields over the redshift range $0 < z < 0.6$ using the phase information of galaxy groups. This approach replaces the explicit theoretical bias correction typically needed to relate galaxy groups to the underlying dark matter density field with a simulation-calibrated statistical mapping, reducing a major source of systematic uncertainty and making the method directly applicable to spectroscopic redshift surveys such as the DESI Bright Galaxy Survey (BGS). We evaluate the performance of our MTV reconstruction pipeline with mock redshift surveys that include a comprehensive set of observational selection effects. The galaxy groups used as tracers are identified with an extended halo-based group finder applied to the DESI mock galaxy catalogue with an apparent magnitude limit of $m_z < 19.65$, yielding a galaxy number comparable to that of the DESI BGS faint sample ($m_r < 20.175$). Our tests show that the reconstructed velocities are accurate and unbiased, with a residual dispersion of $\sim 120\ \mathrm{km\,s^{-1}}$ across the redshift bins. The recovered velocity field allows us to shift galaxy groups to their real-space positions, thereby correcting for the Kaiser effect. By iteratively applying this Kaiser correction to the galaxy groups, we further reconstruct the tidal field and the mass-density distribution. The reconstruction is stable with respect to the grid resolution. Overall, our results demonstrate that this group-based phase-space reconstruction provides a robust pathway to recovering the dark matter MTV fields, with strong prospects for application to DESI BGS data.

Classical Cepheids are fundamental distance indicators that anchor the cosmic distance ladder through their Period-Luminosity, Period-Luminosity-Color, and Period-Wesenheit relations, while also providing valuable constraints on young stellar populations. Recent advances in space-based near-infrared observations highlight the need for theoretical calibrations of Classical Cepheid observables in modern photometric systems optimized for precision distance-scale applications. This work provides a homogeneous theoretical framework for Classical Cepheids in the JWST/NIRCam and Roman/WFI systems and explores their use as standard candles in the near-infrared. We exploit an extensive grid of nonlinear convective pulsation models spanning a broad range of masses, effective temperatures, periods, metallicities, convective efficiencies, and mass-luminosity relations. We derive mean magnitudes, colors, pulsation amplitudes, amplitude ratios, and new theoretical Period-Luminosity-Color, Period-Wesenheit, and metal-dependent Period-Wesenheit relations. The theoretical Wesenheit relations are compared with recent JWST observations of Classical Cepheids in six SN Ia host galaxies from the SH0ES collaboration. We find excellent agreement between predictions and observations, with the largest systematic effects arising from the adopted mass-luminosity relation. Theoretical distance moduli are fully consistent with empirical JWST determinations when a mildly overluminous mass-luminosity relation is adopted instead of the canonical one. These results provide a self-consistent pulsation-based calibration for JWST and Roman observations and a theoretical framework for future precision studies of the local distance scale.

We investigate whether a Hayward regular black hole can be observationally distinguished from a Schwarzschild black hole through strong gravitational lensing when effective field theory (EFT) corrections to photon propagation are taken into account. We derive the modified photon propagation law induced by non-minimal couplings between the electromagnetic field and spacetime curvature, and analyze the resulting photon trajectories in the Hayward spacetime. Using the strong deflection limit, we derive the corrections to the photon sphere and the logarithmically divergent part of the deflection angle. Although the EFT corrections are parametrically small, their effects can be enhanced near the critical propagation region, where the deflection angle exhibits a logarithmic divergence. We evaluate the strong-deflection observables for representative values of the Hayward parameter and compare them with the Schwarzschild case. We find that EFT corrections to photon propagation can leave characteristic imprints on strong-lensing observables. Furthermore, the contribution of the EFT corrections becomes more pronounced as the Hayward parameter $g^3$ approaches its critical value, indicating that curvature-dependent corrections to photon propagation can become particularly relevant in the near-critical regime. These results suggest that strong gravitational lensing may provide a means of distinguishing Hayward regular black holes from Schwarzschild black holes.

In this work we study gravitational perturbations of black holes carrying primary scalar hair in shift-symmetric beyond-Horndeski gravity. By focusing on the axial sector, we study the geometric structure of the perturbation equations and perform time-domain evolutions both in the physical and in the effective metric descriptions, in order to quantify differences in the response characteristics of solutions with different hair. In the frequency domain, we then explore the QNM spectrum in the physical-metric formulation, thus verifying and extending existing approximate calculations derived in the effective-metric approach. We also compute the corresponding greybody factors and absorption cross-sections. Finally, we use localized deformations of the effective potential as a diagnostic of the relative stability of quasinormal frequencies and greybody-factor observables.

We investigate a bimetric cosmological model in which the gravitational and matter sectors are described by distinct metrics related through a time-dependent function $α(t)$. This relation leads to dynamical gravitational parameters, with $G(t)\proptoα^{-3}$ and $c_{\rm grav}(t)\proptoα^{-1}$, where $c_{\rm grav}$ denotes the propagation speed of gravitational interactions. We consider a class of solutions for which $α\rightarrow\infty$ at a finite cosmic time, resulting in $G\rightarrow0$ and $c_{\rm grav}\rightarrow0$. For an analytically tractable solution, we show that the final singularity occurs at finite conformal time and possesses a conformal structure compatible with a possible transition between successive cosmological cycles. The Tipler and Królak criteria indicate that the singularity is strong. We further discuss the physical consequences of the weakening gravitational interaction, including the dissolution of gravitationally bound structures and the shrinking of black-hole horizons. These effects suggest a possible mechanism leading toward an effectively radiation-dominated final state, which may be relevant for Conformal Cyclic Cosmology. The results provide a motivation for extending the analysis to more general scalar--tensor theories of gravity.

We present discussions on the possibility of emerging hidden scale symmetry, as a pseudo-conformal phase in super dense baryonic matter, using the velocity of sound as a criterion for a scale symmetry window in hadronic dense matter. In the density dependent mean field approach à la Brown-Rho scaling, it has been observed that the interplay between vector mesons and $ χ$, one of the strongly correlated effects between hadrons, is nontrivial such that the trace of the energy momentum tensor becomes density-independent in the super dense regime and the sound velocity approaches the conformal sound velocity for the pseudo-conformal phase. It is suggested that in the pseudo conformal phase the rearrangement terms induced by density dependent couplings do not spoil the hidden scale symmetry in the compact star matter. We elaborate further on the astrophysically observable quantities of the compact stars and the implications for the parity doubling and the quark-hadron transitions.

This paper is aimed at investigating electromagnetic energy extraction, charged-particle collisions, charged-field superradiance, and horizon stability in a Lorentz-violating dyonic Kalb-Ramond black hole. The geometry differs from the dyonic Reissner-Nordström solution through a modified asymptotic normalization and an effective charge combining electric and magnetic sectors. We derive the dynamics of electrically and magnetically charged probes, including monopole-induced conical motion, and formulate electric and magnetic Penrose processes as decay or collision mechanisms in which a negative-energy fragment falls into the horizon while another escapes with enhanced energy. The electric and magnetic channels are controlled by \(Φ_H=Q/[(1-\ell)r_+]\) and \(Ψ_H=p/[(1-2\ell)r_+]\), respectively. Negative-energy states arise from electromagnetic canonical energy rather than from a geometrical ergoregion. We also obtain escape conditions for the outgoing products, test the possibility of overcharging or overmagnetizing the black hole, and analyze the center-of-mass energy of charged-particle collisions. Nonextremal same-direction collisions remain finite, whereas head-on or near-critical configurations can become large. In the extremal limit, electrically or magnetically critical particles may generate the Bañados-Silk-West divergence when the radial reachability condition is fulfilled.

We investigate static spherically symmetric vacuum solutions in higher-dimensional $f(R)$ gravity, beginning with the five-dimensional Starobinsky model governed by the action $f(R) = R + αR^2 - 2Λ$. By enforcing the ghost-free stability criterion $f'(R) > 0$ on constant scalar curvature spacetimes, we show that a stable effective cosmological constant cannot be dynamically generated from pure $R^2$ geometric corrections in five dimensions; its existence is inextricably tied to a bare cosmological constant. Generalizing this analysis to arbitrary dimensions $D$ and single-term curvature corrections $f(R) = R + αR^n$ with a vanishing bare cosmological constant, we derive a universal stability bound, $n > D/2$, required for the existence of stable emergent vacua. Finally, we demonstrate that expanding the gravitational action to a multi-term polynomial hierarchy circumvents this strict limitation. By including curvature corrections up to $\mathcal{O}(R^3)$, the extended geometric degrees of freedom simultaneously satisfy the trace constraint and the stability criterion. Furthermore, we establish the exact parameter space boundaries that ensure not only asymptotic vacuum stability but strict global stability ($f'(R) > 0$ for all $R$), allowing for the dynamical generation of exact, globally ghost-free vacuum spacetimes in $D \ge 5$ purely from higher-order geometric terms. The scalaron mass requirement, $m_s^2>0$ is not imposed in full in our analysis. A detailed investigation of its implications on the models considered in this work are left for future study.

While we have learned much from recent surveys of the mm-wave sky, many secondary anisotropies of the CMB remain under-explored. We consider two of these here: the anisotropic screening induced by Thomson scattering of CMB photons off free electrons, and cosmic birefringence, the rotation of CMB polarization due to parity-violating physics beyond the standard model. To measure these effects, we apply state-of-the-art estimators for statistical anisotropy to the ACT Data Release 6 CMB data and construct noisy maps and power spectra of both effects. For anisotropic screening, we find no significant detection with the baseline data, and place an upper limit on its power spectrum that is two orders of magnitude above predictions from current reionization models. We obtain evidence for some foreground contamination at one of our frequencies that is consistent with expectations from extragalactic simulations. For our anisotropic birefringence analysis, we also find no detection and place an upper limit on the signal that is consistent with previous analyses. We report a corresponding upper limit on the Chern-Simons coupling term $g_{a γ} < 4.9 \times 10^{-2} / H_\mathrm{I}$, with $H_\mathrm{I}$ the inflationary Hubble scale. These analyses pave the way toward obtaining improved constraints with the next generation of ground-based CMB surveys.

Non-relativistic objective collapse theories have been remarkably successful in addressing the conceptual problems of standard quantum mechanics. Despite substantial efforts, the project of extending them to the relativistic domain remains burdened by significant conceptual objections and technical challenges, often taken to cast doubt on the viability of the program as a whole. On the conceptual side, relativistic collapse theories have been claimed to face challenges involving tension between instantaneous collapse and relativity, frame-dependence of property values and probabilities, the possibility of superluminal signaling and the failure of narratability. On the technical side, persistent infinities, difficulties in constructing fully covariant frameworks and the apparent need for non-standard fields have hindered the development of workable models. In this paper, we offer a systematic rebuttal of the conceptual objections and provide a structured account of the remaining technical challenges. We conclude that relativistic collapse theories do provide a promising route toward a fully relativistic quantum framework that overcomes the conceptual limitations of standard quantum theory.

We study the weak coupling of two identical two-level atoms to a four-dimensional massless conformally coupled scalar field and compare three settings with the same local temperature but different physical origins: comoving atoms in the Bunch--Davies vacuum of de Sitter spacetime, transversely separated uniformly accelerated atoms in the Minkowski vacuum, and static atoms in a Minkowski thermal bath. We first derive the two-atom cross spectra in the three settings and then obtain the single-atom local spectra uniformly from the zero-separation limit. When $a=H$ and $β=2π/H$, the complete local Wightman kernels and local spectra are identical in all three cases. At finite separation, a genuine thermal bath produces a $\sinc(ΩL)$ spatial factor, whereas de Sitter spacetime and the transversely accelerated vacuum produce a hyperbolic geometric factor. In the long-distance limit, the cross correlations in de Sitter spacetime and the transversely accelerated vacuum decay as $L^{-2}$, whereas those in the Minkowski thermal bath decay only as $L^{-1}$. The de Sitter and transversely accelerated vacuum cross spectra coincide when $a=H$ and the separations are instantaneously matched. For a fixed experimental setup, the physical separation between the two comoving atoms in de Sitter space evolves with cosmic expansion, whereas the pulled-back cross correlations in the uniformly accelerated and thermal Minkowski configurations are stationary.

With the rapid development of modern atomic clock technology, their unprecedented precision elevates them from timekeeping tools to gravitational potential sensors, thereby fostering the highly interdisciplinary field of Relativistic Geodesy. Given the potential for high-precision clock networks to detect periodic gravitational variations, it is imperative to assess their capability to invert solid Earth tide parameters via space-to-ground links in the presence of complex observational noise. To this end, we incorporate Earth's gravitational potential, direct lunisolar tidal potentials, and solid Earth tide effects into a high-precision relativistic framework for space-to-ground clock comparisons. By employing a three-link Doppler cancellation configuration to isolate the target signal, we perform numerical simulations for an inclined geosynchronous orbit satellite to analyze the effects of clock instability and colored precise orbit determination errors on parameter extraction. Our findings reveal that while high orbital altitudes cause severe collinearity between individual Love numbers, an effective parameter combining the $h_2$ and $k_2$ Love numbers successfully converges to a stable estimate within a 30-day continuous observation window. Furthermore, sensitivity analysis demonstrates that extraction accuracy is currently limited by clock stability rather than radial precise orbit determination errors.

For a relativistically decoupled thermal relic, the dark-matter energy-to-entropy ratio scales as $ρ_{\rm DM}/s\sim 10^{-3}m_{\rm DM}$. This is the familiar hot-dark-matter abundance relation. Immediately after weak-sphaleron freeze-out, the baryon asymmetry can be parametrized as $ρ_{\rm baryon}/s\sim 10^{-4}|μ_B|$. Here $μ_B$ is the baryon-number chemical potential. We propose a new class of models, called chemical-potential-matched dark matter ($μ$DM), in which $|μ_B|\sim m_{\rm DM}$, thereby providing a new route to the baryon-dark matter coincidence. As a concrete example of $μ$DM, we consider an Affleck-Dine field whose excitations constitute dark matter. We also briefly discuss spontaneous electroweak baryogenesis associated with axion-like-particle walls. In the absence of entropy dilution, this coincidence mechanism generically points to dark matter in the eV-keV mass range. Its momentum distribution can nevertheless be cold if thermal production is dominated by Bose-enhanced stimulated emission. Alternatively, entropy dilution after both the dark-matter and baryon yields are fixed allows masses up to the MeV scale.

Thermodynamic significance of near-horizon symmetries remains an important open question in black hole physics, particularly in the context of black hole evaporation and information recovery. In this work, we investigate the role of horizon-adapted Bondi-van der Burg-Metzner-Sachs (BMS)-like supertranslations in the thermodynamic description of a dynamical Schwarzschild black hole. Working in a near-horizon Rindler coordinate system, we construct a class of diffeomorphisms that preserve the horizon structure and promote the associated supertranslation parameter to a Goldstone-like mode arising from the breaking of horizon symmetry. By expanding the Einstein-Hilbert action around the background geometry, we obtain the effective action for the Goldstone mode and identify the corresponding conserved horizon charge from the surface contribution of the action. The relevant horizon is defined at the future outer trapping horizon, while the surface gravity is computed using the Kodama-vector construction appropriate for dynamical spacetimes. We show that the horizon supertranslation mode contributes non-trivially to the surface gravity and modifies the thermodynamic description of the black hole beyond the stationary limit. Using the associated Noether charge, we derive the entropy of the horizon-BMS transformed geometry and find that the Bekenstein-Hawking area law is recovered at leading order, while subleading corrections depend explicitly on the supertranslation sector and the dynamical evolution of the black hole.

We revisit the carbon abundance in the primitive iron-poor star J0815+4729 ([Fe/H] $=$ $-$5.49). J0815+4729 is so carbon-enhanced and oxygen-rich that, in addition to CH and C$_{2}$ molecular lines, weaker neutral atomic transitions are measurable in its spectrum, providing a unique opportunity to explore the line formation of C I and O I transitions in such an extremely iron-deficient star. Based on high-resolution spectroscopy and newly constructed atomic models for carbon and oxygen, we present the first investigation to derive carbon and oxygen abundances of this hyper iron-poor star from neutral atomic transitions under non-local thermodynamic equilibrium (NLTE) conditions. The carbon abundance derived from C I lines is A(C)$_{\rm NLTE} = 8.08 \pm 0.07$, consistent with that inferred from C$_{2}$ lines (A(C)$_{\rm LTE} = 7.96 \pm 0.08$), but around 0.6 dex higher than the value from CH lines (A(C)$_{\rm LTE} = 7.47 \pm 0.17$). Since CH lines are the most commonly-used carbon abundance indicator for extremely metal-poor stars, this result casts doubt on the quantitative interpretation of CH absorption in the most primitive stars. We confirm that J0815+4729 is one of the most carbon-enhanced stars with [Fe/H] $\le$ $-$4.5 known to date. We have also determined A(O)$_{\rm NLTE}$ as 7.25 $\pm$ 0.11, with NLTE corrections of around $-$0.11\,dex. The carbon-to-oxygen ratio of the star is [C/O] $=$ 1.06 $\pm$ 0.11.

It is well known that the Einstein equations are tensor equations for a Lorentzian metric. Hence, choosing a suitable gauge condition is crucial for solving them. As a powerful gauge condition, maximal foliations have played a pivotal role in two groundbreaking works in general relativity: the global stability of Minkowski spacetime and the bounded L^2 curvature conjecture. Nevertheless, if the initial hypersurface fails to be maximal, the usual elliptic approach for constructing maximal foliations encounters fundamental difficulties when solving the Einstein equations. The purpose of the paper is to provide a construction of maximal foliations around any initial asymptotically flat Cauchy surface satisfying vacuum Einstein constraint equations, under a suitable smallness condition on the mean curvature.

We present a first-principles derivation of an effective inflaton potential with a bumpy feature. We consider the coupling of a spectator field χto the inflaton φsuch that χis massive enough for almost all values of φ, but becomes massless when the inflaton crossed the value φ_\star. This gives rise to a burst of χ-particle production at φ=φ_\star. By taking the backreaction of the particle production, we obtain a bumpy feature in the effective potential.

We establish a bottom-up model for studying the `would-be' de-Sitter/interface CFT (dS/ICFT) correspondence on the gravity side, using the Coleman-De Luccia instanton. After absorbing the region where the inflaton changes dramatically by a brane, the spacetime consists of two de Sitter spacetimes with different radii joined by a brane, which is expected to be dual to an interface CFT provided that the dS/CFT conjecture is correct. Our goal is to analyze existing prescriptions for the holographic computation of pseudo-entropy, i.e. the extremal surface in such a spacetime and argue that some of them would cause parodox with the results expected the universal properties of interface CFT and compare our results with that of Anti de Sitter/interface CFT correspondence (AdS/ICFT) after analytic continuation. We also read off the $g$-function from the entanglement entropy associated with a subregion perpendicular to the interface.

We prove that the notion of mass recently introduced by Mazurowski and Yao for continuous metrics coincides with Huisken's isoperimetric mass on smooth Riemannian $3$-manifolds with nonnegative scalar curvature whose metric has sharp $C^0$-asymptotically flat behavior. As a consequence, we deduce a Riemannian Penrose inequality for the mass parameter of asymptotically Schwarzschildian $3$-manifolds.

We present NGTS measurements of rotation period distributions for FGKM stars in seven young open clusters spanning ~40--700 Myr within PLATO's first long-stare LOPS2 field. We measure 1063 rotation periods, of which 479 are newly analysed as part of cluster specific rotation studies, whilst 63 are unique periods not reported in the recent TESS All-Sky Rotation Survey. Of the 1063 rotation periods, 285 are identified as likely binary or higher order multiple systems using colour-magnitude diagrams and Gaia astrometry. These are the first comprehensive rotation period distributions for Trumpler 10, NGC 2451B and Alessi 3, while extending existing distributions for NGC 2451A, NGC 2516, Collinder 135 and IC 2391, to create a fuller picture on the rotation state of young stars in PLATO's LOPS2 field. We find that main-sequence solar-mass stars in the ~40 Myr old NGC 2451B cluster, form a slow sequence that can be distinguished from their counterparts at ~70--80 Myr, thereby significantly reducing the age at which young stellar groups can be relatively aged via their rotation sequences. We also observe stalled spin down from the age of NGC 2451A to at least that of NGC 2516 (~70--150 Myr) at masses $\gtrsim1$ $M_{\odot}$, supporting previous predictions that angular momentum redistribution and removal should result in a wave of stalled spin down that propagates as a function of both mass and age. Finally, we provide a new age estimate for Alessi 3 of 687$\pm$106 Myr using differential gyrochronology age dating.

We present the Tomo-e Gozen Bright Metal-Poor Star Survey (TeMPS), a wide-area narrow-band photometric survey for bright metal-poor stars in the northern sky. The survey uses the Tomo-e Gozen camera on the 1.05 m Kiso Schmidt telescope with four narrow-band filters centered on the \ion{Ca}{2} H and K lines, the CH $G$ band, H$α$, and a reference wavelength region. We review the survey strategy, photometric processing, and calibration of metallicity and carbon abundance estimates derived from narrow-band colors. We further present medium-resolution spectroscopic follow-up with Nayuta/MALLS to validate the photometric selection and identify new metal-poor stars. The current data set covers $\gtrsim 22,000$deg$^{2}$ in all four bands with a total integration time of $\sim 100$ hr. The median limiting magnitudes at S/N = 20 correspond to $G\sim12.5$. By combining narrow-band photometry with archival broad-band photometry and {\it Gaia} distances, we estimate $T\_{\rm eff}$, $\log (g)$, metallicity, and carbon abundance. Calibration against literature abundances derived from high-resolution spectra shows typical scatters of $< 0.3$ dex in metallicity and $< 0.4$ dex in carbon abundance. We estimate metallicities for $\sim 1.7$ million stars and identify $\sim 16,000$ very metal-poor candidates with [M/H]$_{\rm NB,fin}<-2$. We show that Nayuta/MALLS medium-resolution spectra provide metallicities consistent with high-resolution measurements, with a scatter of $\sim 0.27$ dex. Among 32 photometrically selected candidates followed up with MALLS, 24 are confirmed to have [M/H]$_{\rm MALLS}< -2$, including one newly identified star with [M/H]$_{\rm MALLS}\simeq -3.4$. These results demonstrate that Tomo-e Gozen narrow-band photometry and MALLS medium-resolution spectroscopic follow-up efficiently select bright metal-poor stars for future high-resolution abundance studies. (abbreviated)

The nonlinear relation between density and expansion is usually formulated for a homogeneous spherical top hat, which has a single local Hubble rate. A smooth spherical profile instead expands differently along ($H_{\parallel}$) and across ($H_{\perp}$) the radial direction-a direct signature of radial inhomogeneity. We show that, for growing-mode pressureless matter with a cosmological constant, the complete shellwise response is nevertheless fixed by one top-hat function. Given the local density contrast $δ(t,r)$ and the enclosed contrast $Δ(t,r)$, a transverse response and its derivative determine $H_{\perp}$ and $H_{\parallel}$. The linear and second-order limits are algebraic maps whose only dynamical input is the usual growth rate $f$. An exact equal-age construction supplies the nonlinear response without integrating an evolution equation. We check that it reconstructs full $Λ$LTB profiles to numerical precision. We also provide a derivative-aware, cosmology-independent three-term symbolic fit that requires only $f$, $δ$, and $Δ$. Across a representative set of matter-curvature-redshift combinations and for a shell located in the compensated transition, the maximum relative errors are $0.3\%$ and $0.7\%$ in the transverse and radial responses, respectively. This compact formulation separates the production of a density profile from its expansion response and makes the effect of radial gradients explicit.

Can a finite quantum episode populate a classical future that the same prepared state never reaches deterministically? We show that it can, using inflation as a cosmological laboratory. In our supergravity realization, two classical continuations already exist, but the prepared state reaches only one without noise. A stable entropy direction temporarily becomes light without a tachyonic instability, allowing mode-derived phase-space fluctuations to access the competing continuation. Finite-horizon first passage assigns it a \(0.33\)--\(0.40\) conditional weight across three nearby CMB-normalized realizations, while suppressing the stochastic source restores its exact deterministic zero. The entropy sector then becomes heavy again, and the selected histories continue classically. A finite quantum episode can thus leave a lasting redistribution among classical futures.

Debris disks, composed of rocks, boulders, planetesimals, and the dust produced in their collisions, present the most readily observable components of mature planetary systems. They also serve as valuable diagnostic tools, enabling studies of planetary dynamical interactions and mineral compositions. Observed from optical to radio wavelengths, each band reveals unique information about the dust populations. Optical and near-infrared observations are specifically sensitive to light scattered off the surfaces of the micron-sized particles. Here, we present results from the JWST/NIRCam GTO program 2780, designed to observe five disk systems previously identified to be exceptionally bright at optical wavelengths (HD 10647, HD 32297, HD 61005, HD 107146, and HD 181327) with six filters using the NIRCam coronagraphs. The NIRCam data complement previous shorter-wavelength images of these same systems. They reveal scattered light from the disks and from the extended halos of tiny grains under the influence of radiative forces, at high resolution and signal to noise. All the systems show evidence for water ice, although it can have differing radial distributions and tends to show stronger signatures in the halos. In the two cases we could analyze, the scattering phase function in the disks resembles the behavior of dust in the Solar System with evidence for enhanced forward scattering in the halos, consistent with the latter being composed of tiny grains. MIRI images for two systems are more centrally concentrated than the shorter wavelength ones, suggesting a role for dragged-in larger grains.