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Learned surrogates for expensive inner solver blocks are a widely pursued route to faster multiphysics simulation. We report a controlled, end-to-end negative result and identify three structural barriers, none of them a deficiency of the network we trained. The testbed is the implicit Newton solve coupling energy-dependent neutrino radiation to matter in a general-relativistic radiation-hydrodynamics code, its most expensive physics routine per call. First, per-call cost and share of runtime are different quantities, and only the second bounds acceleration. An exclusive self-time profile puts the target block at 16.9% of critical-rank wall clock, capping any surrogate at ~1.2x by Amdahl's law. A surrogate 5.8x cheaper per call merely ties the solver, and the configuration stable enough to run without fallback reaches only parity. Second, offline accuracy cannot rank surrogates for deployment: across fourteen networks the pooled Spearman error-versus-survival correlation (rho = +0.73) is a between-family confound that vanishes under control (rho = -0.04). Third, a correct out-of-distribution gate cannot accelerate a loop that leaves its training distribution. We give the break-even deferral fraction in closed form: because the visited states sit 73x off the data manifold, the gate defers 96.8 to 99.7% of cells, almost invariant to surrogate quality. Including its own cost, the gated loop is a 0.94 to 0.96x slowdown. We further separate stability from fidelity: a never-crashing gated run accumulates a linear -19.9% density bias over 6000 steps. The error is a directed, ballistically accumulating bias, not the variance-driven divergence the autoregressive literature targets.

The large-scale dipole anisotropy observed by the Pierre Auger Observatory above $8$~EeV provides important clues about the origin of ultra-high-energy cosmic rays (UHECRs). In this work, we investigate whether gamma-ray active galactic nuclei (AGNs) from the Fermi-LAT Fourth AGN Catalog (4LAC) can explain the observed dipole and examine the contribution of nearby starburst galaxies (SBGs). Incorporating source fluxes, redshift distributions, GZK attenuation, and a mixed-composition framework, we demonstrate that single-source populations of AGNs and SBGs alone fail to reproduce the observed Auger dipole. To resolve this, we construct a hybrid AGN+SBG model. The best-fit solution for the $8$--$16$~EeV interval requires a $23$\% AGN contribution and $45$\% SBG contribution, which gives a dipole amplitude of $5.03$\% while maintaining the dipole direction of ($104.1^\circ$, $-25.1^\circ$) with angular separation of $7.10^\circ$. In the other energy intervals also, we find that the nearby SBGs increasingly dominate the anisotropic component while AGNs provide a subdominant contribution alongside a quasi-isotropic background.

Electron-capture supernovae (ECSNe) are a promising formation channel for low-mass neutron stars, but the minimum gravitational mass of the neutron stars they produce depends on both the progenitor core structure and the neutron-star matter equation of state (EOS). We compute the electron-capture (EC) threshold gravitational mass ($M^\star_{WD}$) of cold white-dwarf-like O--Ne--Mg cores with representative compositions and map the baryon number onto cold neutron-star configurations constructed from a Bayesian ensemble of unified crust--core EOSs. Although the EC threshold density is sensitive to the concentrations of the O--Ne--Mg mixture, the threshold baryon number of the O--Ne--Mg core varies only weakly, producing a narrow remnant-mass window. In the baseline case with no baryonic mass loss during the transition from the EC threshold mass O--Ne--Mg core to the remnant neutron star, standard ECSNe yield remnant neutron stars with gravitational masses of $1.24$--$1.265\, M_\odot$, with only a small EOS-induced spread. Small baryonic mass losses of $0.01$--$0.02\, M_\odot$ shift this range modestly downward, but the $1.174\, M_\odot$ companion of PSR J0453+1559 would require an extreme mass loss close to $0.10\, M_\odot$, which is not favored by current ECSN simulations. We find that the residual EOS dependence of the remnant mass is controlled mainly by the pressure around nuclear saturation density, while the corresponding tidal deformability remains sensitive to the remnant radius and compactness. Thus, low-mass double neutron star systems can in principle connect ECSN-like formation channels with gravitational-wave constraints on the EOS. Our results show that ECSNe naturally form low-mass neutron stars, but within a restricted mass range; the lightest observed neutron stars likely require low-mass iron-core collapse, ultra-stripped supernovae, or other nonstandard channels.

We report results from a search for an isotropic stochastic gravitational-wave background using data collected by the LIGO--Virgo--KAGRA Collaboration. The analysis uses data from the first observing run through April 1, 2025, during the fourth observing run. New frequency-domain cuts are implemented to address a class of non-stationary spectral noise features that were not effectively identified and mitigated by existing data-quality checks in past analyses. Consequently, previously analyzed data from the fourth observing run are re-processed with the updated cuts. We find no evidence for a stochastic background signal and place upper limits on the gravitational-wave energy density. In particular, for a background following a power law with spectral index 2/3 as predicted by inspiralling compact binaries, we find $Ω_\mathrm{GW}(25\,\mathrm{Hz}) \leq 2.0 \times 10^{-9}$, while scale-invariant backgrounds are constrained to $Ω_\mathrm{GW}(25\,\mathrm{Hz}) \leq 2.8 \times 10^{-9}$, both at the 95\% credible level for a log-uniform prior on $Ω_\mathrm{GW}$. Relative to the constraints from previous data recomputed with the new frequency-domain cuts, these limits improve by a factor of 1.4. We also update bounds on alternative gravity scenarios predicting non-standard polarization modes, and we verify that correlated magnetic noise sources remain below the sensitivity of this search. Combining these observational constraints with population models of compact binary coalescences informed by the latest gravitational-wave transient catalog, GWTC-5.0, we predict the amplitude of the compact binary background to be $Ω_\mathrm{CBC}(25\,\mathrm{Hz}) = 6.3^{+5.0}_{-2.2} \times 10^{-10}$ at the 90\% credible level.

We investigate how cosmological environment regulates the evolution of primordial black hole (PBH) seeds in the early universe using a suite of hydrodynamical simulations. Unlike traditional seeding channels, PBHs provide early BH seeds without the need of special conditions for gas collapse/fragmentation that favor overdense regions, thus covering diverse large-scale environments. We find that PBHs follow distinct evolutionary pathways depending on the gas supply and halo assembly history. In underdense regions, limited inflow suppresses both accretion and star formation, producing faint, metal-poor systems, while in overdense environments, sustained gas inflow drives rapid BH growth and the formation of compact, centrally concentrated stellar components. These differences lead to large variations in BH-to-stellar mass ratio, metallicity, and morphology, collectively bracketing a range of possible evolutionary pathways for PBH-seeded systems. We show that compact, BH-dominated sources resembling recently observed Little Red Dots naturally arise as one phase within these evolutionary pathways before evolving into more extended galaxy--AGN systems. Our results suggest that both the initial seed properties and cosmological environment jointly shape early BH--galaxy co-evolution, while subsequent environmental regulation can erase the memory of the initial seeding channel.

Due to their different nucleosynthetic origin, a stellar population produces oxygen (O) and iron (Fe) on different timescales, and their relative abundance can deviate strongly from solar. Galaxy formation models should treat these elements separately, as they play a distinct role in shaping physical phenomena. For example, oxygen mainly sets the gas cooling rate, while the iron abundance sets stellar atmosphere opacities impacting stellar evolution, spectra, and feedback. Observations of star-forming galaxies usually only constrain gas-phase oxygen abundance, vastly limiting our capabilities of separating the cosmic evolution of oxygen and iron. Here, we present an observationally motivated framework to scale the cosmic evolution of O and Fe abundances. We applied the relation between the α-enhancement and galaxies' specific star formation rate ([O/Fe]--sSFR) to derive the Fe and O-dependent cosmic star-formation history (cSFH). We find that star formation with near-solar O/Fe is rare: at least 70% of the integrated cosmic stellar mass forms at non-solar O/Fe. The cosmic average metallicity is generally lower in [Fe/H] than in [O/H] (by up to a factor 3), with the offset increasing from z = 0 to redshifts z ≈ 3 and then approaching the core-collapse O/Fe ratio. We validated our results against samples that probed the Fe-dependent cSFH in different regimes such as absorption-derived ⟨[Fe/H]⟩ from long gamma-ray bursts. Our results impact the interpretations of stellar and galaxy spectra and the predicted rates of transients, especially those linked to metal-poor progenitors (e.g. black hole mergers).

Periodic density structures (PDS) are trains of plasma enhancements carried into the heliosphere, but their repeated timing does not by itself identify where the cadence is set. We test whether PDS selected independently in the outer corona can be traced back to a low-coronal clock, and whether that organization survives through the EUVI--COR1--COR2 observing chain. We analyze 12 STEREO-A/SECCHI events on 2008 January 11--14 along a nonradial path. EUVI 171 A at 1.10--1.20 R_sun shows an unusual concentration of power in the predeclared 80--130 minute band relative to filter-matched red-noise controls, while the ensemble is most strongly organized in upper COR1 at 2.5--3.0 R_sun. These signatures identify a low-coronal modulation candidate and an intermediate-height organization domain, but they do not form a unique phase-preserving clock extending into COR2. Instead, the more persistent observable is spatial order. Events 9 and 12 retain expansion-stable outward ordering. In polar r--theta maps, intersections of oppositely inclined, expansion-aware matched-filter ridge supports form repeated X-/diamond-like patterns. The morphology is compatible with stationary or quasi-stationary shock-cell processing, although the brightness diagnostics are insufficient to establish a stationary MHD shock branch. The observations therefore favor an intermittent source--gate--transfer picture: a low-coronal cadence may modulate plasma release, while event-dependent propagation progressively destroys exact phase coherence. The outward ordering of the density structures can survive, providing a more persistent signature of the source-to-wind transfer than phase locking itself.

We present phase-connected timing and polarization measurements for two long-period FAST-CRAFTS pulsars, PSRs J0000+6252 and J2131+3642, and extend single-pulse emission-state analysis to a five-source sample including PSRs J1903+1407, J1502+4653, and J2112+4058. FAST L-band timing baselines span 414 to 511 days; the two pulsars have spin periods of 1.11 to 1.55 s, period derivatives of $(3.99$ to $4.06)\times10^{-15}~{\rm s~s^{-1}}$, characteristic ages of 4.34 to 6.16 Myr, surface magnetic fields of $(2.15$ to $2.52)\times10^{12}$ G, and spin-down luminosities of $(4.21\times10^{31}$ to $1.16\times10^{32})~{\rm erg~s^{-1}}$. Their rotation measures are $70.2\pm26.7$ and $-44.6\pm7.6~{\rm rad~m^{-2}}$, with linear polarization fractions of 17.6\% to 27.2\%. PSR J2131+3642 shows a short monotonic PA segment permitting a formal rotating-vector-model fit, though limited longitude coverage leaves the geometric parameters poorly constrained; PSR J0000+6252 has too few PA points for such a fit. Gaussian mixture modeling (GMM) of single-pulse energy distributions identifies null, weak, and burst components in PSRs J0000+6252, J1903+1407, J1502+4653, and J2112+4058, while J2131+3642 shows only weak and burst states. We also identify bright single pulses (peak intensity $\geq10\times$ the integrated average profile): 42 in J0000+6252, four each in J1903+1407 and J2112+4058, and none in J2131+3642 or J1502+4653. These bright pulses occur within the main emission window with no evidence of periodic recurrence, consistent with sporadic enhancements of the normal radio-emission beam. For J2112+4058, we measure a scattering timescale $τ_{\rm sc}=5.84\pm0.18$ ms at $ν_{\rm ref}=1.25$ GHz. Together, these results highlight the diversity of magnetospheric variability among slowly rotating neutron stars.

The Solar wind is a continuous flow of charged particles emanating from the solar surface and governed by complex, interacting magnetohydrodynamic processes. Accurate specification of inner-boundary conditions is essential for heliospheric modeling and solar-wind prediction. In many practical applications, only a subset of interacting multi-field variables is directly available, but for a comprehensive view of solar wind prediction and downstream magnetohydrodynamic simulations, a more complete boundary state is required. In this work, we study the problem of learning the multi-field multi-scale solar magnetohydrodynamic state at 30 solar radii ($R_\odot$) using operator learning. Specifically, given the radial velocity and radial magnetic field, we aim to reconstruct the non-radial velocity and magnetic field components, radial and non-radial current density, thermodynamic density, and pressure components. This mapping is highly nonlinear, spatially coupled, and multi-scale, making it a challenging task for data-driven scientific machine learning. To address this problem, we employ a Local Neural Operator (LocalNO) that learns mappings between input and output function spaces while retaining locality and resolution-awareness. Unlike conventional regression models and autoencoder models, neural operators are better suited for learning structured field-to-field transformations arising from physical systems. The resulting predictions along with inputs are intended to serve as boundary condition variables for future inner-heliospheric modeling pipelines.

Cosmological first-order phase transitions (FOPTs) in the early Universe are an exciting prediction of many beyond the Standard Model scenarios. Owing to the stochastic nature of bubble nucleation, some causal patches may remain trapped in the false vacuum long after the surrounding regions have completed the transition and started to redshift. Under suitable conditions, these delayed patches can become sufficiently overdense to collapse into primordial black holes (PBHs). One therefore expects characteristic parameters, such as the PBH mass and PBH formation time to be sensitive to the underlying phase-transition dynamics. Here, we adopt a model-independent framework with a fixed mean bubble separation scale ($R_*$) to directly compare the impact of exponential and Gaussian nucleation profiles on PBH formation. We further incorporate the effects of friction on the bubble walls arising from interactions with the surrounding plasma. We find that friction significantly modifies the bubble dynamics, leading to significant changes in the PBH mass and formation time for both nucleation histories. Moreover, the stochastic gravitational-wave spectrum generated by the FOPT can provide a complementary probe of the underlying dynamics, allowing the effects of friction on the bubble wall evolution to be distinguished from the frictionless case.

We present the capabilities of the Fluorescence detector Array of Single-pixel Telescopes (FAST) observatory in the single telescope configuration as an important step towards the possible future large-field observatory for detecting ultra-high-energy cosmic rays. Reconstruction of main shower physics parameters are explored on noise-free simulated events using machine learning techniques in the challenging domain of a low-intensity transient signal. We find a very good correlation between the true and reconstructed energy of the shower even with the information from just the four photomultipliers of the single FAST telescope, and a reduced performance for the maximum of the shower development Xmax, using various architectures of artificial deep and convolutional neural networks, with a comparison to a benchmark gradient boost regression model. The resolution in the energy is found at the sub-percent level, while in Xmax it is~$5\%$. The relative difference between predicted and true values is under one percent for energy, while for Xmax it ranges from $-8\%$ to $+17\%$, which can be attributed to the limited information from the single FAST telescope configuration. The results constitute an important capabilities verification and a lesson learned with implications for established FAST prototypes as well as for more complex configurations of the FAST observatory under construction.

Background: Localized structure in the equilibrium squared sound speed can affect neutron-star observables, but modifying $dP/dε$ requires thermodynamically consistent reconstruction and assessment over the complete declared domain. Purpose: We quantify the response to positive and negative localized sound-speed changes around a unified BSk24 baseline while keeping the deformation geometry, admission criteria, reconstruction, and fixed-mass comparison protocol fixed. Method: A Gaussian profile with quintic smootherstep activation is added to $c_{\rm s}^2$. Proposals violating $P>0$ or $0<c_{\rm s}^2\leq1$ anywhere in the retained domain are rejected before reconstruction. Passing cases are reconstructed as effective cold one-fluid barotropes and evolved with the Tolman-Oppenheimer-Volkoff and tidal equations. Results: The $A=0$ case reproduces the undeformed control exactly. A localized change produces a persistent pressure offset, relocates the fixed-mass central state, and yields a mass-dependent, nonlinear, sign-asymmetric response. For $A=-0.8$ and $+0.8$, respectively, $ΔΛ/Λ_0=(-46.02\%,+35.33\%)$ at $1.4\,M_\odot$ and $(-63.52\%,+87.17\%)$ at $2.0\,M_\odot$. The reached stiffened feature remains core connected at the reporting endpoints, whereas the softened feature becomes an off-center shell at high mass. Conclusions: These phenomenological BSk24-anchored effective barotropes illustrate how a local sound-speed intervention propagates through thermodynamic reconstruction and stellar structure. They are not BSk24 predictions or evidence for a microscopic composition or phase transition.

While no signal originating at a black hole's event horizon can reach future null infinity, information about the horizon and its immediate vicinity can be encoded in asymptotic properties of waves emitted by matter or field perturbations falling toward a growing/forming horizon. Within a response-filtered framework, "direct wave" denotes source-sensitive plunge and remnant-formation information revealed by filtering the black-hole response. Unlike a stationary damped sinusoid with a fixed complex frequency, the direct wave follows the evolving source and has an evolving instantaneous complex frequency tied at late times to the remnant horizon angular velocity $Ω_H$ and surface gravity $κ_H$. Using rational filters, we remove quasinormal modes from numerical-relativity waveforms to study this evolution. Before numerical contamination, trajectories depend on remnant spin: the real frequency evolves toward $2Ω_H$ from above for lower spins ($χ_f\lesssim0.7$) and from below for higher spins ($χ_f\gtrsim0.7$), while the instantaneous decay rate increases toward $\sim2κ_H$ and, in some high-spin cases, beyond it. As in particle-plunge results, the horizon-controlled value is approached only at late times, as frame dragging controls near-horizon motion. For $χ_f\sim0.7$ remnants of non-precessing, comparable-mass binaries, the early-time real frequency is close to $2Ω_H$ because the binary orbital frequency transitions smoothly to the remnant horizon frequency. Finite-time deviations therefore carry information about merger/collapse dynamics rather than undermining the horizon connection. Full direct-wave evolution requires numerical-relativity calibration. We further show that approximate pole-zero pairing in the Kerr response motivates a spin-dependent minimal filter set that suppresses quasinormal-mode features while revealing source-trajectory information.

We present a detailed observational analysis of BLG723.31.34651, the fourth reported eclipsing member of the class of Roche lobe-filling hot subdwarfs, using time series spectroscopy and multi-band photometric observations from the Goodman High Throughput Spectrograph on the 4.1-m SOAR telescope. Our light curves show periodic photometric variability at $P=64~{\rm min}$ from the tidally-deformed hot subdwarf donor and deep eclipses of the donor by an opaque accretion disc. We report two light curve model solutions that agree with each other to within $2σ$, both suggesting that BLG723.31.34651 contains a typical hot subdwarf donor, a white dwarf accretor, and an irradiated opaque accretion disc. Our spectroscopic analysis identifies statistically significant phase-dependent variations in the effective temperature and surface gravity coinciding with the phase-dependent variations seen in our observed light curves. We attribute these variations to a combination of intrinsic variations in local atmospheric parameters due to tidal deformation of the donor and unmodeled spectroscopic flux contributions from an accretion disc.

Atmospheric photochemical models incorporating the impacts of stellar flares often assume a $\sim$9,000 K spectrum at ultraviolet-optical wavelengths. Recent multiwavelength observations, however, reveal a more complex picture with temperature measurements spanning 4,000-40,000 K, although the occurrence rates for flares with different temperatures remain unknown. Here, we model the evolution of a Proterozoic Earth-like world with 0.01 bar of O$_2$ under repeated flaring to identify the impact of flare effective temperatures. We explore four scenarios - two host star types (K2V and M2.5V) and two flare temperatures (9,000 K and 19,000 K) - selected to bound the potential parameter space. The hotter flares have a larger impact on O$_3$ photochemistry for both stellar types. M-star planetary atmospheres are more volatile and exhibit rapid changes in their O$_3$ production and destruction rates. Meanwhile, K-star planetary atmospheres are more stable and are only impacted by the hottest flares, proving advantageous for biosignature searches. We simulate 0.2-1.0 $μ$m reflected light spectra for all four scenarios, and find that 19,000 K flares can result in either production or destruction of O$_3$ depending on the host star spectral type increasing the 0.2 $μ$m feature by $\sim$2$\times$ for the K2V star but decreasing it by 50% for the M2.5V star. Future missions such as the EVE SMEX mission concept will provide robust flare temperature constraints for young FGKM stars, which will serve as inputs to improve photochemical models to inform future HWO observations.

General relativity predicts the presence of a thin, bright ring of light superimposed on the image of a black hole (the ``photon ring''), whose geometry is sensitive to the spin of the black hole. Detecting this photon ring in observations of nuclear supermassive black holes would yield insight into gravity in the strong-field regime as well as their growth histories. The Black Hole Explorer (BHEX) is a mission being developed to detect the photon ring for the first time, raising the question of how precisely we can constrain spin from its observations. We use the method of Fisher information matrices (FIMs) along with a simple dual-cone semi-analytic emission model to forecast BHEX spin posterior widths for the primary science targets (M87* and Sgr~A*) in a variety of configurations. We find that BHEX can constrain the dimensionless spin of its targets to a precision of $σ_{a_*} \ll 0.1$ after 30 orbits, even in the presence of significant systematic errors. We validate our results via numerical checks for stability and robustness, as well as synthetic data cross-validation to assess handling of covariances and prior information in the FIM against a full posterior exploration.

Closure phase is robust to station based phase errors, yet its connection to image structure is often interpreted only qualitatively and a nonzero closure phase is still interpreted mainly through qualitative symmetry arguments or source specific models. We develop, to our knowledge, the first unified response theory for an arbitrary brightness distribution and an arbitrary zero free observing triangle. Away from visibility zeros, closure phase is naturally a log-visibility observable. This structure yields an exact first order master kernel for image perturbations and a Cartesian moment hierarchy that separates the spatial content of the source change from the sensitivity of the sampled triangle. For a fixed image, the same framework gives a local expansion in Fourier plane displacement, providing a direct description of closure phase evolution along observing tracks. We then identify the boundary of this regular regime: at a true visibility null, the log-visibility expansion fails and the phase behavior is controlled by the local properties of the null, including its order and winding charge, distinguishing removable phase wraps, non-null visibility minima, and nulls for which the visibility phase is multivalued. We validate these expansions using structured ring models. We then apply both branches of the theory to EHT data of M87*, and demonstrate the ability to measure asymmetric image structure associated with higher image moments directly from the evolution of the closure phase. Finally, we show that the same formalism extends naturally to Phase Closure Nulling in optical and infrared interferometry. The resulting framework turns closure phase structure into a quantitative probe of faint and asymmetric source features across interferometric regimes.

In asymptotically flat spacetimes far from an isolated source, gravitational waves (GWs) undergo nonlinear interactions with themselves and with the parts of the spacetime curvature related to the time-dependent four-momentum and angular momentum of the spacetime. These spacetime nonlinearities produce distinctive offsets in the GW strain and its time integrals, which have been referred to as the displacement memory effect for the strain and ``higher'' GW memory effects for the integrals of the strain. There are existing data analysis pipelines that search for evidence for the displacement memory effect in the population of binary-black-hole mergers observed by current GW detectors (though conclusive evidence for the effect has not yet been found). The first set of higher memory effects include the spin and center-of-mass effects (collectively, ``drift'' memory), and the second is the ``ballistic'' memory effect. Prior work has shown that next-generation, ground-based GW detectors could find evidence for the spin memory effect in the large population of binary black holes that these detectors will be capable of observing. In this paper, we investigate how well a detector network of two Cosmic Explorer detectors can measure the displacement through ballistic memory signals. We first formulate what are appropriate notions of time-dependent GW signals associated with these memory effects. With these definitions of displacement and higher memory signals, we find that the Cosmic Explorer network is capable of detecting the displacement memory from tens of individual mergers per year and capable of finding evidence for the spin and center-of-mass memory effects in a population of mergers after a roughly one-year observation run at design sensitivity. [Abstract abridged]

Under normal circumstances, an observer falling into a black hole feels nothing special upon crossing the horizon. In this paper we find an intriguing exception: For most of the parameter range of the extremal Kerr-Newman (KN) spacetime, horizon co-rotating perturbations are enhanced near the horizon, such that an infalling observer experiences an anomalously large tidal deformation as they enter the black hole. Such perturbations arise when there is a persistent source outside the black hole that is either co-rotating itself or has discrete Fourier support at the associated frequencies $ω=mΩ_H$ (where $m$ is the azimuthal number and $Ω_H$ is the horizon frequency). The enhancement is formally infinite at precise co-rotation, and we work with a nearly co-rotating mode to keep perturbation theory under control. The underlying physics is the emergence of discrete self-similarity in the extremal limit, with complex scaling weights of the form $-1/2\pm iα$ for real $α$. It is analogous to the black hole Meissner effect, except that the near-horizon field is enhanced rather than screened. We numerically calculate the scaling exponents for coupled gravitoelectromagnetic (GEM) perturbations of extremal KN black holes and show that the complex exponents arise in the parameter range $Q < Q_*$ with $Q_*\approx.93M$. These exponents also predict the decay and growth rates (Aretakis effect) of generic GEM perturbations of the KN spacetime, both on and off the horizon. The enhancement of co-rotating perturbations can be viewed as a driven Aretakis instability.

We present a framework to study the orientation distribution of nearby double white dwarf binaries with LISA, explicitly accounting for the fourfold degeneracy in the polarization angle. Exploiting the beltlike sky distribution of the nearby sample along the Galactic plane, we show that the low-multipole extraction can be recast as an effectively one-dimensional Fourier analysis along Galactic longitude, within a spherical-harmonic expansion based on the Wigner-<inline-formula><mml:math><mml:mi>D</mml:mi></mml:math></inline-formula> formalism. The symmetry with respect to the Galactic plane governs the sensitivity: modes even under reflection across the plane remain accessible, while odd modes are strongly suppressed. For a fiducial sample of about 500 binaries within <inline-formula><mml:math><mml:mo>≲</mml:mo><mml:mn>4</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>kpc</mml:mi></mml:math></inline-formula>, corresponding to roughly 5% of the resolved Galactic population, the method yields an uncertainty of order 0.02 for the reflection-even coefficients with <inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>≤</mml:mo><mml:mn>3</mml:mn></mml:math></inline-formula>, and is readily applicable to the forthcoming LISA catalog.

By monitoring the times of arrival of radio pulses from millisecond pulsars, pulsar timing arrays (PTAs) serve as unique gravitational wave (GW) laboratories in the nanohertz band. To date, the primary astrophysical sources of GWs targeted in this frequency range have been inspiraling supermassive black hole binaries (SMBHBs) on circular and eccentric orbits. In this work, we demonstrate that, thanks to the so-called pulsar term in the timing residual waveform of GW signals, PTAs can probe individual SMBHBs that merged before timing observations began. We refer to the latter as zombie binaries. Using SMBHB population models consistent with current PTA constraints, we find that while the probability of detecting such systems in existing PTA datasets remains low, the Square Kilometer Array observatory is expected to achieve sufficient sensitivity to have a few zombie binaries with optimal matched filter signal-to-noise ratios exceeding 3 in its data. Although their confident identification might be challenging, this new class of PTA sources opens a novel window for studying the most massive SMBHBs in our local universe.

We investigate the impact of spatial curvature <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mi>k</mml:mi></mml:msub></mml:math></inline-formula> and dynamical dark energy on the cosmological constraints of the neutrino mass sum, <inline-formula><mml:math><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mi>ν</mml:mi></mml:msub></mml:math></inline-formula>. Using a joint analysis of the latest cosmic microwave background (Planck and ACT DR6), baryon acoustic oscillation (DESI Data Release 2), and Type Ia supernovae (DESY5 and DES-Dovekie) datasets, we perform a systematic exploration of the neutrino mass parameter space. To mitigate prior-driven biases near the physical boundary, we implement a symmetric extension wrapper that allows for effective negative masses. We find that the inclusion of spatial curvature modifies the posterior distributions, exhibiting a smooth transition across the <inline-formula><mml:math><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mi>ν</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:math></inline-formula> threshold. In the <inline-formula><mml:math><mml:mi>Λ</mml:mi><mml:mi>CDM</mml:mi><mml:mo>+</mml:mo><mml:msub><mml:mi>Ω</mml:mi><mml:mi>k</mml:mi></mml:msub><mml:mo>+</mml:mo><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mrow><mml:mi>ν</mml:mi><mml:mo>,</mml:mo><mml:mi>eff</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> framework, we obtain <inline-formula><mml:math><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mrow><mml:mi>ν</mml:mi><mml:mo>,</mml:mo><mml:mi>eff</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:mn>0.01</mml:mn><mml:msubsup><mml:mn>1</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>0.050</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.052</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>, reducing the tension with the terrestrial lower limit of 0.06 eV from <inline-formula><mml:math><mml:mn>2.59</mml:mn><mml:mi>σ</mml:mi></mml:math></inline-formula> for the <inline-formula><mml:math><mml:mi>Λ</mml:mi><mml:mi>CDM</mml:mi><mml:mo>+</mml:mo><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mrow><mml:mi>ν</mml:mi><mml:mo>,</mml:mo><mml:mi>eff</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> model to <inline-formula><mml:math><mml:mn>1.17</mml:mn><mml:mi>σ</mml:mi></mml:math></inline-formula>. For the most flexible scenario <inline-formula><mml:math><mml:msub><mml:mi>ω</mml:mi><mml:mn>0</mml:mn></mml:msub><mml:msub><mml:mi>ω</mml:mi><mml:mi>a</mml:mi></mml:msub><mml:mi>CDM</mml:mi><mml:mo>+</mml:mo><mml:msub><mml:mi>Ω</mml:mi><mml:mi>k</mml:mi></mml:msub><mml:mo>+</mml:mo><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mrow><mml:mi>ν</mml:mi><mml:mo>,</mml:mo><mml:mi>eff</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula>, we find <inline-formula><mml:math><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mrow><mml:mi>ν</mml:mi><mml:mo>,</mml:mo><mml:mi>eff</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mo>-</mml:mo><mml:mn>0.07</mml:mn><mml:mo>±</mml:mo><mml:mn>0.11</mml:mn></mml:math></inline-formula> with a tension of <inline-formula><mml:math><mml:mn>1.13</mml:mn><mml:mi>σ</mml:mi></mml:math></inline-formula>, illustrating how the increased parameter freedom notably degrades the precision of the mass estimate compared to simpler extensions. Our results demonstrate that current cosmological bounds on <inline-formula><mml:math><mml:mo>∑</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mi>ν</mml:mi></mml:msub></mml:math></inline-formula> are influenced by boundary effects and geometric degeneracies.

Precision observations of orbital systems have recently emerged as a promising new means of detecting gravitational waves and ultralight dark matter, offering sensitivity in new regimes with significant discovery potential. Such searches rely critically on precise modeling of the dynamical effects of these signals on the observed system, however, previous analyses have mainly only relied on the secularly averaged part of the response. We introduce here a fundamentally different approach that allows for a fully time-resolved description of the effects of oscillatory metric perturbations on orbital dynamics. We find that gravitational waves and ultralight dark matter can induce large oscillations in the orbital parameters of realistic binaries, enhancing the sensitivity to such signals by orders of magnitude compared to previous estimates.

Direct detections of gravitational waves provide a unique opportunity to probe the astrophysical origin of compact binary mergers. The formation channels of these systems remain highly debated, and a fraction may originate in dynamical environments or active galactic nuclei. Binaries formed in such environments are expected to experience line-of-sight acceleration from their surroundings, which can imprint characteristic signatures on the observed gravitational wave signal. Here, we rederive the line-of-sight acceleration effects and implement them in state-of-the-art quasicircular waveform models with precession and higher-order modes. We also implement the corrections in eccentric waveform models, applying them consistently to all contributing harmonics. Using this model, we investigate the impact of these effects on the inference of line-of-sight acceleration and analyze several selected events from the Gravitational-Wave Transient Catalog observed during the third observing run of LIGO and Virgo. We find no substantial evidence for line-of-sight acceleration in these events. We also show that an inconsistent treatment of line-of-sight acceleration between higher harmonics can lead to biased conclusions. Our model provides a robust framework for uncovering line-of-sight acceleration in current and future gravitational wave observations, enabling more accurate probes of environmental signatures in compact binary formation.

In the presence of a weak gravitational wave (GW) background, astrophysical binary systems act as high-quality resonators, with efficient transfer of energy and momentum between the orbit and a harmonic GW leading to potentially detectable orbital perturbations. In this Letter, we develop and apply a novel modeling and analysis framework that describes the imprints of GWs on binary systems in a fully time-resolved manner to study the sensitivity of lunar laser ranging, satellite laser ranging, and pulsar timing to both resonant and nonresonant GW backgrounds. We demonstrate that optimal data collection, modeling, and analysis lead to projected sensitivities that are orders of magnitude stronger than previously appreciated, opening up a new possibility for probing the physics-rich but notoriously challenging-to-access <inline-formula><mml:math><mml:mi>μ</mml:mi><mml:mi>Hz</mml:mi></mml:math></inline-formula> frequency band. This improvement arises from retaining accumulated orbital dephasing in the measured residual and scales quadratically with the ratio of the observing time to the unperturbed binary period, up to eccentricity-dependent factors. We also discuss improved prospects for the detection of the stochastic fluctuations of ultralight dark matter, which may analogously perturb the binary orbits.

AU-scale structures in the interstellar medium have become relevant across several areas of astrophysics, from ISM microphysics to pulsar timing arrays and cosmic-ray (CR) propagation. Current sheets formed at magnetic-field reversals have drawn particular attention because they can deflect the dominant GeV CR population through large angles. Pulsar scintillation, an outstanding probe of plasma density, resolves these structures---but not their magnetic configuration. Conventional Faraday rotation measures (RM) are only marginally sensitive at AU scales and are further compromised by ionospheric systematics. We propose a method that isolates birefringence-induced polarization---the signature of magnetic reversal---by applying phase retrieval to polarized data. Pulsar scintillation further resolves ray paths separated by $\sim 1\,\rm{AU}$, enabling differential RM measurements that suppress the ionospheric common mode while preserving the AU-scale magnetic signal. We demonstrate the method on an archival observation of PSR B0834+06. Phase retrieval resolves the 1-ms scattering feature into two branches in Doppler-delay space. Under the corrugated current-sheet interpretation, these branches correspond to ray paths sampling opposite sides of an AU-scale current sheet. We measure a branch-to-branch RM difference of $(-9.3\pm3.2)\times 10^{-3}\text{ rad m}^{-2}$ at 2.9$σ$ across a 10-MHz band. The signal is consistent with a field reversal of $|Δ\langle B_\parallel \rangle| \simeq4.4\pm2.1\,μ\rm G$, sufficient to sustain a long-lived sheet. With only six independent subbands, we present this as a proof of concept rather than a definitive detection. If it proves scalable, its full implementation could constrain cosmic-ray transport and inform models of propagation-induced noise in pulsar timing arrays.

Recent observations have revealed gas-rich debris disks around intermediate-mass stars at ages of tens of Myr. The origin of this gas remains unclear: it may be primordial, retained from the protoplanetary phase, or secondary, released from volatile-rich solids. Secondary-origin models reproduce CO emission but often overpredict neutral carbon. Recent observations and disk-evolution models suggest that primordial gas may survive longer than previously assumed, motivating thermochemical tests of the primordial-remnant scenario. We test the previously unexplored possibility that primordial-origin disks satisfy the observational constraints on gas-rich debris disks. Specifically, we determine under what conditions a disk around a $2\,M_{\odot}$ star reproduces substantial CO, low CI/CO ratios, and weak HCO+ emission consistent with current non-detections. We post-processed 20-40 Myr structures from 1D disk-evolution models with Cloudy, varying irradiation geometry, dust-to-gas mass ratio (DTG), and cosmic-ray ionisation rate. In the dust-poor models (DTG $=10^{-4}$), CO remains optically thick around $R\sim100$~au. The models yield low disk-integrated CI/CO mass ratios. Our model produces CO radial intensities of the observed order of magnitude, but its CI-emitting region is more extended than observed. The standard CR model overproduces HCO+, whereas the weak CR model brings its predicted luminosity within current observational limits. These results demonstrate that a primordial origin remains chemically viable for CO-rich debris disks. The main remaining tension is the excessive radial extent of the CI emission, although it may reflect our simplified modelling. Further testing of the primordial-origin scenario will require multidimensional, self-consistent modelling, spatially resolved CI observations, and deeper searches for HCO+.

We provide a geometric foundation for the kinetic theory of photons. Although the induced metric $\hat h$ on the light cone bundle $Γ_0^+$ is degenerate, which causes the corresponding volume element to vanish, we can still use a method similar to the Hodge dual to construct a volume element $η_{Γ_0^+}$ for the light cone bundle. Based on this geometric structure $(Γ^+_0,η_{Γ_0^+},\hat h)$, we establish the fully covariant Boltzmann equation for photons. More importantly, the volume element and the induced metric are linked in a nontrivial way, which allows the physical distributions to be defined consistently. This yields the corresponding hydrodynamic quantities and their divergences, which take the same form as in the case of massive particles.

We study cosmological perturbations in multi-field inflation in which the scalar fields couple to the Gauss-Bonnet terms through a general coupling function $f(φ^a)$. We derive the complete quadratic action for the field perturbations in spatially flat gauge and find that, besides corrections to the kinetic, gradient and mass matrices, the Gauss-Bonnet coupling induces an antisymmetric velocity coupling that vanishes in Einstein gravity. Decomposing the field perturbations into curvature and isocurvature modes, we show that the effective mass of the curvature perturbation and the non-derivative mixing mass vanish identically, $\mathcal{M}_{\mathcal{R}}^2 = \mathcal{M}_{\rm mix}^2 =0$ which are justified by the Weinberg's adiabatic mode. We then compute the curvature power spectrum in two limits. First, when the isocurvature mode is heavy, it can be integrated over all scales. This yields an effective single-field theory of the curvature perturbation with a modified sound speed. Second, when the gradient of the coupling function is aligned with the background trajectory ($f_N =0$), the derivative mixings disappear and the superhorizon transfer of isocurvature into curvature perturbations is governed by the turn rate, which is modified by the Gauss-Bonnet corrections. We also present the tensor power spectrum and the tensor-to-scalar ratio at linear order in the coupling.

In the present paper, the cases of a uniformly accelerated reference frame and a Schwarzschild black hole in two and four dimensions are discussed focusing on formal aspects of the corresponding quantum scalar field theories. The main emphasis is made on building consistent quantum field theories and highlighting the problems that are not considered elsewhere. In particular, some pathologies in the case of a uniformly accelerated reference frame are discussed in detail: it is shown that some quantum states that are well defined in Minkowski spacetime possess an infinite energy when considered in Rindler spacetime. In the case of a Schwarzschild black hole in four dimensions, it is shown that the standard theory providing the Unruh vacuum state turns out to be inconsistent. The problem of consistency of quantum scalar field theory in Kruskal-Szekeres spacetime is also discussed.

The central challenge in microscopic quantum-gravity models is to derive geometric and gravitational observables from one refining quantum family without changing dynamics or fitting a new coefficient at each interface. We construct a controlled benchmark on a declared fixed physical Fourier band. For every spatial regulator, one finite-Hilbert interacting Hamiltonian defines a unitary completely positive process and a Euclidean Schwinger functional. Its spectrum and source derivatives determine the Lorentzian gap, connected four-response, mixed matter--geometry response, dynamic geometry kernel, induced Newton coefficient, and quantum-matter stress entering a semiclassical FLRW evolution. Two inequivalent spatial symbols obey analytic second- and fourth-order bounds and converge to the same limiting Hamiltonian. Duhamel and Weyl inequalities, a uniform gap, analytic perturbation theory, and a kernel-remainder estimate yield a common fixed-band regulator limit. Within the declared local covariant FLRW two-derivative sector, the metric source uniquely fixes a positive induced Newton coefficient rather than leaving it as an interface fit. Direct diagonalization independently yields a nonzero connected fourth response and a non-fitted dimensionless spectral combination. The excitation gap supplies a conserved stress tensor for a semiclassical Friedmann trajectory. This is a same-family fixed-band closure theorem, not an all-band QFT or completed quantum gravity. The two-mode band, finite oscillator cutoff, covariant two-derivative sector, and semiclassical Einstein equation are inputs. It does not construct a quantum metric Hilbert space, quantum BV measure, topology-change control, or autonomous sector selection. The result provides a reproducible benchmark and isolates the further theorems needed for nonperturbative closure.

Primordial non-Gaussianities are at their largest when the curvature perturbation is coupled to a massive field by a mixing too strong to be treated perturbatively. We show that this regime, naturally reached in multifield inflation, is solved analytically by the Laplace-space representation we recently introduced, in which every mode becomes a superposition of plane waves weighted by a single kernel. The bispectrum then follows as a rapidly convergent series of elementary functions, in perfect agreement with independent numerical results. We find that the cosmological collider signal at strong mixing is larger than widely believed, with only half of the usual Boltzmann suppression. Its amplitude further grows with the number of time derivatives of the curvature perturbation in the interaction, singling out the most promising observational targets.

The inner disk truncation radius, $R_T$, plays a crucial role in the regulation of star-disk interaction and the early evolution of star-disk-planet systems; however, measuring this parameter is observationally challenging. We present a new method for determining $R_T$ in young accreting systems that hinges on the color dependence of the accretion shock emission in multi-band time-domain surveys. Based on the accretion simulations of Robinson et al. (2017, 2021), we produce synthetic color-magnitude diagrams at near-UV and optical wavelengths that predict the loci of accreting stars as a function of $R_T$. We test these model predictions on young stars with interferometric $R_T$ estimates, finding very good agreement in our results. We apply this novel technique to a pilot survey of 26 classical T Tauri stars in Taurus and Upper Scorpius. We find a predominance of sources with small truncation radii, $R_T < 4\ R_\star$, and an overall distribution of $R_T$ that is statistically similar to that inferred from interferometric studies, while differing from those inferred from emission line modeling. Finally, we discuss the application of this technique to NASA's mission concept EVE, with the goal to provide simultaneous measurements of inner disk truncation radii, corotation radii and mass accretion rates for hundreds of young stars across the Galaxy. The unprecedented survey of inner disk properties that the mission would produce would enable the first stringent test of angular momentum evolution theories in young stars and reveal the impact of the inner disk conditions on early planet architectures.

Light spectator fields during inflation can acquire superhorizon fluctuations that cross a potential barrier between positive and negative regions of their potential. Motivated by the Standard Model Higgs instability, in this work we study the subsequent evolution of patches where this occurs in the radiation era after inflation ends for a Higgs-like spectator field. We utilise fully nonlinear, spherically symmetric numerical relativity. Across the black hole forming configurations in our investigation we find a robust two-stage evolution. First, the central negative potential region reverses its expansion, becomes kinetic dominated, and forms a primordial black hole that hides the runaway core. The positive potential barrier that survives outside this first horizon then determines one of two late-time branches. In the subcritical branch the original apparent horizon grows smoothly and engulfs the remaining scalar structure. In the supercritical branch, however, the potential energy of the barrier dominates the local evolution. The result is a transient wormhole throat, a bifurcating trapping horizon, and an inflating child universe branch. In both branches the parent radiation dominated universe is ultimately left with an ordinary primordial black hole whose subsequent growth is governed by radiation accretion.

We calculate the dynamical tidal response coefficients (TRCs) of 2+1D analogue black and white holes generated by draining and fountaining bathtub flows, respectively. The parameter space is characterized by the frequency and azimuthal number of the Fourier modes and the rotation of the analogue black or white hole. In general, the TRC is a complex-valued function of these parameters. Its real and imaginary parts are defined as the tidal Love number (TLN) and the tidal dissipation coefficient (TDC), respectively. The TRC of the analogue black hole (ABH) exhibits several interesting features. At certain points in the parameter space, including the static case of vanishing perturbation frequency, the TLN exhibits logarithmic running, while the TDC vanishes. Unlike in Einstein gravity, fluid dynamics allows for the physical existence of an analogue white hole (AWH) event horizon. Time-independent, torque-free, barotropic, inviscid, axisymmetric fluid dynamical equations can yield a pair of transonic background flow solutions. The solutions in the pair, corresponding to an ABH-AWH pair, share the same angular momentum and Bernoulli constant but have opposite mass flow rates, all of which are conserved quantities. For such an ABH-AWH pair, the TRC of the AWH is the complex conjugate of the TRC of the ABH.

Massive stars characteristically form in clustered environments. Characterising young massive 'protoclusters' is therefore crucial to constraining the mechanism(s) of massive star formation, and of the assembly of stellar clusters. We present 1.3 mm continuum results from the ALMA EGO-10 imaging survey, targeting ten Spitzer GLIMPSE Extended Green Objects (EGOs) - massive protostars with active outflows traced by extended 4.5 $μ$m emission. Our sensitive 1'.6$\times$1'.6 mosaics reveal rich protoclusters associated with all targets. With a mean spatial resolution 2200$\times$1600 AU, we identify 570 cores - between 13 and 135 per field. We quantify protocluster structure with the $Q$-parameter, finding structural diversity with 0.5 $\lesssim Q \lesssim$ 0.9. The sample is notable for the wealth of complementary high-resolution multiwavelength data available. Correlating our cores with these observations, we find only 2%, 5% and 4% of cores host 6.7 GHz CH$_3$OH masers, 22 GHz H$_2$O masers and cm-$λ$ continuum sources, respectively. The massive protostars traced by 6.7 GHz masers typically reside near protocluster centres (median offset 0.045 pc), and all at $d<$ 3 kpc are found in clustered locales, with $>$10 cores within 10,000 AU. Using VLA cm-$λ$ continuum observations, we construct a new evolutionary indicator: the ratio of protocluster cm-$λ$ continuum luminosity to the mass of the associated ATLASGAL clump ($L_\text{cm}/M_\text{AGAL}$). This ratio correlates positively with $Q$, with the correlation driven primarily by the cm-$λ$ continuum emission from MYSOs. This suggests dynamic protocluster structure, evolving from subclustered to centrally condensed, consistent with the global collapse in hierarchical, clump-fed models of massive star formation.

The Euler product of the Riemann zeta function is the partition function of a free bosonic gas whose mode energies are the logarithms of the primes. We show that this arithmetic gas, combined with the assumption that the entropy--geometry correspondence holds, leads to a quantum-corrected black-hole metric. The prime gas is a Hagedorn system. Its entropy is linear in the energy, which is exactly what an entropy linear in the horizon area requires, and the simple pole of the zeta function at $β=1$ fixes the coefficient of the logarithmic correction to the area law. The nontrivial zeros cannot play this role: their level density grows only logarithmically, and far too slowly to be extensive. Demanding that the reconstructed geometry reduce to Schwarzschild at large radius then fixes the map from arithmetic energy to horizon area and yields the closed-form metric $f(r)=1-2GMr/(r^{2}+\lz^{2})$ with $\lz^{2}=αG/π$. This describes a two-horizon black hole with Reissner--Nordström horizon structure but no Coulombic hair, a positive-energy anisotropic source obeying the null energy condition, a softened central singularity, a bounded Hawking temperature, and a cold extremal remnant that ends the evaporation. The same length scale follows independently from requiring that the first law hold exactly with the corrected entropy. The nontrivial zeros survive only as exponentially suppressed log-periodic ripples in the area, which suggests a physical interpretation of the Riemann hypothesis as the statement that arithmetic corrections to black-hole thermodynamics are as small as they can be.

Strong gravitational lensing contains key information about cosmic acceleration. Modern and next-generation galaxy imaging surveys are expected to provide high-quality data on $\mathcal{O}(10^5)$ galaxy-galaxy lensing systems. The plethora and complexity of the data are likely to present computational challenges for parameter inference methods for fitting high-dimensional likelihoods, which are often analytically intractable. Neural Ratio Estimation (NRE) efficiently computes individual likelihood ratios that can be combined into population-level posteriors. We use simulations to study the capacity of NRE to jointly predict the dark energy equation-of-state parameter $w$ and the total matter density $Ω_{m}$ from lensing images and companion spectroscopic information. We also introduce a post hoc posterior coverage calibration procedure that mitigates the model overconfidence that is typically found in neural density estimation applications. Our experiments show that the errors on both parameters decrease with increasing inference population sizes. In particular, for 100 lenses in a standard $Λ$CDM Universe, our calibrated NRE model achieves median fractional uncertainty of $22.8\%$ in $w$ and $2.9\%$ in $Ω_{m}$. This proof of concept demonstrates a potentially scalable approach for efficient cosmological parameter inference with large populations of galaxy-scale lenses observed in future surveys.

As alternative gravity theories with respect to the general relativity, the Brans-Dicke (BD) scalar tensor model is well known for which the BD parameter is controller the best fit observational data and so its cosmological model is studied in the literature well. On the other side, the Coleman Weinberg (CW) self-interaction scalar field potential coming from radiation corrections of Feynman diagrams is used to describe the best fit cosmic inflation phase and reheating phase. This has a logarithmic term with respect to the Higgs potential and it is applicable even for massless bosons. From particle physics point of view, this is used usually to describe the Higgs mechanism and creation of massive Goldstone bosons. As a basic model to describe the cosmic inflation the cosmological constant is used to describe the de Sitter space where the cosmological constant play as dark energy density. Since, the cosmological constant is a hierarchy problem and it is added in the Einstein equation without to describe that where that is come originally? at a first time the Albert Einstein himself proposed a uni-modular frame for which the cosmological constant generate from a integral constant. This idea is very well because it resolve `fine tuning` problem of the cosmological constant parameter. Hence we use this idea for the BD theory in presence of the CW potential and obtained suitable solutions of the field equations for a flat Robertson-Walker space time by regarding the slow roll parameters of cosmic inflation and then we find the best fit correspondence between the theoretical predictions of the parameters of the solutions and the Planck2018, DESI2024, and ACT2025 observational data.

Coronal holes (CHs) are regions of predominantly open magnetic field and important sources of high-speed solar wind streams. We present a manually compiled catalogue of newly emerging CHs during Solar Cycle 24 (2010-2019), identified from daily CHIMERA maps provided by SolarMonitor. Unlike existing area-based catalogues, our dataset tracks the daily emergence of new CHs in northern, southern, and equatorial latitude zones. The emergence rate of equatorial CHs is anti-correlated with the International Sunspot Number (Pearson r = -0.523, p = 6.2 x 10^-9), indicating reduced formation of low-latitude CHs near solar maximum. The hemispheric asymmetry of CH emergence is also anti-correlated with the hemispheric asymmetry of sunspot activity (r = -0.461, p = 5.2 x 10^-7), suggesting that new CHs preferentially emerge in the hemisphere with lower sunspot activity. The total CH emergence rate is lowest near solar maximum and highest during the declining phase. These results provide new observational constraints on the evolution of open magnetic flux over the solar cycle.

The Little Red Dots (LRDs) are a population of compact, red sources, detected abundantly at $4 \lesssim z \lesssim 9$. While their nature is still debated, many models suggest they are powered by accreting massive black holes. Multiple studies have detected blue companions in their vicinity, motivating our analysis: we compare $253$ LRDs uniformly selected across several JWST deep fields against $>7000$ matched controls in the same mosaics. We find that $27.7\%$ of the brightest LRD quartile host a blue ($β<-1$) companion at $0.5-2$ kpc, versus $11.6\%$ of redshift-matched controls: a $\times 2.39$ excess, significant at $3.3σ$. This phenomenon is limited to the brightest LRDs, as the full LRD sample pairs at a rate indistinguishable from controls. Moreover, the excess is confined to the close-pair window ($< 2$ kpc), outside which LRD environments are ordinary. The companions of the brightest LRDs are overwhelmingly ($92\%$) blue, and significantly bluer, with a median $β\approx-2.32$, versus $β\approx-1.92$ for those of controls, rejecting a common parent distribution at $3.4σ$. The signal is thus directional in three independent ways: luminosity, separation, and companion color, all expected for synchronized-pair seed formation. Therefore, we conclude that the association between the brightest LRDs and their close blue companions is physical, i.e., the two phenomena are causally connected: either one drives the other, or both emerge from the same synchronized formation event. A natural explanation for why the most luminous LRDs have nearby, young, star-forming neighbors is that the radiation from these neighbors enabled the direct collapse of the massive black holes powering the LRDs themselves.

Extensions of the Standard Model scalar sector can render the electroweak phase transition first-order and thereby provide the departure from thermal equilibrium required for electroweak baryogenesis, while at the same time sourcing a stochastic gravitational wave (GW) background in the milli-Hertz range. In this work, we investigate electroweak phase transitions in the CP-conserving type-I two-Higgs-doublet model (2HDM), focusing on the interplay between collider constraints at the HL-LHC and the projected sensitivity of LISA. We compute the expected GW background due to strong first-order electroweak phase transitions and perform extensive Monte Carlo scans over the collider-viable model parameter space. We find that GW signals within reach of LISA arise almost exclusively in regions of parameter space that necessarily predict observable deviations at the HL-LHC, in particular through the $H \to ZZ$ decay channel of the heavy CP-even and neutral Higgs state with $m_H \simeq 180\text{--}250 \, \text{GeV}$. Our results highlight the decisive complementarity between collider and GW probes and show that the largest parts of the 2HDM parameter space relevant for phase transition signals at LISA can already be tested by HL-LHC. A possible future discovery of 2HDM states at the HL-LHC, however, would not allow conclusive statements about LISA being able to find a GW background due to the amount of parameter tuning required for an observable GW signal. In order to evaluate possible caveats of this statement we study the theoretical uncertainties related to the GW predictions in a two-fold approach using both state-of-the-art tools, BSMPT and TransitionListener, and also allow for model realizations in which the electroweak symmetry is not restored in the high-temperature limit, which are the ones combining the loudest GW signals with the weakest collider coverage.

The pulsar timing arrays (PTA) collect the times of arrival of radio signals from the millisecond pulsars, and gravitational waves can modulate their arrival times. Although the PTA observable involves successive radio pulses propagating along different light paths, its standard theoretical description at linear order in perturbations is equivalent to the Sachs-Wolfe formula for the observed redshift, which describes the fractional change in photon frequency along a single geodesic. While this equivalence is well known at linear order, its validity beyond first order has not been systematically addressed in the PTA literature. Here we show that, in the limit of vanishing proper-time separation between successive emission events, the timing modulation is exactly equal to the observed redshift, without expanding the spacetime geometry. For a finite emission interval, we derive the exact relation between the timing modulation and the observed redshift, and show that their difference is controlled by the ratio between the emission interval and the characteristic timescale over which the observed redshift varies.

Mass loss during the advanced evolutionary stages of low- and intermediate-mass stars is traditionally attributed to dust-driven winds, whereas the influence of magnetic fields remains poorly constrained. However, recent observations show massive directional mass ejections from the nearby asymptotic giant branch (AGB) star Mira~A that appear not to be driven by radiation pressure on the dust. We observationally investigate whether magnetic fields are responsible for the mass loss ejections from Mira A. We obtained full-polarisation ALMA observations of the SiO $v=1$, $J=5\rightarrow4$ maser transition at 215 GHz with angular resolutions of 1.5-2.6 au. The observations reveal a highly magnetized loop-like mass ejection traced by strongly polarised SiO masers extending to approximately eight stellar radii. The morphology, polarisation pattern, energetics, and kinematics indicate a magnetically driven eruption analogous to a stellar coronal mass ejection. The observations demonstrate that magnetic eruptions can dominate mass loss from evolved stars and imply that magnetic fields must become an integral component of future models of AGB, and potentially other cool giant, winds.

Evolved stars on the asymptotic giant branch (AGB) lose mass through stellar winds launched from their dynamic extended atmospheres. Recent high-angular-resolution observations of the archetypal AGB star Mira A (o Ceti) have revealed compact hotspots on its submillimetre (submm) surface, whose origin remains unclear. We aim to characterise the submm photosphere of Mira A and investigate the properties and temporal behaviour of the compact hotspots detected on its surface. We analysed 19 epochs of high-angular-resolution observations obtained with the Atacama Large Millimetre/submillimetre Array in Bands 4, 6, 7, and 9, ranging from 134.34 to 669.25 GHz. The complex visibilities were fitted with parametric models to derive the flux density, size, morphology, and brightness temperature of the stellar disc and an additional more compact component across multiple pulsation phases and observing frequencies.Our results show that the flux density of the stellar disc increases with frequency while its apparent size decreases, in agreement with the expected stratified radio photosphere. In Band 6, both the flux density and the size of the stellar disc vary with pulsation phase, reaching maximum values close to optical maximum light and decreasing toward minimum light. Compact hotspots are detected in Bands~6 and 7 near maximum light, with their brightness temperatures exceeding those of the stellar disc by several thousand kelvins, reaching T>8600 K in Band 6. They are highly energetic and persist on timescales of weeks but evolve over longer timescales, showing changes in both position and intensity. Hotspot activity appears to occur repeatedly across multiple pulsation cycles and may be linked to strong shocks, magnetic activity, and/or episodic mass ejections, and could therefore play an important role in the mass-loss process in AGB stars.

Originally proposed as a solution to the strong CP problem and later understood to be a good dark matter candidates, axions have appeared in a variety of beyond-the-standard-model theories and are imbued with rich phenomelogical consequences. In this dissertation, we will examine two of these consequences: the emission of axions from cosmic strings and the superradiant mechanism in black hole physics. The spontaneous symmetry breaking of a global symmetry in the early universe can give rise to a network of cosmic strings, which emit ultra light, axion-like particles that can contribute to the dark matter density. We will discuss how the associated density fluctuations of these particles can be computed from first principles by treating them as a collection of plane waves. We will then calculate the density power spectrum and compare it to well-established cosmological observables, as well as to observables from future surveys, to derive constraints on the mass of the particles and the symmetry breaking scale. These light axion-like particles can also form a gravitational atom around a spinning black hole through the superradiance process. Considering the black hole to be part of a binary system, the tidal potential of the companion periodically perturbs the gravitational atom such that an atomic transition occurs between two of its energy states. Gravitational waves are emitted by the cloud during this transition. We will derive the analytical formulae of both the strain waveform and frequency spectrum of the signal and identify the systems that would be the most promising for detecting it in future, space-based gravitational wave observatories.

We examine particle creation, evaporation, scalar wave absorption and scattering, and geodesic motion in the asymptotically flat regular black hole supported by a phantom Dirac-Born-Infeld field. For massless bosonic and fermionic fields, the Bogoliubov transformations yield thermal spectra whose Hawking temperature decreases as the regular core becomes more prominent. Energy-conserving tunneling recovers the same temperature in the low-energy limit, whereas recoil and the DBI contribution introduce nonthermal corrections and suppress particle emission. In the high-frequency regime, the enlargement of the cross section does not compensate for the reduction in temperature, resulting in a lower luminosity and a longer evaporation time. A numerical partial wave analysis shows that the total scalar absorption increases with the regular core scale, approaches the horizon area at low frequencies, and oscillates around an enlarged geometric capture limit at high frequencies. The scattering phase shifts modify the multipolar amplitudes nonuniformly and displace the interference fringes toward larger angles. Furthermore, both null and timelike trajectories experience stronger deflection as the regular core contribution increases.

We compute the deuterium-production reaction $n+p \to d+γ$ in an effective field theory whose degrees of freedom are the nuclear states themselves: the amplitude is assembled from on-shell three-point vertices, glued across its factorization channels, and completed by the contact terms consistent with the symmetries. The deuteron enters through the $d$-$n$-$p$ vertex, normalized to the measured asymptotic normalization coefficient. Rescattering of the nucleon pair is resummed dispersively, leaving two short-distance constants, an isovector magnetic and an electric dipole contact interaction. A joint Bayesian fit to the thermal capture measurements and the SLEGS photodisintegration data finds both of natural size and determines the thermonuclear rate to 0.22-0.24% across the nucleosynthesis window, including systematics spanning the defensible treatments of the SLEGS data and of the $P$-wave rescattering. Truncating the expansion is bounded separately at 0.12%, of which the next order of contact terms -- degenerate with the two fitted constants -- supplies 0.03%, for a total theory uncertainty of 0.25-0.27%. Propagated through a BBN network, the rate shifts the predicted primordial deuterium by -0.06% and cuts this reaction's contribution to the D/H uncertainty from 0.089% to 0.050%, retiring it from the primordial D/H error budget for practical purposes.

We study scalar field perturbations of a static and spherically symmetric spacetime describing a dyonic black bounce geometry. Depending on the electric and magnetic charges, the spacetime can represent either a regular black hole or a horizonless compact object. We derive the effective potential for massless scalar perturbations and analyze its structure for different parameter regimes. In the black hole phase, the quasinormal mode spectrum is calculated using the Wentzel-Kramers-Brillouin (WKB) approximation and compared with the eikonal formula derived from null geodesics. The two approaches show good agreement in the large multipole limit. When the charge parameters exceed a critical value, the event horizon disappears, and the spacetime becomes a horizonless compact object. In this case, the effective potential develops a characteristic double-barrier structure that forms a trapping cavity for perturbations. The time-domain evolution reveals the presence of the echoes produced by repeated reflections of waves inside this cavity. These results suggest that the dyonic black bounce geometry can naturally generate echo signals without introducing artificial boundary conditions, providing a possible observational probe to distinguish horizonless compact objects from classical regular black holes in gravitational-wave observations.

Reducing orbital eccentricity in numerical relativity simulations of binary black holes is essential for producing astrophysically relevant gravitational wave models, as many of these systems are expected to be near circular in nature. Standard eccentricity-reduction procedures rely on iterative schemes, often requiring four or more trial simulations to achieve desired thresholds. This approach is computationally expensive because each trial simulation adds <inline-formula><mml:math><mml:mo>∼</mml:mo><mml:mn>10</mml:mn><mml:mo>%</mml:mo></mml:math></inline-formula> to the total simulation run-time of multiple weeks to months. We introduce a data-driven approach that accelerates this process by learning the values of the initial orbital frequency, <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula>, and radial velocity, <inline-formula><mml:math><mml:msub><mml:mover><mml:mi>a</mml:mi><mml:mo>˙</mml:mo></mml:mover><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula>, that yield an evolution with small eccentricity. This is done using a Gaussian process regression model trained on an archive of previously eccentricity-reduced numerical relativity simulations. For all configurations tested, using the trained model consistently reduces the number of required eccentricity-reduction iterations to just zero or one, significantly lowering computational costs relative to post-Newtonian initial guesses. These results demonstrate the power of data-driven methods in accelerating expensive numerical relativity simulations.

We develop a unified framework for testing gravity beyond general relativity (GR) with continuous gravitational waves (CWs) from individual supermassive black hole binaries. These long-lived, nearly monochromatic nanohertz signals offer unique strengths for precision tests of gravity, since their coherent phase evolution and inter-pulsar correlations in pulsar timing arrays (PTAs) retain detailed information about departures from GR over cosmological propagation distances. We consider three representative classes of deviations from GR: additional polarization states, modified dispersion relations, and parity-violating birefringence. For each, we derive the interpulsar cross-correlation, the modified antenna response, and the propagation-induced pulsar-term phase delay. For nontensorial polarizations, the CW cross-correlation scales linearly in the alternative-polarization amplitude, compared to the quadratic scaling of the gravitational-wave background, provided the beyond-GR modes are subdominant. PTAs are also competitive for modified dispersion relations, where low frequencies enhance both the antenna-pattern modification and the pulsar-term phase delay. Birefringence, by contrast, is suppressed at nanohertz frequencies for most parity-violating theories. We validate the framework with injection-and-recovery simulations for breathing-mode and massive-graviton signals at current observational limits, recovering the injected beyond-GR parameters and distinguishing the CW signal from both correlated and uncorrelated background models. We further show that a pure-GR CW template recovers source parameters without significant bias when beyond-GR physics is present in the data, supporting a two-stage analysis strategy: identify candidates under GR, then test for deviations.

The so-called ``cosmic dipole tension'' challenges the Cosmological Principle by positing a discrepancy between the Solar System's peculiar velocity inferred from the Cosmic Microwave Background (CMB) dipole and that derived from large-scale structure number-count dipoles. Here we provide a high-precision determination of the kinematic dipole using the redshift-dipole method applied to the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI). By exploiting the Doppler-induced modulation of observed redshifts, this estimator is intrinsically less sensitive to imaging systematics and selection-function uncertainties that can bias traditional number-count measurements. We conduct a tomographic analysis of four tracer populations, Bright Galaxy Sample, Luminous Red Galaxies, Emission Line Galaxies, and quasars, spanning $0.1<z<2.1$. Survey geometry and statistical uncertainties are quantified using 1,000 \texttt{EZmock} realizations. We find that the high-redshift QSO sample implies a peculiar velocity of $v = 357.95_{-48.47}^{+55.05}\,\mathrm{km\,s^{-1}}$, in excellent agreement with the CMB-inferred value of $369.82 \pm 0.11\,\mathrm{km\,s^{-1}}$. By contrast, a complementary number-count analysis yields a significantly enhanced dipole amplitude, which we attribute to leakage of large-scale power and to incompleteness within the DESI DR1 footprint. These results indicate that the redshift dipole provides a cleaner and more reliable probe of the kinematic rest frame, offering strong support for the standard kinematic interpretation at high redshift and helping to resolve the apparent dipole anomaly.

We study the one-loop relation between an effective-field-theory extension of Poincaré gauge theories of gravity with propagating torsion and its unimodular counterpart. In the Einstein representation, we consider two representative quadratic torsion subsectors, namely totally antisymmetric (axial) torsion and the vector component of hook-antisymmetric torsion. On maximally symmetric metric backgrounds with vanishing background torsion, the diffeomorphism-invariant and genuine unimodular theories yield identical one-loop determinants in both sectors, reproducing the same local logarithmic divergences. We further examine flat backgrounds with homogeneous axial or vector torsion, where metric-torsion mixing is present. In each case, the torsion-dependent one-loop effective action in unimodular gauge and in the genuine unimodular formulation is controlled by the same physical determinant. Thus, for the actions and backgrounds considered, the unimodular constraint does not modify local one-loop torsion dynamics. We also delineate the limitations of this equivalence and the extensions needed for generic torsionful curved backgrounds.

We investigate the Joule-Thomson expansion of a non-commutative Schwarzschild-AdS black hole, whose space-time geometry is described by the Lie-algebraic $κ$-deformed space-time, and show that the $κ$-deformation parameter induces Joule-Thomson expansion in an uncharged-AdS black hole. We find the ratio of minimum inversion temperature to critical temperature $T_i^{(min)}/T_c \simeq 0.493$, is independent of the deformation parameter, and is in close agreement with the value for Reissner-Nordstrom black hole. Furthermore, we show the existence of a minimum black hole mass (depending on the $κ$-deformation parameter) below which the Joule-Thomson expansion vanishes.

At a deeper level the principle of least action is interpreted as the law of least entropy increase consistent with Prigogine's principle of minimum entropy production, and the implications of quasistatic information quantization rule (Proc. R. Soc. A 480: 20240024), are explored for the conjectured second law of quantum complexity. It is thus shown that the conjectured second law of complexity is derivable from the information quantization rule such that long after heat-death the quantum state complexity evolves as $C(t)=C_{\rm max}\exp(-1/t)$, increasing with time to a saturation value $C_{\rm max}$ that is exponential in the equilibrium entropy $S_{\rm max}$. For the out-of-equilibrium circumstances, however, the quantum complexity can decrease with the time, asymptotically tending to a minimum determined by the distance from equilibrium.

Gravitational-decoupling provides a systematic framework for constructing black hole geometries from known (seed) solutions (such as the Schwarzschild), whose exterior properties closely resemble those of the seed solution while still retaining potentially distinguishable features. Using this approach, we study a class of static, spherically symmetric hairy black holes and examine how such deformations affect their horizon structure, photon spheres, shadow scales, and scalar quasinormal modes (QNMs). We first determine the physically controlled regions of the parameter space by imposing the relevant horizon and energy condition requirements, while retaining configurations outside these domains only for exploratory comparison. To isolate geometric effects, we compare solutions at a common horizon radius, while accounting explicitly for the distinction between the mass parameter entering the seed geometry and the ADM mass measured asymptotically. The resulting ADM normalized shadow scales are used for an illustrative comparison with the characteristic angular scales of M87* and Sgr A*. We also compute massless scalar QNMs using the sixth-order Wentzel-Kramers-Brillouin approximation and the improved Asymptotic Iteration Method, finding close agreement over the modes considered. Our results show that fixing the horizon radius does not uniquely determine either the optical or perturbative properties of the spacetime, and that mass normalization can significantly alter the interpretation of geometric trends. Horizon structure, null geodesic observables, and scalar QNMs therefore provide complementary probes of gravitationally-decoupled black hole geometries.

The Sun is one of the fundamental benchmarks in astronomy, and there is ever growing interest in precise and accurate determinations of its chemical composition. We present the current state-of-the-art in spectroscopic determinations of the solar elemental abundances, that now routinely employ three-dimensional (3D) radiative-hydrodynamic simulations of the solar photosphere and post-processing in non-local thermodynamic equilibrium (non-LTE). We critically review the recent literature and present recommended present-day and protosolar abundances of the 83 long-lived elements, and discuss their implications vis-à-vis primitive meteorites, helioseismology, neutrino fluxes, and the Solar Modelling Problem.

As a natural extension of the single-field fuzzy dark matter (FDM) model, multifield FDM has attracted increasing attention in recent years. This scenario is motivated both by the axiverse scenario predicted by string theory and by the possibility that multifield FDM may provide a better match to astrophysical observations than its single-field counterpart. In this work, we perform high-resolution numerical simulations to systematically investigate the dynamics of wave structures in multifield FDM halos. In particular, we study the oscillatory and stochastic motions of the central core, the evolution and statistical properties of granules, and the resulting dynamical heating of embedded stellar systems. We find that the frequency spectra of the core density oscillations develop multiple peaks and shift toward higher frequencies relative to the single-field case. The centers of different field components undergo nearly synchronized random walks, while subdominant components exhibit larger random-walk amplitudes. We further show that the suppression of granule density fluctuations with increasing number of fields is largely insensitive to the fractional abundance of each component over a broad parameter range. Moreover, using self-consistent simulations, we find that the dynamical heating induced by granules is progressively suppressed as the number of fields increases. However, once the contribution from the central core is taken into account, this trend would become much less pronounced.

Motivated by the old idea of treating gravity as a gauge theory, we review the gauge-theoretic construction of Conformal Gravity and of Noncommutative (Fuzzy) Gravity. We then outline how these gauge formulations of gravity can be unified with internal interactions described by an SO(10) Grand Unified Theory, based on the critical observation that the dimension of the tangent-space symmetry group of a curved manifold need not coincide with the spacetime dimension.

The deflection angles of rays diverge logarithmically near outside and inside of a photon sphere and they could be detected by near-future space observations on black hole shadows. We show that the deflection angles of the rays near a photon sphere at the throat of an example among some commonly used types of wormholes or one of the simple wormholes, which is often studied by the Event Horizon Telescope collaborations, does diverge not logarithmically but in power. This is because it has a marginally unstable photon sphere at a $Z_2$-symmetric wormhole throat and the wormhole has not a common nor simple but a special property. We investigate numerically gravitational lensing of rays with the deflection angles diverging in power outside and inside of a marginally unstable photon sphere at $Z_2$-symmetric wormhole throat in strong deflection limits. We apply our numerical method to the commonly-used simple wormhole spacetime, a Damour-Solodukhin wormhole spacetime, and a Simpson-Visser black-bounce spacetime. Our numerical results imply universal property of the deflection angle by the marginally unstable photon sphere on $Z_2$-symmetric wormhole throat in the strong deflection limits as similar to the deflection angles of ray bent by a marginally unstable photon sphere off a throat in the strong deflection limits which have a different power. We also correct the coefficients of power-divergent terms of deflection angles of the rays bent nearly outside of the marginally unstable photon sphere at the $Z_2$-symmetric wormhole throat in a semi-analytic approach investigated by the author and others.

We prove the black hole uniqueness conjecture in the axially symmetric, stationary, vacuum setting: there is no regular asymptotically flat configuration with more than one horizon component when every degenerate component has nonzero angular momentum. More precisely, it is shown that in any such multi-horizon configuration, the logarithmic angle defect along every finite axis rod is strictly negative, and hence the interaction force along such axis rods is always attractive. The proof is based on a refined asymptotic analysis of the associated singular harmonic maps, in part from the companion paper [16], together with a global bound for the Weyl conformal factor arising from a certain scalar curvature related differential inequality. Furthermore, as a corollary of the extremal version of this uniqueness result, we obtain the conjectured mass-angular momentum inequality for multiple dynamical black holes using the main theorem of [17].

I prove that a collapsing charged scalar field with scalar charge $\mathfrak{e}$ that forms an exactly extremal Reissner--Nordström black hole à la Kehle--Unger with radius $r_+$ must satisfy $|\mathfrak{e}|r_+ > \frac{1}{3}$. In contrast, I also show that no such bound exists for subextremal collapse: a scalar field with an arbitrary $\mathfrak{e} \ne 0$ can form a subextremal black hole with an arbitrary (subextremal) charge-to-mass ratio. Complementing the extremal bound, I prove that a Schwarzschild black hole can become extremal if $|\mathfrak{e}|r_+ > \sqrt{\frac{3 + \sqrt{33}}{3}}$. The fact that $|\mathfrak{e}|r_+$ can be taken to be of order unity could be considered evidence that the third law of black hole mechanics in vacuum is false.

In multi-fluid, matter-type gravity, the Bianchi identity constrains the divergence of the total effective stress tensor but does not, by itself, determine the currents assigned to its constituent sectors. Those currents are selected only after the off-shell matter action and equations, or an additional phenomenological closure, have been specified. We formulate this distinction and examine a spatially flat two-fluid background inspired by scale-independent EMSG. The case study combines an algebraic perfect-fluid prescription with a vanishing contracted-Hessian contribution, together with separate conservation of the total conventional and modification sectors. Because neither assumption is derived here from a concrete off-shell fluid action, the resulting system is an effective background closure rather than a microscopic EMSG model. For unequal EoS and every $α\ne0$, the closure yields a Barrow-Clifton system whose transfer coefficients depend on $(w_1,w_2)$ but not on $α$. Hence the nonzero-$α$ family is singular: its $α\to0$ limit does not recover the uncoupled GR conservation laws. We solve the two density eigenmodes and derive the associated modified Liénard equation for $H$. We also prove that the discriminant governing rank loss of the density-reconstruction map is strictly positive for every finite $w_1\ne w_2$. Thus each genuinely quadratic case has two distinct real rank-degenerate couplings. Exact vacuum and stiff-fluid families illustrate modal cancellation. When today's conventional densities are positive, the intervals on which both remain positive generally terminate at finite endpoints in the parameter ranges analyzed explicitly. These results provide a consistency diagnostic for separating action-level predictions from closure artifacts in multi-fluid, matter-type gravity; they do not establish an observationally viable EMSG model.

Light spectator fields provide a compelling inflationary origin for scalar dark matter, but their primordial fluctuations often generate excessive isocurvature perturbations on large scales. We develop a novel, exactly solvable filtering mechanism based on a tower of $δ-$function contributions to the spectator mass, which we call kicks, whose dense limit converges to a smooth Coulomb-like profile. The filter produces a isocurvature spectrum during inflation that is strongly suppressed on large scales, rises with momenta as $k^3$ in the infrared, and, after a rapid turnover, smoothly approaches a constant plateau at small scales. This construction is motivated by a discrete translational invariance, admits an EFT regime in which radiative corrections can be systematically controlled, and can find a concrete realization in terms of an inflationary massive-clock model. We embed the filter in a minimal dark-matter scenario in which a spectator condensate oscillates about the minimum of a quadratic potential and behaves as pressureless matter. In this framework, the spectator dark-matter mass is directly related to the turnover scale of the inflationary isocurvature spectrum. We determine the resulting dark-matter abundance and analyze the relevant isocurvature and backreaction constraints. The most restrictive realization, in which the spectator condensate is generated during inflation, favors a relatively low inflationary scale.

We develop a (Wilsonian) functional-renormalisation-group framework for scalar cosmology in which quantum fluctuations of a scalar field are coarse-grained on cosmological spacelike hypersurfaces. Integrating out quantum fluctuations with wavelengths smaller than the Hubble radius $H(t)^{-1}$, we obtain an effective scalar potential $U(φ(t),H(t))$ that evolves in time. We derive a non-perturbative flow equation for this potential, together with the coupled set of (modified) Friedmann equations. We then apply this formalism to the simplest possible case of a de Sitter vacuum when the scalar field is at rest, in two situations which satisfy exactly our flow equation: $(i)$ A flat potential, for which we find that the only pure de Sitter solution is unstable and corresponds to a saddle point. $(ii)$ A quadratic potential with curvature $m^2>0$, for which we find that the presence of the mass term stabilises the system. The latter case leads to a de Sitter attractor either at the Hubble scale when the mass is larger than the Hubble scale at initial time, or by introducing a new attractor located at $H=m$ in the case the mass is smaller, which may be particularly relevant to the phenomenological study of dark energy and inflation theories.

Many approaches to quantum gravity predict logarithmic corrections to the Bekenstein-Hawking entropy. Within the spacetime-thermodynamic description of gravity, such corrections lead to modified gravitational field equations. We study their effect on primordial perturbations during standard single-field slow-roll inflation. We derive the evolution equation for the comoving curvature perturbation and show that it retains the standard Mukhanov--Sasaki form, with the quantum-gravity correction entering through the time-dependent effective frequency. We compute the scalar and tensor primordial spectra at next-to-next-to-next-to-leading order ($\mathrm{N}^3\mathrm{LO}$) in the Hubble-flow parameters, keeping the leading contribution from the logarithmic correction. The scalar spectrum remains nearly scale invariant, but the quantum-gravity correction shifts its tilt and runnings. Free tensor modes still propagate as in general relativity, although on the quantum-gravity-corrected background. This different response of the two sectors shifts both the tensor-to-scalar ratio and the single-field consistency relation, which provide the starting point for tracing the model's signatures through the post-inflationary evolution and into the CMB.

The double copy relates gravitational data to gauge-theory data, most sharply through color-kinematics duality for flat-space scattering amplitudes. This review surveys the extent to which this idea can be formulated beyond asymptotically flat backgrounds. After recalling the flat-space ingredients needed for comparison, we describe curved-space constructions for boundary correlators and scattering observables in AdS, dS, and other non-trivial backgrounds. We then review classical and field-level maps, including Kerr--Schild, Weyl, Cotton, twistor, and convolutional double copies. A separate part is devoted to self-dual sectors. Throughout, we emphasize both the common structures that survive from flat space and the obstructions introduced by curvature.

Understanding a composite system through its constituents is a fundamental practice in physics. In the context of quantum reference frames (QRFs), however, combining the usual quantum-theoretic notion of composition with QRF perspectives gives rise to subtle issues, such as the 'paradox of the third particle'. Here we study in depth how to compose subsystems in QRF perspectives, building on a recent formalism for QRFs [E.Castro-Ruiz and O.Oreshkov, 2025]. We first show how the frames of external observers can be internalised and treated within the framework. This establishes a consistent hierarchy of QRF perspectives, which we use to define the adding and removing of subsystems. We then explain how, owing to the so-called 'extra-particle' degrees of freedom, the formalism gives a consistent treatment of subsystems, avoiding any paradoxes by construction. Consistency then implies that composing subsystems in a QRF perspective differs from, and generalises, the standard quantum-theoretic case. In particular, not every state can be appended in tensor-product form relative to a QRF, and we characterise the set of compatible states. Finally, we introduce 'classicalisation', a procedure that recovers a classical-like perspective from a general QRF. This procedure circumvents the aforementioned state restrictions but carries a different operational meaning, which we study through a concrete example.

The combination of cosmic microwave background (CMB) measurements with distance measurements from baryon acoustic oscillations (BAO) and type Ia supernovae (SNIa) suggests that dark energy is dynamical, with an equation of state crossing the phantom divide at low redshifts. This feature can not be described within canonically normalized, minimally coupled, self-interacting scalar field models. We investigate the possibility of achieving phantom crossing by introducing an interaction in the dark sector such that the dark matter particle mass is dependent on the dark energy field $φ$ as $m \propto φ^α$. We consider a self-interacting potential of the inverse power-law form $V \propto φ^{-β}$. We implement this model both at the background and perturbative levels in a Boltzmann solver and use a bayesian framework to constrain its parameters using using CMB, BAO and SNIa data. We find best-fits that have similar goodness-of-fits as the commonly used phenomenological $w_0-w_a$ parameterization, providing a more fundamental understanding of the dark sector.

Herbig stars are intermediate mass pre-main sequence stars of 1.5 to $\sim15$~M$_\odot$ and are in between low and high-mass star formation. This work provides a catalog of all 243 known bona-fide Herbig stars with clear infrared excess and in many cases accretion signatures within 1~kpc. It contains archival, recalibrated, and newly derived stellar parameters, accretion rate properties, and disk dust mass measurements. We derive the latter by using newly obtained millimeter photometry and literature values. We find that 50\% of Herbig dust disks are more massive than 10~M$_\oplus$, while this is only true for 20\% and 5\% of the T~Tauri disks in Lupus and Upper~Scorpius respectively. Furthermore, the Herbig disk dust mass distribution is bimodal, it consists of a high and low disk mass population with a mean dust mass of 21~M$_\oplus$ and 0.45~M$_\oplus$ respectively. We find that the catalog is near complete for within 300~pc, however, this decreases to 24\% out to 1~kpc. We observe a flat relation for Herbig disks due to objects with short disk lifetimes, caused by a combination of low disk masses and high accretion rates. Furthermore, we find the peak of the occurrence rate of massive (>10~M$_\oplus$) disks to occur at $\sim$1-2~M$_\odot$ stars, coinciding with the peak in occurrence rate of giant exoplanets. The small number of Herbig stars with low accretion rates ($<10^{-8}$~M$_\odot$~yr$^{-1}$) or low disk dust masses ($\lesssim1$~M$_\oplus$), combined with the lack of an age dependence in the disk dust mass distribution, suggests that the observed Herbig star population represents the surviving disk-hosting phase in the optically visible late stages of intermediate-mass star formation. We argue that this is either due to the formation of deep dust traps in these disks or replenishment by late-infall, or a combination of the two.

Small-scale dark matter (DM) structure encodes key information about the particle nature of DM and therefore provides a sensitive test of competing models. Yet, it remains hidden from electromagnetic surveys and is instead inferred through its gravitational effects. Stellar aberration, the apparent shift in a light source's position induced by the observer's motion, offers a largely unexplored channel to access such signatures. DM subhalos can perturb the observer's motion, imprinting characteristic, spatially correlated shifts in stellar positions across the sky. We show that the Transiting Exoplanet Survey Satellite (TESS), with its long temporal baseline, wide sky coverage, and high-cadence observations, is well suited to search for these aberration signals. We derive Fisher-matrix-based sensitivity estimates for constant observer accelerations, forecasting a sensitivity down to $6.3\times 10^{-9}\,\mathrm{m/s^2}$ from the combined sample of TESS stars with magnitude $\mathrm{Tmag}\leq 10$. This sensitivity allows TESS to probe concentrated DM subhalos over a broad parameter space, from $\gtrsim 10^{-6}\,\mathrm{M_{\odot}}$ at AU-scale distances to $\gtrsim 10^{7}\,\mathrm{M_{\odot}}$ at $\mathcal{O}(10\,\mathrm{pc})$. TESS's sector-based observing strategy further provides intrinsic temporal resolution of potential DM-induced aberration signals. Moreover, we briefly discuss challenges for future data analysis, including the modeling of instrumental systematics and stellar astrometric foregrounds, such as parallax and proper motion. Our results establish stellar aberration as a novel probe of DM substructure, paving the way for dedicated searches in TESS and next-generation wide-field surveys.

In a generally covariant theory whose Lagrangian density contains no derivatives of the metric, the spacetime geometry is fixed algebraically by the scalar fields, and the simplest quadratic potential is known to generate exponentially expanding, recollapsing and big-rip cosmologies. We extend this emergent-gravity Higgs cosmology by one new spacelike scalar field - the vacuum-expectation-value generating field - coupled to the Higgs particle through the Standard Model potential, so that the electroweak vacuum expectation value becomes a dynamical quantity evolving with cosmological time. We show that the broken-symmetry valley of the potential is an exact and stable solution of the full nonlinear system, on which the background is solved in closed form in terms of a single conserved charge. The charge feeds the Friedmann equation with a radiation-like component of purely geometric origin, diluting with the inverse fourth power of the scale factor although no radiation fluid is present; as a consequence the universe is born at a finite cosmological time in a radiation-dominated big bang, and for vanishing constant term in the potential the expansion history realizes a spontaneous radiation-to-dark-energy sequence. The sign of the Higgs kinetic term is not an assumption of the model: the requirement that the broken phase be the physical vacuum fixes it dynamically, and the stable choice reproduces the Standard Model relation between the Higgs mass, the self-coupling and the vacuum expectation value with no adjustable parameter. Oscillations about the valley describe a Higgs condensate which is frozen at early times, begins to oscillate through a misalignment mechanism, obeys a parameter-free adiabatic amplitude law, and backreacts on the expansion history as a definite renormalization of the background constants that feeds the radiation-like component.

Paleo-detectors are a proposed approach to the direct detection of dark matter (DM) based on the observation of DM-induced nuclear recoils. Rather than relying on large detector masses required for conventional detectors, paleo-detectors exploit extremely long integration times, using small samples of naturally occurring minerals that have resided deep underground and accumulated damage tracks over geological timescales of order a gigayear. In this contribution, we review recent theoretical predictions for the sensitivity of paleo-detectors to WIMP-nucleus interactions within the framework of a Non-Relativistic Effective Field Theory (NREFT). We discuss both elastic and inelastic scattering processes and consider isoscalar couplings, taking into account backgrounds from neutrinos and radiogenic sources. We present projected sensitivities of paleo-detectors for different readout and exposure scenarios and compare the reach of paleo-detectors to that of conventional direct-detection experiments. For DM masses in the range $1\ \mathrm{GeV}/c^2$-$10\ \mathrm{GeV}/c^2$, paleo-detectors are expected to achieve sensitivities exceeding those of conventional experiments for WIMP-nucleus interactions mediated by all NREFT operators, with only a weak dependence on the read-out scenario or target mineral. For larger DM masses, $10\ \mathrm{GeV}/c^2$-$5\ \mathrm{TeV}/c^2$, paleo-detectors are projected to attain sensitivities comparable to or surpassing conventional experiments for interactions mediated by several NREFT operators, depending on the specific read-out scenario and choice of target mineral.