高能暂现源 · 每日文献综述

Context. Since its launch in 2008, the Fermi Large Area Telescope (LAT) has detected thousands of sources, many of which remain unassociated. Some may be extended sources represented in the catalog by multiple point-like entries. The reinterpretation of HESS J1813-178 as a single extended source motivates a systematic search for further missed extended sources. Aims. We search for clusters of unassociated Fermi-LAT sources and test whether single extended-source models describe them better than multiple catalog sources. Methods. We apply the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) algorithm to 4FGL sources with a linking scale of $ε= 0.3^\circ$ over the 5 GeV to 1 TeV energy range. Each cluster contains at least one unassociated source and up to one extended or point-like associated source from selected categories, including pulsars, pulsar wind nebulae, and supernova remnants. Using Fermipy, we compare extended- and multiple-source models and characterize each candidate spectrally and morphologically, focusing on the Galactic plane. Results. We identify 48 clusters containing 124 sources, each with at least one unassociated source. For all eight clusters passing our quality selection, an extended-source model is statistically preferred. At linking scales of $0.4^\circ$ and $0.5^\circ$, all eight retain their core sources. Cross-matches with the Second Fermi Galactic Extended Sources Catalog (2FGES) and the HESS Galactic Plane Survey (HGPS) show that five overlap known extended sources. The other three, including one with morphology dependent on the interstellar emission model, have no counterpart in either catalog and are new extended-source candidates. Conclusions. Spatial clustering combined with likelihood-based model comparison can uncover extended sources missed in the Fermi-LAT catalog and complements existing searches.

We apply a geometric network diagnostic -- the nodal Forman-Ricci (FR) curvature of natural visibility graphs (NVG) -- to the two largest Forbush decreases (FDs) of solar cycle 25: the Gannon storm of 2024 May 10-11 and the 2025 June 1 event. Using 30-min NMDB records from six NM64 stations spanning cutoff rigidities $R_c \simeq 0$-$7\,$GV, we show that nodal FR curvature exhibits a sharp, coherent minimum at FD onset in both events, with onset-to-quiet median curvature ratios of 2.5-5.2 (Gannon) and 3.3-8.2 (June 2025). A placement (block-permutation) test that preserves the full autocorrelation structure confirms the onset signature at $p \lesssim 2 \times 10^{-3}$ (the floor of the test) for all 24 station-phase combinations, with $|z| = 4.7$-$23.7$. The signature is robust to edge weighting of the Forman curvature and to sliding-window (6-24 h) graph construction, and an amplitude-blind (horizontal-visibility) control confirms that it is specific to the amplitude geometry of the decrease. Nodal FR curvature nearly doubles the effect size obtained from the NVG degree sequence alone (mean Cliff's $δ$ of 0.89 vs 0.54), because it encodes two-hop (neighbor-degree) structure. Folding the station amplitudes through the Dorman coupling function yields FD spectral indices $γ= 0.80$ (Gannon) and $0.50$ (June 2025) with $A_{10} = 11.2\%$ and $18.8\%$: the deeper event is also spectrally harder. Across the 12 station-event pairs the curvature extreme scales with FD amplitude as $|\mathcal{F}_{\min}| \propto A^{0.57}$ (Spearman $ρ= 0.75$). These results establish local graph curvature as a compact, rigidity-resolved descriptor of FD morphology.

Future electromagnetic telescopes, such as $\textit{NewAthena}$, $\textit{CASTOR}$, and an $\textit{AXIS}$-like mission, along with milli-Hz gravitational-wave (GW) detectors such as $\textit{LISA}$, are expected to unearth the population of Galactic ultra-compact binaries (UCBs). Joint multi-messenger detections will probe the uncertain formation, evolution, and observables of mass-transferring UCBs such as AM CVns, but theoretical tools need to be advanced to anticipate future data challenges. Motivated by this, we present a new forward model for the continuum emission of AM CVn binaries that connects source binary parameters to X-ray, optical, and ultraviolet observables. The model assumes GW-driven mass transfer with physically motivated prescriptions for accretion energetics, emission geometry, absorption, and instrumental response. Combining this with $\textit{LISA}$ observations and the output of binary population synthesis enables exploration of the multi-messenger properties of AM CVns. Although uncertain, our model predicts that approximately one per $7000$ AM CVn binaries will permit a joint multi-messenger detection with $\textit{LISA}$, $\textit{CASTOR}$, and $\textit{AXIS}$. We also develop a framework for inferring binary parameters from the inverse model with a convolutional neural net and normalizing flows. Testing the trained flow with our synthetic AM CVn population, we find mean absolute fractional error on the inferred accretor mass of $0.05$ M$_{\odot}$, donor mass of $0.26$ M$_{\odot}$, orbital period of $0.1$ s, and distance of $0.2$ pc, while Spearman's rank shows strongly correlated true and predicted distributions except for the donor mass. These efforts lay a foundation for follow-up studies that will explore detailed binary astrophysics and observational requirements for effective multi-messenger scientific discovery in the coming decade.

A 2016 XMM-Newton X-ray light curve of the Seyfert 1.5 galaxy NGC 6814 exhibited clear eclipsing behaviour, with distinct ingress and egress, during half of the observation. Here, we report on the periodic behaviour in the light curve prior to the eclipse. We use timing and spectral analysis techniques to quantify the behaviour and examine the characteristics of the periodic signal. A superlet transform of the X-ray light curve reveals a period of ~45-50 $μ$Hz in the initial 60 ks of the observation with a detection significance at the >90% level in both the broad (0.3-10 keV) and soft (0.3-1.0 keV) bands. The period is confirmed by fitting a sinusoid, and is also evident in the highest energy bands with diminished significance because of reduced signal-to-noise. There appear to be distinct changes in the variability behaviour during the eclipse as the measured period is modulated (stretched) at all energies. From phase-resolved spectra, we investigate possible physical causes of this periodic behaviour and find that it can be interpreted as changes in the covering fraction or a non-standard inner accretion flow (e.g. truncated disc and misaligned flow). The non-standard inner flow appears consistent with previous reports of a truncated inner disc and compact corona in NGC 6814.

The precise mass and radius measurements of neutron stars by NICER have significantly advanced our ability to constrain the properties of matter at supranuclear densities. In this work, we develop a physics-informed regularized conditional Invertible Neural Network (cINN) that bijectively maps mass--radius posterior distributions directly onto the corresponding central energy density and pressure, eliminating the need for explicit high-dimensional parameter sampling. The physics-informed regularisation guarantees that all inferred solutions satisfy causality and thermodynamic stability, ensuring physically consistent predictions without explicit forward modelling. We demonstrate that the framework accurately reconstructs central EoS posteriors for NICER-like observations while preserving the mapping between macroscopic stellar observables and the microscopic properties of dense matter. Exploiting the computational efficiency of the cINN, we perform a systematic optimisation study of 62,400 simulated mass--radius observations to identify the most informative targets for constraining the high-density EoS. We find that the constraining power depends strongly on the location of the observation in the mass--radius plane, with an optimal strategy that alternates between compact high-mass stars and extended intermediate-mass stars, reducing the uncertainty in the inferred EoS by up to $\sim 9\%-10\%$ relative to the current NICER baseline. These results establish physics-informed invertible neural networks as a powerful framework for rapid, physically consistent inference of dense-matter properties from present and future multi-messenger observations.

The orbital eccentricity of massive black hole binaries (MBHBs) at binary formation shapes the stochastic gravitational-wave background (GWB) detectable by pulsar timing arrays (PTAs). Previous $N$-body simulations show large run-to-run scatter in this quantity, dominated by Poisson noise, raising the question of whether physical substructure adds genuine astrophysical stochasticity. We test this with high-resolution re-simulations of a major merger from IllustrisTNG100-1, evolved with the Griffin $N$-body code. A no-perturber control is compared with two matched suites in which $f_{\mathrm{target}}=0.1$ of the primary bulge mass is redistributed into equal-mass perturbers of $10^7,M_\odot$ ($μ_{\mathrm{p}}\approx3.2\times10^{-3}$) and $10^8,M_\odot$ ($μ_{\mathrm{p}}\approx3.2\times10^{-2}$), with four realisations per scenario. The control gives $σ_e\approx0.11$, consistent with the Poisson noise floor at this resolution. The $10^7,M_\odot$ case gives $σ_e\approx0.115$, indistinguishable from the control, whereas the $10^8,M_\odot$ case gives $σ_e\approx0.26$, a factor of $2.4$ above the floor, although statistically marginal given only four realisations. This excess scatter coincides with larger event-aligned residuals in orbital energy and angular momentum and stronger torque spikes, consistent with near-impulsive perturber--MBHB encounters. In binary--single scattering theory, the transition is set by the perturber--MBHB mass ratio $μ_{\mathrm{p}}$: the $10^7,M_\odot$ case remains diffusive, whereas the $10^8,M_\odot$ case approaches the near-impulsive regime. Because the expected perturber population in massive ellipticals lies mostly below this regime, perturber-driven eccentricity randomisation is unlikely to affect GWB-relevant MBHB mergers.

The possible presence of hyperons in the cores of massive neutron stars has important implications for their microscopic transport properties and thermal evolution. Despite recent progress in modelling core transport, a dedicated analysis regarding the thermal conductivity of a specific \(npΛ\) mixture remains absent from the existing literature. In this work, we investigate the thermal conductivity of dense, \(β\)-equilibrated \(npΛ\) matter within the framework of the variational linearized Boltzmann kinetic approach, employing the density-dependent DDME2 equation of state across a baryon density range of \((0.5\text{--}4.5)\,n_0\). We find that neutrons still dominate thermal transport, while the onset of \(Λ\) hyperons induces only a remarkably small reduction in neutron conductivity compared to pure nucleonic matter. These results suggests that the core thermal relaxation timescale remains practically unaltered in the presence of $Λ$ hyperons.

We study quadrupolar $f$-mode oscillations of dark-matter-admixed neutron stars (DANSs) in full general relativity (GR). The ordinary component is described by microscopic Brueckner-Hartree-Fock matter matched to the Shen2020 crust, while the dark matter (DM) component is treated as a cold self-interacting fermion fluid coupled to ordinary matter only by gravity. For fixed-DM-fraction sequences we solve the polar two-fluid perturbation equations with an outgoing gravitational-wave (GW) boundary condition, obtaining complex eigenfrequencies rather than only real mode frequencies. The spectrum contains two principal $f$-like sequences. Their local character can be ordinary-matter-led, DM-led, or mixed, and is diagnosed using the component kinetic energies, the displacement overlap, and the cancellation of the matter quadrupole. A main result is that, for intermediate DM fractions, one of the two-fluid branches can become weakly radiating, with damping times enhanced by several orders of magnitude. The same calculation gives the outgoing Zerilli amplitude and the GW damping time, which we use to estimate the GW energy required to reach a prescribed detector threshold. Thus the analysis extends previous two-fluid Cowling studies by retaining metric perturbations and the radiative boundary condition.

Primordial Black Holes (PBHs) are an attractive candidate for Dark Matter (DM) and there has been extensive experimental efforts to look for them. The asteroid-mass window, $M_{\text{PBH}} \sim 10^{17} - 10^{23}$ g, is particularly interesting, since PBHs in this range may still constitute all of DM. In this work, we study a scenario in which rotating PBHs are surrounded by superradiantly produced boson clouds that emit an approximately steady and nearly monochromatic flux of neutrinos in the few MeV range. We compute both the Galactic and extragalactic neutrino fluxes from such PBH populations and compare them with existing low-energy antineutrino limits from Borexino, KamLAND, and Super-Kamiokande. For scalar bosons, we find that these neutrino searches can strongly constrain a significant part of the asteroid-mass window and extend to somewhat larger masses. For instance, for rapidly rotating PBHs with spin ã = 0.9 and gravitational fine-structure coupling $α_g=0.25$, the strongest bound on dark matter fraction reaches approximately $f_{\text{PBH}} \sim 10^{-7}$ around $M_{\text{PBH}} \sim 2 \times 10^{22}$ g for a Yukawa coupling $g_{νφ} = 10^{-4}$. These constraints can be significantly stronger than existing microlensing limits in the same mass range. Our results provide a complementary neutrino probe of PBH DM, distinct from previous neutrino constraints based mainly on Hawking evaporation from lighter PBHs.

We consider low-mass black holes (BHs) moving in regular orbits in the cores of non-rotating gaseous star clusters, representative of proto-stellar clusters or the centers of protogalaxies. We argue that as the BH's sphere of influence -- the Bondi sphere -- is advected along the BH trajectory, the transverse velocity shear between the inner and outer hemispheres injects angular momentum, driving the formation of an accretion disk. Coriolis forces oppose angular momentum injection, delaying disk formation but not preventing it. The disk lies in the BH orbital plane and is counter-rotating with respect to the orbital BH motion. We verify this picture with 2D and 3D hydrodynamic simulations in the non-inertial frame of the orbiting BH. We find that the disk-formation timescale following a disruption event is of order the Bondi crossing timescale, $τ_{\rm d} \sim R_{\rm B}/V_{\bullet}$, and that the disk radius is of order $R_{\rm d} \sim ω_{\bullet}^2 R_{\rm B}^4/ G m_{\bullet} $, set by the circularization radius of gas captured in the Bondi sphere. For the case of a BH with $m_{\bullet} = 50\,{\rm M}_\odot$, inside the core of a typical compact proto-stellar cluster, these values read $τ_{\rm d} \sim 0.1 P_{\rm orbit}$ and $R_{\rm d} \sim 10^{-3} R_{\rm B}$.

The SKA telescopes will revolutionise our ability to do cosmology at radio wavelengths, via both their own data and in synergy with other wavelengths. SKAO will be the first instrument able to conduct large-scale cosmological surveys as done in the last decades in the optical and near-infrared. This complementarity will be vital as cosmology hits the limit of systematic uncertainties. Radio cosmology surveys will have radically different systematics, allowing data combinations across surveys to calibrate systematics and increase overall constraining power. Neutral hydrogen (HI) intensity mapping surveys are now reaching maturity, as demonstrated by the progress made by the MeerKLASS survey with MeerKAT. Along with continuum galaxy surveys, they will provide detailed maps of the Universe covering large fractions of the sky, allowing us to answer questions about fundamental physics which can only be measured on the largest scales. In combination with weak lensing and HI galaxy probes, HI intensity maps will also measure the distributions of matter and velocities to give precisions tests of the $Λ$CDM model, including its foundational assumptions of isotropy and homogeneity. In combination with gravitational wave observations and fast radio bursts, they will also help us measure the expansion history and baryon content of the Universe. Here we provide an overview of the achievements of precursor surveys and the progress towards SKA cosmology, starting with AA* and reaching full maturity with AA4 telescopes.

We present a comprehensive analysis of optical photometry and medium-resolution spectroscopy for Type Ic supernovae (SNe) SN 2020akf and SN 2021mxx. Our study covers the evolution of SN 2020akf from -5 to 109 days and SN 2021mxx from -3 to 115 days relative to their B-band maximum. Their peak quasi-bolometric luminosities are estimated at logL_bol = 42.37 +- 0.02 and 42.21 +- 0.02 erg/s, respectively. The velocities of the ejecta, derived from the Fe II 5169 A line at maximum light, are approximately 15000 km/s for SN~2020akf and about 10000 km/s for SN~2021mxx, which are consistent with those observed in other Type~Ic SNe. Using the semianalytical Arnett model, we estimate that SN 2020akf has a kinetic energy of E_k = 6.33 (+0.68)(-0.62) x 10^51 erg and an ejected mass of M_ej = 4.71 (+0.50)(-0.46) M_sun, while for SN 2021mxx, we get E_k = 0.54 (+0.08)(-0.12) x 10^51 erg and M_ej = 0.90 (+0.14)(-0.18),M_sun. The mass of Ni56 synthesized in the explosion is estimated at 0.10 +- 0.02 M_sun for SN 2020akf and 0.05 +- 0.01 M_sun for SN 2021mxx. The metallicities of the host galaxies near the SN regions are ~0.81 Z_sun for SN 2020akf and ~0.76 Z_sun for SN 2021mxx, where Z_sun indicates solar metallicity. Our detailed analysis suggests that SN 2020akf falls into the category of a transitional Type Ic SN, having spectral properties between normal and broad-line Type Ic SNe, while SN 2021mxx is a normal Type Ic SN with an extremely low value of the M_ej / E_k ratio.

Pulsar timing arrays can probe the population physics of supermassive black-hole binaries through the nanohertz gravitational-wave background. We construct a phenomenological forward model that follows source abundance, binary residence time, the high-mass population, finite-source strain moments, and the pulsar timing response. The simulated observables constrain three standardized population coordinates: $β$, which controls the residence-time and spectral response; $φ_{eff}$, which describes source normalization after accounting for its covariance with $β$; and $m_{eff}$, which is dominated by the high-mass cutoff. In the evaluation ensemble, the posterior-mean correlations with the simulated values are $0.928$, $0.926$, and $0.884$, with central 90 per cent coverages of $0.938\pm0.015$, $0.871\pm0.021$, and $0.898\pm0.019$, respectively. Frequency-resolved observables are most important for $β$, and strain moments beyond a common-process power law provide sensitivity to the normalization and high-mass coordinates; the fourth strain moment identifies $m_{eff}$ with rare, massive binaries. These coordinates quantify the relative sensitivity of the adopted PTA summaries within this population model, for which nearby population realizations retain substantial posterior overlap.

With current advances in gravitational wave (GW) detection made by the worldwide LIGO-Virgo-KAGRA (LVK) network of detectors, ever-more sensitive tests of gravity in the strong-field regime are now possible. This enables one to test gauge theories beyond Einstein-Hilbert action, such as Weyl gauge theories of gravity. The only anomaly-free (quantum) gauge theory of a space-time symmetry beyond Poincaré is based on Weyl gauge group (of dilatations and Poincaré symmetry) with Weyl conformal geometry as its natural underlying geometry. This gauge theory has spontaneous breaking of Weyl gauge symmetry to Einstein-Hilbert and Proca actions, plus a positive cosmological constant. We investigate the GW polarisation modes of Weyl (quadratic) gauge theory of gravity in Weyl geometry and compare our findings to the most recent experimental data. We show how the geodesic deviation equation from Riemannian geometry translates to Weyl geometry, and explain why it is crucial to perform the analysis around de Sitter background, which is the correct low-energy limit of Weyl quadratic gravity, to not alter the GW content, and then compute the polarisation modes. In addition to the two transverse-traceless tensor modes predicted by Einstein-Hilbert action, we find two additional vector modes induced by the transverse fluctuations of the Weyl gauge field. If detected, these vectors modes would be important evidence for Weyl gauge symmetry.

The high-luminosity, high-mass and likely super-Eddington quasar PDS 456 is known for its extremely dynamic environment. A wealth of observational features from the millimetric up to the X-ray band shows energetic outflows at all scales, from accretion disc up to galaxy scales. Broad Emission Lines in the optical and UV show significant widths and line-of-sight velocities. Moreover, NIR interferometry revealed a partially outflowing Broad Line Region. Thanks to the unprecedented energy resolution of the X-ray microcalorimeter Resolve onboard XRISM, we detect a neutral Fe K$α$ line, the main tracer of cold matter around AGNs. The line blueshift ($v_{\rm out}=2700$ km s$^{-1}$) and width ($σ\leq 700$ km s$^{-1}$) are on the lower bound of the range of values of the optical-UV emission lines, possibly suggesting a stratified medium. The derived column density, $N_{\rm H} \approx 10^{22}$ cm$^{-2}$, is significantly lower than what expected for Broad Line Regions, again in agreement with a composite emission. The very small Equivalent Width (9 eV) marks one of the smallest measurements in the literature and probes the high-luminosity end of the so-called Iwasawa-Taniguchi (or X-ray Baldwin) effect.

Active galactic nuclei (AGN) feedback via black hole-driven jets and winds plays a key role in redistributing matter across megaparsec scales. However, the implementation of jet feedback in cosmological simulations remains highly prescriptive, leading to uncertainties in the predicted state of the Warm-Hot Intergalactic Medium and other extragalactic observables. We investigate how variations in AGN jet velocity, orientation, and delayed hydrodynamic coupling impact the thermodynamic state of gas surrounding galaxy clusters. We aim to identify observational signatures in the thermal Sunyaev-Zel'dovich (tSZ) effect and galaxy properties that can constrain these models. We employ zoom-in hydrodynamic simulations centered on three galaxy clusters from The Three Hundred, utilizing the SIMBA-C model and various feedback parameters. We use filament-finding algorithms and oriented stacking to probe the effect on large-scale structure and compare our results to observational data for brightest cluster galaxies from eRASS1, black hole-halo mass relations and baryonic mass fractions. Jet velocity is the dominant tested parameter in heating low-density environments at $z > 1$. At lower-$z$, higher velocity jets quench star formation, expel baryonic matter, and prevent black hole growth. While the tSZ signal within the central cluster and surrounding filaments is only affected by $\sim10\%$, the signal in under-dense regions outside filaments is enhanced by $\sim100\%$. In this case study, high velocity AGN jets provide the best match to galaxy properties from eRASS1 and other X-ray surveys. We find that hot gas in low-density regimes is a sensitive probe of AGN feedback. Future high-resolution tSZ surveys like the Simons Observatory and spectral-distortion experiments like FOSSIL have the potential to probe the thermal state of the gas outside clusters to distinguish between these feedback models.

The collision of black holes at relativistic speeds probes gravity in its most extreme dynamical regime. While the maximum gravitational recoil from \emph{grazing} high-energy collisions ($\approx28\,562$~km/s, i.e., $\sim0.1c$) and the maximum radiated energy $E_{\rm rad}$ and remnant spin $α_f^{\max}$ from such encounters ($E_{\rm rad}/M_{\rm ADM}\approx32\%$ where $M_{\rm ADM}$ is the ADM mass, and $α_f^{\max}\approx0.987$) have been established previously~\cite{Healy:2022jbh,Healy:2024lhl}, here we focus on the \emph{head-on} high-energy collision of equal-mass spinning black holes and on the detailed structure of the resulting recoil. Performing a sequence of full numerical simulations for spin magnitudes $s=0.5,0.65$, and $0.8$ over a range of initial momenta $γv$, we characterize the peak recoil $V_p$, the final recoil $V_f$, and the antikick $ΔV\equiv V_f-V_p$, and we provide phenomenological fits of their dependence on $γv$ and $s$. We complement these results with a zero-frequency-limit (ZFL) analysis of the radiated energy and momentum, a quasinormal-mode model of the antikick, and a superposed boosted double-Kerr close-limit estimate. We find that in the relativistic regime ($γv>1$) the peak and final recoil are directly proportional, $V_p\approx7.4\,V_f$ (equivalently $ΔV \approx-6.4\,V_f$), largely independent of both the initial momentum and the spin magnitude, pointing to a common post-merger relaxation. While the ZFL predicts a leading linear-in-spin dependence, the close-limit analysis predicts a leading $s^3$ dependence of the recoil amplitude; with the three spin magnitudes studied here the empirical exponent is $s^{1.27\pm0.08}$, motivating an even higher energy collision spin sequence study.

We derive a set of recursion relations for the perturbative evaluation of the scattering of two black holes in Einstein gravity. To illustrate, we solve the equations up to third post-Minkowskian order in the gravitational coupling constant G, recovering the correct result, including the back-reaction from gravitational radiation.

The routine measurements of the Stokes $IQUV$ signals of emission lines in the solar corona is still a challenging endeavor, particularly for observations using small-aperture instruments. Therefore, recent studies have explored the use of coronal seismology and propagating Alfvénic waves as alternative diagnostics of the coronal magnetic field. In particular, Yang et al. (2024) showed that plane-of-sky (POS) phase-speed measurements provide a direct and consistent diagnostic of the POS magnetic-field component (B$_{\text{POS}}$). Building on recent theoretical advances in solar coronal polarization, we devise a novel inversion scheme and extended the CLEDB software package (Paraschiv & Judge 2022) to exploit Stokes $IQU$ observations of two coronal emission lines combined with coronal-seismology derived B$_{\text{POS}}$ estimations. Using this framework, we perform a comprehensive statistical analysis demonstrating that this combination of diagnostics allows us to infer both magnetic field orientation and strength with an accuracy comparable to that of Stokes $IQUV$ spectropolarimetry. This type of diagnostics is particularly suited for the Fe XIII 1074.7 nm and 1079.8 nm line pair routinely observed by new-generation instruments such as DKIST Cryo-NIRSP, DL-NIRSP, MLSO CoMP/UCoMP, and targeted by the future COSMO and CORSAIR efforts.

We present deep X-ray observations of the nearby, X-ray luminous galaxy cluster Abell 1795 obtained with the Einstein Probe Follow-up X-ray Telescope (EP-FXT), with a total exposure time of 480 ks. Exploiting the large field of view and low particle background of EP-FXT, we directly measure the radial temperature profile of A1795 out to $R_{200}$ with full azimuthal coverage, which increases with radius within 6 arcmin and then gradually declines toward larger radii. The surface-brightness residual map and 2D thermodynamic maps reveal a clockwise spiral structure extending from the cluster core toward the southeast, which traces low-temperature, low-entropy gas and is consistent with sloshing-induced cold fronts. In the northwest, the surface-brightness-enhanced region exhibits an arc-like high-temperature feature, and both the temperature-derived and density-derived Mach numbers support that it is a weak shock. These substructures can be explained by a binary merger scenario: the perturbing subcluster induces sloshing during its first passage past the primary core, and its subsequent return passage through the ICM may drive the shock toward the northwest. Our results indicate that relaxed galaxy clusters such as A1795 can still retain signatures of dynamical activity.

We investigate the role of parity-violating interactions between supernova neutrinos and chiral molecules in nearby interstellar molecular clouds as a potential source of biomolecular homochirality. We introduce neutrino interactions into the autocatalytic chemical reactions in a far-from-equilibrium noise-induced system. These interactions create a directional bias between L and D enantiomers in the racemization reactions, which is amplified by autocatalysis and stochastic fluctuations. We solve the stochastic equations within the Ito sense to obtain the dynamics of the probability distribution of the enantiomeric excesses, offering an astrophysical scenario for the delivery of homochirality seeds to Earth by meteorites. In spite of the weak interactions of supernova neutrinos, our framework introduces an amplification mechanism to yield a considerable enantiomeric excess of more than $10\%$, in agreement with the latest chemical analysis of the meteorites. Moreover, we scan over the parameter space of the model, inferring from the observational values in order to explore the window of opportunity to generate the initial seeds of homochiral states in interstellar molecular clouds.

We present PSRDISP, a novel approach to modeling deterministic and stochastic dispersive processes in pulsar timing datasets using high-precision epoch-wise dispersion measure (DM) estimates, with a Gaussian Process based approach. Unlike the conventional single-pulsar noise analysis methodology, which is applied to frequency-resolved times of arrival (ToAs) of pulses, this technique is applied to epoch-wise DMs which are derived from these ToAs. It can also be applied to wideband DMs measured simultaneously with wideband ToAs. Therefore, this framework provides a paradigm-agnostic approach to characterise single-pulsar dispersive processes. This method is expected to minimise the impact of achromatic red noise processes while characterising these dispersive effects. We substantiate the discussed technique with representative examples using simulated narrowband and wideband datasets with realistic noise injections. We found the recovery to be in close agreement with the injections, and agnostic to the estimation technique. Our method applies to pulsar timing experiments where precise, epoch-wise DM estimates are possible, such as the Indian Pulsar Timing Array. This technique can serve as a powerful diagnostic tool for validating single-pulsar noise analyses, which is crucial for precision pulsar timing experiments, such as Pulsar Timing Arrays.

Self-interacting dark matter turns the structure of galactic halos into a direct requirement on a low-energy scattering amplitude. We show that, for weakly coupled scalar dark matter, this requirement implies a much stronger mass bound on the dark matter particle than partial-wave unitarity alone. Using analyticity, crossing symmetry, locality and partial-wave unitarity, we compute the maximal allowed threshold amplitude with a dispersive primal S-matrix bootstrap, assuming only a weakly coupled EFT below a scale $Λ$ and allowing arbitrary UV particle content above $Λ$. For the benchmark self-interaction cross section $σ_{\rm self}=10^{-24}(M/\mathrm{GeV})\mathrm{cm}^2$, the mass of a generic weakly coupled scalar satisfies $M\lesssim 0.3\,\mathrm{GeV}$ in the controlled EFT regime. If dark matter is a derivative-dominated pseudo-Nambu-Goldstone boson, the mass bound is lowered to the MeV scale or below, depending on the hierarchy $M/Λ$.

We use our newly constructed three-dimensional red supergiant (RSG) stellar model, which also mimics nuclear energy production and photospheric emission, to calculate the stochastic component of the angular momentum of the mass that a companion spiraling within the RSG's envelope accretes during common envelope evolution (CEE). The accreted mass has a fixed-direction angular-momentum component arising from the density gradient in the RSG envelope and orbital motion. The angular momentum component with a stochastically varying direction results from vigorous envelope convection. We do not include the companion's influence on the RSG envelope during the CEE and consider an undisturbed, non-rotating RSG stellar model. We find that the fluctuating angular momentum amplitude can be several times the fixed-axis angular momentum. The total specific angular momentum of the accreted mass easily forms intermittent accretion disks around neutron stars and black holes, but it is only marginally sufficient, or not at all, to form accretion disks around main-sequence stellar companions. The intermittent accretion disks we expect to form will launch wobbling jets with varying axes. We discuss aspects of wobbling jets in the CEE and the grazing envelope evolution (GEE), which might precede the CEE or replace it altogether. Studies have claimed that jets are a crucial ingredient in many cases of CEE, and the standard CEE should include jets that the companion launches, before (like the GEE), during, and/or at the exit from the CEE. Our study supports this claim and emphasizes the importance of wobbling jets.

Magnetic activity cycles provide a fundamental constraint on stellar dynamos, but remain difficult to identify beyond the Sun. However, recent studies have shown that flares offer a unique tracer of activity cycle behavior. In this study, we use seven years of short-cadence observations from the Transiting Exoplanet Survey Satellite (TESS) for over 14,000 stars in the Continuous Viewing Zone to search for long-term changes in flare activity. For each star, we perform injection and recovery tests and provide well-characterized completeness limits for flare detection thresholds, and flare finding results. From this search, we identify 17 stars with evidence of long-term variability in flare rate, synonymous with activity cycle behavior. These candidates span a range of effective temperatures, rotation periods, and flare-variability morphologies. One G-type star, TIC 167344043, stands out as the clearest solar-like case, despite rapid rotation and superflare activity. Our results identify candidate activity cycles in stars that are more rapidly rotating and flare-active than the typical stellar-activity-cycle targets in the literature. These systems probe a new regime where stellar dynamos are still evolving, providing critical constraints on when cycle-like magnetic variability first emerges.

Gravitational waves affect the propagation of electromagnetic waves in laser cavities, modulating the frequency of emitted photons. We use this effect to search for high-frequency gravitational waves between 100 kHz and 100 MHz using optical precision spectroscopy. Our limits constrain much of this frequency range for the first time. We discuss future improvements of the technique, which we expect to enhance the sensitivity by eight orders of magnitude, and to extend the frequency coverage up to at least 1 GHz.

Studies of the oldest and most metal-poor stars in the Milky Way have confirmed a common abundance signature of high carbon enrichment, coupled with a subsolar abundance pattern of neutron-capture elements. The so-called CEMP-no stars have been speculated to be bona fide population II stars, potentially tracing the nucleosynthesis of the very first stars formed in the universe. However, constraining the binary nature of CEMP-no stars is crucial for understanding their abundance patterns. In previous radial-velocity monitoring of CEMP-no stars, the binary fraction has tentatively been found to vary with carbon enhancement. Here we present the results of radial-velocity monitoring of 30 CEMP-no stars over five years. Combined with literature data, this yields a total sample of 90 CEMP-no stars with constrained binary statuses, providing a larger statistical sample to investigate the CEMP-no binary fraction as a function of carbon enrichment. We find an overall binary frequency of $50^{+13}_{-13}\%$ among high-carbon ($A(\mathrm{C}) \ge7.3$) CEMP-no stars, compared to $18^{+5}_{-4}\%$ for low-carbon ($A(\mathrm{C}) <7.3$) stars, establishing for the first time a statistically significant increase in the CEMP-no binary frequency as a function of carbon at the 2$σ$ confidence level. Of the confirmed binary systems, we derive orbital parameters for four new ones, which, combined with literature data, amount to a total of 12 CEMP-no binaries with constrained orbits. We discuss these results in the context of the progenitors of CEMP-no stars; in particular, we explore the nature of the companion in these binary systems and the possibility of mass transfer.

The large-scale environment is thought to play an important role in setting galaxy properties. The Milky Way (MW) and Andromeda (M31) reside in the Local Group, embedded in the Local Sheet (LS). To study the sheet's influence on the dark matter (DM) halo shapes, spins, and disks orientations of MW and M31 analogues, we use a new suite of constrained Local Group simulations that reproduce the observed configuration of the two main halos and the LS. We determine their shapes and alignments relative to the LS analogues, and the effect of infall and coalescence of massive mergers. We find that the DM halo shapes of our MW and M31 analogues are on average slightly rounder than literature reports for similar-mass galaxies in random environments. We find preferential alignment between the sheet normal and the halos' minor axes, but not with the halos' spins. The present-day disk angular momenta ($L_{\rm disk}$) closely align with the halos' minor axes (median $15^{+15}_{-8}$ degrees at $R_{\rm vir}$) and with the halos' spins. The direction of $L_{\rm disk}$ is often set by one of the two highest mass ratio mergers during the last 8-10 Gyr. While recent ($\leq 2$ Gyr) massive mergers can reorient the outer halo's minor axis leading to twisted shapes, $L_{\rm disk}$ retains the imprint of the earlier accretion event. These results can explain the peculiar alignment of the MW's disk and DM halo shape and their orientation relative to the LS. The prolate-like morphology and orientation of the MW's outer halo can be explained by the Magellanic Clouds (and perhaps Sagittarius) accreting from within the LS. As their orbital planes are nearly perpendicular to the Galactic disk, the disk orientation must be set earlier, possibly by the GES merger. This merger's estimated infall direction, highly inclined relative to the present-day LS, is broadly consistent with the LS's direction of maximum collapse.

I derive the charged-particle spectral edge from the quantum instrument of soft QED. I use two projections of that instrument. Tracing over unresolved photons gives the reduced hard-sector channel. Pushing the outcomes to total energy gives the inclusive energy distribution. Its Laplace exponent is fixed by the diagonal soft intensity. For ${\rm d} N_h(ω)=η_h{\rm d}ω/ω+{\rm d} N_{h,\mathrm{reg}}(ω)$, I obtain $ρ_{\mathrm{inc}}(s)\sim Cθ(s-m^2)(s-m^2)^{-1+η_h}$. I retain the coherence kernel and derive hard-sector dephasing and the spectral edge from two contractions of one soft environment. The diagonal coefficient $κ_{aa}$ fixes the endpoint exponent, while $\frac12(κ_{aa}+κ_{bb}-2\operatorname{Re}κ_{ba})$ fixes the dephasing exponent between hard alternatives. I then classify infrared energy marginals, derive the finite-resolution residue $Z(μ)=(μ/Λ)^{η_h}$, prove stability under infrared-integrable perturbations, and separate the bath exponent from a hard threshold exponent. For the one-electron spectral measure, the hard threshold factor is regular. The resulting edge has the local power law of a gapped unparticle spectrum, while its exponent remains a response coefficient of the unresolved photon sector.

The Dynamic Eclipse Broadcast (DEB) Initiative citizen science program observed coronal visible continuum brightness during the 2024 April 8 total eclipse from locations across North America. We present results from 11 DEB sites spanning 2700 km of distance and showing 49 minutes of evolution. The coronal brightness radial profiles from these telescopes are tightly correlated from 1.2 to 4.0 solar radii and comparable to published photometric coronal intensities. The coronal flattening parameter is measured from 1.4 to 2.8 solar radii. A Ludendorff index of 0.0761 +/- 0.0007 is computed but the extrapolation techniques used by some to calculate this index are shown to disagree with this direct measurement, and alternate structure parameters are suggested. Measured radial velocities are compared with an MHD model of the corona during the eclipse from Y. Li et al. (2026). A polar downflow is measured with an average radial velocity of -37 +/- 3 km s-1 and a deceleration of 14 +/- 3 m s-2 at a speed and position which agrees with the model. The predicted mixture of outflows and downflows at low heights is seen, as well as outflows in two western regions of the corona. The fastest observed outflow has a radial speed of 105 km s-1 and is likely associated with a transient event not predicted by the model. Future DEB Initiative eclipse experiments can more tightly constrain models of the inner corona by using both coronal intensity and radial velocity measurements.

WR 25 is a massive colliding-wind binary in the Carina nebula comprising a WN6ha primary with an O5 companion in an eccentric 208-d orbit. Recent spectroscopic analysis estimates the total binary mass to approach $100\,M_\odot$, and a primary-to-secondary mass ratio of $q= M_1/M_2\approx2$. The presence of additional spectroscopic signatures from a third, intruder star was also noted, making it a candidate hierarchical triple system. In this study, we present a VLTI/PIONIER interferometric observation of WR 25, spatially resolving all three components for the first time. For the inner WN6ha + O5 binary, we find an angular separation of $1.68\pm0.02$ milliarcseconds (mas). Leveraging the fortunate timing of the VLTI observation, which was obtained when the two components were passing the line of nodes, we determined the semi-major axis $a=3.11\pm0.20$ au. Subsequently, the newly constrained total dynamical binary mass is $93\pm18\,M_\odot$, with a primary mass $M_1=62\pm13\, M_\odot$ and secondary mass $M_2=31\pm7\, M_\odot$. We detect the tertiary with an angular separation of $27.69\pm0.02$ mas from the primary, with a chance alignment probability lower than $10^{-4}$. Using newly obtained brightness ratios between all components, we revisit archival spectroscopic data of WR 25 to disentangle spectra for individual components and derive their stellar parameters. The tertiary, which has a spectral type O7, is coeval with the inner binary and has an evolutionary mass $M_3=25.6^{+2.8}_{-2.3}\,M_\odot$. Based on simulations, we estimate the tertiary period to be in the range 19 - 82 yr. The newly confirmed triple nature of WR 25 makes it an important benchmark system to measure accurate dynamical masses of the inner binary and potentially the tertiary, to calibrate stellar evolution and atmosphere models, and to study its formation and stability as a hierarchical triple system.

Residing in the center of the Milky Way, Sgr A* is the closest massive black hole (MBH). Its vicinity has allowed measuring individual stellar orbits around it. The stars act as test particles and probe the gravitational potential around the $4.3 \times 10^6 M_\odot$ MBH. These observations have determined the central mass to sub-percent precision, and the mildly relativistic motions of stars have given access to the dominant relativistic corrections, the gravitational redshift, the transverse Doppler effect, and the prograde precession imposed by the Schwarzschild metric nature of the potential. These effects are of order $β^2 = (v/c)^2$ (for velocity $v$ and speed of light $c$). The Kerr metric for a rotating black hole leads to corrections of order $β^3$. Here, we report the discovery of a faint main-sequence star ($m_K = 19.3$), S301, on a 8.7-year orbit and with small enough a pericenter distance, such that the star's peak velocity reaches $25000\,$km/s. Within the measurement capabilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, S301's motion is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart via the Hills mechanism.

Ultralight axion-like particles coupled to photons rotate the linear polarization of distant sources. We propose using the three-dimensional two-point correlation of galaxy polarization-rotation angles to measure not only the amplitude of this birefringence field but also its spatial coherence scale. For a nonrelativistic ALP component with an isotropic Gaussian velocity distribution, the equal-time field correlation has an $e^{-1}$ scale $L_{\rm G}^{\rm phys}=\sqrt{6}/(m_a v_a)$, where $v_a$ is the three-dimensional rms ALP velocity dispersion. A detected turnover in the galaxy-pair correlation therefore measures the characteristic momentum scale $m_a v_a$, while the correlation amplitude constrains $g_{aγ}\sqrt{Ω_a/Ω_{\rm DM}}$. For a fiducial survey with $10^6$ polarized galaxies over a quarter of the sky to $z=2$ and effective per-galaxy scatter $σ_{\rm tot}=10^\circ$, we find $5σ$ sensitivity to sub-degree correlated rotations over $m_a(10^{-3}c/v_a)\sim10^{-29}$--$10^{-27}\,{\rm eV}$, with $σ[\log_{10}(m_a v_a)]\lesssim0.1$ for detected signals across much of this range. This provides a geometric late-time probe complementary to CMB birefringence and structure-formation constraints.

We study the 4-point out-of-time-ordered correlator (OTOC) of scalar fields using the eikonal approximation to gravity around a de Sitter background. It can be defined in a gauge-invariant way by dressing the operators to an observer. We consider de Sitter space in any dimension and any bulk masses of the scalar fields. At tree level we find the maximal Lyapunov exponent of $2 π/β_{\rm dS}$. However, we describe an IR problem in this calculation at the loop level associated with the vector part of the graviton propagator. Regularizing this divergence leads to the vanishing of all odd powers in Newton's constant in the asymptotic expansion, so that the leading answer comes with twice the maximal Lyapunov exponent in simple operator configurations. We find that the perturbative OTOC in de Sitter can initially grow, which is not allowed by the quantum bound on chaos. Interestingly, we find that this growth is mediated by large diffeomorphisms in the graviton propagator.

The Hubble tension has reached a nominal significance above $7σ$, while new high-precision measurements of the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) sharpen the test of proposed solutions. Using a common framework, we compare fourteen representative alternatives to the standard $Λ$ Cold Dark Matter ($Λ$CDM) model in light of up-to-date CMB, BAO and supernovae data to gauge their ability to resolve the tension. The models span late-time modifications, modified recombination, and exotic pre-recombination expansion histories driven by additional radiation or a localized dark energy injection. We evaluate each proposal with complementary frequentist and Bayesian measures of the residual calibration tension and of the improvement in the joint fit. Both approaches identify the same broad hierarchy. Early dark energy and early modified gravity models perform best, shifting the $H_0$ inference without local measurement priors toward $70\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and reducing the residual discrepancy with SH0ES to approximately $2.5-3.6σ$, depending on the model and statistic, while receiving strong support over $Λ$CDM in the combined fit. Varying the electron mass at recombination yields an intermediate improvement, whereas the enhanced-radiation and late-time scenarios do not improve over $Λ$CDM. This Letter summarizes the group stage of the competition; in a companion paper (Paper II) we present the results of an exhaustive set of analyses and assess their robustness to variations in modeling assumptions and datasets.

In this work we reconstruct two subclasses of theories from the Horndeski class, the most general scalar-tensor theories of gravity with second-order equations, according to a variety of late-universe cosmological data: model independent measurements of Hubble rate from Cosmic Chronometers, as well as Hubble rate and distance measurements from Baryonic Acoustic Oscillations. In the modified gravity family considered, the scalar field is non-minimally coupled to gravity leading to time variations in the gravitational constant at cosmological scales. The theories considered are also consistent with constraints on the propagation speed of gravitational waves. The reconstruction is carried out by using the Gaussian Processes bayesian technique either for the Brans-Dicke and the Cubic Galileon subclasses of Horndeski theory. We find that the most likely Gaussian Process reconstruction for the data favors a cosmology from the Brans-Dicke subclass over $Λ$CDM in the range $z \in (0.4, 1.7)$, and for the Cubic Galileon subclass we find that the most likely reconstruction indeed favors modifications of the scalar field to gravity with time evolving dark energy density in the whole range of redshift observations.

In this study we introduce a new simulation campaign designed to understand how parameters that control star-formation and AGN feedback processes in cosmological hydrodynamical simulations impact observables such as the galaxy stellar-mass function (GSMF) and the gas fractions in large dark matter halos. These simulations are zoom-ins to halos selected from the MillenniumTNG (MTNG) simulation, and are run employing a novel multi-zoom approach which simultaneously re-simulates several sub-regions of a given large volume at a higher resolution than the background, thus reducing computational cost and imbalances in parallelization. We measure the GSMF and gas-fractions in halos for each of the re-simulations, and train Gaussian-process emulators on these quantities. The resulting emulators predict the GSMF and gas-fractions in halos with $\sim0.1\,\mathrm{dex}$ and $\sim 10\%$ precision respectively. Using the emulators we can simultaneously fit recent measurements of both quantities, in particular the lower gas fractions now observed even for comparatively massive clusters. Interestingly, we find a combination of parameters of the MTNG galaxy formation model that provides a qualitatively good fit to both the measured GSMF and gas fractions. This combination of parameters differs from the fiducial one mainly by requiring that stellar-feedback is significantly less energetic, and that kinetic AGN feedback events are significantly more energetic and rare. This finding implies that the MTNG model can be consistent with scenarios of strong feedback that remove large amounts of gas from groups and clusters, albeit we caution that we have not extensively examined the effect of these new parameters on many quantities for which MTNG made successful predictions.

Modifications to the dispersion relation of electromagnetic (EM) waves are a central probe in the search for quantum gravitational effects. In this work, we construct a general framework for the interaction between the EM field and a quantum background geometry, employing an extended Born-Oppenheimer approximation. This leads to a quasi-phenomenological model for EM wave propagation in curved spacetime. Unlike previous semi-classical approaches for mode-dependent dispersion relations, our framework naturally reproduces chromatic dispersion effects analogous to those observed in light-matter interactions in nonlinear optics. As a concrete application, we analyze EM wave propagation on a flat quantum Friedmann-Lemaitre-Robertson-Walker (FLRW) background, combining analytical techniques with numerical simulations to extract observable signatures of the prism-like behavior induced by quantum light-geometry interactions. Crucially, it remains valid across all energy regimes, enabling access to quantum gravitational corrections beyond the semi-classical limit.

We investigate the late-time expansion history of the Universe using a model-independent spline reconstruction of cosmological distances based on the latest DESI DR2 baryon acoustic oscillation (BAO) measurements and the DES Dovekie Type Ia supernova compilation. Comparing the reconstructed expansion history with the prediction of the Planck 2018 $Λ$CDM model, we identify a localized deviation over the redshift interval $0.3\lesssim z\lesssim0.6$, reaching a maximum significance of approximately $3.5σ$ at $z\simeq0.47$. We demonstrate that this feature persists under substantial variations of the reconstruction methodology, dataset composition and sound-horizon calibration. Mock analyses further show that the reconstruction is unbiased and that the observed anomaly is unlikely to arise from reconstruction bias or miscalibrated uncertainties. If confirmed by future observations, this localized feature could point to previously unrecognized late-time physics or reveal subtle inconsistencies between early and late Universe cosmological probes.

A non-minimal coupling between gravity and the inflaton field is a generic contribution in inflationary cosmology. In this work, we consider models of inflation with a non-minimal coupling $-ξφ^2 R$ and study their observational predictions for the tensor-to-scalar ratio $r$ and the spectral index $n_s$. Unlike the conventional approach of working in the Einstein frame, we perform the analysis in the Jordan frame where the underlying dynamics, particularly the competition between the potential force and the coupling-induced friction, are more transparent. For suitable values of $ξ$, the system exhibits an extended constant-roll regime, in which this friction counteracts the potential force. Focusing on monomial potentials $V(φ) \propto φ^n$, we find that negative $ξ$ systematically reduces $r$ in the $(n_s, r)$ plane, while the shift in $n_s$ depends on the power $n$: for $n \geq 4$, the spectral index increases with $|ξ|$, whereas for $n < 4$, it decreases. Notably, the quartic model $V(φ) = λφ^4/4$ with $ξ\lesssim -0.1$ shows good agreement with the ACT DR6 data and exhibits a distinct $n_s(ξ)$ dependence compared to other monomial cases. Our results demonstrate that non-minimally coupled models can effectively reconcile Planck and ACT observations, providing a unified framework for interpreting current CMB constraints.

The GeV-scale QCD axion -- where Peccei-Quinn (PQ) symmetry is broken by the QCD condensate at $f_a\sim\mathcal{O}(1)$~GeV -- faces a structural isospin problem: the PQ spurion coupling to light quarks ($u,d,s$) breaks $\mathrm{SU}(2)$ isospin, generating an unacceptable $\sim 15\%$ $π^0$--$π^\pm$ mass splitting. We show that coupling the PQ scalar to the charm quark instead eliminates this violation entirely, and lowering $m_φ\sim 3$--$4$~MeV makes the charm Yukawa $κ_c\propto m_φ$ perturbative ($κ_c<1$). The resulting $f_a\sim\text{GeV}\ll M_{\rm Pl}$ solves the axion quality problem: even the lowest-dimension $d=6$ Planck-suppressed operator gives $m_{\rm PQ}/m_a\sim 10^{-14}$, without any additional symmetry. The model predicts a distinctive {\it negative} $ΔN_{\rm eff}\sim -0.1$ for $m_φ\sim 3$~MeV, testable by CMB-S4. The $B\to Kσ$ penguin predicts $\mathrm{BR}\sim 2\times 10^{-5}$, consistent with the Belle~II evidence for $B^+\to K^+ν\barν$ at $(2.3\pm0.7)\times 10^{-5}$~\cite{BelleII:2024knv}. All ten classes of experimental, astrophysical, and cosmological constraints are satisfied in the viable window $m_φ\sim 3$--$4$~MeV, with the $B_s$ mixing constraint pending a dedicated lattice calculation.

K 1-6 has long been classified as a planetary nebula (PN) hosting a binary central star, yet it has remained poorly studied due to its faintness. The central star exhibits pronounced photometric variability whose origin has so far been unclear. We aim to present a comprehensive characterisation of the K 1-6 system, including the physical properties of its stellar components and the nature of the surrounding nebulosity. We conducted a multi-wavelength analysis combining optical and UV spectroscopy obtained with the Gran Telescopio Canarias, the Telescopio Nazionale Galileo, the Nordic Optical Telescope, and the Hubble Space Telescope. We also present long-term multi-band ground- and space-based photometry, including high-cadence data from the Transiting Exoplanet Survey Satellite, narrow-band imaging, and the latest astrometric constraints from Gaia. Our results show that the nebula is not a remnant PN, but instead consists of interstellar medium photoionised by a hot white dwarf, which is relatively evolved. It has a cooling age of 1-2 Myr, implying that any original PN has long since dissipated. We further find that the central object is a hierarchical triple system, comprising an inner binary with an orbital period likely of the order of thousands of days and a distant tertiary companion on a timescale of tens of thousands of years. The optically dominant cool component of the inner binary is an inflated K-type star displaying extreme magnetic activity, including large-amplitude variability and flaring. Its properties resemble those of BY Dra-type binaries and Abell 35-type systems, and are difficult to reconcile with single-star evolution, pointing instead to a history of binary interaction.

This thesis investigates theoretical and observational aspects of cosmic acceleration, focusing on dark energy models and modified gravity frameworks. The goal is to analyse their viability, dynamical behaviour, and perturbative stability to identify models capable of describing the accelerated expansion of the Universe. Chapter 1 reviews the essential background General Relativity, and mathematics of teleparallel and symmetric teleparallel geometries, basic cosmological models, matter components, and the key observational probes. The chapter concludes with a summary of modified gravity theories, including f(R), f(T) and f(Q). Chapter 2 studies a dissipative Chaplygin gas cosmology in f(Q) gravity. The model is constrained using the CC and Pantheon+SH0ES dataset, and its performance is assessed through information criteria and diagnostic tools such as Om and statefinder analysis. Chapter 3 reconstructs the dynamics of Dirac-Born-Infeld (DBI) dark energy using Hubble and DESI observations via Gaussian Processes. The reconstructed potential is fitted to theoretical models using chi-square and MCMC techniques, giving constrains on DBI scalar field. Chapter 4 analyses canonical scalar-field cosmology in coincident f(Q) gravity using dynamical systems. Chapter 5 extends the dynamical analysis to the DBI scalar field in f(Q) gravity. Chapter 6 examines scalar-field evolution at both background and perturbation levels. Perturbation equations for key gauge invariant variables are derived, and an extended phase space combining background and perturbation dynamics is constructed. Finally, chapter 7 summarizes the main results along with scope for future research. Further mathematical details including derivation of field equation and cosmology equations and foundational issues related to f(Q) gravity are compiled in the following four sections: Appendix A, Appendix B, Appendix C and Appendix D.

We investigate the reliability of the symbiotic star class in the general variability classification of Gaia DR3 and search for new genuine symbiotic systems among the objects assigned to this category. The sample contains 649 sources, including 246 previously known symbiotic stars, 61 literature candidates, and 339 new candidates proposed by the Gaia variability pipeline. Diagnostics based on the Gaia colour-magnitude diagram, near-infrared photometry, and the pseudo-equivalent width of H$α$ indicate that a large fraction of the new candidates are likely contaminants, predominantly pulsating red giants. To quantify the contamination, we constructed a Random Forest classifier trained on confirmed symbiotic stars and on Mira and semi-regular variables, using Gaia photometry, variability parameters, H$α$ measurements from XP spectra, and infrared colours. The classifier reaches a balanced accuracy of $\approx$0.94 and efficiently separates most symbiotic binaries from single evolved stars, leaving only eight strong candidates among the 339 newly proposed objects. Follow-up spectroscopy confirms three new symbiotic stars through the presence of high-excitation emission lines, while several additional objects remain possible symbiotics. Our results show that the Gaia DR3 variability classification efficiently recovers known symbiotic stars but has low purity due to overlap with pulsating red giants. The small number of newly confirmed systems implies that the discrepancy between predicted and observed Galactic symbiotic populations remains unresolved, although Gaia provides a powerful basis for future searches combining variability, spectroscopic indicators, and multi-wavelength data.

Symbiotic stars are long-period interacting binaries composed of an evolved giant and a hot compact companion. Their complex spectra and variability make them both astrophysically valuable and observationally challenging. We investigate how known symbiotic stars are represented in Gaia DR3 and the Focused Product Release (FPR), assess the reliability of these data, and evaluate their usefulness for candidate analysis and searches for new systems. We crossmatched Gaia DR3 and the FPR with confirmed symbiotic stars from the New Online Database of Symbiotic Variables and examined their astrometric, photometric, and spectroscopic data, along with derived products such as astrophysical parameters, orbital solutions, variability properties, and emission-line classifications. Astrometric data reliably constrain the position of symbiotics in the color-magnitude diagram, aiding searches for new systems, while RUWE is generally not a reliable indicator of their binarity. Most symbiotics are variable in Gaia photometry. Mean and epoch radial velocities, as well as inferred orbital solutions, are broadly consistent with the literature, and we provide the first tentative orbital solutions for two systems. Effective temperatures and metallicities are unreliable due to contamination from nebular continuum and strong emission lines. H$α$ emission is detected in nearly all symbiotics, making it a robust diagnostic. Additionally, resolved companions were identified for two systems, and one previously confirmed symbiotic star was reclassified as non-symbiotic. Gaia DR3 provides a rich dataset for the study of symbiotic binaries. The forthcoming DR4 promises a major leap forward with its longer time baseline, new data products, and epoch data.

Context: The origin of magnetic fields and the dynamo mechanism in red giants are still not fully understood. Aims: We aim to interpret the dynamo behaviour of global 3D simulations of red giants using mean-field dynamo models. Methods: We use mean-field models constrained by the differential rotation profile extracted from 3D simulations. We perform $α^2$, $αΩ$, and $α^2Ω$ mean-field dynamo simulations, varying the strength of the $α$ effect and the differential rotation. Results: The mean-field models can reproduce the growth rate of several 3D runs. The morphology of the large-scale magnetic field is better reproduced for the slowly rotating 3D cases than for the rapidly rotating ones. Faster rotation enhances dynamo action and it also modifies the dominant mode of the dynamo. Rapidly rotating 3D runs produce predominantly non-axisymmetric equatorial dipoles instead of axisymmetric fields at slower rotation. Mean-field models are supercritical even in the absence of differential rotation, indicating $α^2$ dynamo action. By contrast, models using the $αΩ$ approximation require sufficiently strong differential rotation to become supercritical. Conclusions: Our results suggest that the magnetic field in red giants requires dynamo action. In our mean-field runs, differential rotation speeds up the magnetic decay, disfavouring the idea of a persistent fossil magnetic field in red giants.

Field redefinitions connect many formulations of the same physics, but the standard equivalence theorem is an on-shell result and cannot be used to decide when two quantum descriptions are equivalent off shell. This paper develops an operational criterion for that problem in terms of the Vilkovisky--DeWitt effective action. The central idea is that equivalence should be tested on scalar observables built by pairing configuration-space tensors with admissible probes. This makes the criterion sensitive to the observable class under consideration and separates the usual on-shell notion of equivalence from the stronger off-shell notions needed in gravity, cosmology and non-equilibrium quantum field theory. Metric $f(R)$ gravity and a scalar field theory example serve as prototypes, showing how the formal criterion distinguishes local, branchwise and genuinely global equivalence. In particular, we show that metric $f(R)$ gravity and its auxiliary-field reformulation yield the same quantum theory when the auxiliary constraint is enforced in the path integral. This is distinct from quantizing the corresponding scalar--tensor action with the metric and scalar treated as independent integration variables, which defines a different quantum theory. Apparent quantum inequivalences can then be traced to comparisons between different quantum objects, rather than to a failure of actual equivalences. This leads to general and precise notions of local and global equivalence under both field redefinitions and auxiliary-variable extensions.

We investigate the dynamics of magnetically charged spinning test particles in the spacetime of the Bardeen regular black hole, sourced by nonlinear electrodynamics and featuring a magnetic monopole charge parameter g. Employing the Mathisson-Papapetrou-Dixon equations supplemented by the Tulczyjew spin condition and extended to include magnetic interactions via the generalized Lorentz force, we derive the effective potential governing the radial motion in the equatorial plane. We analyze the properties of circular orbits, including the location and parameters of the innermost stable circular orbit, and examine how they are modified by the particle's spin s, specific magnetic charge lambda, and the black hole's magnetic charge g. Prograde spin and attractive magnetic interactions reduce the ISCO radius, whereas repulsive interactions and retrograde spin shift it outward. We further impose timelike constraints to exclude unphysical superluminal trajectories, delineating the admissible parameter space. Finally, we explore high-energy particle collisions near the horizon, computing the critical angular momentum and the center-of-mass collision energy. Due to the regular core of the Bardeen spacetime, the Bañados-Silk-West effect is significantly suppressed or capped at finite values, in contrast to singular black hole solutions. These results highlight distinctive phenomenological signatures of regular black holes and offer potential observational probes of nonlinearity in electrodynamics and of magnetic monopoles through accretion processes, extreme-mass-ratio inspirals, and ultra-high-energy particle interactions.

Galaxy cluster samples based on X-ray and Sunyaev-Zel'dovich (SZ) observations are affected by selection biases. These catalogs preferentially include systems with high gas content and surface brightness. Excluding objects with depleted gas content, low-gas-fraction clusters (LGFCs), could lead to an incomplete sampling. We aim to investigate the abundance and the properties of the LGFCs population using The Three Hundred hydrodynamical simulations, focusing on the Gadget-X code. In particular, we study outliers in the $f_{\mathrm{g},500} - M_{500}$ relation, environmental influences, and their behavior in key scaling relations, with a focus on the Compton-Y observable. We analyze a sample of $N_{\mathrm{tot}} = 9858$ simulated objects from The Three Hundred, in the redshift band $z \in [0;0.817]$. LGFCs are selected statistically as outliers of the $f_{\mathrm{g},500}-M_{500}$ relation. To analyze environmental effects, we compare the gas density and temperature radial profiles of LGFCs against the No-LGFCs population. Finally, we study how the temperature, entropy, and spherical Compton parameter scaling relations are affected by the inclusion of LGFCs. We find that LGFCs are preferentially found at the low-mass end and their abundance increases toward low redshift. Radial profiles of LGFCs show lower gas concentrations in the core regions and higher temperatures, suggesting a more diffuse and heated ICM. This behavior is also reflected in the entropy scaling relation, where LGFCs are extreme positive outliers. Contrary to observations, the $Y_{\mathrm{sph},500}$ values of LGFCs show no significant deviation from the general population. Nevertheless, we cannot rule out that these differences are partly driven by the mass incompleteness at the low-mass end and the environmental bias of our simulated sample.

In this paper, we study the decay of a particle in Kerr--Newman spacetime. Both theoretical analysis and numerical simulations show that when a particle splits in Kerr--Newman spacetime, its mass is inevitably reduced, and this mass deficit is transformed into kinetic energy in the center-of-mass frame. We denote the parent particle as $O$ and the daughter particles as $A$ and $B$. We also find that as the charge parameter $Q$ increases, the specific angular momenta of the parent particle $O$ and the daughter particle $B$ become smaller, the masses of the two daughter particles become larger, the absolute values of their specific energies become smaller, the relative Lorentz factors among the three particles become smaller, and their four-velocities also become smaller. These trends are opposite to those observed when increasing the cosmological constant in Kerr--de Sitter spacetime.

The influence of dynamical (phantom) dark energy on light propagation within the framework of the Zeldovich-Kantowsky-Dyer-Roeder (ZKDR) approximation is discussed. This effect is considered in terms of the redshift-dependent parameter $α(z)$ of the ZKDR model, which is defined as the ratio of the mean matter density (baryonic matter, dark matter, and dark energy in the form of a cosmological constant) to the total density, including its fluctuations. We demonstrate that the observational manifestations of $α(z)$ admit two equivalent interpretations in the framework of both (1) a dynamical dark energy model, and (2) a model considering weak gravitational lensing in a universe with a cosmological constant. Thus, a degeneracy arises between these two physical scenarios. Finally, a simple statistical test, independent of cosmological probes, is proposed for breaking this degeneracy and testing the dynamical nature of dark energy. The test is based on the expected isotropy of dynamical dark energy at a fixed redshift, in contrast to stochastic inhomogeneities in the matter distribution induced by weak gravitational lensing.

Radio interferometers suffer from the missing short-spacing problem, losing large-scale diffuse emission. This missing flux underestimates gas mass and biases key metrics like star formation efficiency. Quantifying this scale-dependent loss currently relies on computationally intensive mock observations, lacking an analytical image-domain framework. We introduce the Constrained Diffusion Decomposition (CDD) method to decompose an input image ($I_{\mathrm{in}}$) into $n$ continuous scale-space components, denoted as $I_l = \mathrm{CDD}_l(I_{\mathrm{in}})$ for $l \in [1, n]$, and apply it to simulated Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Perseus molecular cloud across multiple array configurations. We find that the interferometric spatial filtering response can be mathematically decoupled: the scale-dependent flux recovery fraction follows a one-dimensional error function (\texttt{erf}), defined as $R(l) = \frac{B}{2} \left[ 1 - \mathrm{erf}\left( \frac{l - c_{\mathrm{recover}}}{w} \right) \right]$, where compact structures are effectively recovered, while extended emission decays monotonically as scales approach the maximum recoverable scale. The proposed CDD--\texttt{erf} framework predicts the spatially filtered interferometric image $I_{\mathrm{pred}}$ directly in the image domain, bypassing visibility simulations, mapping the true sky brightness distribution via the equation $I_{\mathrm{pred}} = \sum_{l=1}^{n} [ \mathrm{CDD}_l(I_{\mathrm{in}}) \times R(l)]$. This provides a quantitative bridge between model and interferometric observations.

In this work, we constrain the phenomenological interacting dark energy $ξ$IDE model using baryon acoustic oscillation measurements from DESI Data Release 1 and Data Release 2, combined with cosmic chronometer measurements, compressed CMB likelihoods, and three different Type Ia supernova compilations: Pantheon$^+$, DES-Dovekie, and Union3. We constrain the parameters of the $ξ$IDE model using a Markov Chain Monte Carlo analysis, using the nested sampling algorithm implemented in the \texttt{dynesty} package through the cosmological inference code \texttt{SimpleMC}. Compared to $Λ$CDM, the $ξ$IDE model favors slightly higher values of the Hubble constant, leading to a reduction in the Hubble tension. However, the tension is not completely resolved for any dataset combination. The parameter $ξ$ is consistently constrained to values close to its $Λ$CDM prediction, indicating no statistically significant deviation from the standard cosmological scenario. Similarly, except for the CMB + DESI DR1 + CC dataset combination, the dark energy equation-of-state parameter prefers values of $w_{\rm X}>-1$, showing a preference for quintessence-like behavior and providing evidence for dynamical dark energy. The interaction parameter combination, $ξ+3w_{\rm X}$, is found to be negative for all dataset combinations, corresponding to energy transfer from dark energy to dark matter. The strongest preference is obtained for the CMB + DESI DR2 + CC + Union3 dataset combination, yielding $ξ+3w_{\rm X}=-0.035\pm0.023$, which corresponds to a deviation of only $1.52σ$ from the non-interacting limit. Therefore, the current data provide only a weak indication of a non-zero dark-sector interaction. Model comparison based on the $Nσ$ statistic shows weak evidence in favor of the interacting scenario, while Bayesian evidence consistently prefers the $Λ$CDM model.

In contrast to potential-driven warm inflation models, this paper presents a new inflationary scenario driven purely by noncanonical kinetic terms. We derive the evolution equations and the associated slow-roll approximations specific to the kinetic case. The model incorporates a nonminimal derivative coupling that enhances gravitational friction; when combined with thermal damping, this leads to a significantly slower evolution of the pure kinetic inflaton. The resulting slow-roll approximations differ fundamentally from those of potential-driven inflation. The attractor behavior of this warm $k$-inflation with nonminimal derivative coupling is explored, confirming that slow-roll solutions can approach a strict exponential expansion attractor under relaxed slow-roll conditions. We further calculate the density fluctuation equations and obtain analytic expressions for the power spectrum, spectral index, and tensor-to-scalar ratio. Compared to standard inflation in general relativity, the energy scale at horizon crossing is lower, and the tensor-to-scalar ratio is significantly reduced due to the combined effects of thermal damping and nonminimal derivative coupling. The field excursion remains comfortably sub-Planckian. The model's predictions are in excellent agreement with the latest Planck 2018 data, offering a novel and successful extension of the warm inflation paradigm.

Cosmology stands at a crossroads. The Hubble tension has reached a nominal significance above $7σ$, while analyses combining DESI BAO and Type Ia supernova data show emerging hints of departures from $Λ$CDM. Meanwhile, high-precision CMB measurements from ACT and SPT enable a timely and more stringent reassessment of proposed solutions to the tension. In this paper, we revisit the $H_0$ Olympics, a systematic contest comparing proposed alternatives to $Λ$CDM using common datasets, likelihoods, and statistical criteria. In this updated edition, the $H_0$ World Cup, we subject fourteen representative solutions to a common analysis of current CMB, BAO, and SN data. The contenders span four broad mechanisms: late-time modifications of the expansion history, modified recombination, additional pre-recombination radiation, and early non-radiative energy injection. Relative to the original analysis, the present competition includes models and mechanisms proposed in the intervening years and evaluates all contenders using both Bayesian and Frequentist tests of tension and model performance, letting the neutrino mass sum vary. We further test if late-time extensions through curvature or the Chevallier-Polarski-Linder (CPL) dark energy parametrization can aid the success of the models. Finally, we subject the leading contenders to dedicated robustness tests involving alternative CMB likelihoods and multipole cuts, supernova samples, large-scale-structure information, and big-bang nucleosynthesis constraints. This framework assesses both the ability of each mechanism to ease the Hubble tension and the robustness of our conclusions to datasets and analysis choices.

We present high-resolution (R~45,000), high signal-to-noise (S/N>200) H and K-band spectra of BD+44 493, obtained with the newly commissioned IGRINS-2 instrument on the Gemini North Telescope. BD+44 493 is a well-known carbon-enhanced (CEMP; [C/Fe]=+1.40) ultra metal-poor (UMP; [Fe/H]~-4.0) star that has been extensively studied with high-resolution spectroscopy from the ultraviolet to the infrared. From the IGRINS-2 data, we derive new estimates of the abundances of C, Mg, Si, and Ca by measuring the equivalent widths of several atomic absorption features and comparing the values and line profiles with synthetic models. We measure, for the first time, a weak atomic carbon absorption feature (C I) in a UMP star in the H-band. Our estimated abundance agrees well with other values derived from CH molecules in the optical. The abundance values for the other elements are also in good agreement with previous estimates derived from ultraviolet, optical, and near-infrared observations, confirming the speculations that BD+44 493 is a bona fide second-generation star. With these data, we demonstrate that IGRINS-2 is a powerful new resource for studying and characterizing low-metallicity stars in the Milky Way and beyond.

We derive generalized effective Friedmann and Raychaudhuri equations for a bouncing polymer quantized Bianchi-I spacetime. We further prove that the relation numerically derived in the literature between the matter energy density and the anisotropic shear at the bounce holds exactly for any type of matter content, and derive explicit expressions for the constants that appear in that relation.

We study entanglement harvesting with particle detectors coupled to the renormalized energy density of a massless scalar field. Our analysis identifies the distributional mechanism underlying the persistent ultraviolet divergences previously observed for quadratic detector couplings and discusses that the energy density coupling exhibits the same underlying structure. We show that these divergences are entirely controlled by the switching correlation at coincident interaction times. Consequently, pointlike detectors are UV finite whenever the switchings do not overlap. For Gaussian-smeared detectors, the harvesting correlations are automatically finite in 1+1 and 2+1 dimensions, while in higher dimensions the remaining divergences are removed by non-overlapping switchings. Finally, we derive general expressions for arbitrary zero-mean Gaussian states and illustrate the formalism with a thermal field state.

Dynamical dark energy offers an alternative to a cosmological constant with distinct observational signatures. However, the small energy density scale, Hubble-sized mass, and Planckian excursions make simple models fine-tuned and unnatural. In this work, we show that a weak version of the axion, identified with the phase field of the anomalous $U(1)_{B+L}$ of the Standard Model, can generate the scale hierarchies expected for dark energy. The axion potential is controlled by sources of explicit baryon and lepton number violation and is radiatively stable. We show that the leading contribution comes from two inequivalent Weinberg operators, one $B+L$-conserving and one $B+L$-violating, which generate the axion potential. We propose a flavor selection rule based on a spontaneously broken $S_3$ permutation symmetry in the lepton sector that simultaneously removes the quadratic divergence and dominant temperature-dependent contributions. The resulting potential first appears at quartic order in the axion-dependent neutrino masses and, for the observed departure from tribimaximal mixing, its amplitude is parametrically close to the dark-energy density. The dominant uncertainty comes from $δ_{\rm CP}$ and $θ_{23}$, so experiments like Hyper-K and DUNE can directly test the model in the future. Cosmologically, the field behaves as thawing quintessence for $f$ close to $M_{pl}$, stays frozen by Hubble friction until late times and never enters the adiabatic regime.

We investigate the timelike entanglement first law in holographic conformal field theories whose bulk dual is Lovelock gravity. Using the double Wick rotation formulation of timelike entanglement entropy together with the Jacobson-Myers entropy functional, we compute the linear variation of holographic timelike entanglement entropy for hyperbolic subregions. For cubic Lovelock gravity, we explicitly show that a single, universal multiplicative renormalization factor governs how higher curvature interactions enter the variations of both the entropy and the modular Hamiltonian, leading to $ΔS=Δ\langle H\rangle$ for low-energy thermal excitations. We then extend the analysis to Lovelock gravity of arbitrary order around the anti-de Sitter spacetime in the Fefferman-Graham gauge. For normalizable perturbations, the variation of the Jacobson-Myers functional reduces to the Einstein gravity's result multiplied by the same coupling-dependent factor that renormalizes the effective Newtonian constant in the linearized field equations of Lovelock gravity. We further show that the boundary contribution vanishes in the conformal limit for the class of perturbations considered. Consequently, the timelike entanglement first law is equivalent to the linearized field equations of Lovelock gravity about the maximally symmetric background, within the hyperbolic and perturbative regime considered in the present paper.

We study the entropy of dynamical black holes in Einstein--Maxwell theory. Starting from a stationary electrovacuum background with a bifurcate Killing horizon, the apparent horizon is constructed perturbatively in Gaussian null coordinates, and the area of its cross section is derived to second order. Within the covariant phase space formalism, we introduce a modified canonical energy that incorporates contributions from external matter and is naturally related to the second-order variation of the entropy through the balance law. Under the null energy condition and the asymptotic stationary condition, the entropy is proportional to the area of the apparent horizon up to second-order and satisfies the second law. Without the null energy condition, establishing this area law requires additional assumptions.

The long-standing puzzle of the CO/CH$_{4}$ transition in brown dwarfs endures. Although the bulk spectral evolution across an atmosphere can be accounted for through thermal equilibrium cloud models, the behaviour in the NIR remains unaccounted for, indicating that additional, non-thermal processes may influence atmospheric chemistry alongside conventional pathways. We explore cloud-driven electrical activation, where low-energy sparks to full lightning discharges, unlocks non-equilibrium reaction pathways inaccessible under thermal conditions alone. To quantify this, the aim of this paper is to model the electron-moderated atmospheric chemistry with SPARCKS, a bespoke zero-dimensional code that solves the coupled set of particle balance equations for substellar plasma activation and reaction kinetics, focusing on the key CO-CH$_{4}$ electron-moderated chemistry across the parameter ranges $T_{\rm gas} \in [700, 1600]$ K} and $T_{e} \in [2, 5]$ eV. We simulate a 1 microsecond pulse, representing a short dart-stepped leader; and, two pulsed systems with $(t_{\rm on}, t_{\rm off})$ = ($10^{-8}$ s, $10^{-6}$ s) and $(10^{-9}$ s, $10^{-9}$ s), representing small-scale inter-grain discharges, consistent with a typical characteristic substellar atmosphere. Our results show that even modest, physically plausible energies can strongly perturb atmospheric composition: an electron energy of 3.0 eV is sufficient to halve the CH$_{4}$/CO ratio in our sample atmosphere within one microsecond. Beyond the CO-CH$_{4}$ system, electron-moderated plasma chemistry exerts a far-reaching influence on substellar atmospheric composition.

We investigate whether viable Hu-Sawicki-like $f(R)$ models can produce deviations from $Λ\mathrm{CDM}$ that can be tested against current background cosmological data. We adopt a machine-learning approach based on Genetic Algorithms (GA) to reconstruct analytical perturbations around the Hu-Sawicki class of models. We develop a pipeline that interfaces the \texttt{GATO} GA code with the \texttt{CANDI} cosmology code. Each $f(R)$ function generated by the GA is first tested against theoretical viability conditions, including stability, the recovery of a standard matter-dominated epoch, the General Relativity limit, and chameleon screening mechanism. Viable candidates are then passed to \texttt{CANDI} to reconstruct the corresponding background cosmology and are tested against DESI DR2 BAO measurements and the Pantheon+ Type Ia supernova catalogue. %\newline The deviations we find are largest at late times, where the lower curvature makes modified-gravity effects more relevant, and are rapidly suppressed at higher redshift, in agreement with the imposed matching to the matter-dominated era. To further quantify deviations from the standard cosmological model, we compute the $Om(z)$ diagnostic. It shows only a very small departure from the constant $Λ\mathrm{CDM}$ behaviour. The effective dark energy equation of state associated with the reconstructed $f(R)$ function also evolves only weakly, showing a mild transition from an effective quintessence-like nature to an effective phantom-like regime. Overall, our results indicate that, within perturbations around the Hu-Sawicki class of models, current background data allow only limited deviations from $Λ\mathrm{CDM}$.

Wineglass wormholes mediate the nucleation of baby universes out of an asymptotically Anti-de Sitter or flat spacetime. Upon materialization, the new universe naturally undergoes an inflationary phase. Here we study the quantum state of tensor and probe scalar field fluctuations that these wormhole geometries prepare, finding that they reproduce the Bunch-Davies vacuum for short-wavelength modes but lead to deviations from Bunch-Davies on large scales. These deviations, which depend on the charge of the wormhole, cause a small shift as well as oscillations in the angular power spectrum generated by an ensuing inflationary phase, and thus provide a distinctive cosmological observable. The wormholes are supported by either axionic or magnetic charges. In the limit of vanishing charge, they undergo a topological transition after which they contain no-boundary instantons. We find that the small-charge limit suppresses deviations from the Bunch-Davies state for both scalar and tensor fluctuations, indicating that the topological transition is smooth.

We analyze the circular velocity curves of 215 non-active galaxies from the Calar Alto Legacy Integral Field spectroscopy Area survey. Using radar-chart analysis, we compare two classification schemes: the circular velocity curve classification and the quenching classification. We find a systematic progression from slow-rising to flat, round-peaked, and sharp-peaked curves that mirrors the transition from star-forming to fully retired systems. Star-forming galaxies are mainly slow-rising and flat, whereas fully retired galaxies are predominantly round-peaked and sharp-peaked, with intermediate stages showing a gradual shift. Overall, advanced quenching stages are associated with more centrally peaked curves and higher outer velocities (i.e., velocities along the flat part of the curve), indicating increasingly concentrated central mass distributions and more massive disks of the galaxies.

Cosmic Microwave Background (CMB) polarization measurements are pushing instrumental sensitivities to levels where calibration systematics become a dominant limitation. The large dynamic range between cosmological and Galactic emission prevents future experiments from relying only on diffuse sky measurements or standard celestial calibrators. To address this challenge, the COSmological Microwave Observations Calibrator (COSMOCal) project proposes an artificial calibration source deployed as a guest payload on a geostationary satellite, scheduled for launch by the Eutelsat group by 2030. This source will provide stable, well-characterized polarized microwave signals accessible to multiple ground-based observatories. In this work, we present the status of the project, the updated development timeline, and the refined scientific and technical requirements, defined with the observatories that plan to use this calibration source. Furthermore, we investigate the interplay between instrumental systematics and component separation in the presence of complex models of interstellar dust emission. We discuss in this paper how this can impact the recovery of the primordial signal, and whether residual calibration errors can degrade the performance of foreground cleaning algorithms.

In this work, we investigate an interacting viscous generalized QCD ghost dark energy model in the framework of reconstructed $f(G)$ gravity proposed in Phys.\ Lett.\ B 631, 1--6 (2005). The interaction between dark matter and dark energy together with bulk viscosity is incorporated to describe a more realistic cosmic evolution. A hybrid expansion law is adopted to reconstruct the modified Gauss-Bonnet function, which naturally connects the early matter-dominated epoch with the present accelerated expansion of the universe. Since an exact analytical reconstruction is difficult, the $f(G)$ function is obtained numerically in both the early- and late-time regimes. Motivated by the numerical reconstruction, a reconstruction-inspired power-law form of $f(G)$ is also considered to examine the cosmological implications of the model. The results show that the reconstructed $f(G)$ function evolves smoothly throughout the cosmic history, while the effective equation of state gradually approaches the de Sitter phase at late times. The thermodynamic behavior of the model is further examined using Barrow entropy following Eur.\ Phys.\ J.\ C 81, 644 (2021). The non-negative evolution of the total entropy shows the validity of the generalized second law of thermodynamics. The study finally concludes that the interacting viscous generalized QCD ghost dark energy model in reconstructed $f(G)$ gravity provides a viable and thermodynamically consistent framework for explaining the late-time accelerated expansion of the universe.

The Blazhko effect -- a quasi-periodic modulation of pulsation amplitude and phase observed in all subtypes of RR Lyrae stars -- has defied physical explanation for over a century. Here we show that pulsation harmonics of the Blazhko RRab star V783 Cyg, traditionally regarded as passive replicas of the fundamental mode, encode a direct observational signature of the effect's origin. Using Kepler short-cadence photometry, we identify a subset of harmonics -- here termed ``disharmonized harmonics'' -- whose amplitude variations are anti-correlated with those of lower-order harmonics and of the fundamental mode. These disharmonized harmonics map precisely onto the base of the convective envelope, where convection--pulsation interaction is strongest, and their behaviour directly traces the strength of turbulent convection. We interpret the Blazhko modulation as arising from quasi-periodic variation of turbulent convection in the stellar envelope, which regulates both the efficiency of energy transport and the thickness of the $κ$-excitation zone. This interpretation naturally accounts for the occurrence of the Blazhko effect across RR Lyrae subtypes and its absence when the convective envelope extends only shallowly into the He~I/H ionization zone, establishing an observationally grounded framework that links harmonic behaviour to the convective dynamics underlying this long-standing puzzle.