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Neutrinos can undergo substantial flavor evolution between their production in astrophysical sources-such as the Sun and core-collapse supernovae-and their subsequent detection in terrestrial detectors. This flavor evolution is strongly influenced by the environment that the neutrinos interact with, making them useful astrophysical messengers capable of potentially carrying information about the properties of the source wherein they are produced. In this work, we apply the framework of statistical data assimilation (SDA) in order to ascertain the extent to which a neutrino signal at detection may contain information about matter density fluctuations along their path from the source. Using simplified models of neutrino flavor evolution and propagation in the sun and in a core-collapse supernova (CCSN), we find that the SDA method is able to extract information about the amplitude of density fluctuations in both the solar and CCSN scenarios, with the method showing relatively greater reliability in the solar neutrino case. Nonetheless, even in the CCSN neutrino model, the method proved effective at high fluctuation amplitudes

Magnetic reconnection converts stored magnetic energy into kinetic energy, heat, and radiation. While extensively studied in fully ionized plasmas, observations show that ionization and recombination also play a role by modifying the local plasma resistivity and enabling additional heating channels. This study examines magnetic reconnection in the partially ionized solar chromosphere, analyzing its morphology and energy releases with attention to elastic collisions, ionization, and recombination. The MAGNUS code, originally built for ohmic resistivity and heat transfer, was modified to handle elastic and inelastic collisions. The simulations are 2.5D resistive MHD with two-fluid effects (charged + neutrals), adapted to handle interactions via these collision terms. A mixed explicit-implicit scheme was implemented to manage the stiffness of these terms. Simulations were carried out for three magnetic field strengths: 100~G, 110~G, and 120~G. These values correspond to the low- to mid-chromosphere under quiet-Sun conditions. We found that reconnection heats the plasma components by 18\% to 110\%. Although plasma beta rises only slightly, the energy release grows far more, indicating that charged--neutral collisions, not just beta or available magnetic energy, drive the enhanced reconnection. Furthermore, ionization and recombination become most significant in regions of peak temperature, particularly where particles are accelerated. Maximum reconnection rates from temporal analysis are 0.226, 0.253, and 0.279, respectively. Finally, in terms of energy, our results show that in a chromospheric volume of $0.4 \times 0.01 \times 0.4$~Mm$^{3}$, the energy released ranges from $10^{22}$ to $10^{23}$.

Inspiraling binary systems of compact objects probe gravity in strong-field regimes, thereby exploring potential higher curvature corrections to General Relativity. Parity-invariant quadratic corrections can be described by scalar-Gauss-Bonnet (sGB) theory, which involves a scalar field dynamically coupled to curvature scalars and can give rise to scalar condensates around black holes. Considering a mass for the scalar field is natural and leads to new phenomenology related to this additional scale. We compute the dynamics of a binary system of nonspinning black holes in massive sGB using the post-Newtonian (PN) approximation. We obtain solutions for the equations of motion, center-of-mass transformation, and binding energy for circular and eccentric orbits up to 1PN order, where for the first time the higher curvature coupled to scalar mass corrections are included. While most of our calculations are valid for generic scalar masses, the final explicit expressions assume that the mass is small compared to the total mass of the binary and expand to quadratic order in this ratio. We show that the scalar mass corrections to the gauge-invariant binding energy come with same and opposite sign order terms, contributing an overall opposite sign contribution in the perturbative limit, decreasing the binding energy slightly. The effects are largest for binary systems with high mass ratio and large eccentricity. Our methods and results will also be useful as a basis for computing the gravitational waves sourced by such systems.

Two standard thermometers applied to the same quiet-Sun corona disagree by a stable factor: radio brightness reads about 0.6 MK while ionization- and scale-height-based diagnostics read about 1.5 MK, a ratio of 2.4 +/- 0.3 over eight years. We argue this discrepancy is not an anomaly awaiting reconciliation but a kinetic measurement, and we read it three times: for the shape of the electron distribution, for its origin, and for the energy where it must end. First, a temperature diagnostic is a projection onto the Maxwellian family; the quiet-Sun distribution carries a power-law tail of index kappa ~ 2.5, so different projections return different temperatures, and their disagreement decomposes exactly into a temperature gap fixed by the measured ratio alone and a residue no scalar temperature can carry: a fifth of the electron thermal energy, stored in shape, whose presence means no local conductive closure exists for this plasma. Second, reading one model atmosphere two independent ways returns kappa = 2.52 from its temperature-density structure at the height where the diagnostics read 2.57, while the local electric field on the same column is 39-56 times too weak to seed the tail in place: the tail was transported, not made there. Third, a transported tail terminates at the energy whose collisional stopping column equals the column traversed; the tabulated column puts that termination at 1.8-3.5 keV, overlapping the 1.7-3 keV bracket independently required by the diagnostic ratio and the hard X-ray limits, with no tuned parameters, and the computed termination reproduces the measured ratio: the spectrum's edge is a pressure gauge on the tail's loading layer. The premise is audited, a deciding within-ion test on archived spectra is specified, and nine falsification conditions are stated. The discrepancy, read kinetically, returns more than either thermometer was built to measure.

We present a systematic X-ray spectral analysis of 27 giant radio galaxies (GRGs) using archival \textit{XMM-Newton} and \textit{Chandra} data. Roughly 44\% of the sample show intrinsic absorption ($N_{\mathrm{H,int}} \gtrsim 10^{21}\,\mathrm{cm}^{-2}$), while several exhibit narrow Fe K$α$ emission lines with equivalent widths up to $\sim$570 eV. Soft X-ray features are also common, including thermal plasma emission (kT$\sim 0.2$--0.8 keV) and ionized absorbers, in some cases consistent with high-velocity outflows. The photon indices are typically hard (median $ Γ\sim 1.6$), in line with radio-loud AGN. A comparison between nuclear X-ray luminosity and extended radio power shows that many sources host relatively strong X-ray cores compared to their lobes, pointing toward possible restarted nuclear activity. For 10 GRGs with published black hole masses, we explore preliminary relations with X-ray luminosity, Eddington ratio, and photon index. We find a positive trend between $M_{\rm BH}$ and $L_{2-10\,{\rm keV}}$, while Eddington ratios show no clear dependence on mass. A tentative negative relation between $λ_{\rm Edd}$ and $Γ$ hints at inefficient accretion, although confirmation will require larger samples. Overall, GRGs display diverse nuclear conditions and accretion states, supporting scenarios where obscuration, episodic or restarted activity, and low-efficiency accretion shape their long-term evolution.

(abridged) Aims. We aim to generate large samples of CVs selected from the SRG/eROSITA surveys to address fundamental questions about binary evolution, the role of magnetic fields in CV evolution and their contribution to the Galactic Ridge X-ray Emission (GRXE). Methods. We have trained a Random Forest (RF) classifier based on combined X-ray and optical properties to select CV candidates from the first eROSITA X-ray all-sky survey (eRASS1). Follow-up spectroscopy to securly identify the objects was performed on a subset of the selected candidates using telescopes at the Northern and the Southern hemisphere. Newly identified CVs were further analyzed to determine their likely subtype with the help of dedicated follow-up photometry, spectroscopy and archival resources. Results. We have identified 156 CVs of almost all subtypes covering a wide range of distances (between 170 pc and several kpc), absolute magnitudes (G= 4.5-12) and X-ray luminosities (log LX (0.2-2.3 keV)= 29.6-33.3 erg/s). For most objects, the nature as a CV is reported here for the first time. For 40 objects periods were determined, which were regarded as their likely orbital periods. These range from 79.2 min, at the CV minimum period, to almost 22 hours. Seven objects were found to be eclipsing, a lower limit to the actual number due to the current lack of dedicated follow-up observations. A further five objects show cyclotron or Zeeman features, that allowed to determine their field strengths. A soft component was found in 14 CVs, all were regarded being magnetic. The sample comprises X-ray luminous objects, excellent candidates for being new Intermediate Polars, that are thought to be main contributors to the GRXE. The new method of selecting CVs from eRASS1 was found to be superior to other methods described in the literature ...

Supernova remnants (SNRs) are key tracers of stellar feedback and the chemical and dynamical evolution of the interstellar medium. However, identifying SNRs in complex regions remains challenging, particularly when only archival multi-wavelength data are available. We aim to search for previously unrecognized SNR candidates in the Circinus constellation by combining optical imaging, radio continuum data, and available SRG/eROSITA all-sky survey X-ray observations, to provide a prioritized list of targets for future confirmation. Candidate remnants were classified according to the strength of the combined diagnostics. This joint analysis yields improved constraints on the morphology and spectral properties of known remnants in the region, including MSH 15-52 and G320.6- 1.6. In addition, we identify a new promising SNR candidate, G320.2-3.6. Our study provides a first multi-wavelength search of SNR candidates in the Circinus region. In addition to G320.2-3.6, three objects (G319.8-2.0, G321.8-1.1, and G320.5-1.8) emerge as promising targets for follow-up radio, spectroscopic optical, and deeper, spatially resolved X-ray observations to confirm their nature.

In this paper, we calculate the tidal Love numbers of multi-state boson stars (MSBSs) composed of ground state and first excited state complex scalar fields. Under synchronized and nonsynchronized frequency conditions, the background solutions of MSBSs are classified into single-branch and double-branch types. The field functions, ADM mass, and binding energy of different solutions are discussed. We then calculate the quadrupolar ($\ell=2$) electric and magnetic tidal Love numbers for branches containing stable solutions. Our results show that the electric tidal Love numbers are initially positive and then suddenly transition to negative values. This phenomenon occurs when the parameters satisfy $\tildeμ_1 > 0.891$ or $\tildeω_0 > 0.777$; for smaller values of these parameters, the electric Love numbers remain positive. The magnetic tidal Love numbers are always negative, with absolute values smaller than those of the electric tidal Love numbers.

Based on the observation of the active galaxies NGC 1068 and TXS 0506+056, and on additional evidence for the sources NGC 4151, CGCG 420-015, NGC 7469, and the Circinus Galaxy emerging from IceCube data, we make the case for the production of high-energy neutrinos within a few gravitational radii of supermassive black holes surrounded by a dense plasma radiating X-rays. X-rays represents the target for the production of neutrinos by protons accelerated near the black hole; they also absorb the gamma rays from the decay of neutral pions produced in the same interactions. Neutrinos with energies of tens of TeV and above originate in photoproduction interactions with X-rays of $0.1 \sim 1$\,keV energy on the $Δ$ resonance, $p + γ\rightarrow Δ\rightarrow n + π^+$. Our analysis of the multimessenger data points to gamma-ray-obscured sources with a characteristic neutrino flux linearly proportional to the X-ray flux originating within $\sim 10$ gravitational radii of the black holes, with lower values preferred. We speculate on such sources producing the diffuse flux of neutrinos and cosmic rays of extragalactic origin.

The universal I-Love-Q relations for compact stars have significant applications in gravitational-wave astronomy, but thermal effects can break these relations in low-mass white dwarfs. In this work, we employ the stellar evolution code MESA to construct realistic models of $0.15 \, M_{\odot}$ helium-core and $0.6 \, M_{\odot}$ carbon-oxygen core white dwarfs at various temperatures. By utilizing the Clairaut-Radau equation, we quantitatively extract the radial variation of the eccentricity of internal isodensity surfaces. Our numerical results demonstrate that higher central temperatures amplify the eccentricity variation, causing the I-Love relations to deviate from the zero-temperature Chandrasekhar model, whereas subsequent cooling restores them. This confirms that the temperature-induced violation of the universal relations is fundamentally driven by the loss of self-similarity in isodensity surfaces, providing key insights into the applicability conditions of I-Love-Q relations in compact objects.

Gravitational-wave observations reveal a population of binary black hole (BBH) mergers with primary masses above ${\sim}40\,\mathrm{M}_\odot$, extending into and potentially beyond the pair-instability mass gap, with a possibly flat mass-ratio and broader χ_\mathrm{eff} distribution. We investigate whether super-Eddington accretion during stable mass transfer in isolated binary evolution can produce BBH mergers consistent with these properties across primary BH mass, mass-ratio, and χ_\mathrm{eff} distributions. Using POSYDON, we simulate BBH merger populations with primary BH masses above ${\sim}40\,\mathrm{M}_\odot$, under three BH accretion efficiencies: Eddington-limited, GRRMHD-informed, and fully conservative. We additionally vary the natal kick strength, including strong kicks at high BH masses. We find that super-Eddington accretion does not suppress BBH mergers in the high-mass regime. Fully-conservative accretion leads to an increase of BBH mergers in POSYDON with a strong kick-independent peak at $χ_\mathrm{eff}=0.6$ and a sharp mass-ratio peak at $q\sim0.5$, whereas observations favor $χ_\mathrm{eff}=0.0$ and a flatter mass-ratio distribution. The GRRMHD-informed and Eddington-limited accretion are compatible with the observed primary BH mass and mass ratio distribution, but require natal kicks to populate negative χ_\mathrm{eff}. A joint analysis of the primary BH mass, mass ratio, and χ_\mathrm{eff} distributions provides strong constraints on binary evolution physics, and disfavor fully-conservative BH accretion as the dominant formation mechanism for high-mass BBH mergers. The Eddington-limited and GRRMHD-informed prescriptions with modest kicks can explain part of the high-mass population, but an additional formation channel is still needed to account for the high fraction of negative χ_\mathrm{eff} systems and high secondary BH spins.

One might expect the most massive stars to also be the largest by size, yet they are not, and this has puzzled astronomers for decades. The Eddington limit sets an upper bound to stellar mass through the balance between radiation pressure and gravity, but a second empirical boundary on stellar radius has equally far-reaching consequences for modern Astrophysics: it affects the black-hole masses and merger rates inferred by LIGO and Virgo, determines whether a star remains a hot, compact ionising source or inflates into a cool supergiant, and thereby shapes how spectra of distant, unresolved stellar populations are interpreted. Half a century ago, this radius limit was shown to trace a characteristic kinked shape in the Hertzsprung-Russell diagram known as the Humphreys-Davidson limit, yet a predictive, first-principle physical explanation has remained elusive. Here we show that this kink is the evolutionary manifestation of a transition from classical stellar outflows to an Eddington-enhanced mass-loss regime implemented self consistently in evolutionary models. With our models accurately reproducing the empirical constraints on the radii of the most massive stars, we now have a framework that can be incorporated into binary population synthesis, black-hole mass predictions, gravitational-wave event rates, and the interpretation of high-redshift James Webb spectra.

Recent analyses of the central compact object in the HESS J1731--347 supernova remnant suggest an unusual combination of a low mass and small radius, while its thermal emission indicates a relatively high surface temperature at its estimated age. Reconciling these structural and thermal properties within a unified theoretical framework may provide important constraints on the equation of state and composition of dense matter. In this work, we investigate the thermal consequences of negatively charged kaon condensation, an exotic phase that softens the equation of state and facilitate the reproduction of the inferred bulk properties of HESS J1731--347. We find that the onset of kaon condensation strongly accelerates the thermal evolution, leading to surface temperature substantially below the observationally inferred range. Within the adopted cooling framework, kaon condensation therefore cannot simultaneously account for the structural and thermal properties of HESS J1731--347.

Abell 3266 (A3266) is a dynamically active galaxy cluster embedded in a dense environment of galaxy groups and clusters at similar redshift. Data from the Spektrum Roentgen Gamma (SRG)/eROSITA all-sky survey enable the study of faint X-ray emission in cluster outskirts. We investigate the previously unexplored outskirts of A3266 out to $3R_{100}$, characterize its nearest neighboring galaxy group, and search for connecting filaments using X-ray emission and galaxy number density. We performed X-ray imaging, surface brightness, and spectral analyses in selected regions and sectors. These were complemented by the distribution of member galaxies from the NASA/IPAC Extragalactic Database NED and by comparison with the cosmological simulation Simulating the LOcal Web (SLOW). We detect an X-ray filament connecting A3266 to its nearest northwestern group over a 3D length of $L_{R_{200}\text{--}R_{200}} = {1.1}_{-0.1}^{+0.5},\mathrm{Mpc}$ with a significance of $3.6,σ$. The group exhibits cool-core properties and is embedded within the filament. The filament has a temperature of $T={1.2}_{-0.2}^{+0.3},\mathrm{keV}$, metallicity $Z={0.07}_{-0.05}^{+0.09},Z{\odot}$, and, assuming a simple geometry, an electron number density of $n_{\rm e}={8}_{-2}^{+1}\times 10^{-5},\mathrm{cm}^{-3}$. Our findings reveal a coherent network of galaxy groups around A3266, tracing its ongoing assembly along the large-scale structure. The filament is hotter and denser than expected for pristine warm-hot intergalactic medium, consistent with gas processed in the cluster environment and influenced by the infall of the northwestern group. Comparison with SLOW shows that the observed group distribution and filamentary connections are qualitatively consistent with an actively accreting cluster embedded in the cosmic web.

We define a covariant variation of tensor fields by combining its Lie derivative with the metric variation. This operator preserves the metric, contractions, and Hodge duality, but its commutator is not closed due to an anomaly. We derive its algebraic and geometric properties, and compare it with the Kosmann derivative. Combining the covariant variation with Kosmann derivative gives total covariant variation for the fields with both spacetime and Lorentz structure, all of which belong to the metric Lie derivative. Moreover, we introduce families of extended operators which contain the affine connection, Lie derivative, and covariant variation. From the anomaly of the covariant variation along the superrotation, an electromagnetic helicity flux appears at null hypersurfaces in four dimensions. We also apply the covariant variation and its anomaly to tensor fields in arbitrary spacetime dimensions, and especially focus on the $p$-forms in $d=2p+2$ dimensions.

The physics, geometry, and kinematics of the gas and dust surrounding the central engine of active galactic nuclei (AGN) are not well understood. In some sources, this material obscures the coronal X-ray emission, allowing its line-of-sight column density ($N_H$) to be measured. Many sources display $N_H$ variability across multiple epochs. Studying this variability can provide information about the distribution and dynamics of the obscuring material. Particularly, previous works have shown that the probability of observing $N_H$ variability increases with the number of observations available. Additionally, the observed magnitudes of the $N_H$ variability depend on the timescale between observations. In this work, we present the $N_H$ variability analysis of NGC 1142, which has a total of nine X-ray observations over $\sim20$ years, including five from a NuSTAR+XMM-Newton monitoring campaign performed in 2024-2025 and reported here for the first time. We use the physically-motivated torus models UXCLUMPY, XSKIRTor, and RXTorusD to model the spectra. Our results confirm the previously observed trends, and we compare these trends to simulated obscuration curves assuming different locations and distributions of obscuring clouds.

Coronal mass ejections (CMEs) are inherently three-dimensional (3D), but traditional white-light coronagraphs provide only two-dimensional (2D) projections. While the Polarization Ratio Technique (PRT) utilizes Thomson scattering to infer proxy 3D plasma locations, its results are typically limited to 2D topographical maps, hindering geometric interpretation. To address this, we introduce PRICM3 (Polarization Ratio Integrated CME Mapper in 3D), a framework that converts PRT-derived maps into explicit 3D point-cloud reconstructions. This tool enables intuitive visualization, multi-viewpoint comparison, and direct validation against geometric models such as the Graduated Cylindrical Shell (GCS). As a proof-of-concept, we analyzed a limb CME observed on 28 October 2021 using data from Solar Orbiter/Metis, SOHO/LASCO, and STEREO-A/COR2. The independent reconstructions converged into a consistent 3D volume that aligned remarkably well with the corresponding GCS geometry. These results prove that the 3D information embedded in PRT topographical maps can be successfully recovered through spatial reconstruction. PRICM3 offers a powerful, intuitive method for interpreting polarimetric coronagraph observations, providing a vital tool for current and future solar missions such as Proba-3/ASPIICS and PUNCH.

Cold, collisionless dark matter successfully explains a wide range of observations, including the formation of large-scale structure. Nevertheless, tensions remain on small, galactic scales, most notably the cusp-core, or inner-mass-deficit, problem and the diversity of inner rotation-curve shapes and central densities at fixed halo mass. These observations suggest that additional dark-sector physics may affect the inner structure of halos, although no generally accepted explanation has yet emerged. Here, making use of a toy but representative model, we show that interacting dark-matter Q-balls -- non-topological solitons stabilized by a conserved charge -- can provide a natural mechanism for halo cusp flattening and may contribute to observed diversity of inner halo profiles. Produced in the early Universe in the dark sector, these Q-balls grow in the dense central regions of halos, while their interaction cross section decreases as the soliton mass increases. This process operates preferentially in halo centers, converting part of the rest-mass energy stored in massive Q-balls into relativistic dark-sector particles and thereby modifying the inner mass-density profile. The resulting density-dependent, self-regulating energy loss provides a dynamical mechanism for flattening halo cusps while leaving the outer halo largely unaffected.

Aims. Our main goal is to determine whether the lower stellar density observed in the Carina─Sagittarius Arm region (320° ≤ l ≤ 325°, |b| ≤ 1°) is an intrinsic structural feature of the spiral arm or a consequence of high interstellar reddening and extinction along the line of sight. Methods. We performed a systematic search for new open clusters using the HDBSCAN algorithm on Gaia DR3 astrometric data. The physical reality of the candidates was validated through a multiwavelength analysis, integrating Gaia photometry with the near-IR photometry of the 2MASS catalog, the mid-IR photometric information of the WISE catalog to identify young stellar objects (YSOs), and the radio continuum emission at 843 MHz with the SUMSS survey to identify and associated interstellar material. Results. We report the discovery of five new open clusters, the ESFERA sample. Our analysis reveals four young systems (10─50 Myr), with ESFERA 3 hosting YSOs, while ESFERA 5 is identified as a 2 Gyr old cluster. Although the region lacks massive star-forming complexes, these clusters trace active star formation within the region, although at a lower efficiency. Conclusions. Our results suggest that this region represents an intrinsic star formation valley or low-density node (the string in a morphology of beads on a string) and is not an effect of extinction. The discovery of these clusters demonstrates that while the local surface density is below the critical threshold for massive complexes, star formation persists in isolated small-scale pockets throughout the arm.

The discovery of supermassive black holes (SMBHs) at redshifts <inline-formula><mml:math><mml:mi>z</mml:mi><mml:mo>≳</mml:mo><mml:mn>10</mml:mn></mml:math></inline-formula> by JWST challenges the standard Eddington-limited growth models. We propose a novel mechanism for the rapid assembly of primordial black hole (PBH) seeds through scalar-induced critical accretion during early universe phase transitions. By solving the generalized relativistic Euler equations with a non-minimal coupling between dark matter and a background scalar field <inline-formula><mml:math><mml:mi>ϕ</mml:mi></mml:math></inline-formula>, we demonstrate that a "scalar fifth force" significantly shifts the sonic point outward. This mechanism yields an accretion enhancement factor <inline-formula><mml:math><mml:mi>ϒ</mml:mi><mml:mo>≍</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mi>β</mml:mi><mml:msup><mml:mo>)</mml:mo><mml:mn>2</mml:mn></mml:msup></mml:math></inline-formula>, enabling a <inline-formula><mml:math><mml:mn>10</mml:mn><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula> seed to reach <inline-formula><mml:math><mml:msup><mml:mn>10</mml:mn><mml:mn>6</mml:mn></mml:msup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula> by <inline-formula><mml:math><mml:mi>z</mml:mi><mml:mo>≍</mml:mo><mml:mn>15</mml:mn></mml:math></inline-formula>, far exceeding standard General Relativistic predictions. Furthermore, this rapid mass growth predicts a distinctive spectral redshift in the stochastic gravitational wave background (SGWB), shifting the peak from the LIGO/Virgo band toward the Einstein Telescope sensitivity window (1─10 Hz).

We report on a blinded search for dark matter (DM) using ionization-only (S2-only) signals in XENONnT with a total exposure of <inline-formula><mml:math><mml:mrow><mml:mn>7.83</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mtext>Tonne-year</mml:mtext></mml:mrow></mml:math></inline-formula> over 579 days in three science runs. Dedicated background suppression techniques and the first complete S2-only background model in XENONnT provide sensitivity to nuclear recoils of <inline-formula><mml:math><mml:mrow><mml:mo>[</mml:mo><mml:mn>0.5</mml:mn><mml:mo>,</mml:mo><mml:mn>5.0</mml:mn><mml:mo>]</mml:mo><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:msub><mml:mrow><mml:mi>keV</mml:mi></mml:mrow><mml:mrow><mml:mi>nr</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula> and electronic recoils of <inline-formula><mml:math><mml:mrow><mml:mo>[</mml:mo><mml:mn>0.04</mml:mn><mml:mo>,</mml:mo><mml:mn>0.7</mml:mn><mml:mo>]</mml:mo><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:msub><mml:mrow><mml:mi>keV</mml:mi></mml:mrow><mml:mrow><mml:mi>ee</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula>. No significant excess over the expected background is observed, and we set 90% confidence level upper limits on spin-independent DM-nucleon and spin-dependent DM-neutron scattering for DM masses between 3 and <inline-formula><mml:math><mml:mrow><mml:mn>8</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>GeV</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mi>c</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>, as well as on DM-electron scattering, axionlike particles, and dark photons, improving on previous constraints. For spin-independent DM-nucleon scattering, we exclude cross sections above <inline-formula><mml:math><mml:mn>6.0</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>45</mml:mn></mml:mrow></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:msup><mml:mrow><mml:mi>cm</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> at a DM mass of <inline-formula><mml:math><mml:mrow><mml:mn>5</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>GeV</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mi>c</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>, pushing the XENONnT sensitivity closer to the region where coherent elastic neutrino-nucleus scattering becomes an irreducible background.

We investigate the horizon structure, constrained thermodynamics, and geodesic properties of an effective Kerr-like black hole in a non-commutative background. Deformation modifies the radial geometry through a mass-dependent charge-like contribution, while preserving the separability of the geodesic equations. We determine the conditions for horizon existence, identify the extremal zero-temperature configuration, and analyze the stationary-limit surfaces and the ergoregion. Special attention is paid to the thermodynamic interpretation of the model, where the geometric Hawking quantities are distinguished from the conjugate variables associated with the constrained state space at fixed non-commutative deformation parameter. The canonical and grand-canonical heat capacities are derived to characterize their ensemble-dependent local thermal behavior. We also obtain the spherical photon region, equatorial light rings, and shadow boundary, showing that the deformation shifts the characteristic photon orbits inwards and reduces the overall size of the shadow. Timelike circular motion is studied through the innermost stable circular orbit, where non-commutative correction produces an inward shift of both the prograde and retrograde branches. Finally, invariant photon frequency shifts are obtained by treating the emitter's orbital direction and the photon's tangential emission direction as independent physical choices.

In this work, we analyze the differences and similarities between two recent constructions, which are distinct in their methodologies for extending the Wald entropy of stationary black holes to non-stationary situations in general higher-derivative gravity. One of them, denoted by $S_\text{Wall}$, is constructed by exploiting the boost symmetry of the near-horizon geometry, whereas the other, denoted by $S_\text{dyn}$, is obtained from a covariant phase-space analysis based on the Wald-Iyer Noether charge formalism. While $S_\text{dyn}$ is, by construction, defined only for linearized fluctuations around a stationary black hole solution, $S_\text{Wall}$ does not require such a linearization for its construction. Although the linearization is necessary to interpret $S_\text{Wall}$ as a well-defined notion of entropy, the construction itself naturally contains terms that are higher order in the dynamical fluctuations. By comparing the technical structures underlying the two constructions, we clarify the fundamental differences between the methods on which they are based. We demonstrate that while the construction of $S_\text{dyn}$ given the $S_\text{Wall}$ is straightforward, the converse is more subtle. We develop an algorithm to obtain a local expression for $S_\text{Wall}$ from a known expression for $S_\text{dyn}$ in a generic diffeomorphism-invariant theory of gravity, provided certain technical conditions are satisfied. We justify our analytical findings with explicit demonstrations in a particular case: the Riemann-squared example of the higher-derivative theory of gravity.

We present results of long-term high-resolution spectroscopic monitoring of the extremely low-metallicity star PS Gem (=HD 52961), associated with IRAS 07008+1050. The spectra were obtained with the 6-meter BTA telescope at a resolving power of R=60000 during 1997--2025, providing a homogeneous dataset spanning nearly three decades. Quantitative measurements of the H$α$ two-peaked profile reveal repeatable phase-dependent variations of the peak-intensity ratio $I_{\rm V}/I_{\rm R}$ and the absorption-core velocity over multiple orbital cycles, whereas the peak separation remains nearly constant at $ΔV_r = 78\pm 4$ km s$^{-1}$. The heliocentric radial velocity measurements yield a systemic velocity of $γ=7.46\pm 0.64$ km s$^{-1}$ and an orbital semi-amplitude of $K=2.49$ km s$^{-1}$. No significant long-term secular drift is observed over the monitoring interval. The stellar luminosity, estimated from the equivalent width of the OI 7773 Ȧ triplet ($W_λ=1.16\pm$ 0.02 Ȧ), corresponds to $\log(L/L_{\odot})\approx$3.5. The average radial velocity of diffuse interstellar bands is consistent with the location of PS Gem in the Local Arm. These observations provide rare multi-decade time-domain constraints on the dynamical behavior of this evolved metal-poor star.

The faint infall regions surrounding the virial radius of galaxy groups remain largely unexplored due to their low X-ray surface brightness. Using the large statistical power of SRG/eROSITA survey observations, we present the first spectroscopic measurement of the intragroup medium (IGrM) in the infall regions of a large sample of low-mass galaxy groups ($M_{\rm tot}<1\times10^{14}\,M_{sun}$), extending to $\sim2\,R_{200m}$ (2.2 Mpc). Through spectral stacking of 827 nearby groups from the first eROSITA All-Sky Survey catalog, we detect diffuse emission and measure the thermodynamic properties of gas at densities previously inaccessible to X-ray observations. The stacked spectra are well described by a Gaussian differential emission measure model, yielding a temperature distribution with a mean temperature of $0.96_{-0.04}^{+0.05}$ keV and width of $0.28_{-0.10}^{+0.10}$ keV, and a metal abundance of $0.21_{-0.04}^{+0.06}$ A$_{sun}$, consistent with expectations for group outskirts. The inferred electron densities decrease from $(4.8\pm1.3)\times10^{-5}$cm$^{-3}$ at $(0.7-2)\,R_{500c}$ to $(5.5\pm2.0)\times10^{-6}$ cm$^{-3}$ at $(2-4)\,R_{500c}$, demonstrating eROSITA's ability to probe the low-density outskirts of galaxy groups. Residual emission in the spectra suggests the presence of an additional spectral component. While a secondary thermal interpretation requires an unexpectedly hot, metal-poor plasma, a non-thermal inverse Compton model provides an equally plausible explanation, contributing $\sim30\%$ of the thermal flux. Assuming that the additional component is produced by inverse Compton emission from a common population of relativistic electrons, the inferred magnetic field strength would be in the sub-$μ$G regime.

Nearby young moving groups (NYMGs) provide benchmarks for studying the evolution of magnetic activity and planetary environments at early ages. The Volans-Carina association (VCA), as a pre-main sequence association at a distance of ~90 pc, occupies a sparsely sampled region of parameter space between younger nearby associations and older distant open clusters. We have re-evaluated the membership probabilities of the VCA catalog presented in Gagne et al. (2018a) with BANYAN as well as updated kinematics from Gaia DR3. Stellar parameters were derived from multi-wavelength spectral energy distributions, with the obtained bolometric luminosity and effective temperature being used in combination with pre-main sequence evolutionary models to obtain an isochronal age of the association. We joined data from the SRG (Spectrum Roentgen Gamma)/eROSITA and ROSAT all-sky surveys with a dedicated XMM-Newton Large Programme to construct a comprehensive X-ray catalog. Our updated census of the VCA comprises 29 highly probable members, 30 candidate members, and four uncertain objects. Isochronal analysis of the association yields an age of 80+-20 Myr, consistent with previous estimates. We detect X-ray emission from 56 objects, reaching ~90% completeness, one of the most complete X-ray censuses achieved for a NYMG. Low-mass members and candidates (<0.6 Msolar) have X-ray to bolometric luminosity ratios around the canonical saturation limit (Lx/Lbol~10^-3). X-ray luminosity functions show the VCA to be consistent with the similarly aged Pleiades (in the F5 to M3 range) and significantly more active than the older Hyades (in the F5 to K4 range). The combination of an updated membership census and a nearly complete X-ray census establishes the VCA as a particularly robust calibration point for studies of the early evolution of coronal X-ray activity.

A comprehensive measurement of stellar X-ray emission has important implications for our understanding of stellar dynamos, exoplanet atmosphere loss, and evaporation of protoplanetary disks. We present a catalogue of X-ray detections of the FGK-type main-sequence stars within 10 pc, which accounts for stellar multiplicity and optical loading of the detectors. To build the X-ray catalogue, we analysed all observations from SRG (Spectrum Roentgen Gamma)/eROSITA and XMM-Newton for stars in the FGK 10pc sample, and we cross-matched the sample with ROSAT catalogues. We fit thermal plasma models to the X-ray spectra to derive X-ray fluxes and coronal temperatures. We investigate the relation between coronal temperature and X-ray surface flux and find the result to be consistent with the ranges covered by different types of solar coronal magnetic structures. The lower end of the temperature-brightness relation is defined by the Maunder minimum star HD 166620 and three other stars whose positions identify them as possible Maunder minimum candidates. We identify systematic differences between the temperature-brightness law derived from XMM-Newton and eROSITA data that we attribute to their different instrumental response functions. We study the impact of both short-term flaring and long-term activity cycles on the evolution of coronal temperature along with X-ray brightness. With the exception of three stars, a remarkably small scatter is observed in the temperature-brightness relation across the whole sample. A natural explanation of the empirical kT-FX relation is provided by the RTV scaling law defined for solar magnetic loops. The small spread of the observed relation indicates a universal scaling between the length and the filling factor of coronal loops. The three observed outliers deviate from this invariance of the ratio of loop length to filling factor for an as of yet unknown reason.

During their secular evolution, cataclysmic variable stars (CVs) evolve toward shorter orbital periods ($P_{\rm orb}$) until reaching a minimum near $P_{\rm orb}\sim80$ min, after which they evolve back toward longer periods. CVs that have evolved past this evolutionary turning point are known as period-bouncers (PBs). Despite predictions that 40-80% of all CVs should be PBs, only 3-25% of the observed CV population is composed of PBs, a discrepancy likely due to their intrinsic low luminosities. We aim to investigate the evolutionary status of 213 SRG/eROSITA-selected PB candidates. The sample also includes 19 previously confirmed PBs, which serve as benchmarks for evaluating the candidates. We confirmed 24 new CVs through the identification of Balmer emission lines in optical spectra from the Sloan Digital Sky Survey V (SDSS-V) and of dwarf-nova outbursts in archival photometric surveys. By fitting hydrogen-rich atmosphere models to the SDSS-V spectra, we estimated the effective temperature and secular mass accretion rate of the WDs. We also measured the Balmer decrements, used as diagnostics of the physical conditions of the accretion disc, to assess whether they are consistent with known PBs. In addition, we analysed archival light curves from the Transiting Exoplanet Survey Satellite (TESS) to determine $P_{\rm orb}$ for a subset of systems, and compiled multi-wavelength photometry to construct and model spectral energy distributions (SEDs), from which we inferred approximate donor spectral types. Our analysis of the new CVs indicates that they are consistent with being PBs, potentially increasing the population of confirmed PBs by $\sim 50\%$. Our results suggest that a substantial fraction of the PB population may remain hidden in WD catalogues.

We investigate the hydrodynamics of expanding bubbles in first-order phase transitions with non-conformal thermodynamics. We analyze a broad class of equations of state interpolating between bag-model descriptions, commonly used for electroweak transitions, and QCD-like theories. As a concrete benchmark, we determine the bubble solutions for pure SU(3) Yang-Mills theory. We uncover a new set of hydrodynamic obstructions to bubble expansion. These obstructions arise both at the bubble wall and along the fluid flow, and can partially or completely eliminate otherwise allowed solutions. We also identify a new class of solutions, which we dub "shocked detonations", consisting of ordinary detonations with an additional shock inserted in the rarefaction wave. As a consequence of these obstructions, the space of admissible bubble wall velocities is significantly constrained, with gaps appearing between different expansion regimes. For example, for QCD-like theories, all detonation solutions, including shocked detonations, are excluded. We show that these effects can strongly impact the kinetic energy budget of the fluid and, therefore, the resulting gravitational-wave signal, potentially suppressing the most efficient configurations. Our results highlight the importance of non-conformal dynamics for accurately modeling phase transitions and the resulting gravitational-wave spectrum. The code used to construct the bubble solutions is publicly available. For completeness, we also show how the interpolation between the bag-model and QCD-like limits can be realized holographically by varying the backreaction of matter fields on the geometry.

Solar $^8\mathrm{B}$ neutrinos offer a unique probe of solar interior dynamics and neutrino electromagnetic properties. We present a systematic, multi-method periodogram analysis of the 22-year Super-Kamiokande solar neutrino dataset (1996--2018), comparing nine algorithms. Through hierarchical temporal segmentation, we disentangle astrophysical signals from detector systematics. The Generalized Lomb-Scargle (GLS) method provides the most statistically robust detections by correctly handling heteroscedastic uncertainties, whereas classical Lomb-Scargle systematically underestimates significance. The Lafler--Kinman method generally fails, whereas independent algorithms like MHAOV and PDM1 recover consistent periodicities, providing vital cross-validation. In pre-2001 and SK-I data, seven algorithms provide \textit{weak evidence} ($\ln B > 0$) for a $\sim 38.8$ d periodicity. However, this signal is entirely absent in the highest-statistics SK-IV modified flux data, where the Bayes factor decisively favors the null model ($\ln B \ll -5$), indicating it is a transient feature of the early low-statistics era. Conversely, a $\sim 24.3$ d signal in post-2001 raw flux is decisively rejected by the Bayesian framework and vanishes in modified flux, confirming its seasonal systematic origin. Furthermore, no evidence is found for an $\sim 11$-year solar cycle modulation, yielding a stringent amplitude upper limit of $<0.2\%$ of the mean flux. By highlighting the stark contrast between frequentist significance and Bayesian model selection ($\ln B$) in low signal-to-noise regimes, we establish a rigorous, multi-metric best-practice framework for periodicity searches. This work provides a direct methodological blueprint for next-generation observatories like Hyper-Kamiokande and JUNO.

We show that gyrokinetics in a spatially uniform equilibrium (or `astrophysical gyrokinetics') possesses a previously unknown quadratic invariant, which we call the gyrokinetic helicity. We derive the limiting forms of the gyrokinetic helicity in five subsidiary expansions of astrophysical gyrokinetics: the isothermal electron fluid (ITEF) approximation, kinetic reduced magnetohydrodynamics (KRMHD), electron reduced magnetohydrodynamics (ERMHD), finite-Larmor-radius magnetohydrodynamics (FLR-MHD), and the kinetic reduced electron heating model~(KREHM). We subdivide the real part of the gyrokinetic helicity into `magnetofluid' and `phase-space' components, which are separately conserved in KRMHD, ERMHD, and FLR-MHD (in which there is no phase-space helicity). The magnetofluid helicity is equivalent to the Alfvénic part of the cross helicity in KRMHD, the magnetic helicity in~ERMHD, and the generalized helicity in FLR-MHD and~KREHM. We derive an analytic expression for the rate at which magnetofluid helicity is converted into phase-space helicity at length scales comparable to the proton gyroradius in the ITEF approximation. We also explain how this conversion circumvents the helicity barrier and enhances turbulent heating in coronal holes and the near-Sun solar wind.

Numerical relativity (NR) simulations of compact binaries in theories beyond general relativity (GR) will be pivotal for the continued development of future tests of gravity with gravitational waves (GWs). In this Letter, we show that the combination of spectral methods and the "fixing-the-equations" approach allows us to produce the longest waveforms in the literature for a genuine beyond-GR theory, thus bringing NR methods for alternative theories of gravity closer to the state-of-the-art in GR. For concreteness, we focus on the well-known shift-symmetric version of scalar Gauss-Bonnet gravity, a theory postulating the existence of an additional dynamical scalar and describing black holes (BHs) different from the Kerr solution. We extract the gravitational and scalar waveforms at future null infinity for equal-mass, nonspinning, eccentricity-reduced BH binaries, and quantify the phase errors to be $\lesssim$ 1 rad after 40+ GW cycles (20+ orbits). We also show that the GW phase corrections in this alternative theory are distinguishable from Einstein's theory and lead to an earlier coalescence time than in GR. Obtaining such waveforms is a stepping stone to perform precise comparisons with Post-Newtonian theory and to calibrate waveform models beyond GR.

We implement the "fixing-the-equations" approach [Phys.Rev.D 96 (2017) 8, 084043] in spectre, an NR code using a pseudo-spectral discontinuous Galerkin scheme, to produce long and accurate NR waveforms in the well-known shift-symmetric version of scalar Gauss-Bonnet (sGB) gravity. To achieve this, we introduce a new family of comoving driver equations that exploits the approximate symmetries of quasicircular binary systems and is designed to recover the exact (quasi-)stationary solutions of the fully-coupled theory. We validate our single black hole (BH) solutions against analytic predictions and show that, even for binary BHs in the early inspiral, the intrinsic BH quantities are relatively insensitive to the timescales entering the driver equation. Attention is given to the prescription of driver equations for tensors, for which we give an example of how treating tensor components as scalars can lead to undesired behaviour over long timescales, including spurious growth of the BH spins. A more appropriate generalization to the tensor case is given for the comoving driver, which is shown to avoid these issues. Overall, our implementation leverages state-of-the-art methods for eccentricity reduction and wave extraction with Cauchy Characteristic Evolution to simulate systems with eccentricity $\lesssim 10^{-3}$. We obtain waveforms with phase errors $\lesssim \mathcal{O}(1) \, \mathrm{rad}$ over almost 40 GW-cycles, which naturally incorporate memory contributions.

The evolutionary pathways and ultimate fates of very massive stars are governed primarily by mass loss through radiatively-driven winds. We present a new theoretical mass-loss prescription for (very) massive stars, capturing the complex dependence on the Eddington parameter $Γ_e$, luminosity, temperature, and hydrogen abundance. We calculated an extensive grid of 178 hydrodynamically consistent wind-atmosphere models in non-local thermodynamic equilibrium using the PoWR-HD code, predicting wind properties such as the mass-loss rate and terminal velocity self-consistently. The grid spans masses $M_*$ = 16-500 Msun, luminosities $\log(L_*/L_\odot) = 5.5-6.8$, inner boundary temperatures $T_* = 12-50$ kK, and hydrogen mass fractions X = 0.01-0.9, at a fixed metallicity of Z=0.02. We confirm the presence of a mass-loss kink in the $\dot{M}-Γ_e$ relation across the explored parameter space. The kink marks the transition from a shallow scaling ($\sim 2.8$) at low $Γ_\mathrm{e}$ for optically thin O-star winds to a steeper scaling ($\sim 10$) for optically thick winds at high $Γ_e$. We derive comprehensive fitting relations capturing both the kink behaviour and two bistability jumps arising from iron ionisation changes, and provide auxiliary relations for implementation into stellar evolutionary calculations. Our prescription correctly reproduces the model-independent transition mass-loss rate in the Arches Cluster, confirming the accuracy of our predicted rates at the O-to-WNh transition. Application of our recipe to the Zero Age Main Sequence provides excellent agreement with recent empirical $\dot{M}-Γ_e$ relation obtained for a wide range of temperatures and Eddington parameters. We provide a physically motivated, continuous, and empirically anchored mass-loss recipe for (very) massive stars, suitable for stellar evolution calculations in the 20-500 Msun range.

We study stationary nonlinear clouds of a complex scalar field on fixed Kerr and Schwarzschild backgrounds, with an unbounded scalar potential. On Kerr backgrounds we find finite-amplitude Q-clouds satisfying the synchronisation condition $ω=mΩ_H$. They occupy two-dimensional regions of the ($Ω_H,M_K$) plane and reduce to the corresponding linear clouds as the scalar field amplitude tends to zero. The quartic domain can continue to $Ω_H=0$, where both the synchronised frequency and the Noether charge vanish and the solutions become static nonlinear scalar clouds on a fixed Schwarzschild background. We also compute the existence domain for a normalised, axion-inspired cosine potential.

We investigate the height variation of the line-of-sight (LOS) magnetic field in solar active-region (AR) plage from the upper photosphere to the upper chromosphere using co-spatial ultraviolet (UV) and infrared (IR) spectropolarimetric observations from the Sunrise III stratospheric balloon flight. The Ca II K and Ca II 8542 Å lines provide complementary chromospheric diagnostics, while nearby Fe I lines sample photospheric layers. The LOS magnetic field is inferred from the intensity and circular polarization profiles, applying the weak field approximation for the Ca II lines and the center-of-gravity method for the Fe I lines. The photospheric Fe I lines reveal strong, finely structured magnetic fields of the order of a kG. In contrast, the chromospheric Ca II diagnostics yield systematically weaker fields, typically in the $\sim$ 100-400 G range, with a more diffuse spatial distribution. Magnetic field maps inferred from the Ca II K line, formed higher in the chromosphere, are smoother and more extended than those derived from the Ca II 8542 Å line, which samples lower heights. We find that the magnetized area increases by a factor of $\sim$ 2 from the photosphere to the chromosphere. These results provide direct quantitative evidence that magnetic fields in AR plage weaken and expand with height, evolving from compact kG photospheric concentrations into weaker and more spatially extended structures in the upper chromosphere.

We study the observational signatures of kinetic gravity braiding (KGB) models in relativistic cosmological probes constructed along the past light cone. Using the relativistic $N$-body code KGB-evolution, we generate light-cone outputs and compute several observables that directly probe the gravitational field, including weak gravitational lensing convergence, Shapiro time delay, the integrated Sachs-Wolfe and Rees-Sciama (ISW-RS) effects, and gravitational redshift. Full-sky maps and angular power spectra of these quantities are constructed and compared with $k$-essence models and predictions from linear perturbation theory. We find that the derivative coupling between the scalar field and the metric modifies both the amplitude and the time evolution of the gravitational potentials, producing scale-dependent deviations ranging from a few percent to tens of percent. In particular, the ISW-RS signal exhibits the largest fractional response, as the slower decay of the Weyl potential suppresses the KGB signal in the ISW-dominated regime, whereas nonlinear evolution reverses this trend at higher multipoles, producing differences of tens of percent relative to $k$-essence. Weak gravitational lensing also provides a strong complementary probe and, for the model considered here, exhibits clear deviations from the $k$-essence prediction at small scales with enhancements up to $\sim 10$-$12\%$ at multipoles $\ell \sim 10^2$-$10^3$. Our results show that linear perturbation theory accurately describes the large-scale behaviour, while nonlinear effects become important at smaller scales, particularly for the ISW-RS signal and, more moderately, for the convergence, and must therefore be included for reliable theoretical predictions.

We study properties of domain walls (DWs) arising in the model with a double well potential assuming different early universe cosmologies: from dust through stiff matter domination to the limit of effective Minkowski space. Using lattice simulations we demonstrate that evolution of DW networks exhibits an approximate universality with respect to the cosmological equation of state (EoS). Namely, the wall area inside a given large volume is mainly determined by the particle horizon, while details of cosmic expansion play a subdominant role. As it follows, particle horizon rather than the inverse Hubble rate is pivotal in DW evolution defining the network correlation length and hence its phenomenology, e.g., the characteristic wavelength of emitted gravitational waves (GWs). Formation of closed DWs is shown to be very sensitive to the cosmological EoS and hence violate the universality, but their contribution to the total network is too small to change the overall picture. The universality breaking is also observed in the spectral properties of GWs from annihilating (biased) DWs: i) the IR slope of the spectrum depends on the EoS parameter; ii) there is a plateau in the UV part of the spectrum, which gets more pronounced as one stiffens the EoS. However, the dependence on the EoS is rather weak in the near peak region, which is most relevant from the viewpoint of pulsar timing arrays and other searches for the stochastic GW background. For a selection of primordial cosmologies, we provide fitting formulae for the spectra near the peak.

These notes are an introduction to non-linear perturbation theory for cosmological large-scale structure. They are aimed at undergraduate and beginning graduate students and do not require any cosmology or quantum field theory background. All necessary concepts are developed from scratch. The lectures are intended to explain all key ingredients needed to model the observed clustering of galaxies in real and redshift spaces. After a brief pedagogical introduction to the ideas of effective field theory (EFT), we develop large-scale structure EFT in the context of Newtonian cosmology using symmetry principles. We discuss in detail the shortcomings of Standard Perturbation Theory, the non-linear evolution of baryon acoustic oscillations and its relation to the equivalence principle, counterterms and renormalization of the loop diagrams, and stochastic effects. Then we develop EFT for galaxy bias and redshift space distortions. We also highlight some important facts about redshift-space stochasticity, relevant for ongoing and future galaxy surveys. Finally, we introduce Lagrangian Perturbation Theory.

We derive the Boltzmann equation for electroweak baryogenesis within a field-theoretical framework. We first algebraically solve the Kadanoff--Baym equations by incorporating the effects of gradients of the CP-violating bubble-wall background order by order. We then apply approximations and assumptions appropriate for electroweak baryogenesis and identify the on-shell parts of the solutions, which describe quasiparticle states propagating in a plasma. Using these on-shell solutions, we derive the Boltzmann equations for flavor-diagonal quasiparticle modes, while neglecting the off-diagonal components describing quantum coherence between different flavor modes. Our derivation generalizes previous field-theoretical treatments by retaining self-energy corrections, and our results are consistent with the known Boltzmann equations when these corrections are neglected. This framework provides a systematic and transparent derivation applicable to multi-flavor bosonic and fermionic systems.

While the $Λ$ cold dark matter ($Λ$CDM) model can successfully reproduce the measurements of many cosmological probes, some discrepancies have recently emerged. Therefore, it is necessary to test the standard cosmological model for consistency. An important stress test is to separate the influence of different cosmological regimes on the parameter inference. We treat three regimes separately here: geometry, growth, and the early universe. The geometrical regime concerns the expansion and curvature history, while the growth regime governs structure formation and the early-universe regime affects physics prior to recombination. Previous analyses have performed the split between geometry and growth, whereas we also consider the early universe influence separately. We perform this split for the present day matter density parameter $Ω_{\rm m}$ using multiple cosmological observables. The used data are galaxy clustering and weak lensing statistics (3x2pt) from the Dark Energy Survey (DES), cosmic microwave background (CMB) data from Planck, spectroscopic baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI), type-Ia supernovae (SNe Ia) samples from Pantheon+, and redshift-space distortions (RSD) from a collection of galaxy surveys. For each of these probes, we introduce a phenomenological split into these three regimes. This work shows a strong correlation between the geometric and the early regime for the matter density, but no strong correlation between the growth regime and the others. All regimes are compatible in the posterior distribution, however the difference between the geometry and the early regimes, $ΔΩ_{\rm m}^{\rm geo,early}$, is 2$σ$ apart from 0.

Context. Galaxy clusters sometimes host synchrotron radiation on scales of ~100 kpc to ~1 Mpc, with surface brightness only a few times the image noise. This diffuse cluster radio emission is a sensitive probe of magnetic fields and intracluster medium dynamics, but disentangling the underlying physical processes requires statistically large samples spanning a wide range of cluster masses, dynamical states, and redshifts, together with sufficient sensitivity to low-surface-brightness emission. Aims. We explore two techniques for improving detection of diffuse emission in galaxy cluster images, relative to a baseline classifier: the scattering transform (ST) and squeeze-excitation (SE) attention. Methods. We integrate an ST encoder into a dual-branch classifier (DualSSN) and a scattering network (ScatterNet). We incorporate SE attention into the DualSSN and dual-branch convolutional neural network (DualCSN). These classifiers are then benchmarked against a simple CNN, across ten image preprocessing configurations and three cropping strategies. Performance is evaluated on small labelled datasets from the second data release of the LOFAR two-metre sky survey overlapping with the second Planck catalogue of Sunyaev-Zel'dovich sources (LoTSS-DR2/PSZ2). Results. Alongside the multi-branch approach with SE and ST, cropping the image to a fixed number of telescope beams and uv- tapering (smoothing to a coarser angular resolution) improve classification performance, while stacking multiple preprocessed ver- sions of an image does not. Conclusions. Scattering-transform-based multi-branch architectures with beam-normalised cropping are a promising direction for diffuse emission classification in the SKA era.

We examine the relationship between the Null Convergence Condition (NCC) and the Borde-Guth-Vilenkin (BGV) Theorem. We first show that, for an expanding spacetime foliated orthogonally to a timelike geodesic congruence with vanishing shear and vorticity, the BGV Theorem follows when the NCC holds and the spatial curvature is non-positive, ${}^3\mathcal{R} \leq 0$. The situation becomes more complex in the presence of shear, or non-geodesic threading of the spacetime. In these cases, the local expansion that enters the BGV construction depends not only on the local scalar expansion, but acquires terms given by the contraction of the shear and acceleration with locally defined spatial unit vectors. In the general case, null convergence and non-positive curvature are no longer sufficient to guarantee that the BGV Theorem holds. We discuss the result in the context of eternal inflation in the presence of comoving curvature perturbations and state a more general version of the BGV condition relevant to asymptotically past-de Sitter eternal inflation.

We study the impact of spatial curvature ($Ω_k$) and dynamical dark energy (parametrized by $w_0$ and $w_a$) on the spectral index $n_s$ using a combination of cosmic microwave background datasets (Planck, SPT, and ACT), and spectroscopic galaxy samples from DESI, including both BAO and full-shape clustering measurements. We show that a small negative curvature, $Ω_k\simeq 3\times 10^{-3}$, lowers the value of $n_s$, bringing it closer to predictions of the Starobinsky, Higgs, and simplest $α$-attractor inflationary models. In particular, we find $n_s= 0.9667\pm0.0041$ (using Planck and DESI data) or $n_s= 0.9692\pm0.0035$ (adding ACT and SPT). Allowing for time-evolving dark energy also reduces the spectral index, leading to $n_s=0.9716\pm0.0032$ (from the combined dataset), or $n_s=0.9694\pm0.0035$ in combination with a small negative curvature. Our results demonstrate that the tension between current observational data and the Starobinsky, Higgs, and simplest $α$-attractor models holds only for $Λ$CDM, and can be mitigated in extended cosmological models. We discuss implications of these findings for inflationary models in an open universe and/or with dynamical dark energy, including scenarios with quantum tunneling and non-standard topology. Furthermore, we briefly describe a special class of $α$-attractor models, where one can make $n_s$ arbitrarily large, and we describe the $α$-attractor quintessence model. Such models may be of particular relevance when future data from DESI, as well as DESI-II, SPHEREx, Euclid, Rubin, and Roman, becomes available.

Recent small-scale CMB data show a preference for larger scalar spectral index, $n_s$, when combined with DESI data. Monomial $α$-attractor T-models can be reconciled with the new observations if the power of the hyperbolic tangent function is $p\geq 6$, where $p$ is even. The supergravity construction of monomial T-models with $p>2$ relies on the assumption that lower powers remain negligible over the whole field range explored during inflation and reheating. What would be the consequences of going beyond the monomial formulation? As a first step in this direction, we consider the case of a binomial potential, given by the sum of quadratic and quartic terms. When the quartic coefficient, $c$, is $0< c+1/2 \ll 1$, we find that the new model displays non-universal behavior for $n_s$, leading to values as large as $0.965$ and with an $α$-dependence that is qualitatively different from that of monomial potentials. By solving the background dynamics during the first few e-folds of perturbative reheating we show that the quartic term might dominate before the quadratic one eventually takes over as the inflaton oscillations decrease in amplitude. This leads to a time-dependent equation of state, with $\bar w\sim 1/3$ initially before ultimately $\bar w\to 0$. Obtaining a quartic-dominated reheating stage lasting $\sim4$ e-folds requires a substantial hierarchy between the quartic and quadratic terms, $c\sim 10^5$ for $α\gtrsim 0.1$. Our study highlights that models beyond the monomial form can lead to non-trivial deviations of $n_s$ from the predictions of monomial potentials. Furthermore our results for $\bar w$ during reheating call into question the use of monomial models with large $p$; assuming that $\bar w$ is uniquely determined by the $p$-th power relies on substantial fine-tuning of the underlying supergravity potential.

Small-scale cosmic structure provides a powerful test of the fundamental nature of dark matter (DM). A wide range of DM models impact matter clustering on small scales, including warm, fuzzy, and (self-)interacting DM. In these scenarios, DM physics such as free-streaming, wave interference, and self/Standard Model interactions alter the abundance and internal structure of DM halos. Cosmological and astrophysical probes of nonlinear structure---including dwarf galaxies, strong lensing, the Lyman-$α$ forest, stellar streams, and high-redshift galaxies---are therefore sensitive to these effects. Here, we review DM constraints provided by small-scale structure, focusing on observables that probe scales smaller than $\sim 1~\mathrm{Mpc}$, which define the frontier of current measurements. We summarize how these constraints have been translated to limits on microphysical DM models, and we discuss key modeling uncertainties and observational systematics. Finally, we highlight the growing importance of probe combination and simulation-based inference for this field, and we overview upcoming observational facilities that will sharpen small-scale structure tests of DM physics.

We study reconstruction of stochastic gravitational-wave backgrounds from metastable cosmic strings with the Laser Interferometer Space Antenna (LISA). In the vacuum-tunneling benchmark, the network decays via zero-temperature nucleation of monopole pairs on the string worldsheet. Initially following cosmic-string scaling, loop production is suppressed after a decay time linked to efficient loop breaking and network collapse. This finite lifetime imprints an infrared tail and a transition toward a stable-string-like high-frequency plateau. Using synthetic LISA data with instrumental noise and unresolved astrophysical foregrounds, we perform Bayesian analysis in the $(Gμ,κ_{\rm CS})$ parameter space. We map detectability, uncertainties, correlations, and localization to distinguish background detection from parameter reconstruction. Reconstruction is governed by lifetime-dependent spectral features in the LISA band. When LISA samples the transition between tail and plateau, data contain amplitude and shape information, enabling recovery of string tension and metastability scale. Posteriors may remain correlated, reflecting an amplitude--lifetime trade-off, yet occupy a small parameter region. By contrast, featureless spectra constrain only limited parameter combinations or become prior dominated. High-SNR plateau-like spectra can retain partial sensitivity to $κ_{\rm CS}$ through residual lifetime dependence even when the transition is not prominent. Finally, we assess sensitivity to Galactic foreground modeling by comparing a flexible template with a reduced tanh template. Our results show where LISA can move beyond detection to probe the lifetime of the underlying string network.

An axial coupling between an inflaton and an Abelian gauge field can trigger the tachyonic amplification of one gauge-field helicity. For a massless vector, modes with physical momentum $k/a\sim |ξ|H$ are enhanced by approximately $\exp(π|ξ|)$, and sufficiently efficient production can provide substantial friction for the homogeneous inflaton. We extend this mechanism to a vector of mass $m$. The instability is present only for $|ξ|>\bar m\equiv m/H$, and in the heavy regime the mode amplitude scales as $\exp[π(|ξ|-\bar m)]$. Because the amplified modes remain well inside the Hubble radius when $\bar m\gg1$, their contribution to long-wavelength curvature perturbations is power-law suppressed at fixed background backreaction. In the weak-backreaction regime we obtain ${\cal P}^{\rm id}_ζ \propto \bar m^{-2}$, while including the gauge-induced friction of scalar perturbations gives the scaling ${\cal P}^{\rm id}_ζ\propto \bar{m}^{-3}$. These estimates indicate that ${\cal P}^{\rm id}_ζ\lesssim 10^{-9}$ on CMB scales should be compatible with gauge field backreaction for $\bar{m}$ larger than order a few hundred. We test the analytical mode functions and backreaction estimates with the first lattice simulations based on a massive-vector extension of the \texttt{Pencil Code}, including simulations in the strongly backreacting regime.

We consider fourth-order and second-order partial differential operators localized on domains of the sphere in arbitrary dimension. These operators arise as weighted compositions of the linearized Einstein constraint operators and their adjoints, and played a key role in our resolution of the optimal localization problem in general relativity, also referred to as the gravitational shielding problem. To control the asymptotic behavior of solutions to Einstein's constraints in our companion paper (preprint arXiv:2312.17706), we introduced the notions of harmonic, radial, and shell stability. Harmonic stability controls the borderline harmonic modes, radial stability governs the radial evolution of spherical averages, and shell stability controls the coupled radial-angular evolution of solutions. In the present paper, we establish that these stability properties follow from weighted Poincaré, Korn, and Hardy inequalities. Furthermore, in arbitrary dimension, we investigate the behavior of the associated geometric constants, and conclude that the stability conditions hold for a broad class of localization functions; the theory applies to arbitrarily small localization domains, corresponding to gluing cones with arbitrarily small aperture. At the opposite extreme, our conditions also hold on the entire sphere, corresponding to the absence of localization. This completes, for gluing cones of arbitrarily small aperture in every dimension, the program initiated by A. Carlotto and R. Schoen on gravitational shielding and the construction of solutions enjoying super-harmonic decay estimates. In our proofs, we introduce Hamiltonian and momentum functionals, which we call shell functionals, and show that they enjoy monotonicity and semi-coercivity properties; their structure also suggests possible analogies with functionals arising in other curvature-related geometric problems.

It is widely repeated in the popular literature and elsewhere that the singularity at the center of a black hole is a point. It is not true. Two observers who free-fall into a spherical black hole along two different angular trajectories at the same time $t$ do not encounter each other at the central singularity; rather, they lose causal contact with each other already well away from the singularity. Counterintuitively, in general relativity two points can be spatially close yet causally distant. The singularity is a surface, not a point. The story for rotating black holes is more complicated, but the same conclusion holds. For a rotating black hole, the singular surface almost certainly resides at its inner horizon, where even the tiniest classical or quantum perturbations ignite the exponential mass inflation instability, precipitating collapse to a spacelike singular surface. There are implications for quantum gravity. We argue that, whatever the ultimate theory of quantum gravity may be, the quantum states of a black hole probably reside at its effectively 2-dimensional singular surface, which coevolves unitarily with, and in thermodynamic equilibrium with, the hot atmosphere of trapped Hawking radiation that the black hole generates within its event horizon.

We prove a sharp volume-stability theorem for closed hyperbolic three-manifolds. Let \((M,h)\) be closed hyperbolic with \(\operatorname{Ric}_h=-2h\), and let \(g_i\) be smooth metrics on \(M\) satisfying $R(g_i)\geq -6, \operatorname{Vol}_{g_i}(M)\longrightarrow \operatorname{Vol}_h(M)$. After passing to a subsequence, there exist \(Z_i\subset M\), smooth domains \(K_i\subset M\), and diffeomorphisms $ψ_i:K_i\longrightarrow M\setminus Z_i $ such that $\operatorname{Vol}_{g_i}(Z_i)\longrightarrow0, \operatorname{Vol}_h(M\setminus K_i)\longrightarrow0, $ and $ \|ψ_i^*g_i-h\|_{C^0(K_i,h)}\longrightarrow0. $ Thus near-equality in the sharp hyperbolic volume bound forces tensorial \(C^0\)-convergence to the hyperbolic metric outside regions of vanishing volume. As an application, we establish stability of the Fischer--Moncrief reduced Hamiltonian at the Lorentz-cone ground state: after CMC normalization, near-minimizing compact vacuum data in the hyperbolic topological class converge, modulo sets of vanishing volume, to the hyperbolic Lorentz-cone geometry in tensorial \(C^0\). This provides a rigorous volume-dominance formulation of the Fischer--Moncrief asymptotic picture.

We investigate the non-relativistic limit of the generalized Dirac equation in a weak, static, and spherically symmetric background of symmetric teleparallel gravity. The underlying generalized spinor connection incorporates the complete Clifford-algebra basis and introduces additional couplings to the non-metricity sector beyond those of the conventional Dirac theory. Working in the coincident gauge and adopting the weak-field Schwarzschild geometry in isotropic coordinates, we derive the corresponding generalized Dirac Hamiltonian and perform successive Foldy--Wouthuysen transformations up to order $1/m^2$, retaining terms to first order in the gravitational potential and its spatial derivatives. The resulting block-diagonal Hamiltonian contains not only the expected gravitational counterparts of the kinetic, spin--orbit, and Darwin interactions, but also additional operator structures generated by the generalized spinor connection. In particular, direct spin--gravity, anisotropic spin--momentum--gravity, and tidal spin--momentum couplings arise naturally from the generalized metric-affine interaction. We further perform an order-of-magnitude analysis for an electron in the Earth's weak gravitational field to justify the adopted truncation of the inverse-mass expansion. These results demonstrate that the generalized Dirac equation in a symmetric teleparallel background gives rise to new low-energy interaction channels involving the fermion spin, momentum, and spatial derivatives of the gravitational field. The resulting effective Hamiltonian provides a framework for exploring phenomenological constraints on the additional couplings entering the generalized spinor connection.

Red-giant asteroseismology yields precise stellar parameters, making it a powerful tool for studying stellar structure and evolution, as demonstrated by the Kepler mission. However, due to Kepler's limited field of view, it primarily sampled the more populous low-mass red giants found outside of the Galactic plane, leading to limited detections of red giants above $\rm 3\ M_{\odot}$. Here we use the all-sky TESS data to isolate 227 intermediate-mass candidates from large catalogs with a pre-selection based on photometric and spectroscopic data. We optimize TESS light curves using a boutique light curve detrending method with custom apertures. Compared to the MIT Quick Look Pipeline, this yields a 12% average increase in the power-to-background ratio within the oscillation envelope, even in the heavily crowded Galactic plane. We detect solar-like oscillations in 98 targets, including 43 with $\rm M_* > 3\ M_{\odot}$. Our sample also includes 10 stars having masses greater than $5\ \rm{M}_{\odot}$, among the highest-mass solar-like oscillators detected to date. From our detections, we find that the APOGEE DR19 spectroscopic $\log g$ is systematically larger by, on average, 0.23 dex compared to the seismic $\log g$. This offset is possibly due to the lack of intermediate-mass giants observed by Kepler, which was used to calibrate the spectroscopic $\log g$ in the APOGEE pipeline. Extending the same pre-selection criteria to TESS targets with Gaia XP spectroscopic parameters identifies up to 37,000 candidate intermediate-mass solar-like oscillators for follow-up and population studies.

We establish the equivalence between homogeneous and isotropic linearized gravity with minimally coupled degrees of freedom and the Caldeira--Leggett model of open systems. This connects gravitation to an established system-plus-reservoir formalism, which we use throughout. Eliminating the reservoir reduces the equations of linearized gravity to a generalized Langevin equation for the metric perturbation, with the environmental influence condensed in the spectral density. The equivalence also yields a natural setting for reduced quantization of the perturbation and stochastic gravitational dynamics. We analyze the resulting dynamics with Laplace methods, extending results presented in other open-system settings.

We introduce here a general model, under agnostic and minimum assumptions, to uniquely find the entropy area law with a corresponding fixed logarithmic correction term. In this approach, the horizon is discretized into generic Planck-scale cells representing coarse-grained indistinguishable geometric structures, and it is just the statistical combinatorial counting that determines the form of the entropy terms. We highlight the role of the assumptions and their comparison with known models providing fixed logarithmic contributions to the entropy.

We revisit the proof of a $\mathfrak{bms}_3-$integrable hierarchy introduced in Fuentealba et al. (JHEP, 2018), using different structural methodologies. Specifically, from the variational complex on the ring of polynomial symbols, a $\mathfrak{bms}_3$ bi-Hamiltonian structure is constructed on which a Nijenhuis operator can be attached. A similar argument is made for the $\mathrm{AdS}_3$ case, where we check that the flat limit of the latter recovers the asymptotically flat situation. An alternative $τ-$scheme description is also presented. Moreover, a Lie-Poisson description suggests that such a $\mathfrak{bms}_3-$hierarchy is not unique. For a subclass of so-called energy-dependent Schrödinger operators, it is shown that their Lax flows are described by the coadjoint orbits of $\mathfrak{bms}_3$.

Generative machine learning models have recently emerged as powerful tools for producing cosmological simulations. However, many existing emulators do not explicitly enforce the underlying physical dynamics governing cosmological evolution, often leading to artifacts and poor adherence to the evolution equations. In this work, we present a physics-informed generative U-Net framework for fuzzy dark matter (FDM) that addresses two complementary tasks: (i) the evolution of cosmological fields from initial conditions to an arbitrary cosmological scale factor and (ii) the super-resolution of FDM simulations at a specified cosmological scale factor. Our model incorporates a physics-informed loss function that explicitly enforces consistency with the underlying Schrödinger-Poisson (SP) dynamics during training. For the evolution task, we find that the inclusion of this physics-based loss significantly improves the quality of the predicted simulations, even when only a small amount of training data is available. Using only 20% of the training data, the model accurately reproduces the target simulations in a 1 $h^{-1}$ Mpc box. Furthermore, the framework generalizes effectively across previously unseen realizations of the initial conditions. For the super-resolution task, we present, for the first time, a generative super-resolution model trained on FDM simulations obtained by solving the full SP equations, considering both single and multiple realizations of the initial conditions and analyzing the role of the physics-informed loss in each case. Our approach enables modern generative modeling of cosmological simulations while maintaining physical consistency and substantially reducing generative artifacts.