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Pulsar timing arrays (PTAs) observations will detect gravitational waves (GWs) from the early inspiral phase of supermassive black hole binaries (SMBHBs) with orbital periods of weeks to years. Current PTA analyses generally assume circular binaries; however, dynamical interactions with the surrounding environment can prevent complete circularization, allowing SMBHBs to retain appreciable eccentricities. In this work, we present a gravitational waveform model for eccentric binaries based on the effective-one-body (EOB) formalism, designed for continuous GW searches in PTA data. The model is accurate up to the second post-Newtonian (2PN) order for the conservative dynamics and up to postleading order for the radiation-reaction terms. We provide both a numerically precise and a computationally efficient approximate implementation and evaluate the latter's accuracy against the full model over a broad range of eccentricities and initial orbital frequencies. Our results show that a substantial region of the parameter space exhibits pronounced orbital evolution, much stronger than in the circular case. We demonstrate the rich harmonic structure of timing residuals induced by eccentric GWs. Properly characterizing eccentric binaries is an essential step toward detecting GWs in PTA data and interpreting the results, ultimately improving our understanding of the supermassive black hole population in the local Universe.

We determine the metric of a Kerr black hole subject to external tidal fields using metric reconstruction techniques. Working within the Newman─Penrose formalism, we solve the Teukolsky master equation for static, quadrupolar modes associated with a slowly varying tidal environment, and reconstruct the corresponding metric perturbation in the outgoing radiation gauge. As an application, we derive the secular Hamiltonian governing the motion of a test particle in the tidally deformed Kerr spacetime and investigate long-term tidal effects relevant to extreme-mass-ratio inspirals. In particular, we compute tidal-induced shifts of the innermost stable circular orbit and the light ring. We find that these tidal corrections are strongly spin dependent, with significantly larger effects for retrograde orbits around rapidly rotating black holes. Our results provide a fully analytic framework for studying tidal interactions and secular dynamics in rotating black-hole spacetimes, with direct applications to gravitational-wave modeling and tests of gravity in the strong-field regime.

We explore the possibility that the high energy neutrino flux observed by terrestrial telescopes originates from dark matter (DM) annihilation. Specifically, we study a minimal, UV-complete U(1) extension of the Standard Model with a Dirac DM candidate, whose annihilation into neutrinos proceeds exclusively through a Z′ boson. By computing the annihilation cross section and comparing with the observed flux, we derive bounds on the model parameters. Additional constraints are obtained within the freeze-in framework, where the observed relic abundance is reproduced. Considering cosmic string vibrations as a source of gravitational waves, we further constrain the vacuum expectation value of the U(1) breaking. All results are contrasted with perturbativity limits and existing constraints from low- and high-energy experiments.

In this paper, we explain the recently reported a nHz-band gravitational-wave background from NANOGrav 15-year through the merger of binary super-massive black holes with masses of <inline-formula><mml:math><mml:msup><mml:mn>10</mml:mn><mml:mn>9</mml:mn></mml:msup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula> formed by the growth of primordial black holes. When a primordial black hole accretes at a high accretion rate, it emits a large number of high-energy photons. These heat the plasma, causing high-redshift cosmological 21 cm line emission. Since this has not been detected, there is a strict upper bound on the accretion rate. We have found that with the primordial black hole abundance <inline-formula><mml:math><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>14</mml:mn></mml:mrow></mml:msup><mml:mo>≲</mml:mo><mml:msub><mml:mi>f</mml:mi><mml:mrow><mml:mi>PBH</mml:mi></mml:mrow></mml:msub><mml:mo>≲</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> and the mass <inline-formula><mml:math><mml:mrow><mml:mn>1</mml:mn><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub><mml:mo>≲</mml:mo><mml:msub><mml:mrow><mml:mi>m</mml:mi></mml:mrow><mml:mrow><mml:mi>PBH</mml:mi></mml:mrow></mml:msub><mml:mo>≲</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula>, we successfully fit the nHz band gravitational wave background from NANOGrav 15-year while avoiding the 21 cm line emission. In addition, we also discuss the implication for the origins of the little red dots. We propose that future observations of the gravitational wave background and the cosmological 21 cm line can test this scenario.

The IceCube Neutrino Observatory has observed a diffuse flux of high-energy astrophysical neutrinos for more than a decade. A relevant background to the astrophysical flux is prompt atmospheric neutrinos, originating from the decay of charmed mesons produced in cosmic-ray-induced air showers. The production rate of charmed mesons in the very forward phase space of hadronic interactions, and consequently, the prompt neutrino flux, remains uncertain and has not yet been observed by neutrino detectors. An accurate measurement of this flux would enhance our understanding of fundamental particle physics such as hadronic interactions in high-energy cosmic-ray-induced air showers and the nucleon structure. Furthermore, an experimental characterization of this background flux will improve the precision of astrophysical neutrino flux spectral measurements. In this work, we perform a combined fit of cascade-like and track-like neutrino events in IceCube to constrain the prompt atmospheric neutrino flux. Given that the prompt flux is a sub-dominant contribution, treating systematic uncertainties arising from the potential mis-modeling of the conventional and astrophysical neutrino fluxes is critical for its measurement. Our analysis yields a non-zero best-fit result, which is, however, consistent with the null hypothesis of no prompt flux within one standard deviation. Consequently, we establish an upper bound on the flux at <inline-formula><mml:math><mml:mrow><mml:mn>4</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>16</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula> (GeV m<inline-formula><mml:math><mml:mmultiscripts><mml:mrow></mml:mrow><mml:mrow></mml:mrow><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math></inline-formula> s sr)<inline-formula><mml:math><mml:mmultiscripts><mml:mrow></mml:mrow><mml:mrow></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:mmultiscripts></mml:math></inline-formula> at 10TeV.

Test-mass thermal noise is a limiting noise source for current and next-generation ground-based gravitational-wave observatories in their most sensitive frequency band. In addition to ongoing efforts to reduce coating mechanical loss, complementary approaches such as the use of uniform-intensity laser beams, including higher-order Laguerre─Gaussian (LG) and Hermite─Gaussian (HG) modes, have been proposed as alternatives to the fundamental Gaussian beam due to their thermal-noise advantages. As the interferometer operating power increases toward the megawatt regime, as anticipated for next-generation detectors, thermal aberrations arising from absorption in the high-reflectivity coatings of the test masses become an increasingly severe challenge. In parallel, the spatially more extended intensity profiles of higher-order modes, which enable their thermal-noise benefits, modify the thermal loading of the test masses, making it essential to assess their behavior under absorption-induced self-heating. In this work, we quantify the robustness of higher-order incident beams against thermal deformation. We show that, under identical operating conditions, higher-order modes produce significantly more uniform thermally induced mirror distortions compared to the sharply peaked aberrations generated by the fundamental mode. As a result, substantially less thermal compensation power is required for their active correction, with the required optimal curvature correction reduced to 33% of that of the fundamental mode for the <inline-formula><mml:math><mml:msub><mml:mi>LG</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>,</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math></inline-formula> mode and to 24% for the <inline-formula><mml:math><mml:msub><mml:mi>HG</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>,</mml:mo><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:math></inline-formula> mode. In addition, we show that the residual thermal deformation for higher-order modes leads to significantly lower optical power loss and enables larger power buildup and enhanced modal purity in an aLIGO-like optical cavity. We further demonstrate that astigmatism compensation enhances the intracavity modal purity of HG modes under self-heating-induced thermal deformation. These results highlight that higher-order laser modes mitigate thermal noise while intrinsically suppressing beam self-heating-induced thermal distortions, making them more thermally robust and well suited for operation in high-power gravitational-wave interferometers.

Numerical relativity (NR), solving Einstein equations numerically, plays an important role in source modeling for gravitational wave astronomy. Traditional methods for NR including the finite difference method, spectral method, and finite element method have been well developed. But newly developed neural network methods for partial differential equations (PDE) have not been well studied yet for NR. We present a physics-informed neural network (PINN) method to solve the Hamiltonian constraint equation for binary black hole (BBH) initial data in NR. This equation is a highly nonlinear elliptic PDE, posing significant challenges for conventional PINN approaches. To overcome these difficulties, we introduce a set of new techniques. We show that our PINN together with these techniques can successfully solve the Hamiltonian constraint equation for generic BBH systems. Validation against the traditional results demonstrates the high accuracy and robustness of our method, revealing the immense potential of constructing a PINN-based initial data solution to all BBH systems for NR.

Following the recent evidence for a gravitational wave (GW) background found by pulsar timing array (PTA) experiments, the next major science milestone is resolving individual supermassive black hole binaries (SMBHBs). The detection of these systems could arise via searches using a power-based GW anisotropy model, i.e., a cross-correlation search for single sources, or a deterministic template model. In Schult et al. [Expectations for the first supermassive black-hole binary resolved by PTAs. I. Model efficacy, Phys. Rev. D 114, 023034 (2026).PRVDAQ2470-001010.1103/vncm-5j2n], we compared the efficacy of these models in constraining the GW signal from a single SMBHB using realistic, near-future PTA datasets, and found that the full-signal deterministic continuous wave (CW) search may achieve detection and characterization first. Here, we continue our analyses using only the CW model given its better performance, focusing now on characterization milestones. We examine the order in which CW parameters are constrained as PTA data are accumulated and the signal-to-noise ratio (<inline-formula><mml:math><mml:mrow><mml:mi>S</mml:mi><mml:mo>/</mml:mo><mml:mi>N</mml:mi></mml:mrow></mml:math></inline-formula>) grows. We also study how these parameter constraints vary across sources of different sky locations and GW frequencies. We find that the GW frequency and strain are generally constrained at the same time (or <inline-formula><mml:math><mml:mrow><mml:mi>S</mml:mi><mml:mo>/</mml:mo><mml:mi>N</mml:mi></mml:mrow></mml:math></inline-formula>), closely followed by the sky location, and later the chirp mass (if the source is highly evolving) and inclination angle. At fixed <inline-formula><mml:math><mml:mrow><mml:mi>S</mml:mi><mml:mo>/</mml:mo><mml:mi>N</mml:mi></mml:mrow></mml:math></inline-formula>, sources at higher frequencies generally achieve better precision on the GW frequency, chirp mass, and sky location. The time (and <inline-formula><mml:math><mml:mrow><mml:mi>S</mml:mi><mml:mo>/</mml:mo><mml:mi>N</mml:mi></mml:mrow></mml:math></inline-formula>) at which the signal becomes constrained is dependent on the sky location and frequency of the source, with the effects of pulsar terms and PTA geometry playing crucial roles in source detection and localization.

īThe search for dark matter candidates includes primordial black holes (PBHs) as possible constituents. Recent studies show that some PBHs can survive to the present epoch by gaining angular momentum through Hawking radiation of photons and becoming extremal before complete evaporation. While this provides a plausible model in a two-derivative theory of gravity, additional issues arise in effective field theory (EFT)-corrected theories of gravity. In such theories, a rapidly spinning black hole can lead to extremely high tidal forces on a near-horizon observer, with possible observational consequences. In this work, by modeling Hawking radiation as a biased random walk within an EFT of gravity, we show that nearly the same fraction of PBHs survives as in Einstein's two-derivative theory of gravity. We argue that the resultant near-horizon tidal effects should be detectable in future gravitational-wave observables.

One of the main features of nonlinear electrodynamics is the existence of an effective geometry that describes the geodesic motion of photons. A detailed analysis of the properties of the effective geometry is of utmost importance for a better understanding of nonlinear electrodynamics theories and their possible imprints on physics, especially in the context of black holes. We consider a nonlinear electrodynamics model that depends on the two electromagnetic scalar invariants and obtain that the motion of photons in nonlinear electrodynamics exhibits vacuum birefringence, i.e., photons can propagate along two distinct paths, depending on their polarization. As a consequence of this phenomenon, we show that static black hole solutions sourced by nonlinear electrodynamics can admit two distinct unstable light rings, leading to the formation of two distinct shadows. Moreover, to explore the potential astrophysical relevance of our results, we compare them with the astrophysical observations for the shadow radius of Sagittarius A*. We place upper limits on the charge-to-mass ratio of the nonlinear electrodynamics-sourced black hole. We also show that the motion of photons in this context can be interpreted as nongeodesic curves subjected to a four-force term from the perspective of an observer in the spacetime metric, generalizing previous results in the literature for nonlinear electrodynamics models that depend on a single electromagnetic scalar invariant.

Plasma surrounding a black hole modifies light propagation and can alter the observed shadow, potentially affecting the interpretation of Event Horizon Telescope data. We study the effects of dark matter and nonmagnetized pressureless plasma on the shadow of a Kerr-like black hole by analyzing null geodesics in both homogeneous and inhomogeneous plasma distributions. For the homogeneous plasma profile, the asymptotic Bardeen coordinates acquire a refractive normalization factor arising from the leading-order coupling between the metric function <inline-formula><mml:math><mml:mi>∆</mml:mi><mml:mo>(</mml:mo><mml:mi>r</mml:mi><mml:mo>)</mml:mo></mml:math></inline-formula> and the radial plasma function <inline-formula><mml:math><mml:msub><mml:mi>f</mml:mi><mml:mi>r</mml:mi></mml:msub><mml:mo>(</mml:mo><mml:mi>r</mml:mi><mml:mo>)</mml:mo><mml:mo>∝</mml:mo><mml:msup><mml:mi>r</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:math></inline-formula>. We show that increasing the black hole spin generally enlarges the shadow radius and increases its deformation, while moving the observer away from the equatorial plane decreases both quantities. For the parameter ranges considered, astrophysically reasonable dark matter densities in this model do not produce appreciable changes in the photon trajectories. Plasma effects, however, are significant; increasing the plasma density increases the shadow radius and deformation for homogeneous plasma, but decreases them for inhomogeneous plasma. The energy emission rate likewise depends strongly on the plasma model, with homogeneous plasma producing a substantially larger rate as the plasma strength increases. As an illustrative benchmark, we compare the resulting geometric critical-curve sizes with EHT-inferred shadow-size intervals for M87* and Sgr A*.

We study hierarchical triple systems formed by a compact binary orbiting a supermassive black hole (SMBH), focusing on the role of relativistic magnetic tidal interactions. Extending previous analyses of precession resonances to 0.5 post-Newtonian order, we incorporate quadrupolar magnetic tidal moments, which have no Newtonian counterpart. We find that magnetic tides introduce new resonances absent at lower order, leading to additional eccentricity excitations and significantly modifying the binary's long-term evolution. Numerical solutions of the Lagrange planetary equations confirm these analytical predictions and reveal how resonance strength depends on orbital eccentricity and inclination. The resulting dynamics accelerates the binary merger and imprints distinctive signatures on gravitational waves, potentially observable by LISA. Our findings identify magnetic tidal coupling as a novel strong-gravity effect and establish its importance for the resonant dynamics of compact-object binaries near SMBHs.

GW231123 is a short-duration, low-frequency gravitational-wave signal consistent with a binary black hole coalescence and dominated by the merger-ringdown regime due to the high mass of the source. We demonstrate that fits of this ringdown signal using two quasinormal modes are statistically preferred over single-mode fits, for a broad range of fit start times. We also find that two-mode fits give remnant mass and spin measurements consistent with those of the inspiral-merger-ringdown model NRSur7dq4, whereas one-mode fits struggle to do so. Agreement of our fits with those of NRSur7dq4 is achieved by labeling the two quasinormal modes as the <inline-formula><mml:math><mml:mo>(</mml:mo><mml:mo>ℓ</mml:mo><mml:mo>,</mml:mo><mml:mi>m</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mo>(</mml:mo><mml:mn>2</mml:mn><mml:mo>,</mml:mo><mml:mn>2</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> and (2, 0) Kerr prograde fundamental modes. However, we find some indications that fits with the (2, 1) quasinormal mode instead of the (2, 0) mode may describe the data better, hinting at possible NRSur7dq4 error or other systematics. When fitting at early times near the estimated peak strain, we find that the inclusion of a third mode, an <inline-formula><mml:math><mml:mo>(</mml:mo><mml:mo>ℓ</mml:mo><mml:mo>,</mml:mo><mml:mi>m</mml:mi><mml:mo>,</mml:mo><mml:mi>n</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mo>(</mml:mo><mml:mn>2</mml:mn><mml:mo>,</mml:mo><mml:mn>2</mml:mn><mml:mo>,</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> prograde overtone, improves consistency with fits at later times. Finally, we perform a test of general relativity by searching for deviations from the Kerr frequency spectrum. Setting issues of systematics aside, we validate the Kerr frequency and damping rate spectrum to within <inline-formula><mml:math><mml:mo>±</mml:mo><mml:mn>10</mml:mn><mml:mo>%</mml:mo></mml:math></inline-formula> at the 90% credible level using a fundamental mode fit, and we also report <inline-formula><mml:math><mml:mo>±</mml:mo><mml:mn>8</mml:mn><mml:mo>%</mml:mo></mml:math></inline-formula> constraints using a model with fundamental modes and an overtone fit at times near the peak strain. Understanding the systematic errors that may be affecting the most accurate analyses of GW231123 is crucial in the context of population and binary formation studies—our (2, 1) mode fits return a significantly higher remnant mass and spin than all available inspiral-merger-ringdown models including NRSur7dq4, and this difference in parameter estimates may have astrophysical implications.

Using a combination of Hubble Space Telescope and James Webb Space Telescope imaging, a runaway supermassive black hole was recently identified with an inferred velocity of <inline-formula><mml:math><mml:mrow><mml:msubsup><mml:mrow><mml:mn>954</mml:mn></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>126</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>110</mml:mn></mml:mrow></mml:msubsup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>km</mml:mi><mml:mtext> </mml:mtext><mml:msup><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula>, likely ejected from a compact star-forming galaxy at <inline-formula><mml:math><mml:mi>z</mml:mi><mml:mo>≍</mml:mo><mml:mn>0.96</mml:mn></mml:math></inline-formula>. Assuming the runaway black hole originated from a gravitational-wave-driven merger of two supermassive black holes (SMBHs), we combine its measured recoil velocity with gravitational-wave recoil predictions from numerical relativity and black-hole perturbation theory to constrain the mass ratio and spin configuration of the progenitor binary that overcame the final-parsec problem and merged <inline-formula><mml:math><mml:mo>∼</mml:mo><mml:mn>70</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>Myr</mml:mi></mml:math></inline-formula> ago. We find that the progenitor binary must have been precessing, with a mass ratio <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>1</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>≲</mml:mo><mml:mn>6</mml:mn></mml:math></inline-formula>, and that the more massive SMBH likely possessed a high dimensionless spin magnitude (<inline-formula><mml:math><mml:mo>∼</mml:mo><mml:mn>0.75</mml:mn></mml:math></inline-formula>) in order to generate a recoil of this magnitude. Such SMBH mergers could represent an interesting source population for the upcoming Laser Interferometer Space Antenna mission, with characteristic signal-to-noise ratios of order <inline-formula><mml:math><mml:mo>≳</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>3</mml:mn></mml:msup></mml:math></inline-formula>. Furthermore, the inferred progenitor SMBH properties suggest that the compact galaxy likely originated from a major, gas-rich ("wet") merger between two galaxies of comparable mass, with a mass ratio <inline-formula><mml:math><mml:mo>≲</mml:mo><mml:mn>4</mml:mn></mml:math></inline-formula>.

The properties of a remnant black hole can be probed by analyzing the gravitational waves emitted during its ringdown phase. This signal provides a direct test of general relativity in the strong-field regime. In this study, we apply both a time-domain <inline-formula><mml:math><mml:mrow><mml:mi>F</mml:mi><mml:mtext>-statistic</mml:mtext></mml:mrow></mml:math></inline-formula> framework and full Bayesian time-domain sampling to the ringdown of GW231028_153006. We report decisive evidence for multimode content, specifically identifying both the first overtone (<inline-formula><mml:math><mml:mrow><mml:mi>ℓ</mml:mi><mml:mo>|</mml:mo><mml:mi>m</mml:mi><mml:mo>|</mml:mo><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>221</mml:mn></mml:mrow></mml:math></inline-formula>) and the higher order (<inline-formula><mml:math><mml:mrow><mml:mi>ℓ</mml:mi><mml:mo>|</mml:mo><mml:mi>m</mml:mi><mml:mo>|</mml:mo><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>210</mml:mn></mml:mrow></mml:math></inline-formula>). The detection of the 221 mode is statistically significant, achieving a Bayes factor of <inline-formula><mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mn>189.2</mml:mn></mml:mrow></mml:math></inline-formula> and an amplitude exclusion from zero at <inline-formula><mml:math><mml:mrow><mml:mo>&gt;</mml:mo><mml:mn>7</mml:mn><mml:mi>σ</mml:mi></mml:mrow></mml:math></inline-formula> credibility for an analysis beginning at <inline-formula><mml:math><mml:mrow><mml:mn>10</mml:mn><mml:mi>M</mml:mi></mml:mrow></mml:math></inline-formula> postpeak. These findings are rigorously validated against the numerical relativity injection SXS:BBH:1282, which confirms that such multimode features are physically expected for a remnant with the inferred high spin and mass ratio. The inclusion of the overtone mode allows for precise constraints on the remnant's properties, yielding a redshifted final mass of <inline-formula><mml:math><mml:mrow><mml:mn>246</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>2</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>22.4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>22.3</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math></inline-formula> and a final spin of <inline-formula><mml:math><mml:mrow><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>81</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.10</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.07</mml:mn></mml:mrow></mml:msubsup></mml:mrow></mml:math></inline-formula> (at <inline-formula><mml:math><mml:mrow><mml:mn>90</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math></inline-formula> credibility), consistent with full inspiral-merger-ringdown predictions. A test of the no-hair theorem, enabled by this robust multimode detection, shows consistency with general relativity.

The vacuum persistence probability, encoded in the imaginary part of the in-out effective action, is a basic measure of vacuum instability and particle production. However, its evaluation may appear prescription-dependent: the Bogoliubov prescription and the Green's function prescription can yield different expressions. We show that this apparent ambiguity arises because the Green's function prescription omits the nontrivial contribution from the endpoint vacuum wavefunctionals, which should be present in the complete in-out amplitude. Including this contribution accomplishes the boundary completion of the Green's function prescription. The ambiguity is thereby resolved, and the complete result agrees with the Bogoliubov expression.

Indigenous narratives have long preserved observations of celestial phenomena, offering insights that resonate with modern astrophysical research. Deeply embedded in cultural traditions, these stories describe events such as stellar variability, supernovae, eclipses, and planetary alignments. Indigenous communities have also used the stars for navigation and calendar systems, reflecting a sophisticated understanding of celestial patterns. These narratives not only complement historical records but offer human-centric perspectives on stellar life cycles that often parallel modern models. Drawing from traditions from diverse Indigenous communities - including Aboriginal Australians, Pueblo peoples, Inuit, and Polynesians - this paper highlights the profound connections between Indigenous knowledge and stellar astrophysics. Integrating these traditions acknowledges their value, safeguards their relevance in the Space Age, and fosters mutual learning through cross-cultural collaboration. We explore how such narratives can enrich education, public engagement, and even inspire scientific hypotheses, bridging cultural heritage and modern astrophysics.

We numerically investigate networks of global $\mathbb{Z}_N$ strings with multi-string junctions in a scalar field model whose vacuum manifold is $\mathrm{PSU}(N)=\mathrm{SU}(N)/\mathbb{Z}_N$. The construction of the model is motivated by the Higgs vacua of mass-deformed $\mathcal{N}=4$ supersymmetric Yang-Mills theory, commonly known as $\mathcal{N}=1^*$ theory, and provides a tractable effective description of string networks with baryon-vertex-like junctions. We perform classical lattice simulations of the formation and evolution of these networks in a radiation-dominated universe. For $N=2,3,4,5,$ and $8$, we find that the networks approach a scaling regime rather than becoming frustrated. The normalization of the string density grows in proportion to the dimension of the adjoint representation, $N^2-1$, while non-minimal-charge components remain subdominant. These results imply that, at least for $N\lesssim 8$, the amplitude of the gravitational-wave energy density generated by the cosmic-string network scales as $Ω_{\rm GW}\propto μ^2 (N^2-1)^2$, where $μ$ is the tension of a unit-charge string.

We study the effect of shear viscosity in stationary magnetized accretion tori on synthetic images of non-rotating black hole shadows. Shear viscosity and spacetime-curvature contributions are introduced perturbatively in the tori through first and second-order transport coefficients within a second-order causal theory of non-ideal relativistic hydrodynamics. Synthetic black hole shadow images at 230\,GHz are obtained via general relativistic radiative transfer computations assuming thermal synchrotron emission and for a wide range of plasma magnetization parameters, viewing inclination angles, electron-temperature prescriptions, and viscosity parameters. A comparative pixel-by-pixel analysis using two normalized metrics shows that the largest image differences occur for strongly magnetized tori. While shear viscosity induces only minor changes in the overall shadow morphology, its combined effects with spacetime curvature are more evident in localized modifications of the synchrotron emission and pixel-wise flux distribution. These effects become increasingly pronounced at higher inclination angles, with the largest brightness differences between viscous and non-viscous configurations occurring for larger values of the electron-temperature parameter. Overall, our results indicate that shear viscosity and spacetime curvature leave only modest imprints on black hole shadow images produced by stationary thick disks, with differences remaining at the level of a few $μ$Jy. Since our analysis is limited to stationary tori, the effects of shear viscosity might however be more significant in fully dynamical accretion systems.

The origin of ultra-high-energy cosmic rays (UHECR) remains a central open problem in astroparticle physics. The observed large-scale anisotropy of UHECR arrival directions above several EeV, together with the spectral suppression at the highest energies, suggests extragalactic sources confined to a limited volume of the nearby Universe, though the effective spatial scale of this contribution remains poorly constrained. We aim to constrain the effective propagation horizon of UHECR with energies E >= 8 EeV by studying how well the observed large-scale anisotropy is reproduced by the distribution of nearby galaxies at different distances, accounting for Galactic magnetic-field deflections. We construct volume-limited galaxy samples from the GLADE+ catalogue extending to 10000 km/s (~150 Mpc) and use both dipole comparisons and angular cross-correlation analyses with the Pierre Auger Observatory UHECR flux map (E >= 8 EeV) to characterize the distance dependence of the anisotropy signal, further weighting the galaxy-UHECR correlations using deflection maps derived from a Galactic magnetic field model. We find that the angular separation between the galaxy and UHECR dipoles is minimized for galaxies within cz < 4000 km/s (< 60 Mpc), where the galaxy dipole amplitude is also maximal. The galaxy-UHECR cross-correlation signal is dominated by this nearby population and weakens rapidly at larger distances. Accounting for Galactic magnetic field deflections enhances the correlation amplitude by more than a factor of two, indicating that magnetic effects significantly shape the observed anisotropy. Our results indicate that the observed anisotropy of UHECR of 8 EeV and above is primarily driven by sources within ~50-60 Mpc, consistent with expectations from energy losses and magnetic deflections, retaining a measurable imprint of the nearby extragalactic matter distribution.

GW231123, if unlensed, is a rare binary black hole merger with high masses and high spins for both progenitors. We show that the signal is better fitted by a lower-mass, lower-spin merger that is diffraction-lensed by an isolated object of redshifted mass $\sim 1000\,\rm M_\odot$, modeled either as a point mass or as a spherically symmetric compact halo. Because diffraction-lensed events are also rare, hypothesis testing should quote the posterior odds ratio rather than the Bayes factor, which requires quantifying our prior belief in the two hypotheses. We adopt the GWTC-5 population distribution as our source parameter prior, and quantify the prior on the lens hypothesis through the lensing optical depth. For a point-mass lens, observational constraints on the abundance of black holes in the Universe yield an upper bound on the optical depth, and hence on the posterior odds, which do not rule out lensing. However, using a predicted mass function of intermediate-mass black holes formed in star clusters gives a low optical depth that strongly disfavors lensing. For a dark matter halo lens, standard collisionless cold dark matter does not form halos compact enough to give the required optical depth, but self-interacting dark matter with a large cross section at low velocities can trigger gravothermal collapse and form them, in which case the posterior odds are inconclusive. In either case, from a frequentist perspective, we show that detecting a lensed event with properties like GW231123 is unlikely. These conclusions apply to isolated lenses, while a lens embedded in an external gravitational potential could change the picture.

White dwarfs that show atmospheric metal enrichment offer a direct window into the bulk composition of exoplanetary material and provide some of the most direct constraints on the building blocks of rocky exoplanetary systems. Recent work has suggested that magnetic fields may influence how this material is accreted, potentially channelling debris along field lines and retaining it near magnetic poles rather than allowing it to be redistributed uniformly across the stellar surface. Here we report the discovery of photometric, spectroscopic, and spectropolarimetric variability in the magnetic DZ white dwarf WD1532+129, revealing a rotation period of about 289 days, by far the longest yet directly measured for any degenerate star. The observed period may result from the combined effect of angular-momentum loss during the evolution leading to the white dwarf stage and an unusually slowly rotating progenitor, perhaps a magnetic Ap star. The white dwarf exhibits surface abundance inhomogeneities, with metals concentrated near both magnetic poles, and the observed modulation is consistent with localised metal-rich regions aligned with the magnetic geometry. WD1532+129 is the third of the four known magnetic DZ white dwarfs within 20 pc to show evidence for polar metal enhancement, indicating that magnetically guided inhomogeneities are a common property of magnetic metal-enriched white dwarfs.

Binary (super-)massive black holes (BHs) are expected to reside in the center of some galaxies. In this work, we re-visit accretion onto binary massive BHs, incorporating recent advances in both accretion theory and the mass transfer rate between the two massive BHs. We focus on relatively bright systems with an Eddington ratio of 0.1 for a binary with total BH mass $10^8\,M_\odot$, but consider a wide range of mass ratios $10^{-4} \le q \le 0.5$. The binary system consists of two mini-disks surrounding two individual BHs and a circumbinary disk surrounding the mass center of binary BHs. Depending on the mass ratio, the two mini-disks can be hot accretion flows, standard thin (cold) disks, or Slim disks. The radiative contributions from all three disks, each potentially in different accretion modes, are taken into account self-consistently. The spectral energy distributions of the binary BH system show universal ``notch'' features from the near-infrared to ultraviolet bands, caused by the gap or cavity in the accretion disk, consistent with previous studies. Binary with different mass ratios exhibit distinct spectral energy distribution properties, offering opportunities for testing (identifying candidates) with future broad band (infrared up to X-rays) observations. We also investigate the evolution of these binary systems, and find that, for systems with initial mass ratios $q \lesssim \text{a few} \times 10^{-3}$, the mass ratio evolves toward an equilibrium value $q \sim 10^{-3}$. For binary BH systems with a larger initial mass ratio, their mass ratio instead evolves toward unity.

There is broad consensus that Type Ia supernovae (SNe Ia) are the thermonuclear explosions of C/O white dwarfs (WDs) in binary systems, but their progenitors and explosion mechanisms remain uncertain. Their earliest light curves probe the outermost ejecta and provide important constraints on the explosion. We analyse the ZTF DR2 sample of SNe Ia to search for objects exhibiting early flux excess (EEx SNe Ia). We employ two complementary approaches: (i) identifying deviations from models without early excess using power-law and SALT2 fits, and (ii) comparing observations with double-detonation (DD) and companion-interaction (CI) models. The latter identify 145 and 199 candidates, respectively, although the methods disagree substantially on both the objects selected and the total number of candidates. Combining all methods, we identify 17 robust EEx SN Ia candidates, including six over-luminous events (five 91T-like and one 03fg-like). The strongest excesses reach ${\sim}10$--17\% of the peak flux. The best candidates have, on average, higher stretch and preferentially occur in lower-mass, bluer host galaxies, suggesting younger stellar populations. We estimate that EEx SNe Ia comprise $\lesssim25\%$ of all SNe Ia, consistent with previous work, with over-luminous subtypes showing a higher relative incidence than normal SNe Ia. Among the DD models, the preferred solutions favour intermediate WD core masses ($1 M_{\odot}$), low shell-burning fractions ($20\%$), and heavier He-burning products ($^{56}$Ni), whereas the CI models show no clear parameter trends. Overall, we find no compelling evidence that either scenario is preferred as the origin of the observed early flux excesses.

We report a clear observational evidence for jet precession and nutation manifested in the gamma-ray emissions of the blazar PKS 1424$-$41. An analysis of $\sim$16 yr of \emph{Fermi}-LAT observations reveals three distinct and superimposed quasi-periodic oscillations (QPOs), with characteristic timescales of $\sim$5.26 yr, $\sim$0.96 yr and $\sim$1.3 yr. The longer timescale is detected with a local significance of $\sim 4.1σ$ ($99.9958\%$ confidence) and a global significance of $\sim 2.45σ$ ($98.5714\%$ confidence). The shorter timescales ($\sim 0.96$ yr and $\sim 1.3$ yr) both show local significances exceeding $3σ$. To interpret the origin of these modulations, we examine two physically motivated scenarios: Lense-Thirring precession of a misaligned accretion disk and orbital-driven jet precession in a binary supermassive black hole (BSBH) system. We model the gamma-ray variability using a compound framework that incorporates jet precession together with intrinsic jet rotation (nutation), which successfully reproduces the observed year-like QPOs. Our results indicate that the long-term, persistent $\sim$5.26 yr oscillation is most consistent with a BSBH origin, while the shorter year-like QPO modulation can be naturally attributed to rotation or wobbling of the jet axis. If PKS 1424$-$41 hosts a supermassive black hole binary, it represents a promising target for future space-borne gravitational-wave observatories such as the Laser Interferometer Space Antenna (LISA), providing an independent probe of supermassive black hole binaries.

We investigate baryogenesis in the framework of rainbow cosmology employing a complex scalar field with a softly broken global $U(1)$-symmetry. The modified dispersion relation of rainbow cosmology leads to energy-dependent modification to the FLRW metric components, that modifies the Friedmann equation and the scalar field dynamics. We thereby obtain analytical solutions for the scalar field evolution in the radiation-dominated epoch, and show that baryon asymmetry is generated dynamically even from an initially baryon-symmetric state. We find that the baryon-to-photon ratio asymptotically approaches a constant value in the long-time limit, which is proportional to the symmetry-breaking coupling strength $λ$, and scales with the scalar field mass as $M^{-5/2}$. Our results demonstrate that requiring consistency with the observed baryon asymmetry constrains the $λ$ and $M$ values within reasonable ranges, thus providing a viable dynamical mechanism for baryogenesis during the radiation-dominated era without invoking supersymmetry.

Hybrid measurements at the Pierre Auger Observatory indicate that most high-energy hadronic interaction models underestimate the average depth of the shower maximum, $\langle X_{\max} \rangle$, at a center-of-mass energy of $\sqrt{s}=97.7 \pm 0.4^{+6.6}_{-6.2}\,\mathrm{TeV}$. In this Letter, the hadronic interaction models are shown to follow a universal relation between the predicted $\langle X_{\max} \rangle$ and the mean values of variables characterizing the energy spectra of secondary particles produced in the first interaction of proton-induced air showers. Assuming the validity of these relations in Nature, we map the values of $\langle X_{\max} \rangle$ favored by Auger data into mean values of these variables. All models favor an increase in the mean elasticity and in the fraction of hadronic energy in proton--air interactions. The latter must be amplified by a factor of $2.8$ to $4.6$ to account for the muon puzzle.

We developed in this work a perturbative technique to compute the deflection angle of light rays in general static and spherically symmetric spacetimes with a homogeneous non-magnetic plasma. The deflection angle is expressed as a power series of $M/b$ with $M$ and $b$ being the spacetime mass and the impact parameter of the ray. The series coefficients are polynomials of the asymptotic expansion coefficients of the metric functions and the reciprocal of the asymptotic refractive index. When the plasma is dilute, the deflection angle can also be expressed as a dual series of $M/b$ and the frequency ratio between the electron plasma frequency and the asymptotic photon frequency. The series result reveals that for general SSS spacetime, the plasma at the leading order always enhances the deflection angle and therefore increases the apparent angles of the gravitational lensing images. These series results of the deflection angle are then shown to have excellent agreement with those obtained using numerical integration. The general formula of the deflection angle is then applied to the Reissner-Nordström, charged Horndeski and charged Galileon spacetimes. The effect of the plasma and characteristic parameter of the spacetimes on the deflection angles in these spacetimes was briefly discussed. In the appendix, the deflection angles in previously attempted spacetimes are re-computed and compared with the literature.

Stellar binary disruptions through the Hills mechanism produce two fossils: a hypervelocity star (HVS), and a star tightly bound to the supermassive black hole. Among known galactic HVSs, only S5-HVS1 has unambiguous galactic-centre origin. Its measured mass and velocity determine a relation between the mass and semi-major axis of its captured companion. GRAVITY has now discovered S301, whose orbit and photometrically inferred mass satisfy this relation, making it the only compelling candidate for the captured companion of S5-HVS1. We build a forward model for the Hills origin and compare it to the null hypothesis. This confirms that S301's orbit aligns much more closely with that of S5-HVS1's companion than a typical S-star. However, the catalog-level Bayes factor remains of order unity and dependent on the probabilities of survival and detection. S301 is thus a compelling candidate, but establishing its association with S5-HVS1 will require improved mass measurements, chemical comparisons and GRAVITY-calibrated selection functions.

We present cosmo_learn, an open-source python-based software package designed to simulate cosmological data and perform data-driven inference using a range of modern statistical and machine learning techniques. Motivated by the growing complexity of cosmological models and the emergence of observational tensions, cosmo_learn provides a standardized and flexible framework for benchmarking cosmological inference methods. The package supports realistic noise modeling for key observables in the late Universe, including cosmic chronometers, supernovae Ia, baryon acoustic oscillations, redshift space distortions, and gravitational wave bright sirens. We demonstrate the internal consistency of the simulated data with the input cosmology via residuals and parameter recovery using a fiducial wCDM model. Built-in learning and inference modules include traditional Markov Chain Monte Carlo, as well as more recent approaches such as genetic algorithms, Gaussian processes, Bayesian ridge regression, and artificial neural networks. These methods are implemented in a modular and extensible architecture designed to facilitate comparisons across inference strategies in a common pipeline. By providing a flexible and transparent simulation and learning environment, cosmo_learn supports both educational and research efforts at the intersection of cosmology, statistics, and machine learning.

This letter presents an assessment of quantum gravity effects on extreme-mass-ratio inspirals (EMRIs) for the rotating quantum Oppenheimer-Snyder (qOS) black hole. Employing the adiabatic evolution, we compute the gravitational wave (GW) dephasing, which quantifies the cumulative phase shift induced by the quantum correction α. We further generate the augmented analytic kludge (AAK) waveform and investigate the faithfulness between the waveforms with and without the quantum parameter α for different values of a. Our results reveal that the quantum gravity effect induces detectable imprints in LISA, while the presence of rotation suppresses these signatures. We further show that the opposite influences of α and a can compensate in the accumulated dephasing, pointing to a potential degeneracy between rotation and quantum corrections. This suggests that rotational degrees of freedom must be carefully accounted for when probing quantum gravity with EMRI observations.

Sterile neutrinos are compelling dark matter (DM) candidates, yet the minimal production mechanism solely based on active-sterile (<inline-formula><mml:math><mml:msub><mml:mi>ν</mml:mi><mml:mi>a</mml:mi></mml:msub><mml:mo>-</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula>) oscillations is robustly excluded by astrophysical observations. Nonstandard self-interactions in either the active (<inline-formula><mml:math><mml:msub><mml:mi>ν</mml:mi><mml:mi>a</mml:mi></mml:msub><mml:mo>-</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>a</mml:mi></mml:msub></mml:math></inline-formula>) or sterile (<inline-formula><mml:math><mml:msub><mml:mi>ν</mml:mi><mml:mi>s</mml:mi></mml:msub><mml:mo>-</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula>) sector are known to alter the sterile neutrino DM production in the early Universe, which could alleviate the tension with astrophysical constraints. Here, we propose a new solution where scalar-mediated nonstandard interactions between active and sterile neutrinos (<inline-formula><mml:math><mml:msub><mml:mi>ν</mml:mi><mml:mi>a</mml:mi></mml:msub><mml:mo>-</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula>) lead to new production channels for sterile neutrino DM, independent of the active-sterile mixing and without the need for any fine-tuned resonance or primordial lepton asymmetries. This framework enables efficient sterile neutrino DM production even at vanishingly small mixing angles and opens up new viable regions of parameter space that can be tested with future x-ray and gamma-ray observations. It also provides a new mechanism for observable neutrino-DM interactions while being consistent with DM relic density and has broad phenomenological implications for astrophysical neutrinos and beyond.

The precise calculation of gravitational waves (GWs) from cosmic string networks is of significant theoretical and experimental interest. The Nambu─Goto (NG) approximation has long been employed to calculate GW emission from such networks; however, its validity has never been systematically verified. We perform large-scale zero-temperature Abelian─Higgs lattice simulations under different gauge couplings and compare them with NG predictions. We find excellent agreement in the power-law region for near-global strings but strong deviation for strongly coupled local strings with <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mi>v</mml:mi></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi>m</mml:mi><mml:mi>s</mml:mi></mml:msub><mml:mo>∼</mml:mo><mml:mn>1</mml:mn></mml:math></inline-formula>, quantitatively establishing the breakdown of the NG approximation. Additionally, we confirm that particle emission significantly dominates the energy loss of the string network, with the ratio of GW energy to particle energy approximately <inline-formula><mml:math><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> to <inline-formula><mml:math><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> for both near-global and local string scenarios.

This manuscript investigates the impact of bulk viscosity on the viability of cosmic bounce solutions in the context of $f(\mathcal{Q})$ theory, where $\mathcal{Q}$ is a non-metricity scalar. To complete this objective, we study the behavior of an isotropic homogeneous universe filled with a perfect fluid and consider a innovative parametrization of bulk viscosity coefficient with an arbitrary constant $ζ_0$ as $ζ=ζ_{0}H$. We consider the particular functional form of this modified theory to explore how this gravitational framework influences the cosmic evolution and explore a range of cosmological parameters to assess the existence and physical viability of bounce solutions. We also investigate the evolution of entropy through the second law of thermodynamics. The positive trend of energy density, negative pressure profile and violation of energy conditions support the existence of viable cosmological bounce scenario and highlights the significance of bulk viscosity in this framework. These results demonstrate that $f(\mathcal{Q})$ gravity provides a compelling alternative to standard cosmic models and provides deep insights into the nature of gravitational interaction and the early cosmos.

This paper presents a convolutional neural network (CNN) approach to classifying the nuclear equation of state (EOS). As illustrative examples, we use five two-dimensional core-collapse supernova (CCSN) simulations that differ only in their EOS. We analyze estimates of the initial slope of the high-frequency feature (HFF) reconstructed in real interferometric data from the O3b LIGO-Virgo-KAGRA (LVK) observing run at Galactic source distances of 1, 5, and 10 kpc. The CNN classifier achieves an overall accuracy of 98.58% at 1 kpc and 52.43% at 5 kpc. At 10 kpc, its ability to distinguish among the EOS classes is effectively lost. The successful EOS classification at 1 kpc suggests that this approach may be scalable to next-generation observatories. The expected order-of-magnitude sensitivity improvements of Cosmic Explorer and the Einstein Telescope could enable comparable classification performance at approximately ten times the current distance. More detailed performance metrics, including the macro-averaged one-vs-rest (OvR) area under the curve (AUC), yield values of 0.97 and 0.98 at 1 kpc. These results indicate strong classification performance both across the complete set of EOS classes and for the individual classes.

In these notes, we extend a formalism for computing the amplitude of gravitational-waves (GWs) beyond null-geodesic propagation, as may arise in alternative theories of gravity. We derive the wavevector divergence governing the amplitude evolution in this more general setting and apply the formalism to GWs with a frequency-independent subluminal group velocity and to massive gravitons. We show that, in both cases, a naive interpretations of the resulting amplitudes can lead to significant apparent distortions. This formalism opens the door for amplitude interactions with extra degrees of freedom in the presence of dispersion.

We report the discovery and characterisation of MKT J032848.4-271904.6, a new radio transient identified in the LADUMA field using SARAO Science Data Processor (SDP) UHF-band images from approximately one year of MeerKAT observations. Using the Transient Pipeline (TraP) and advanced filtering techniques, we identified a number of candidate variable radio sources in the field. All but one show variability consistent with refractive interstellar scintillation, leaving MKT J032848.4-271904.6 as the sole source exhibiting intrinsic variability. In addition, MKT J032848.4-271904.6 shows intrinsic variability at 0.816 GHz, with 13 radio detections across 41 epochs and a peak flux density of 1.041+/-0.043 mJy. We associate this emission with a low-mass M-dwarf star LP 888-63, located 23 pc from the Sun and identified as a companion to the white dwarf binary system LAWD 14 / WD 0326-273. LP 888-63 displays active flaring behaviour across the electromagnetic spectrum, including multi-band optical data from MeerLICHT. TESS photometry reveals a periodic modulation in the blended light curve of 5.780+/-0.507 days, consistent with rotational variability of a mid-M dwarf. Archival ESO spectra reveal Halpha emission, confirming magnetic activity. These findings highlight the capability of MeerKAT's SDP imaging and facilities like MeerLICHT for real-time detection and characterisation of stellar transients.

We present the optical design of VIPER: a high-resolution, multimode fiber-fed, narrowband, cross-dispersed, seeing-limited spectrograph based on a Virtually Imaged Phased Array (VIPA), a spectral disperser that provides higher dispersion and a more compact form factor than conventional diffraction gratings. VIPER is specifically designed to probe exoplanet atmospheric escape through the helium 1083 nm triplet line, with a resolving power of 300,000 over a 10 nm wavelength range. VIPER is intended for operation at the 1.5 m Tillinghast Telescope at the Fred Lawrence Whipple Observatory (FLWO) on Mount Hopkins, Arizona, USA, and is matched to this telescope's 100 $μ$m circular-core multimode optical fiber feed at f/6. VIPER's optical design is specifically optimized for high throughput, which, in a VIPA spectrograph, is more challenging with a multimode fiber feed than a single-mode fiber feed because of the larger etendue. To address this challenge, our design implements a novel use of a cylindrical beam expander and pupil slicer. We validate our design using analytic calculations and simulations in Zemax OpticStudio non-sequential mode. Our results demonstrate the feasibility of multimode fiber-fed VIPA spectrographs as compact, high-throughput alternatives to conventional grating-based spectrographs for exoplanet science and other astronomical applications.

We present py5vec, a Python package for implementing and extending the 5-vector method, used to search for continuous gravitational wave (CW) signals. We also provide a comprehensive theoretical review of the 5-vector method and extend the relative likelihood formalism by marginalizing over the noise variance, resulting in a more robust Student's <inline-formula> <mml:math><mml:mrow><mml:mi>t</mml:mi></mml:mrow></mml:math></inline-formula>-likelihood, and over the initial phase to account for pulsar glitches. py5vec provides a modular architecture that separates data representation, signal demodulation, and statistical inference into independent abstract stages. It supports multiple input data formats and interoperates with existing Python software, providing a bridge between different statistical approaches and data processing pipelines. For example, using a bilby-based interface, py5vec implements Bayesian parameter estimation within the 5-vector formalism for the first time. The modular design also allows for making exact multi-level and direct comparisons between other software, such as cwinpy and SNAG in MATLAB. In py5vec, we implement a multidetector targeted search for known pulsars, validated using LIGO data from the O4a run and hardware injections, demonstrating consistent reconstruction of signal parameters. This package therefore provides a flexible platform for current targeted searches and for future extensions to other CW search strategies.

Primordial Black Holes (PBHs) have emerged as a leading non-particulate candidate for dark matter and a unique cosmological probe, a paradigm shift accelerated by the detection of anomalous binary mergers by the LIGO-Virgo-KAGRA (LVK) collaboration. While the literature is rich with phenomenological constraints, the fundamental quantum and relativistic underpinnings governing PBH genesis and evolution often receive comparatively less emphasis. This review bridges that gap by systematically detailing the physics of PBH formation and their subsequent evolutionary trajectory. We critically examine the hydrodynamic complexity of the early universe, establishing the relativistic thresholds for collapse, the non-linear race against sound in the primordial plasma, and the rigorous mathematical utility of the compaction function. Furthermore, by incorporating the dynamic nature of FLRW backgrounds, higher curvature corrections, and quantum backreaction via the memory burden effect, we challenge the standard Hawking evaporation and show that extreme-curvature environments halt evaporation entirely, leaving Planck-scale relics that evade current extragalactic bounds. Finally, we map the multimessenger observational landscape, highlighting how the imminent search for sub-solar mass inspirals by next-generation gravitational wave observatories—such as the Einstein Telescope and Cosmic Explorer—could yield smoking-gun evidence for the PBH paradigm, ultimately transforming these primordial relics into unparalleled laboratories for high-energy physics.

We explore the Kazakov-Solodukhin quantum deformed black hole spacetime, characterized by a single deformation parameter η that encodes quantum corrections to the classical Schwarzschild solution. The model preserves the correct general-relativistic limit as η → 0, while introducing significant and physically meaningful deviations in the strong-field regime. A central and remarkable feature of the geometry is the regularization of the classical singularity: curvature invariants remain finite near the minimal radius <mml:math><mml:mrow><mml:mi>r</mml:mi><mml:mo>=</mml:mo><mml:mi>η</mml:mi></mml:mrow></mml:math>, effectively replacing the divergent core with a smooth and well-behaved region. This behavior naturally introduces a minimal length scale into the spacetime structure, offering a geometrically motivated resolution of the singularity problem. The deformation modifies the horizon structure, shifts the event horizon location, and alters the mass-radius relation. It also reduces the surface gravity, leading to a lower Hawking temperature and a slower evaporation process, thereby enhancing the thermodynamic stability of the black hole. Photon dynamics are correspondingly affected, resulting in a displaced photon sphere and modified strong-lensing characteristics. While the shadow remains perfectly circular due to spherical symmetry, its size depends sensitively on η. Observational constraints can be expressed through <mml:math><mml:mrow><mml:mrow><mml:mo>|</mml:mo><mml:msub><mml:mi>R</mml:mi><mml:mrow><mml:mi>s</mml:mi><mml:mi>h</mml:mi></mml:mrow></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>η</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mo>−</mml:mo><mml:msub><mml:mi>R</mml:mi><mml:mrow><mml:mi>o</mml:mi><mml:mi>b</mml:mi><mml:mi>s</mml:mi></mml:mrow></mml:msub><mml:mo>|</mml:mo></mml:mrow><mml:mi>⩽</mml:mi><mml:mstyle><mml:mi>∆</mml:mi></mml:mstyle><mml:msub><mml:mi>R</mml:mi><mml:mrow><mml:mi>o</mml:mi><mml:mi>b</mml:mi><mml:mi>s</mml:mi></mml:mrow></mml:msub><mml:mo>,</mml:mo></mml:mrow></mml:math> which places an upper bound on the deformation parameter. In the weak-field limit, the deflection angle acquires a quadratic correction proportional to η<SUP>2</SUP>, ensuring consistency with precision tests while allowing potentially detectable deviations in strong-gravity observations. These features make the model both theoretically appealing and observationally testable.

We present a new constraint on the effective number of relativistic species in the early universe, <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula>, by combining recent primordial helium abundance measurements from the Large Binocular Telescope <inline-formula><mml:math><mml:msub><mml:mi>Y</mml:mi><mml:mi>p</mml:mi></mml:msub></mml:math></inline-formula> Project with primordial deuterium abundance data, CMB observations from Planck, the Atacama Cosmology Telescope, and the South Pole Telescope, and baryon acoustic oscillation (BAO) data from the Dark Energy Spectroscopic Instrument, yielding <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>2.990</mml:mn><mml:mo>±</mml:mo><mml:mn>0.070</mml:mn></mml:math></inline-formula> (68% CL). This is the tightest constraint on <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula> to date, and is in excellent agreement with the standard model prediction of <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>3.044</mml:mn></mml:math></inline-formula>. Furthermore, we constrain excess contributions to <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula> beyond the three neutrino species, finding <inline-formula><mml:math><mml:mi>∆</mml:mi><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub><mml:mo>&lt;</mml:mo><mml:mn>0.107</mml:mn></mml:math></inline-formula> (95% CL). This bound nearly approaches the minimum contribution to <inline-formula><mml:math><mml:mi>∆</mml:mi><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula> from a light spin-<inline-formula><mml:math><mml:mrow><mml:mn>3</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:math></inline-formula> particle that decoupled at any time after inflation ended. Our baseline analysis does not include large-scale Planck polarization information, enabling a fully consistent combination of state-of-the-art CMB and BAO measurements. As a byproduct, we show that current <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula> bounds are essentially insensitive to the inclusion or exclusion of optical depth constraints inferred from large-scale CMB polarization data, making <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula> highly robust in this regard. Our constraints place stringent limits on light particles in the early Universe and on a broad range of models aimed at increasing the CMB-inferred value of the Hubble constant.

In this Letter, we develop a numerically efficient framework for evaluating parameters in metric theories of gravity, and apply it to constrain the horizon-scale magnetic field in the Kerr-Bertotti-Robinson spacetime using the latest Event Horizon Telescope observations. The method's adaptive ray-tracing strategy achieves near-linear computational efficiency without loss of numerical accuracy. This efficiency allows for high-precision black hole shadow modeling at a minimal computational cost and, for the first time, supports statistically robust parameter inference for arbitrary stationary black holes from horizon-scale observations. Applying the framework to the exact magnetized and rotating Kerr-Bertotti-Robinson solution [Phys. Rev. Lett. 135, 181401 (2025)PRLTAO0031-900710.1103/rfgv-ybz5], we evaluate the horizon-scale magnetic fields of M87* and Sgr A*. The analysis yields a field strength of <inline-formula><mml:math><mml:msubsup><mml:mn>93.3</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>23.8</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>14.7</mml:mn></mml:mrow></mml:msubsup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>G</mml:mi></mml:math></inline-formula> for Sgr A*, consistent with the 71 G equipartition estimate from polarized Atacama Large Millimeter/submillimeter Array observations, thereby supporting the predictions of Einstein's gravity.

The space mission LISA, scheduled for launch in 2035, aims to detect gravitational wave (GW) signals in the milli-Hz band. In the context of the ESA Voyage 2050 Call for new mission concepts, other frequency ranges are explored by the Gravitational-Wave Space 2050 Working Group to conceive new proposals for a post-LISA space-based detector. In this work, we give a preliminary estimate of the observational potential of three mission designs proposed in the literature, namely, <inline-formula><mml:math><mml:mrow><mml:mi>μ</mml:mi><mml:mi>Ares</mml:mi></mml:mrow></mml:math></inline-formula>, AMIGO and the Decihertz Observatory. The analysis framework includes astrophysical GW sources, such as massive black hole binaries and extreme mass-ratio inspirals, and compact binaries, such as stellar black holes and white dwarfs. For each detector, we first present a consistent computation of the unresolved gravitational wave background (GWB) produced by the sum of all anticipated astrophysical populations using an iterative subtraction algorithm. We then investigate which types of systems are the most appealing by measuring the number of GW signals detected and exploring the source properties.

Inflationary phase transitions can generate a stochastic gravitational-wave background that probes primordial physics. We study the detectability and parameter reconstruction of such a signal with a space-based gravitational-wave detector. Using a Taiji-like mission as a benchmark, we construct a realistic data-analysis framework that includes instrumental noise, astrophysical foregrounds and backgrounds, and the <inline-formula><mml:math><mml:mrow><mml:mi>A</mml:mi></mml:mrow></mml:math></inline-formula>, <inline-formula><mml:math><mml:mi>E</mml:mi></mml:math></inline-formula>, and <inline-formula><mml:math><mml:mi>T</mml:mi></mml:math></inline-formula> time-delay interferometry channels. The target signal is described in a minimal, model-independent form and analyzed using both Fisher-matrix forecasts and Bayesian inference with nested sampling. We quantify detection significance and parameter-recovery thresholds, showing that, while detection is achievable at moderate signal-to-noise ratios, stronger signals provide more reliable parameter reconstruction. These results offer a realistic assessment of the capability of future space-based missions to probe inflationary phase transitions through stochastic gravitational radiation.

We investigate the gravitational wave spectrum from first-order QCD phase transitions using the parity doublet model at finite baryon chemical potential. The model incorporates the chiral invariant mass <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula>, representing the portion of nucleon mass that persists even when chiral symmetry is restored. Within the model, we identify two first-order phase transition regions: the nuclear liquid-gas transition and the chiral phase transition. By solving the bounce equation and computing the Euclidean action <inline-formula><mml:math><mml:msub><mml:mi>S</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:mi>T</mml:mi></mml:math></inline-formula>, we obtain the gravitational wave spectra from both transitions. The liquid-gas transition yields transition strength <inline-formula><mml:math><mml:mi>α</mml:mi><mml:mo>∼</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> and inverse duration of the phase transition <inline-formula><mml:math><mml:mi>β</mml:mi><mml:mo>/</mml:mo><mml:mi>H</mml:mi><mml:mo>∼</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>10</mml:mn><mml:mo>)</mml:mo><mml:mi>─</mml:mi><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>100</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> near the end point of the first-order line, producing signals with peak frequencies from the millihertz to the nanohertz band that can fit the existing data. In contrast, the chiral transition produces signals suppressed by approximately five orders of magnitude, well below the sensitivity of all current and planned detectors. These results establish, within the parity doublet model, a quantitative link between first-order QCD phase transitions and their gravitational wave signatures, and provide a concrete framework for relating gravitational wave observations to the chiral invariant mass <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula> and thus to the origin of nucleon mass.

Current multiton detectors put stringent constraints on the GeV-scale galactic dark matter, pushing the allowed cross section almost toward the neutrino fog, yet remain mostly insensitive to the light dark matter. Cosmic rays can upscatter the nonrelativistic halo dark matter particles, making a subpopulation of them gain sufficient kinetic energy to be discernible in current direct search experiments. In this work, we explore this alternate strategy to probe sub-MeV electrophilic dark matter boosted by cosmic rays with the latest data of LZ 2025 (WS2024 run). We also incorporate the attenuation effect on the boosted dark matter flux during its propagation through the Earth and perform a full numerical treatment to obtain the resulting event rate. Our result shows LZ 2025 data improve the constraint on the MeV scale dark matter by almost <inline-formula><mml:math><mml:mo>∼</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> compared to the previous XENONnT limit for the energy-independent cross section. Using realistic energy-dependent cross sections, we also analyze such a scenario, where the associated mediator mass plays a crucial role in governing the event rate and hence the expected limits too. With energy-dependent cross sections, our obtained limits also remain stronger than the existing constraints from the XENONnT experiment. Even compared to the limits from neutrino detectors with much larger target masses, LZ 2025 can place stringent constraints in certain regions of the mediator parameter space, particularly in the light-mediator regime, excluding previously unexplored regions.

We investigate the cosmological information in higher-order statistics of the cosmic microwave background (CMB) lensing convergence field for a near-term experiment with noise properties similar to the Simons Observatory (SO). Using a fully field-level forward-modeling pipeline based on ray-traced simulations from the MassiveNuS suite and realistic SO-like CMB lensing reconstruction, we naturally include nonlinear structure formation, post-Born effects, and higher-order reconstruction noise. We measure several non-Gaussian statistics, including Minkowski functionals, peak and minima counts, moments, and wavelet-scattering coefficients. We train Gaussian-process emulators to model each statistic's dependence on the matter density fraction <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mi>m</mml:mi></mml:msub></mml:math></inline-formula>, the scalar power spectrum amplitude <inline-formula><mml:math><mml:msub><mml:mi>A</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula>, and the neutrino mass sum <inline-formula><mml:math><mml:msub><mml:mi>M</mml:mi><mml:mi>ν</mml:mi></mml:msub></mml:math></inline-formula>. We quantify the relative information gain these statistics provide beyond the lensing power spectrum and identify which are most robust to reconstruction noise. We find that morphology-based statistics, particularly Minkowski functionals and peak/minima counts, offer significant complementary constraining power: combining all non-Gaussian statistics with the power spectrum yields reductions of 40% and 38% in the marginalized uncertainties on <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mi>m</mml:mi></mml:msub></mml:math></inline-formula> and <inline-formula><mml:math><mml:msub><mml:mi>A</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula>, respectively, and a 70% reduction in the one-sided uncertainty on <inline-formula><mml:math><mml:msub><mml:mi>M</mml:mi><mml:mi>ν</mml:mi></mml:msub></mml:math></inline-formula>. These gains remain non-negligible even when the power spectrum is extended to larger scales and combined with primary CMB and BAO data, with Minkowski functionals providing an additional 11% improvement in <inline-formula><mml:math><mml:mi>σ</mml:mi><mml:mo>(</mml:mo><mml:msub><mml:mi>M</mml:mi><mml:mi>ν</mml:mi></mml:msub><mml:mo>)</mml:mo></mml:math></inline-formula> and 35% in <inline-formula><mml:math><mml:mi>σ</mml:mi><mml:mo>(</mml:mo><mml:msub><mml:mi>Ω</mml:mi><mml:mi>m</mml:mi></mml:msub><mml:mo>)</mml:mo></mml:math></inline-formula> beyond the extended power spectrum. By contrast, moments and wavelet-scattering coefficients provide more limited gains at SO noise levels. Our results highlight the potential of non-Gaussian statistics to enhance cosmological constraints from SO and future CMB surveys.

The memory effect is known to introduce a permanent displacement in the gravitational wave (GW) detectors after the passage of a GW signal. While the linear memory adheres to the source properties, the nonlinear memory is a secondary effect sourced by the GW itself. In the present work, we discuss GW signals with both these kinds of memory effects, while focusing on the parabolic limit of an encounter. This special case is theoretically intriguing and emerges as a limiting situation for both eccentric and hyperbolic events. However, in this paper, we argue that a simple extrapolation of memory calculations for eccentric or hyperbolic cases to the parabolic case may lead to incorrect estimations. Therefore, we treat the parabola as a special case and use an intrinsic parametrization, with which we calculate gravitational wave signals and their energy spectrum via an effective field theory formalism. Unlike the hyperbolic case, which is known to have linear memory, we notice that parabolic encounters bring out new features in the zero frequency limit. The exactly parabolic case is studied here primarily as an idealized separatrix between bound and unbound motion, and our analysis highlights some of the key challenges and salient aspects of GW memory in this regime.

The cosmic microwave background radiation or particles surrounding a black hole can be modeled as a thermal bath coupled to the black hole. In this study, we investigate the effects of such a thermal bath on black hole radiation. The model involves Hawking radiation particles that are bilinearly coupled to the thermal bath. In this framework, the Hawking radiation particles are treated as the system, and the bath serves as the environment. We analytically derive the effective Keldysh action and the dynamical equation of the system, which are essential for studying the effective dynamics of Hawking radiation. For the specific case of a bath weakly coupled to the radiation particles from a Schwarzschild black hole, we numerically solve the effective dynamical equation. The results indicate that the bath suppresses Hawking radiation. Consequently, an observer located far away from the black hole will detect radiation that is weaker than Hawking's original prediction.

Precise predictions of cosmic-ray antinuclei fluxes, a prime dark matter signature, are limited by the lack of data constraining production rates of antinuclei. We mitigate this bottleneck with a fast, differentiable normalizing-flow surrogate for the femtoscopic source model (CECA), fit to 49 ALICE proton-proton correlation functions, and extrapolated via scaling laws to the low-multiplicity domain relevant for cosmic rays. The surrogate reproduces CECA with sub-percent emulation fidelity, yielding data-constrained source functions that remove the dominant uncertainty in coalescence-based antinuclei production rates. The resulting uncertainties on the coalescence parameters $B_2$ and $B_3$ shrink from factors of 10-100 and ${\sim}$1000 to the few percent and ten-percent level, respectively, with an additional ${\sim}15\%$ wavefunction systematic for $B_{2}$.

Simulation-based inference (SBI) with machine learning is an increasingly important tool for solving inverse problems in science and engineering, including parameter inference and the inversion of detector effects. We provide an overview of the Bayesian and frequentist statistical frameworks, describe how machine-learning-based SBI methods, such as neural posterior estimation and neural likelihood estimation, can be used for parameter estimation within these frameworks, and show that the same methods can also be applied to Empirical Bayes or unfolding tasks. We also discuss how to validate inference results and the limitations of SBI with machine learning.

The Paradox of the Third Particle arises when comparing subsystem descriptions across Quantum Reference Frame (QRF) perspectives. We isolate two distinct origins of the Paradox: the QRF covariance of the partial trace and the failure of the physical Hilbert space to inherit the kinematical tensor-product structure. We give an explicit counterexample to the Relational Trace (RT) resolution: an uncorrelated product state for which the RT statistical condition trivialises. We then introduce a new statistical consistency condition comparing subsystem discarding between external and internal QRFs, together with an associated frame-dependent map, the Perspective Relational Trace (PRT). We argue that our condition captures the operational content of the Paradox: rather than imposing consistency on the whole state space, we characterise exactly the states on which it holds in the Perspective-Neutral (PN) and Quantum-Information (QI) approaches. This separates three levels of description: a PN subsystem of a PN whole, where consistency fails on a characterised set that includes product states; a QI subsystem of a QI whole, where it holds for all states; and a QI subsystem obtained from a PN whole by kinematical partial trace, where the full weakly invariant algebra is recovered, yet consistency holds only on a proper subset. These results show that the PN approach can consistently describe only a closed, isolated system, while the QI approach can accommodate arbitrary subsystems. Tracing out a subsystem from a globally PN state yields a charge-superselected algebra, reproducing in a minimal QRF model the boundary-charge structure of edge modes. We understand the Paradox not as a genuine contradiction, but as the consequence of comparing inequivalent physical layers without tracking which information is externally and which internally accessible.

We study the thermodynamic and dynamical properties of static, spherically symmetric BHs in Einstein-Maxwell theory modified by a non-minimal $\ln(R)F^{2}$ coupling. The Hawking temperature follows from the Hamilton-Jacobi form of the fermionic tunnelling method for spin-$\tfrac12$ particles, and it carries scale-dependent logarithmic corrections. We then analyze the propagation of massless Dirac fields, compute the quasinormal-mode (QNM) spectrum with the third-order WKB approximation, and read off a quality factor whose balance between oscillation and damping depends on the logarithmic coupling in a mode-dependent way. Moving outward from the horizon, we work out the transmission of the fermionic field and its Hawking emission, and we solve the null and timelike geodesic problems to obtain the photon sphere, the shadow radius, the innermost stable circular orbit (ISCO), the associated zoom-whirl bound orbits, and the orbital and epicyclic frequencies that set the twin-peak quasiperiodic-oscillation (QPO) ratio. A photon-sphere reading of the eikonal QNM frequencies ties the geodesic sector back to the field perturbations. On the thermodynamic side, we build the phase space with quantum-geometric corrections through the Barrow entropy, and we characterize the global phase structure with the topological method, where the winding numbers of the off-shell free energy are governed by the interplay of the fractal Barrow deformation, the electric charge, and the logarithmic coupling. We find that the effective pressure vanishes exactly on the topological defect line, which links the pressure sign to the local stability of each branch.

We investigate static, spherically symmetric black hole solutions in a class of higher-curvature gravitational theories described by a Lagrangian that depends on the Ricci and Kretschmann scalars. The modified Einstein equations contain higher-order curvature terms. We develop a perturbative scheme to look for a black hole solution that deviates parametrically from the Schwarzschild geometry. We obtain first-order corrections to the metric by solving the resulting coupled differential equations. The corresponding Iyer--Wald entropy is then evaluated consistently to first order in the perturbative coupling. We show that the higher-curvature contribution gives rise to a power-law correction to the Bekenstein--Hawking entropy.

We study soliton-black hole phase transitions in asymptotically AdS planar spacetimes sourced by a cloud of strings. In the planar background, the string cloud exhibits a brush-like configuration, where the strings are aligned parallel to each other and extend along the radial direction. This leads to a stress tensor with nonvanishing components only along the temporal and radial directions. By comparing the Euclidean on-shell actions of the planar black hole and the corresponding AdS soliton under the same boundary conditions, we obtain the free energy difference between the two phases. Our results show that the string cloud parameter significantly modifies the competition between the black hole phase and the soliton phase. In particular, positive and negative values of the string cloud parameter affect the phase structure in different ways, changing the dominance relation between the black hole and soliton configurations.

We construct and analyze stationary, axisymmetric vacuum metrics obtained by magnetic Ernst inversion of Kerr-NUT with Manko-Ruiz parameter $C$ and pre-inversion twist constant $β$. The transformation lies in the Ehlers orbit; our contribution is the NUT-dependent geometry and the roles of $C$ and $β$ in its axis, horizon, singularity, and azimuthal-CTC structure. The inversion preserves the canonical Weyl radius, the signed WLP numerator $F$, and the sign of $g_{φφ}$ on regular domains. At the selected pole $x=σ$, $C=-σ$, the local axis condition and conicity are controlled by $χ_σ^{(β)}=β-2m(2σa+3l)$. Exterior zeros of the chosen seed Ernst representative obey the exact criterion $-β\in\mathcal{R}_C$; for the sampled families numerical traces yield half-line ranges with closed-form corner endpoints. For $D=a^2+l^2-2alC\ne0$, $Λ_βΣ$ has a finite nonzero seed-ring limit. High-precision calculations find direction-independent finite limits of both quadratic Weyl invariants along the sampled rays, without establishing $C^2$-extendibility. At $β=0$, exterior simple Ernst zeros are found for the sampled $C=-1,0$ cases but not for $C=+1$, consistently with the computed ranges. Near one zero the Kretschmann scalar has a generic sixth-order blow-up; a numerical angular scan identifies exceptional directions of lower order. All sampled points in the regular ($g_{φφ}>0$) exterior are Petrov type I. Candidate horizon locations remain those of Kerr-NUT, and the asymptotics are Levi-Civita type.

Interpreting the observed properties of intermediate-mass stars requires accounting for rotation, binarity, intrinsic population diversity, and measurement uncertainty. We combine these effects in population-synthesis models of 1.4-2.5 M$_{\odot}$ stars that include pulsation properties of $δ$ Sct variables. The synthetic populations successfully reproduce the distributions of stars observed in young associations, validating the mapping between intrinsic and observed stellar properties. Using these models, we infer a semi-empirical $δ$ Sct instability strip by matching synthetic and observed populations. The resulting instability strip is systematically hotter than widely used theoretical prescriptions, implying that $δ$ Sct stars are on average 0.1 M$_{\odot}$ more massive than previously assumed, with 95% of the population spanning approximately 1.50-2.30 M$_{\odot}$. We then investigate the $δ$ Sct period-luminosity relation and reproduce its recently identified second ridge with mixture models in which roughly one-third of stars have the fifth or sixth radial overtone as their dominant mode; the fraction increases in younger populations. In contrast, unresolved binarity does not account for the second ridge. Finally, we predict the population-wide distribution of asteroseismic large separations, $Δν$, finding a peak between 6 and 7 d$^{-1}$, in agreement with recent observations. We also demonstrate that $Δν$ is most reliably measured with the échelle technique, while autocorrelation methods can produce spurious detections when used in isolation. These results show that population synthesis provides a powerful framework for interpreting populations of $δ$ Sct stars and for linking stellar evolution, pulsation, and observation.

Comparison of the spectra obtained at different observing dates shows significant variations in the complex H$α$ and H$β$ line profiles, such as a systematic increase in their emission peak strengths in 2025$÷$2026 compared to those observed in 2013 as well as a strong variability in their wind component. A detected variable radial velocity $V_{\odot}$ from $-20$ to $-68$ km/s with a standard deviation of the mean value of K$\sim$8.0 km/s can be interpreted as a result of pulsations or the presence of a companion. The stationary positions of forbidden lines with a mean radial velocity of $V_{\odot}=-51.38\pm0.26$ km/s are taken as an improved systemic velocity. Overall, a set of properties of IRAS 01005+7910 points to its status as a post-AGB star, which has undergone the hot-bottom burning phase.

We ask when a spinless spherical extended test body in vacuum general relativity moves as its point-particle counterpart. In Newtonian gravity, harmonicity of the external potential gives an all-order cancellation: in source-free regions all spherical multipole forces beyond the monopole vanish. Using Dixon's covariant multipole formalism, with spherical symmetry defined as $O(3)$ invariance in the Tulczyjew-Dixon momentum rest space, we show that the relativistic analogue holds through quadrupole order in any Ricci-flat spacetime. At this order the torque vector vanishes, the force reduces to Ricci contractions, and the representative worldline is geodesic; the spherical octupole is forbidden by symmetry. This universality, however, is not an all-order effacement principle. At hexadecapole (16-pole) order the torque vector still vanishes, so the spinless sector remains dynamically consistent, but curvature-squared terms generate Weyl-driven forces that can survive in vacuum. In Schwarzschild spacetime we compute the resulting force for radial infall and the invariant leading correction to the infall proper time. We also show that periodic modulations of the hexadecapole moments act as an internal drive: Melnikov's method gives transverse homoclinic splitting and local chaotic layers near the geodesic separatrix for generic driving frequencies. The analysis is restricted to the small-body regime of Dixon's finite multipole expansion.

We investigate the spontaneous scalarization of brane-localized charged black holes, focusing on the role of quantum-enhanced tachyonic instabilities. By solving the coupled bulk-brane equations numerically within the Einstein-DeTurck formulation, we construct fully backreacted static black hole solutions and map the system from the semi-classical limit to the strongly coupled quantum-dominated regime. We demonstrate that the holographic quantum effects, parameterized by $κ$, significantly modify the background geometry. Stability analysis, conducted via both effective potential diagnostics and quasinormal mode calculations, confirms that this geometric deformation deepens the negative well of the effective potential, thereby triggering a tachyonic instability. Integrating these results, we establish the global phase diagram of spontaneous scalarization in the $T_h/μ-κ$ parameter space. The results reveal that these quantum-enhanced instabilities promote scalarization by substantially raising the critical temperature, providing a clear quantitative signature of quantum effects on black hole hair formation.

While hundreds of protoplanetary discs have been studied in great detail, the detection of protoplanets still embedded in their native discs remains rare. WISPIT 2 is only the second laboratory allowing for direct study of planet formation while in progress. The recently discovered system hosts two giant protoplanets in a multi-ringed disc. Here, we aim at characterising the WISPIT 2 host star spectroscopically to determine its stellar properties, accretion rate, and inner disc diagnostics, providing a more complete picture of the system. We present optical and near-infrared spectroscopic observations obtained with the ESO VLT/X-Shooter and 2.2 m/FEROS instruments. We model the stellar spectrum to determine the spectral type and effective temperature, analyse the emission lines to estimate the accretion rate, and search for evidence of a close stellar companion using radial velocity measurements. Our observations reveal that WISPIT 2 is a spectroscopic binary. The binary has a period of $4.8\pm 0.1$ days, which corresponds to a semi-major axis of $0.072$ au or $15.54 R_\odot$, assuming co-planarity with the disc and a circular orbit. The binary system consists of a $\sim 0.97 M_{\odot}$ primary of spectral type K3 ($T_{\rm eff} \sim 4700K$), and a $\sim 0.33M_\odot$ secondary (mass ratio $\sim$0.34). We detect weak H$α$ emission, implying an accretion rate of $\sim 2 \times 10^{-11}\,M_{\odot}\,\mathrm{yr}^{-1}$. However, this value is below the chromospheric level, suggesting little to no ongoing accretion onto the young stars. This discovery makes the WISPIT 2 disc the first circumbinary system with directly imaged protoplanets, establishing this system as a unique benchmark for studying planet formation and disc evolution around binary stars.

HRMOS (High-Resolution Multi-Object Spectrograph) is a proposed new instrument for the ESO Very Large Telescope (VLT) developed in the context of the ESO VLT Beyond 2030 call. It is designed to fill a unique and currently unoccupied region in the observational landscape: the combination of very high spectral resolution (R=80,000) with multi-object capability (50-60 simultaneous targets), a radial-velocity (RV) precision of 10m/s, and coverage of three key spectral windows (385-421nm, 480-522nm, 623-677nm). Scientifically, HRMOS will address a rich portfolio of high-priority astrophysical questions spanning from f giant exoplanet formation, to nucleocosmochronology and constraints on cosmological parameter, to probing hierarchical galaxy assembly outside the Milky Way. The instrument is based on a modular approach and it consists in four primary subsystems, the Front End with a hybrid fiber-positioning and atmospheric dispersion correction (ADC) architecture, the Fiber Link with double-scrambling and the image slicers, three spectrographs based on volume phase holographic (VPH) gratings covering the three spectral ranges and a Calibration Unit.

We consider a cosmological model where dark matter and baryons interact via a pure momentum transfer. We present the foundations of the interaction and derive its main equations. In this framework, only the Euler equations for the two interacting components differ from those in the standard scenario. We also explain how this interaction relates with the common Tight-Coupling Approximation schemes typically used in many Boltzmann solver. After that, we implement the model in CLASS and CAMB Boltzmann solvers, which relay on those Tight-Coupling Approximation schemes and in SymBoltz, which is free from any approximation scheme. We demonstrate full consistency among all the implementations in the difference codes. We demonstrate how this type of interactions are useful regarding the $σ_8$ tension by producing a reduced clustering on smaller scales. Using cosmic microwave background, baryon acoustic oscillations, supernovae type Ia data and low-redshift constraints from Sunyaev-Zeldovich cluster counts and from DES year 6 results, we show the interacting scenario is mildly preferred when those low-redshift datasets are included.

For over a century, following the work of Eddington, Kapteyn, Malmquist and others, astronomers have wrestled with selection biases when making inferences from samples of objects. Typically, selection is performed on the same observations used to make the measurements of interest. However, selecting objects using one observable while analyzing another can also lead to a selection bias when the observables are correlated. Within a Bayesian framework, unmodelled selection effects correspond to a misspecified generative model. We derive the likelihood for truncated selection in correlated observables and demonstrate its usefulness by searching for residual selection effects of this form in Cepheid variable star brightness measurements used in recent distance-ladder measurements of the Hubble constant H0. We specifically look for the form of bias where the selection correction depends on the photometric uncertainties, which the Cepheid data can constrain while simultaneously measuring H0. We find only weak evidence for non-zero residual selection corrections, at a significance of (1.2sigma) to (1.9sigma), depending on the distance prior adopted. Including a single extra parameter to model the unknown cut-off lowers the recovered H0 by -0.7km/s/Mpc to -1.1km/s/Mpc, again depending on the distance prior. Allowing for a different selection for each host galaxy can decrease H0 further, although this becomes very sensitive to the distance prior applied. While introducing a new selection correction cannot by itself explain the Hubble tension, it may be a component of a multi-faceted solution that includes the choice of priors on distances and other effects.

The majority of spectroscopic surveys assume local thermodynamic equilibrium (LTE) during the modeling of stellar spectra. This assumption begins to break down for luminous stars, like the red giants targeted by SDSS-V's Milky Way Mapper Survey in its Galactic Genesis program. In this work, we present non-LTE (NLTE) abundances for 360,000 red giant stars in Milky Way Mapper DR19, from infrared APOGEE spectra. We generate NLTE spectra using precomputed departure coefficient grids for Na, Mg, Si, Al, Ca, Ti, Mn, and Ni. To fit APOGEE spectra at scale, we train neural network emulators (NNEs) to synthesize LTE and NLTE H-band spectra. After verifying that the NNEs are accurate, we fit the APOGEE spectra with ASPCAP results that fall within the same parameter range as the training data. We find strong NLTE effects on the order of 0.1\,dex for Al, Mn, and Ti, and smaller effects for Si and Ni. We provide a catalog of the results of our LTE and NLTE fits, as well as NLTE-corrected ASPCAP abundances using a polynomial fit correction.

We present a spectroscopic survey of young stellar objects (YSOs) in Sh2-284, a distant (~4.5 kpc), low-metallicity (Z~1/3 Zsun) star-forming region (SFR) toward the Galactic anticenter. Candidate YSOs were selected using mid-infrared Spitzer/IRAC data with optical and near-infrared photometry. Follow-up spectroscopy was conducted with MODS at the Large Binocular Telescope. We characterize the stellar and accretion properties of the disk-bearing population in a metal-poor environment, probing stellar masses from ~2.95 Msun to ~0.35 Msun. This work constitutes the first wide-field (~45'x45') spectroscopic investigation of YSOs in Sh2-284, providing a comprehensive view of star formation in this Galactic environment. We provide tentative first estimates of iron abundance for three low-mass targets and lithium abundance for a limited number of objects, offering an initial characterization of the chemical properties of these stars. We adopt a multi-diagnostic approach based on nine tracers, exploiting the spectral coverage of the instrument, using H_alpha, H_beta, H_gamma, CaII infrared triplet, Pa_eta, Pa_zeta, and Pa_epsilon emission lines. We derive key accretion properties and compare them with those measured in nearby, solar-metallicity SFRs to explore potential metallicity-driven differences in accretion behavior and disk evolution. We tentatively find indications of a flattening in the flux-flux relations of the metal-poor YSOs compared to solar-metallicity samples, a behavior that is recovered across diagnostics. Our observations indicate that the selected disk-bearing YSOs in this metal-poor environment exhibit resilient accretion activity, spanning from ~2.3x10^(-10) Msun/yr and ~1.0x10^(-6) Msun/yr, and a median rate of ~2.2x10^(-8) Msun/yr. Overall, this demonstrates that gas accretion can be efficiently sustained in sub-solar metallicity environments.

We study inflation driven by a scalar potential arising from composite-sector dynamics, inspired by generalized composite Higgs models. The introduction of a non-minimal coupling, possessing the same functional form as the potential, induces a flattening at large field values that enables successful inflation. We analyze the conditions for ultra-slow-roll inflation, which leads to enhanced curvature perturbations, by combining analytical criteria near the inflection point with comprehensive numerical scans of the parameter space. The region consistent with cosmic microwave background constraints and yielding approximately <inline-formula><mml:math><mml:mrow><mml:msub><mml:mi>N</mml:mi><mml:mi>e</mml:mi></mml:msub><mml:mo>≍</mml:mo><mml:mn>55</mml:mn><mml:mo>-</mml:mo><mml:mn>60</mml:mn></mml:mrow></mml:math></inline-formula> e-folds also predicts primordial black holes with masses in the range <inline-formula><mml:math><mml:mrow><mml:msup><mml:mn>10</mml:mn><mml:mn>3</mml:mn></mml:msup><mml:mo>-</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>5</mml:mn></mml:msup><mml:mspace></mml:mspace><mml:mtext>g</mml:mtext></mml:mrow></mml:math></inline-formula>. Although such ultra-light primordial black holes are typically expected to have evaporated, recent proposals invoking evaporation suppression via memory-burden effects could allow their survival as viable dark matter candidates. Under this assumption, the predicted gravitational wave signal lies in a frequency range currently inaccessible to any existing or proposed detectors. Although no experimental proposals presently reach this frequency band, our results provide strong motivation to push the frontiers of gravitational wave detection towards these unexplored high-frequency regimes.

We investigate gravitational lensing by compact objects embedded in anisotropic Bianchi-I cosmologies using directional Jacobi maps within the thin-lens approximation. The formalism is developed for a general diagonal Bianchi-I spacetime and specialized to the Kasner solution as an analytically tractable background. Using the Ellis--Bronnikov wormhole and the Schwarzschild black hole as representative lenses, we derive anisotropic lens equations, characteristic axis-aligned lensing scales, and the corresponding critical curves. We show that the directional splitting of the characteristic scales depends on the complete source--lens--observer optical propagation and provides a geometric probe of anisotropic expansion independent of the overall lens scale. By contrast, the exact critical curves exhibit a much weaker deformation, indicating that characteristic-scale splitting and critical-curve morphology probe distinct aspects of the lens mapping. The comparison between wormhole and black-hole lenses further reveals that identical anisotropic backgrounds are filtered differently by distinct weak-field deflection laws. These results provide a simple framework for disentangling cosmological anisotropy from the local geometry of compact lenses.

Recent results demonstrate that there exists a sharp, Page-like transition for the complexity of subsystems of Haar-random states as their fractional subsystem size surpasses one half. They further demonstrate that this transition also occurs for the holographic complexity of boundary subregions of eternal AdS black holes, assuming the Complexity$=$Volume (CV) proposal for subregions. We interpret this transition as a crossover from spectrum-dominated to basis-dominated subsystem complexity, reflecting the breakdown of approximate thermality beyond half-system size. We then apply this reasoning to an evaporating AdS black hole coupled to a bath, modeled by a quantum circuit undergoing random evolution on an interior subsystem of diminishing size. Using the basis-spectrum decomposition of subsystem complexity, we argue for a similar Page-like transition in the radiation complexity. We then show, using CV for subregions, that the same transition appears in the holographic complexity of the radiation subsystem through the emergence of an island, whose volume gives the dominant contribution. We argue that the island volume contribution resolves an apparent complexity paradox analogous to the information paradox.

We develop a local and covariant framework for constraining the evolution of generalized entropy along null horizons, combining classical geometric methods with constraints from quantum field theory. Building on the quantum focusing conjecture (QFC), which restricts entropy evolution along null directions, we derive a Raychaudhuri-type differential inequality for the generalized expansion that makes its local dependence on expansion and shear explicit. The resulting relation, $\frac{dΘ}{dλ} \le -θΘ+ \tfrac{1}{2}θ^2 - σ^2$, reveals a direct interplay between expansion and shear in controlling entropy flow: shear contributes negatively and yields a monotonic suppression of the generalized expansion in shear-dominated regimes, while expansion provides a competing geometric source term. This structure provides a local geometric formulation consistent with the QFC and clarifies its interpretation as a constraint on entropy evolution. We further show that quantum extremal surfaces correspond to configurations characterized by $Θ=0$, whose local stability properties are governed by the same geometric data. In addition, we derive a bound on the exponential separation of nearby null generators, indicating that the same combination of expansion and shear also controls geometric instability. Our results provide a local geometric refinement of entropy bounds in semiclassical gravity and establish a direct connection between null geometry, quantum energy conditions, and entropy flow, offering a local geometric perspective on horizon thermodynamics beyond global formulations.

Here, we investigate the observational viability of fractional power law inflationary potentials, $V(\varphi) \propto \varphi^n$ ($n=$ 1/3, 2/5, 2/3), within the effective framework of Loop Quantum Cosmology (LQC) incorporating inverse volume corrections. By employing LQC modifications to the background dynamics and cosmological perturbation equations in the semi-classical regime, we analytically derive the scalar spectral index $n_{\rm s}$ and the tensor-to-scalar ratio $r$. These theoretical predictions are confronted with the latest high precision joint observational constraints, including ACT DR6, Planck 2018, DESI BAO, and BICEP/Keck datasets (P-ACT-LB-BK18). While steeper classical power law models are in severe tension with modern data, our results demonstrate that the inclusion of LQC inverse volume effects induces a prominent negative shift in $n_{\rm s}$. Consequently, for specific viable ranges of the quantum geometric parameters ($σ$ and $δ$), the theoretical predictions are translated horizontally across the $r-n_{\rm s}$ plane. This mechanism successfully steers classically disfavored models back into the tightly constrained 68\% and 95\% CLs, significantly improving their consistency with precision cosmological data.

Classical general relativity allows time travel, but Hawking [Phys. Rev. D 46, 603 (1992)] conjectured that quantum mechanics ultimately prevents it. Here we propose a feasible experiment to test Hawking's Chronology Protection Conjecture in optics. In this scheme, time travel is inherently related to lasing, and the amplified vacuum fluctuations above laser threshold establish the quantum limits of time travel

It was proposed that extra dimensions can acquire large size by higher dimensional inflation connecting two large hierarchies in particle physics and cosmology, namely the weakness of the actual gravitational force to the largeness of the observable Universe, in terms of one fundamental scale. This proposal is consistent with the observed approximate scale invariant power spectrum of primordial density perturbations only for one or two extra dimensions of around the micron size. While a cosmological history connecting the period of higher dimensional inflation to the beginning of the standard cosmology has recently been studied, we investigate here the TransPlanckian Censorship Conjecture in that context and show that it drastically constrains the parameter space of the model.

The Integrated Sachs--Wolfe (ISW) effect probes the late-time evolution of gravitational potentials and provides a complementary test of the nature of dark energy. We investigate whether the redshift evolution of the ISW effect can provide new constraints on the time evolution of dark energy. We consider theoretical predictions for the standard $Λ$CDM cosmology and alternative $w_0w_a$CDM models favoured by recent DESI BAO and DES Y6 constraints. We perform a tomographic cross-correlation analysis of the {\it Quaia} quasar catalogue and \textit{Planck} CMB temperature maps to measure the ISW signal over a broad redshift range, spanning about $(10h^{-1}~\mathrm{Gpc})^3$ comoving volume. We assess the robustness of the inferred ISW amplitude against variations in sky coverage, multipole range, and tomographic binning. We detect the ISW effect at a significance of $2.8 \, σ$, corresponding to an amplitude of $A_{\rm ISW}\simeq1.69\pm0.61$ relative to the \textit{Planck} $Λ$CDM prediction. The inferred signal remains stable against various analysis choices, including various CMB maps (SMICA, NILC, SEVEM), different choices of $\ell_{\rm max}$ and the number of multipole bins, and an alternative quasar bias model. The measured ISW amplitude is moderately stronger than predicted by the fiducial $Λ$CDM cosmology, and the alternative $w_0w_a$CDM models do not account for this discrepancy. Future tomographic ISW measurements with improved quasar catalogues from {\it Gaia} and forthcoming wide-area galaxy surveys such as {\it Euclid} and DESI will help clarify the origin of this difference.

The exponential <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> gravity model provides a theoretically well-motivated extension of General Relativity, introducing a modified gravitational dynamics at late times consistent with a dynamical dark energy scenario, while recovering the <mml:math><mml:mi>Λ</mml:mi></mml:math>CDM-like regime at high redshifts with a smooth transition. Using a Bayesian Markov Chain Monte Carlo (MCMC) analysis, we constrain the parameters of the exponential <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> model in combination with the total neutrino mass <mml:math><mml:mrow><mml:mo>∑</mml:mo><mml:msub><mml:mrow><mml:mi>m</mml:mi></mml:mrow><mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>, employing the latest measurements from cosmic chronometers, the DESI DR2 BAO data, the CMB acoustic scale, and the Pantheon+ supernovae compilation, comparing the results with the <mml:math><mml:mi>Λ</mml:mi></mml:math>CDM and the <mml:math><mml:mrow><mml:msub><mml:mrow><mml:mi>w</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>w</mml:mi></mml:mrow><mml:mrow><mml:mi>a</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>CDM models. Our results show that the exponential <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> model remains consistent with current observations while partially alleviating the Hubble tension and the neutrino mass problem relative to <mml:math><mml:mi>Λ</mml:mi></mml:math>CDM, although the constraints on <mml:math><mml:mrow><mml:mo>∑</mml:mo><mml:msub><mml:mrow><mml:mi>m</mml:mi></mml:mrow><mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> are tighter than those obtained for the phenomenological <mml:math><mml:mrow><mml:msub><mml:mrow><mml:mi>w</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>w</mml:mi></mml:mrow><mml:mrow><mml:mi>a</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>CDM scenario. These results indicate that the interplay between modified gravity and neutrino physics in the late Universe may offer a viable framework for further investigating cosmological tensions.

The EXoplanet Climate Infrared TElescope (EXCITE) is a balloon-borne mission dedicated to measuring spectroscopic phase curves of hot Jupiter-type exoplanets. Phase curve measurements can be used to characterize an exoplanet's longitude-dependent atmospheric composition and energy circulation patterns. EXCITE carries a 0.5 m primary mirror and a moderate resolution diffraction-limited spectrograph with spectral coverage from 0.8 to 3.5 μm. EXCITE is designed to fly from a long-duration balloon. EXCITE will observe through the peak of a target's spectral energy distribution and through spectral signatures of hydrogen and carbon-containing molecules. In this paper, we present the science goals of EXCITE, detail the as-built instrument, and discuss its performance during a 2024 engineering flight from Fort Sumner, New Mexico.

The Hoyle-Narlikar action-at-a-distance formulation for gravity remains one of the most mathematically rigorous attempts to incorporate Mach principle into physics. A lesser known, but no less significant, is the fact that it is also one of the first in which two point functions play a key role in determining spacetime structure, and hence, are more fundamental than the metric. We argue that these ingredients have direct relevance for modern attempts to understand fundamental, emergent, and informational aspects of quantum spacetime. The non-local description of quantum spacetime, with Planck scale $\ell_0$ as a zero-point length, provides a mechanism through which the entire universe can inform local causal structure, `a la Mach, Hoyle, and Narlikar. In this chapter, we highlight certain key aspects of the HN theory and then discuss case studies from modern research that illustrate how the quantum spacetime might realize a version of the quantum gravitational Mach principle.

I argue that dimensional analysis can appropriately be thought of as a gauge theory. This picture naturally leads to the usual quantity calculus through inherent properties of Lie groups, Lie algebras, and representation theory. The gauge theory interpretation is perhaps strongest for non-constant units. I explain how Stevens's classification of scales of measurement can be understood by choices of different gauge groups. Finally, I reinterpret and rephrase the Buckingham-Π Theorem in a way that also applies to typical gauge theories. Counting the number of ''Π''s becomes a well-known problem of invariant theory.

We find a system of differential equations on an initial data surface whose solutions are in bijection with unit vector fields on the Einstein $Λ$-vacuum development that are proportional to a Killing vector. We refer to these conditions as the \textit{unit Killing initial data} (uKID) equations, analogous to the classical \textit{Killing initial data} (KID) equations. The uKID equations can be useful in a setting where only the unit-normalized part of the Killing vector is geometrically distinguished. We eliminate the scaling degree of freedom of a general Killing vector to obtain the space-time equations characterizing unit normalized Killing vector fields and also the uKID equations. These equations are also prolonged to canonical connection form, showing their finite type character. Finally, we obtain an independent derivation of the uKID equations by revisiting the propagation identity method, which has previously been used to characterize the initial data of other geometric equations.

Binary lens microlensing events in which the source crosses a caustic produce sharp, distinctive magnification peaks and can therefore be readily identified. In this paper, we explore the importance of binary orbital motion for the binary lens caustic crossing cross section. If the orbital timescale of the binary system is smaller than the Einstein ring crossing timescale, the caustics sweep out a larger area in the source plane, generally enhancing the cross section. We find that face-on binaries in circular orbits exhibit a substantial increase (up to 4$\times$) in the cross section for caustic crossings. However, highly inclined orbits produce a net decrease relative to a static face-on binary with the same semi-major axis. Using a sample of ~$2800$ synthetic binary microlensing events drawn from a realistic Milky Way population-synthesis model, we calculate the average change in the caustic crossing cross section for individual systems with and without orbital motion. Although orbital motion can significantly alter the cross section for specific geometries, we find that, when averaged over the full population, it produces only a small, negligible increase of ~$0.1\%$ in the average cross section. We also compute the overall rate of caustic crossing binary events in the simulated sample and find that, with sufficiently dense photometric sampling, $6.3 \pm 0.2\%$ of microlensing events with impact parameter $u_0 < 1$ should exhibit caustic crossings. This rate depends on the distributions of binary separation, mass ratio, and multiplicity of stars and compact objects, and can therefore be used to test our understanding of these underlying properties.

We investigate static, spherically symmetric charged black holes in Einstein gravity coupled to Euler--Heisenberg (EH) nonlinear electrodynamics, including purely electric, purely magnetic, and dyonic configurations. Rather than adopting the Hamiltonian formulation based on the auxiliary electromagnetic invariant $\mathcal{P}$, we work directly with the physical electromagnetic invariant $\mathcal{F}$ in the Einstein-Euler--Heisenberg Lagrangian, thereby describing all charged configurations without introducing auxiliary variables. Within this approach, we derive an exact analytical solution for the purely electric case, recover the purely magnetic solution directly from the field equations, and construct the dyonic solutions numerically. We systematically study the horizon structure, causal properties, and thermodynamics of these solutions. While the purely electric branch exhibits the familiar Reissner--Nordström horizon structure, the purely magnetic branch naturally admits a novel three-horizon configuration consisting of one event horizon and two inner horizons. The dyonic solutions continuously interpolate between the electric and magnetic limits and exhibit either one- or three-horizon configurations, depending on the magnetic-to-electric charge ratio and the EH coupling. We further show that the EH nonlinear interaction significantly modifies the horizon structure and thermodynamic properties of charged black holes while leaving the central curvature singularity unresolved. These results demonstrate that EH nonlinear electrodynamics gives rise to qualitatively new causal structures beyond Einstein--Maxwell theory.

We investigate cosmic evolution in generalized Nash's theory of gravity involving the quadratic Ricci invariant $χ=R_{μν}R^{μν}$. The analysis is divided into two complementary branches. First, we study the power-law family $f(R,χ)=R^α+βχ$ as a reduced autonomous system in a flat FLRW background. Because the adopted variables become singular at the Einstein--Hilbert limit $α=1$, the phase-space analysis is restricted to $α\neq1$, with $α=2$ used as a representative quadratic benchmark. This benchmark contains radiation-like boundary configurations, restricted scaling saddles, and de Sitter-like accelerating endpoints (a stable node away from $α=2$ and non-hyperbolic at the benchmark itself), but not a complete regular radiation-to-matter-to-de Sitter sequence. Second, we constrain the regular observational branch $f_{\rm obs}(R,χ)=R-2Λ+βχ$, which reduces exactly to flat $Λ$CDM when $β\to0$. The Hubble rate is obtained from the reduced $Λ$CDM-connected background branch, integrated over $0\le z\le10$ and matched at higher redshift to a standard radiation+matter+$Λ$ background. Using SNe~Ia, BAO, and Planck~2018 compressed CMB distance priors, we find an expansion history very close to $Λ$CDM, with the quadratic correction tightly constrained around the nested standard-model limit. The resulting bound on $β$ should be interpreted as a background-level constraint within this reduced prescription, not as a perturbation-level viability test of the full higher-derivative theory.

We construct numerically two families of static five-dimensional Lorentzian metrics, solutions of vacuum Einstein equations with a negative cosmological constant, one with and one without a black hole region, with a metric at sections of conformal infinity which is conformal to any squashed three-dimensional sphere.

We present a framework for inferring stellar ages from spectroscopic stellar parameters, calibrated with ensemble kinematics by averaging over the median vertical action, $J_z$, for stars with similar atmospheric and chemical properties, yielding self-consistent age estimates across the Hertzsprung-Russell (HR) diagram. We refer to these ages as ensemble kinematic ages as the age scale is calibrated from the average kinematics of ensembles of stars with similar stellar parameters. Individual stellar kinematics are not used in assigning ages. We validate the method against subgiant ages, achieving an accuracy of ~30%, comparable to [C/N]-based estimates. We find a clear age-$J_z$ relations that enable age inference up to ~10 Gyr for both the high- and low-$α$ disks. Applying this framework to 1.5 million LAMOST stars, we derive ages for subgiants and giants with typical uncertainties of ~2 Gyr. The inferred ages agree well with literature age catalogs, with no significant systematic trends as a function of $\log g$. We also demonstrate the potential of empirical isochrones to calibrate theoretical stellar models. We identify an old (~7 Gyr) population within the low-$α$ disk but draw no firm conclusions. Although ensemble kinematic ages are statistical and sensitive to selection effects, Galactic potential assumptions, and Galactic location, they provide a robust population-level tool for Galactic archaeology, complementing traditional age indicators and extending age estimates across the full HR diagram.

In these lecture notes for the Les Houches School on Loop Quantum Gravity 2025, which took place in September 2025, we give a pedagogical review of the basics of the spinfoam framework for a quantum gravity path integral. While spin network states in loop quantum gravity describe the quantum geometry of the 3d space as dynamical networks of entangled quanta of volumes, spinfoams define transition amplitudes for those spin networks using the reformulation of general relativity as an "almost-topological" field theory and tools from quantum BF theory and topological state-sums. The lectures were a short format of three times one hour and a half, only allowing to cover the basics and offer a glimpse of more advanced lines of research. We introduce spin foam path integrals for increasing spacetime dimensions starting with 2d BF theory, then build up to 3d quantum gravity with the Ponzano-Regge state-sum and the Turaev-Viro invariant, and finally the quantization of general relativity in four dimensions.

Spin-orbit misalignment is increasingly observed in binary stars, but its origin remains uncertain. We study the secular spin dynamics of stellar binaries and derive the associated Cassini states. We isolate a spin-spin resonance that arises when the two stellar spin-precession frequencies become commensurate. This resonance can excite the obliquity of the secondary to high values even when the primary is only weakly tilted. Magnetic braking favors the formation of high-obliquity states, whereas tidal dissipation may suppress them. Spin-spin coupling may thus help explain the observed diversity of obliquities in young stellar binaries.