We evaluate the effective action for the Dynamical Casimir Effect (DCE) for a real scalar field in $d+1$ dimensions within the worldline formulation of quantum field theory. The scalar field is coupled to a spacetime-dependent mass term, which here plays the role of the moving medium and imposes imperfect boundary conditions on time-dependent surfaces. Expanding in powers of the departure of the geometry from a planar configuration, the worldline path integral factorizes into simpler, lower-dimensional ones. In the limit of a strong coupling to the surface, we recover the Dirichlet result and derive the systematic corrections in inverse powers of the coupling, calculating the imaginary part of the effective action to arbitrary order in those inverse powers. Finally, we also apply the method to a two-surface configuration.

Recent proposals suggest that detecting entanglement between two spatially superposed masses would establish the quantum nature of gravity. However, these gravitationally induced entanglement (GIE) experiments rely on assumptions about subsystem independence. We sharpen the theoretical underpinnings of such proposals by examining them through the lens of algebraic quantum field theory, distinguishing operational and algebraic notions of independence. We argue that state and measurement independence of subsystems, essential to the experimental logic, is nontrivial in the presence of gauge constraints and gravitational dressing. Using gravitationally dressed fields, we recall that commutation relations between spacelike separated observables are nontrivial, undermining strict Hilbert space factorization. We further explore the implications for entanglement witnesses, investigating the Tsirelson bound when subsystem algebras fail to commute, and showing that the bound persists for a suitably symmetrized Clauser–Horne–Shimony–Holt observable even though the operational status of such “joint” observables becomes delicate when commutativity is lost. Our analysis highlights how, even within linearized covariant quantum gravity, violations of microcausality may affect the interpretation, modeling, and design of proposed laboratory tests of quantum gravity, despite remaining negligible for current experimental regimes. Although we consider GIE-style protocols as a concrete case study, the subsystem-independence issues we highlight are generic to low-energy (perturbative) quantum gravity. Finally, we derive estimates for dressing-induced microcausality violations, which suggest a complementary avenue to current proposals as a probe of the quantum nature of gravity (likely far beyond current experimental sensitivity, though).

Schrödinger symmetry has been shown to emerge in a “fluid limit” from the full superspace to several minisuperspace models. To investigate one aspect of the robustness of this emergent symmetry, we consider two spherically-symmetric static minisuperspace models with matter fields at the classical level: (i) a Maxwell field with a cosmological constant, and (ii) $n$ massless scalar fields. By developing a method based on canonical transformations, we demonstrate that for model (i), three-dimensional Schrödinger symmetry emerges, and the solution is the (anti–)de Sitter Reissner-Nordström spacetime; for model (ii), ($2+n$)D Schrödinger symmetry appears, and the solution is a generalized Janis-Newman-Winicour spacetime and its “interior,” a Kantowski-Sachs–type closed universe. Furthermore, for the vacuum model, we find that two-dimensional Schrödinger symmetry holds with different lapse functions and minisuperspace coordinates, suggesting the potential, yet unconfirmed, covariance of the symmetry. Finally, we propose a physical interpretation of the symmetry under the Hamiltonian constraint $H$: symmetry generators commuting with $H$ map a solution to another one, while those noncommuting with $H$ generate a new theory with the Schrödinger symmetry and the transformed configuration is a solution to the new theory. These results reinforce the robustness of the emergent Schrödinger symmetry and open new frontiers for exploring dynamics of matter and gravity.

Spontaneous scalarization phenomenon in scalar-tensor gravity is known to be a form of phase transition, and it was recently shown that the order of this transition changes depending on the parameters of the theory. There exists a phenomenological description of this result based on Landau theory, but the underlying mechanisms which determine the coefficients of the Landau expansion were unknown. In this study we calculate these coefficients starting from first principles. To this end, we start with an energy functional that describes the nonlinear behavior of the theory, and reduce it to an energy function. This allows us to explain the previously observed, but not well-understood, features of the scalarization phase transition, and enables us to predict which phase transition order will be present for which coupling function or in which regime of the parameter space. The details of the phase transition determine certain astrophysical observables such as signals sourced by transitions from metastable states in first-order scalarization. Thus, predicting these details is an important part of understanding scalarization itself.

High-resolution images of M87* and Sgr A* turn black-hole shadows into practical probes of strong gravity, making it useful to distinguish between observables that encode the overall angular scale of a shadow and those that remain sensitive to the detailed geometry of the underlying spacetime. We compare the Kerr geometry with a static, axisymmetric quadrupolar benchmark spacetime (the Zipoy-Voorhees or $q$-metric) using quantities tied directly to measurement and defined on the local observer sky. In this work, the $q$-metric is used not as a literal astrophysical black-hole model, but as a controlled nonrotating reference geometry that isolates quadrupolar deformation without frame dragging. For the $q$-metric we derive compact finite-distance expressions for the equatorial photon sphere, the critical impact parameter, and the angular radius seen by a static observer, thereby clarifying the domain and limits of shadow formation: the photon sphere exists for $q>-3/2$ with $r_{\mathrm{ph}}^{(q)}=m(3+2q)$, while the requirement that it lies outside the curvature singularity and that $b_{\mathrm{cr}}(q)$ be real and positive restricts observable static shadows to $q>-1/2$, with the angular size decreasing monotonically as $q$ becomes more negative within this domain. In the case of the Kerr spacetime, we use the standard parametric description of spherical photon orbits to compute the shadow at finite distance and arbitrary inclination. To compare the two models operationally, we define $q(a;r_O,{\vartheta }_O)$ by equating a single, well-specified angular radius on the observer’s sky; we adopt the vertical (top) radius to avoid edge degeneracies. In the far field, this yields a single-valued correspondence within the explored domain governed by the critical impact parameters, $b_{\mathrm{K}}(a,{\vartheta }_O)=b_{\mathrm{cr}}(q)$, and a small–deformation approximation provides accurate estimates at moderate spin. We emphasize that this construction defines a controlled size-level correspondence rather than a full degeneracy of the shadow contour. We identify a regime in which static quadrupolar deformations reproduce the Kerr vertical angular scale and show that simple additional observables—the vertical and horizontal radii, the centroid shift and a basic distortion measure—restore discriminating power within the present contour-level benchmark setting. In this way, the analysis separates what can be matched at the level of a single angular-size observable from what remains specifically sensitive to rotation. The framework offers a systematic route from image-plane data to a controlled comparison between Kerr and a static quadrupolar benchmark geometry at finite distance, helping identify which shadow features are associated with scale matching and which remain specific to rotational asymmetry.

Within general relativity, black holes are widely regarded as the ultimate benchmark for compactness in the Universe. Recently, however, neutron star models have been constructed in a higher-curvature theory—quasitopological gravity (QTG)—whose compactness can exceed the black-hole limit [S. Li, H. Lü, Y. Gao, R. Xu, L. Shao, and H. Yu, companion Letter, Neutron stars more compact than black holes as a probe of strong-field gravity, Phys. Rev. D 114, L021504 (2026).]. Here we present a detailed analysis of both the equilibrium structure and radial stability of such configurations in QTG. By examining several representative equations of state and different values of the gravitational coupling constant, we find that in the high-central-density regime the compactness exceeding the black-hole bound exhibits a universal behavior in QTG. We further show that QTG corrections grow increasingly significant at large central densities and can stabilize configurations that are radially unstable in general relativity over a broad parameter range. These results establish ultracompact neutron stars in QTG as theoretically viable strong-field configurations and provide a foundation for further investigations of their dynamical and phenomenological implications.

Cosmology aims to understand the physical properties of our Universe on its largest scales. One such feature is the expansion of the Universe, which currently seems to be dominated by a phenomenon referred to as dark energy. The physical nature and properties of dark energy are one of the main topics of investigation of modern cosmology. Observational cosmology aims to reconstruct the evolution and the equation of state of dark energy, while theoretical cosmology aims to provide methods for such reconstructions and models explaining the nature of dark energy. If the equation of state, defined as the ratio of pressure to density $w=p/\rho$, deviates from $-1$, i.e. $w\ne -1$, then this would imply the existence of some sort of dynamical process behind dark energy. Most investigations assume a specific parametric form of $w$, e.g. $w(z)=w_0+w_{a}z/(1+z)$, with $z$ being redshift and $w_0$ and $w_a$ being constants. The analysis of the data is then reduced to fitting the model to the data. In this work, we take a different approach. Instead of imposing a predefined parametric form for $w(z)$, we reconstruct the equation of state indirectly from the dimensionless comoving distance $D(z)$ and its derivatives. This avoids assuming a specific physical parametrization of dark energy, such as the CPL form, but still requires adopting functional representations for the distance–redshift relation itself. The method should therefore be regarded as a hybrid or semiparametric reconstruction approach: the parametrization is shifted from the equation of state to the observable distance function. Finally we apply the method to the Pantheon+ SH0ES data. The results are consistent with dark energy being the cosmological constant, i.e. $w=-1$. Future surveys such as Legacy Survey of Space and Time will provide more data and narrow down the uncertainty. This in turn will yield tighter constraints on dark energy.

The inflationary $B$-mode signals encode invaluable information about the origin of our Universe, and searching for potential signatures of primordial gravitational waves (PGWs) is one of the major science goals for future precision observations of cosmic microwave background (CMB) polarization. However, dominant $B$-mode signals of both Galactic foreground contamination and gravitational lensing effects prevent direct measurements of the PGW $B$-mode signals. There are existing proposals which can effectively eliminate these two contaminants but issues remain for future high-sensitivity and multifrequency CMB polarization observations, such as spatially varying spectral energy distribution (SED) of polarized foreground and cosmological $B$-mode signals due to primordial magnetic fields. In this work, we investigate inference of PGW $B$-mode signals in the presence of both complexities. We employ a constrained moment internal linear combination method to remove polarization signals arising from spatially varying SEDs. Also, we employ a power-spectrum-based approach to extracting both the Galactic and cosmological $B$-mode components. Two methods have been validated by mock data and different consistency tests have been performed. We apply these two methods to end-to-end simulations for future high-sensitivity and multifrequency polarization observations and investigate the detectability of different $B$-mode signals in the presence of complex polarized foregrounds under different scenarios. This study will be important for new physics studies with $B$-mode signatures.

We assess the impact of nonideal, continuously rotating half-wave plates (HWPs) on cosmic microwave background (CMB) polarization measurements targeting large angular scale signal. Such hardware solutions are used in or planned for multiple modern CMB efforts, both ground-based, for instance, small aperture telescopes of simons observatory or satellite borne, such as litebird. Using a frequency-dependent parametric model based on the Mueller matrix formalism, we characterize the induced mixing of Stokes parameters. Through end-to-end simulations, we propagate these effects from time-ordered data to cosmology via map-making and component-separation stages, quantifying their impact on the $B$-mode power spectrum and the tensor-to-scalar ratio, $r$. Our analysis shows that neglecting the frequency dependence of a three-layer HWP gives rise to significant polarization leakage, biases foreground spectral parameters, and thus leads to residual contamination in the recovered CMB maps. To mitigate these effects, we investigate multiple analysis strategies progressively incorporating a more complete description of the instrumental response. At the map-making level, this requires generalizing the standard pointing matrix, to account not only for the scanning strategy but also for the full time- and frequency-dependent instrumental response. We find that two standard HWP models, referred to as effective and stack HWP models, reduce the biases only down to $r\sim {10}^{-2}$; however, a more advanced approach based on a generalization of both map-making and component-separation procedures, implemented using jax, can suppress it down to $r\sim 7\times {10}^{-4}$. Finally, we extend this approach to a time-domain component-separation framework, enabling a statistically consistent treatment of instrumental response in the presence of time-domain features such correlated noise. We demonstrate its feasibility and validate it by performing a full end-to-end analysis, recovering results in good agreement with the map-based ones. This sets the stage for the full exploitation of this approach’s capability in the future.

In an electron-positron plasma, an imbalance in the number of right- and left-chiral particles can lead to the growth of a helical magnetic field through a phenomenon called the chiral plasma instability (CPI). In the early Universe, scattering reactions that violate chirality come into thermal equilibrium when the plasma cools below a temperature of approximately 80 TeV. Since these reactions tend to relax any preexisting chiral asymmetry to zero as the system approaches equilibrium, the standard lore is that primordial magnetogenesis via the CPI is not viable below 80 TeV. In this work, we propose that the presence of a source for chirality can allow the CPI to operate even below 80 TeV, we explore the implications of this scenario, and we derive predictions for the resultant magnetic field helicity using a combination of analytical methods and direct numerical simulation.

We develop a semianalytical model to describe the cosmological evolution of networks of cosmic strings with small-scale structure by extending the velocity-dependent one-scale model to include an additional length scale describing the typical interkink density. We study the impact of the different physical processes involved in the production and removal of small-scale structure from cosmic strings on the attainment of a full linear scaling regime, in which the characteristic lengths of the network and of the small-scale structure evolve proportionally to physical time, and the root-mean-squared velocity of the network remains constant. We find, using this novel velocity-dependent two-scale model, that quite generally small-scale structure does not prevent the attainment of a linear scaling regime since, even if not enough kinks are carried away when loops are chopped from the network, the effect of gravitational backreaction is generally enough to ensure that the interkink density scales. We find, however, that this regime is characterized by a smaller energy density and root-mean-squared velocity when compared to strings without small-scale structure and that this reduction may be significant when scaling is maintained by gravitational backreaction. In this case, we also find that, before reaching full scaling, the network should evolve in a transient quasiscaling regime, in which its evolution is very similar to that of cosmic strings without small-scale structure.

We show that in multistage axion monodromy inflation an interruption near the end of the penultimate stage can lead to a spike in the gravitational wave background. These gravitational waves are in the frequency range and with an amplitude accessible to proposed terrestrial detectors such as the Einstein Telescope, Cosmic Explorer, and future Levitated Sensor Detector experiments.

Measurements of high degree of optical polarization from blazars provide a strong evidence for the optically thin synchrotron radiation from the relativistic leptons in the jet. This in turn characterizes the presence of a partially ordered magnetic field in the emission region. However, the topology of magnetic field and acceleration of electrons to ultrarelativistic energies within the blazar jet are poorly understood. In this work, we investigate the decade long optical light curve of the well-known blazar 1ES $1959+650$ to probe the role of jet magnetic field in the peculiar behavior of the highly varying degree of linear polarization in the wavelength band 500–700 nm. The optical light curves in V- and bands exhibit a strong flare without any significant change in the contemporaneous degree of linear polarization. However, the degree of linear polarization shows strong positive and negative/anticorrelations with the V- and R-band fluxes measured during the epochs before and after the flare, respectively. This can be satisfactorily explained using the helical magnetic field and transverse shock under the framework of a relativistic jet with modestly varying orientations. During the flare, the fractional linear polarization is explained by the turbulent emission multizone model.

Accretion discs that are tilted with respect to their compact hosts can warp out-of-plane through general relativistic frame dragging. Warp influences disc dynamics in ways that have been studied extensively, especially as regards instabilities that might lead to rapid angular-momentum cancellation between neighboring rings of fluid and mass infall. We provide a review of warped-disc phenomena here, revisiting key hydrodynamical assumptions that impact calculations of the shear viscosity controlling instability thresholds. Relativistic effects at the level of gas-parcel orbits are included, as are external Lorentz forces applied by the compact primary’s magnetic field. Semianalytic analysis reveals that intense magnetic fields can bring about new branches of warp modes and avoided crossings that significantly reduce the perpendicular viscosity at sub-Eddington accretion rates. Critical strengths required for misaligned torques to tear a thin disc may thus relax for systems like neutron star x-ray binaries or radio-loud active galactic nuclei.

We introduce and systematically study the length of the slow stable hybrid star branch as a quantitative measure of the extended stability region that arises in hybrid neutron stars when the hadron-quark phase conversion is slow compared to the radial oscillation timescale. Combining generalized piecewise-polytropic hadronic equations of state of varying stiffness with a constant-speed-of-sound quark-matter model, we construct a large set of hybrid equations of state spanning a broad range of transition pressures, energy-density jumps, and quark-matter speeds of sound. We identify four morphological types for the slow stable branch in the mass-radius plane: waterfall branches that descend monotonically from the hadronic maximum mass, bridges that connect the hadronic branch to a second unconditionally stable hybrid branch, tails that extend briefly beyond the maximum mass of an unconditionally stable hybrid branch, and tail bridges that combine features of the latter two. Their prevalence is governed primarily by the transition pressure and the energy-density jump, while the branch length is also significantly influenced by the stiffness of the hadronic sector and the quark-matter speed of sound. Imposing current astrophysical and microphysical constraints shows that viable long branches are predominantly of a waterfall type, and that stiff hadronic equations of state—strongly disfavored under the rapid-conversion assumption—remain compatible with all current constraints within the slow-conversion framework. In the plane of transition baryon density versus density jump, slow stable configurations open a new region of viable parameter space inaccessible under rapid conversions.

We respond to the comment by Matteo Giordano [1] on our article [2]. We find—and [1] itself acknowledges—that the proposed counterexamples intended to refute our argument violate a crucial assumption of QCD at high temperatures, namely that every gluonic observable is an analytic function of the squared quark mass, $m^2$. We further point out a technical mistake found in [1]. We conclude that the arguments presented in [1] are not valid.

Aoki, Fukaya, and Taniguchi claim that both the spectral density of the Dirac operator at the origin and the topological susceptibility must vanish identically for sufficiently small but nonzero quark mass $m$ in the chirally symmetric phase of quantum chromodynamics with two light quark flavors, under certain technical assumptions on the spectrum and on the dependence of observables on $m$. I argue that a crucial step of their proof is not justified and the validity of these conclusions should be reassessed.

We investigate the production of axionlike particles through nonlinear Compton-like scattering in intense laser fields using the Baier-Katkov operator method. By explicitly constructing the eikonal spinor wave function, we utilize the semiclassical nature of relativistic electrons, which simplifies the theoretical derivation and circumvents the need to evaluate certain operator products. The spin-resolved axion emission rate is obtained within the locally constant field approximation. Explicit calculations demonstrate that the radiation yield is dominated by the electron’s transverse acceleration, driven by the magnetic field and by the component of the electric field perpendicular to the instantaneous electron velocity. The electron spin dependence of the axion emission rate is found to differ significantly from that of photon emission by analytically examining the asymptotic behavior of the radiation in both the weak- and strong-field limits and by numerically exploring the intermediate regime. The derived spin-resolved axion emission rate can be directly incorporated into existing semiclassical Monte Carlo algorithms developed for strong-field photon processes, enabling efficient modeling of axionlike particle generation. Our results provide promising avenues for the experimental detection and control of axionlike particles with high-intensity laser facilities.

Normalizing flows can be used to construct unbiased, reduced-variance estimators for lattice field theory observables that are defined by a derivative with respect to action parameters. This work implements the approach for observables involving gluonic operator insertions in the SU(3) Yang-Mills theory and two-flavor QCD in four space-time dimensions. Variance reduction by factors of 10–60 is achieved in glueball correlation functions and in gluonic matrix elements related to hadron structure, with demonstrated computational advantages. The observed variance reduction is found to be approximately independent of the lattice volume, so volume transfer can be utilized to minimize training costs.

The inclusion of a masslike term for the gluon in Yang-Mills theories quantized in the Landau gauge has proven to be an effective way of reproducing lattice results for gauge-fixed correlation functions within perturbative computations. Since those quantities are gauge dependent, it is natural to question how general this prescription is for describing the infrared behavior of gluon and Faddeev-Popov ghost propagators in different gauges. In this work, we provide a systematic investigation of this issue in the maximal Abelian gauge, which cannot be deformed into the Landau gauge and has been investigated in gauge-fixed lattice simulations. We compute the one-loop non-Abelian and diagonal gluon propagators and perform fits to lattice data in the case of $SU(2)$. Our results show that the transverse component of the non-Abelian gluon propagator as well as the diagonal gluon propagator, are in good agreement with lattice data in the infrared.