Showing new listings for Monday, 10 November 2025

New submissions (showing 11 of 11 entries)

In this study, we explore the reconstruction of a new agegraphic dark energy model in a flat Friedmann-Robertson-Walker spacetime by f(Q) gravity framework, where Q represents non-metricity. We assume that the scale factor follows a power-law and explore how this model aligns with the expanding universe. In this perspective, we develop a new agegraphic f(Q) model and analyze the graphical behavior for cosmic evolution. We analyze physical characteristics of the model using the equation of state parameter, (omega_{D}-omega^{prime}_{D}) and the (r-s) planes. The equation of state parameter indicates a quintessence era characterized by accelerated expansion. The (omega_{D}-omega^{prime}_{D})-plane identifies the freezing region and the Chaplygin gas model is represented in the (r-s)-plane. Finally, we examine the stability of the non-interacting model by evaluating the squared speed of sound. Our findings show that the non-interacting new agegraphic dark energy model effectively resolves the cosmic coincidence problem.

Applications of effective field theory (EFT) and scattering amplitudes to gravitational problems have recently produced many unique results that advanced our understanding of the dynamics of compact binaries. Many of these results were made possible by comparing gravitational scattering amplitudes in EFT with exact expressions from general relativity. However, the latter expressions are not easily available as they require intricate solution techniques for the Teukolsky master equation, such as the Mano-Suzuki-Takasugi (MST) method. In this paper, we develop and present the first public package that enables computations of gravitational scattering amplitudes in black hole perturbation theory within the MST framework. Our package supports both analytic computations to a given post-Minkowskian (PM) order in the low-frequency limit and exact numerical computations for an arbitrary frequency of the perturbing field. As an application, we compute scattering phase shifts and inelasticity parameters for massless spin - 0, 1, and 2 fields resulting from scattering off a rotating Kerr black hole through the third PM order and compare these results with the exact numerical solutions.

We present a new class of expanding and twisting solutions to the Einstein-Maxwell equations of algebraic type D, where the null eigendirections of the Faraday tensor are not aligned with PNDs of the Weyl tensor. After deriving this novel solution, we explore its various metric forms and parameterizations. In suitable coordinates, the solution depends on six physical parameters, namely mass m, Kerr and NUT twist parameters a and l, complex charge c, acceleration alpha, and parameter beta that governs the interplay between electric and magnetic charges in the aligned part of the Faraday tensor. This parameterization, as the Griffiths-Podolský form of the Plebański-Demiański solution, facilitates explicit special subcases, such as Kerr-Newman black holes, and a deeper physical interpretation. Additionally, in the static limit, our solution reduces to previously known cases.

Primordial Black Holes (PBHs) represent one of the more interesting ways to address dark matter, at the interface of both cosmology and quantum gravity. It is no surprise then that testing PBHs is a venue of active interest, with several cosmological and astrophysical probes constraining different mass ranges. In this work, we propose novel Solar System scale searches for PBHs, motivated by the unique precision and coverage of local observables. We show that asteroid to dwarf planet mass PBHs can induce measurable dipolar timing signatures in pulsar timing arrays, while planetary mass PBHs can generate detectable ADAF accretion flares through interactions with Kuiper Belt bodies. Together, these complementary approaches open a new observational frontier for probing PBHs across mass ranges that remain unconstrained by conventional cosmological methods.

The two-parameter inhomogeneous and time-dependent Pimentel solution of Brans-Dicke theory is analyzed to probe the new thermal view in which General Relativity is the zero-temperature (equilibrium) state of scalar-tensor gravity. As the parameters vary, we uncover phenomenology not found before with other exact solutions, nor contemplated thus far in the general theory. In the process, we also discuss the anomalous limit to General Relativity of the Pimentel geometry and show how the Mars solution of the Einstein equations is its Einstein frame version.

In this work, we study two potentials, the single-field and the two-field, from the modified (R+gamma R^n) gravity in D=8 dimensions. From those potentials, we calculate four observable quantities in inflation, including scalar-to-tensor ratio, spectral index, running index and scalar amplitude. Then, we compare them to the experimental data to verify the righteousness of the models. Last but not least, de Sitter conjectures are brought up with these two potentials to investigate that it is possible or not the theory lay in the Landscape of quantum gravity.

We develop a fully relativistic approach for determining the frequency-dependent tidal response of a compact star. The strategy involves matching the solution for the linearised fluid dynamics in the star's interior to the spacetime perturbations in the near-zone surrounding the body, along with an identification of the tidal driving and the star's response. Notably, this identification is exact in Newtonian gravity and we provide strong evidence that it remains robust also in the relativistic case. The argument does not involve a sum over the star's quasinormal modes and hence circumvents one of the obstacles that have held up the development of models for relativistic tides. Numerical results are provided, at the proof-of-principle level, for a realistic matter equation of state from the BSk family, including composition stratification leading to the presence of low-frequency gravity modes. We also sketch the connection with the field-theory inspired approach to the problem, in which the tidal response is expressed in terms of asymptotic scattering amplitudes.

We present a fully covariant and gauge-invariant analysis of linear cosmological perturbations in Energy-Momentum Squared Gravity. Working within the 1+3 formalism, we derive the exact propagation equations for scalar, vector, and tensor modes on FLRW backgrounds, in the case of radiation and dust. Two representative subclasses are examined in detail, in which non-linearity enters through mathcal{O}(etarho^2) corrections or modifications in the equation-of-state parameter and the sound speed. For scalar perturbations, the density contrast can be enhanced or reduced relative to General Relativity, depending on the coupling parameter and the wavelength regime. A similar behaviour occurs for vector modes, allowing for a non-trivial vorticity at early-times. Tensor modes, described by the magnetic part of the Weyl tensor and the shear tensor propagate as damped waves with slowly varying effective masses. All sectors reduce continuously to their GR limits as eta!to!0. The framework isolates robust signatures - early-time scalar tilts, tensor damping shifts, and altered vorticity decay - that can be confronted with CMB and large-scale-structure observations to constrain these theories of gravity.

This work investigates the temporal distribution of glitches detected by LIGO, focusing on the morphological classification provided by the Gravity Spy project. Starting from the hypothesis that these events follow a Poisson process, we developed a statistical methodology to evaluate the agreement between the empirical distribution of glitches and an ideal Poisson model, using the coefficient of determination (R^2) as the main metric. The analysis was applied to real data from the LIGO detectors in Livingston and Hanford throughout the O3 run, as well as to synthetic datasets generated from pure Poisson distributions. The results show that while several morphologies exhibit good agreement with the proposed model, classes such as 1400Ripples, Fast Scattering, and Power Line display significant deviations (R^2 leq 0.6), suggesting that their origins do not strictly follow Poissonian statistics. In some cases, a dependence on the detector or the observing run was also observed. This analysis provides a quantitative basis for distinguishing glitch classes based on their degree of "Poissonness", potentially supporting the development of more effective glitch mitigation strategies in gravitational wave detector data.

Born-Infeld electrogravity is defined through a Lagrangian that couples gravity and electromagnetism within a single determinantal structure. The field equations are derived in Palatini's formalism, where the metric, connection, and vector potential are varied independently in the action. As a result, the gravitational sector reduces to Einstein's equations with a torsion-free, metric-compatible connection. The electrodynamic sector, in turn, admits two equivalent interpretations or pictures: it can be seen either as a standard Born-Infeld electrodynamics in an effective background geometry, or as an anomalous Born-Infeld electrodynamics in the physical metric. We illustrate the dynamics by analyzing the horizon structure and extremality conditions of spherically symmetric dyonic solutions. A comparison with the Reissner-Nordström geometry shows that Born-Infeld electrogravity softens but does not eliminate curvature divergences, and geodesic incompleteness persists.

We identify curvature decay |mathrm{Riem}| sim r^{-3} as a sharp spectral threshold in linearized gravity on asymptotically flat manifolds. For faster decay, the spatial Lichnerowicz operator possesses a purely continuous spectrum sigma_{mathrm{ess}}(L) = [0,infty), corresponding to freely radiating tensor modes. At the inverse-cube rate, compactness fails and zero energy enters sigma_{mathrm{ess}}(L), yielding marginally bound, finite-energy configurations that remain spatially extended. These static modes constitute the linear precursors of gravitational memory and soft-graviton phenomena, delineating the geometric boundary between dispersive and infrared behavior. A complementary numerical study of the radial model [ L_p = -frac{d^2}{dr^2} + frac{ell(ell+1)}{r^2} + frac{C}{r^p} ] confirms the analytic scaling law, locating the same transition at p = 3. The eigenvalue trends approach the flat-space limit continuously for p > 3 and strengthen progressively for p -1 and w_a0 when alpha_B is varied which will require further study to determine whether this is due to systematics or new physics. [Abridged]

We present SGNL, a scalable, low-latency gravitational-wave search pipeline. It reimplements the core matched-filtering principles of the GstLAL pipeline within a modernized framework. The Streaming Graph Navigator library, a lightweight Python streaming framework, replaces GstLAL's GStreamer infrastructure, simplifying pipeline construction and enabling flexible, modular graph design. The filtering core is reimplemented in PyTorch, allowing SGNL to leverage GPU acceleration for improved computational scalability. We describe the pipeline architecture and introduce a novel implementation of the Low-Latency Online Inspiral Detection algorithm in which components are pre-synchronized to reduce latency. Results from a 40-day Mock Data Challenge show that SGNL's event recovery and sensitivity are consistent with GstLAL's within statistical and systematic uncertainties. Notably, SGNL achieves a median latency of 5.4 seconds, a 42% reduction compared to GstLAL's 9.3 seconds.

In this short note we analyze the inflationary dynamics in Weyl-invariant Einstein-Cartan gravity coupled to the Standard Model of particle physics. We take the axion-like particle of gravitational origin to be approximately massless in the early Universe and show how inflation with the Higgs field materializes.

Gravitational waves (GWs) serve as standard sirens by directly encoding the luminosity distance to their source. When the host galaxy redshift is known, for example, through observation of an electromagnetic (EM) counterpart, GW detections can provide an independent measurement of the Hubble constant, H_0. However, even in the absence of an EM counterpart, inferring H_0 is possible through the dark siren method. In this approach, every galaxy in the GW localization volume is considered a potential host that contributes to a measurement of H_0, with redshift information supplied by galaxy catalogs. Using mock galaxy catalogs, we explore the effect of catalog incompleteness on dark siren measurements of H_0. We find that in the case of well-localized GW events, if GW hosts are found in all galaxies with host halo masses M_h > 2 times10^{11} M_{odot}h^{-1}, catalogs only need to be complete down to the 1% brightest magnitude M_i 21.1 (13.8) for the first-order subluminal (superluminal) scenario, and E_{rm{QG}}/10^{11}~text{GeV}> 14.9 (13.7) for the second-order subluminal (superluminal) scenario.

The papers included in this Focus Point collection are devoted to the studies on the cosmological tensions and challenges stimulated by the latest observational data. The first results of the LARES-2 laser ranging satellite on the high precision testing of the frame-dragging effect predicted by General Relativity are presented. The data on the S-stars monitoring in the Galactic center obtained by GRAVITY collaboration were analysed within the Physics-informed neural network (PINN) approach. The results enabled to probe the role of the cosmological constant, of the dark matter, the star cluster in the core of the Galaxy obtaining an upper limit for the star density. The topics include the conversion of high-frequency relic gravitational waves into photons in cosmological magnetic field, cosmological gravitational waves stochastic background generation through the spontaneous breaking of a global baryon number symmetry, observational predictions of the Starobinsky inflation model and other studies.

We present a non-perturbative framework for the dynamics of slow-roll inflation that consistently incorporates quantum corrections, based on an alternative functional renormalisation group (RG) approach. We derive the coupled Friedmann-RG flow equations governing the joint evolution of spacetime, the inflaton field, and its effective potential. Applying this formalism to alpha-attractor E-models, we find that the RG flow induces a dynamical destabilisation of the inflationary trajectory, leading to a premature termination of slow roll. Remarkably, the resulting predictions bring alpha-attractors into full agreement with the latest ACT data without introducing new physics beyond a consistent quantum-corrected treatment of the inflaton dynamics.

Gravitational-wave observations of massive, rapidly spinning binary black holes mergers provide increasing evidence for the dynamical origin of some mergers. Previous studies have interpreted the mergers with primary mass gtrsim45,M_odot as being dominated by hierarchical, second-generation mergers, with rapidly spinning primaries being the products of previous black hole mergers assembled in dense stellar clusters. In this work, we reveal confident evidence of another subpopulation with rapid and isotropic spins at low mass containing the two exceptional events GW241011 and GW241110, consistent with a hierarchical merger hypothesis. Our result suggests the mass distribution of the second-generation black holes is peaked at low primary masses of sim16,M_odot rather than gtrsim45,M_odot in the pair-instability gap. Such low-mass second-generation black holes must be formed from the merger of even lighter first-generation black holes, implying that dense, metal-rich stellar environments contribute to the binary black hole population. By separating the contamination of higher-generation black holes, our result reveals the primary mass distribution of first-generation black holes formed from stellar collapse, which shows a significant dip between sim12,M_odot to sim20,M_odot. This may indicate a dearth of black holes due to variation in the core compactness of the progenitor.

Draining vortices provide a powerful platform for simulating black hole phenomena in tabletop experiments. In realistic fluid systems confined within a finite container, low-frequency waves amplified by the vortex are reflected at the walls, rendering the system unstable. This process, known in the gravitational context as the black hole bomb, manifests as a sloshing motion of the free surface. The analogy, however, becomes more nuanced when a realistic vortex core with a non-singular vorticity distribution is considered. We investigate this by analysing a non-draining Rankine vortex in the shallow-water and inviscid limits. At low circulation, the sloshing corresponds to an instability of the vorticity field, whereas at high circulation where fluid is expelled from the vortex core, the destabilising mechanism coincides with that of the black hole bomb. Our variational framework distinguishes the energetic contributions of vorticity and irrotational perturbations, offering new insight into the rotating-polygons instability reported by, e.g. Jansson et al. (2006). From the analogue-gravity perspective, we identify hollow core vortices as an optimal regime for exploring black-hole-like instabilities in fluids.

Extending the single-angular-momentum case analyzed in our previous work, we investigate the solution-generating technique based on the Breitenlohner-Maison (BM) linear system for asymptotically flat, stationary, bi-axisymmetric black hole solutions with two angular momenta in five-dimensional vacuum Einstein theory. In particular, we construct the monodromy matrix associated with the BM linear system for the doubly rotating Myers-Perry black holes and the Pomeransky-Sen'kov black rings. Conversely, by solving the corresponding Riemann-Hilbert problem using the procedure developed by Katsimpouri et al., we demonstrate that the factorization of the monodromy matrix precisely reproduces these vacuum solutions, thereby reconstructing both geometries.

The quantum induced stress tensor of 3+1-dimensional Einstein gravity, with conformally coupled matter, is studied in an effective field theory approach. In this context, Riegert's non-local effective action is sufficient to reproduce the trace anomaly in curved spacetime but in general the effective action can include additional non-local but scale invariant terms that influence the semiclassical physics without affecting the trace anomaly. Here, a truncated model, with only one additional term involving the square of the Weyl tensor, is used to find the induced stress tensor in a black hole background. With suitable physical conditions, a solution of the resulting 4th order equations leads, in a static limit, to a unique quantum state matching expected properties of the Unruh state.

Leveraging the computational framework presented in reference [JHEP 07, 062 (2024)], we evaluate the analytic scattering waveform in General Relativity to second order, G^3 M^3 /r b^2 and to all orders in velocity. This new representation of the next-to-leading order waveform is well-suited for numerical evaluation. Integrating the [modulus square of the] waveform over the angles on the celestial sphere, we also compute the power spectrum of the radiation to order G^4 numerically.

We propose a resolution to the longstanding problem of perturbative normalizability in canonical quantum gravity of the Lorentzian Chern-Simons-Kodama (CSK) state with a positive cosmological constant in four dimensions. While the CSK state is an exact solution to the Hamiltonian constraint in the self-dual formulation and semiclassically describes de Sitter spacetime, its physical viability has been questioned due to apparent nonnormalizability and CPT asymmetry. Starting from a nonperturbative holomorphic inner product derived from the reality conditions of the self-dual Ashtekar variables, we show that the linearization, in terms of gravitons, of the CSK state is perturbatively normalizable for super-Planckian cosmological constant. Furthermore, we demonstrate that a rotation in phase space, a generalization of Thiemann's complexifier, can render the full perturbative state normalizable for all Lambda by analytically continuing the non-convergent modes in phase space. This provides the first concrete realization of a CPT-breaking, yet normalizable, gravitational vacuum state rooted in a nonperturbative quantum gravity framework. Our results establish the CSK state-long thought formal-as a viable candidate for the ground state of quantum gravity in de Sitter space.

We investigate the stability of homogeneous minimal submanifolds in two families of closed Einstein manifolds, the Page space mathbb{CP}^2 # overline{mathbb{CP}^2} and the Sasaki-Einstein spaces Y^{p,q}, which are equipped with cohomogeneity-one Einstein metrics admitting the isometric action of SU(2) times U(1) and U(1) times U(1) times SU(2) respectively. We determine all the homogeneous, minimal hypersurfaces and explicitly compute the spectrum of their associated stability operators and determine their index.

Replacement submissions (showing 28 of 28 entries)

In this work, we study the gravitational lensing by a Lorentz-violating (LV) black hole inspired by the recent contribution [1]. Explicitly, we concentrate on a specific application: we perform the computation of gravitational lensing effects under the strong field limit. In particular, we analytically derive the deflection angle so that the lens equation can also be addressed. This methodological approach yields physically measurable outcomes, including the determination of relativistic image positions and their corresponding magnifications. As an application of this methodology, we consider the gravitational lensing by Sagittarius A{}^* and obtain the corresponding observables expressed as functions of the LV parameter.

We obtain generally covariant operator-valued geodesic equations on a pseudo-Riemannian manifold M as part of the construction of quantum geodesics on the algebra D(M) of differential operators. Geodesic motion arises here as an associativity condition for a certain form of first order differential calculus on this algebra in the presence of curvature. The corresponding Schrödinger picture has wave functions on spacetime and proper time evolution by the Klein-Gordon operator, with stationary modes being solutions of the Klein-Gordon equation. As an application, we describe gravatom solutions of the Klein-Gordon equations around a Schwarzschild black hole, i.e. gravitationally bound states which far from the event horizon resemble atomic states with the black hole in the role of the nucleus. The spatial eigenfunctions exhibit probability density banding as for higher orbital modes of an ordinary atom, but of a fractal nature approaching the horizon.

The stochastic gravitational wave background (SGWB) is one of the main detection targets for future millihertz space-borne gravitational-wave observatories such as the ac{LISA}, TianQin, and Taiji. For a single LISA-like detector, a null-channel method was developed to identify the SGWB by integrating data from the A and E channels with a noise-only T channel. However, the noise monitoring channel will not be available if one of the laser interferometer arms fails. By combining these detectors, it will be possible to build detector networks to search for SGWB via cross-correlationthis http URLthis work, we developed a Bayesian data analysis method based on ac{TDI} Michelson-type channel. We then investigate the detectability of the TianQin-LISA detector network for various isotropic SGWB. Assuming a three-month observation, the TianQin-LISA detector network could be able to confidently detect SGWB with energy density as low as Omega_{rm PL} = 6.0 times 10^{-13}, Omega_{rm Flat} = 2.0 times 10^{-12} and Omega_{rm SP} = 1.2 times 10^{-12} for power-law, flat and single-peak models, respectively.

We revisit the one-parameter generalization of the C-metric derived by Ernst, which solves the vacuum Einstein equations. Resolving conflicting claims in the literature, we determine the correct value of the parameter that ensures the regularity of the metric on the axis. This "regularized C-metric" describes a pair of accelerating black holes without the line source present in the original C-metric. Additionally, this generalization changes the Petrov type from D to I. We use the Gauss-Bonnet theorem to analyze the nodal singularities, the line source, and their relation to the horizon topology. Both the black hole and acceleration horizons are found to be embeddable in mathrm{E}^3. We examine various geometric and asymptotic properties in detail using several coordinate systems and construct the corresponding 2D and 3D conformal diagrams. This process is more involved than for the original C-metric due to the presence of the exponential factors. These exponential factors also introduce curvature singularities at infinity, which obstructs asymptotic flatness. Contrary to Bonnor's expectation, we demonstrate why Bondi's algorithm for obtaining the standard Bondi form fails for the C-metric, despite its asymptotic flatness. We also show that Ernst's solution-generating prescription in boost-rotation symmetric coordinates is a symmetry of the wave equation.

By resolving the Riemann curvature into electric and magnetic parts, Einstein's equation can accordingly be written in terms of electric (active and passive) and magnetic parts. The electrogravity duality is defined by the interchange of active and passive parts. It turns out that in static and stationary spacetimes, there is a subset of the equations (that identifies the effective vacuum equation) that is sufficient to yield the vacuum solution. In spherically symmetric spacetime, the electrograv dual of the effective equation solves to give the Schwarzschild black hole with a global monopole. Interestingly, this is not so for axial symmetry, where the Kerr vacuum solution turns out to be electrograv self-dual. However, in the asymptotic limit where the effect of rotation dies out, the situation reverts to the static case, admitting a global monopole. This is also what follows when we apply the Newman-Janis transformation to the static black hole with a global monopole.

We present simulations of the Einstein-Maxwell-Klein-Gordon system on compactified hyperboloidal slices. To the best of our knowledge, this is the first time that this setup is evolved with a common formulation like BSSN/Z4. Hyperboloidal slices smoothly reach future null infinity, the only location in spacetime where radiation (such as gravitational waves) is unambiguously defined. We are thus able to reach null infinity and extract signals there. We showcase the capabilities of our implementation in spherical symmetry with the evolution of a charged scalar field perturbing a regular spacetime and near an electrically charged black hole. We also present the collapse of a charged scalar field into a Reissner-Nördstrom black hole.

Quasinormal modes and power-law late-time decay tails of a charged scalar field in a charged black hole background have been studied, but never in the fully non-linear regime, as far as we know. In this paper, we study the dependence of these properties on the charges of scalar field and black hole. For the quasinormal modes, a fit of the spherical fundamental mode is shown for the purely uncharged case and compared to the charged one. We also see for the first time the transition from gravitational decay to pure electromagnetic decay, and show disagreement with the oscillation frequency between real and imaginary parts of the scalar field prescribed in the literature. Full non-linear evolutions of hyperboloidal slices in spherical symmetry were used to obtain our results, allowing for the extraction of signals at future null infinity.

Using general relativistic radiative transfer (GRRT) simulations, we investigate the bright ring features and polarization structures in images of the Kerr-Sen black hole associated with Sgr A*, as illuminated by 230 GHz thermal synchrotron emission from radiatively inefficient accretion flows (RIAF). Our findings reveal that an increase in the dilaton parameter leads to a shrinking of the bright ring, accompanied by enhancements in both its width and brightness. As the disk thickness grows, the bright ring's diameter and width both decrease. The brightness enhancement induced by the disk thickness is less prominent than that driven by the dilaton parameter. Comparing with the Event Horizon Telescope (EHT) observational data of SgrA*, we present the allowed ranges of black hole parameters, and find that effects of the disk thickness on the allowed parameter space are more strong than those of the observer's inclination. Furthermore, we analyze the coefficient beta_2 to probe the polarization structure of the black hole images, and reveal that effects of the disk thickness on beta_2 are much weaker than those from the dilaton parameter.

We introduce and study a new solution describing a static, spherically symmetric and electrically charged black hole embedded in a charged quintessence fluid, which corresponds to an electric generalization of the Kiselev geometry. We derive the effective potential and analyze the various types of orbits followed by charged particles. A special attention is given to circular orbits and their stability. We found that for uncharged particles the periapsis shifts for bounded orbits is always prograde. However, for charged test particles the periapsis shifts can become retrograde in some cases.

Effective models inspired by loop quantum gravity typically resolve the central singularity by replacing it with a bounce of the matter density in the Planckian regime. In the specific model analyzed here, this bounce is generally followed by the formation of shell-crossing singularities. The purpose of this work is to provide a physically meaningful extension of spacetime beyond the shell-crossing singularity. To this end, we derive the dynamics of a dust thin shell within the effective Hamiltonian framework. The motion of the shell remains timelike throughout: after undergoing a quantum-gravitational bounce, it expands and eventually emerges from the white-hole vacuum region.

In bouncing cosmological models, either classical or quantum, the big bang singularity is replaced by a regular bounce. A challenging question in such models is how to keep the shear under control in the contracting phase, as it is well-known that the shear grows as fast as 1/a^{6} toward the bounce, where a is the average expansion factor of the universe. A common approach is to introduce a scalar field with an ekpyrotic-like potential which becomes negative near the bounce, so the effective equation of state of the scalar field will be greater than one, whereby it dominates the shear in the bounce region. As a result, a homogeneous and isotropic universe can be produced after the bounce. In this paper, we study how the ekpyrotic mechanism affects the inflationary phase in both loop quantum cosmology (LQC) and a modified loop quantum cosmological model (mLQC-I), because in these frameworks inflation is generic without such a mechanism. After numerically studying various cases in which the potential of the inflaton consists of two parts, an inflationary potential and an ekpyrotic-like one, we find that, despite the fact that the influence is significant, by properly choosing the free parameters involved in the models, the ekpyrotic-like potential dominates in the bounce region, during which the effective equation of state is larger than one, so the shear problem is resolved. As the time continuously increases after the bounce, the inflationary potential grows and ultimately becomes dominant, resulting in an inflationary phase. This phase can last long enough to solve the cosmological problems existing in the big bang model.

We propose that black holes are emph{soliton-esque} objects, where gravitational collapse is balanced by quantum vacuum dispersion, modeled via (R+alpha R^{2}) gravity. Classical singularities are replaced by oscillating, finite-radius cores, thereby evading static no-go theorems. The event horizon is replaced by the textit{Lamarina}, a surface of maximum redshift whose surface geometry yields Hawking-like radiation with corrections. The Raychaudhuri equations impose a Dyson-type ceiling on the maximum radiated power ((P_{infty} lesssim c^{5}/G)), while effective field theory matching dictates a universal minimum Lamarina radius set by the dispersion scale.

We show that the requirement that the trace of the stress-energy tensor of matter must vanish if invariance under Weyl rescalings is a symmetry of a given gravitational theory is not universal. This requirement holds wherever the masses of timelike fields are constant parameters that are not transformed by Weyl rescalings, or if the energy density of perfect fluids transforms as rhorightarrowOmega^{-3}rho. In contrast, if the masses of timelike fields are point-dependent quantities transforming under the Weyl rescalings as mrightarrowOmega^{-1}m, and the energy density of perfect fluids transforms as rhorightarrowOmega^{-4}rho, the Weyl symmetry does not require the vanishing of the trace of the matter SET. This result is demonstrated in two different ways. In consequence, any matter fields, regardless of whether the trace of their stress-energy tensor vanishes or not, can be coupled to gravity. The phenomenological consequences of the novel result are drawn.

In curved space-time, a scalar field phi is generically expected to couple to curvature, via a coupling of the form xiphi^2R. Yet in the study of Hawking emission from regular black holes (RBHs), where scalar fields are often introduced as simple probes of the geometry, and the Ricci scalar is generically non-zero, this non-minimal coupling is almost always ignored. We revisit this assumption by studying scalar Hawking emission from four representative RBHs (the Bardeen, Hayward, Simpson-Visser, and D'Ambrosio-Rovelli space-times), within two benchmark cases: the conformal case xi=1/6, and a large negative value xi=-10^4 motivated by Higgs inflation. We compute the graybody factors and emission spectra, showing that the latter can be either enhanced or suppressed, even by several orders of magnitude. A crucial role is played by the sign of the term xi fR, with f(r)=-g_{tt} in Schwarzschild-like coordinates, as it determines whether the non-minimal coupling suppresses or enhances the geometric potential barrier. For the D'Ambrosio-Rovelli case with large negative xi, the low-energy emission spectrum is enhanced by up to five orders of magnitude, since xi fR<0 throughout the space-time, leading to a deep potential well which broadens the transmissive window. The deviations we find can be particularly relevant in the case where primordial RBHs are dark matter candidates, given the impact of the non-minimal coupling on their evaporation history.

In this paper, we utilize the ray-tracing method to conduct an in-depth study of the observational images of asymmetric thin-shell wormholes in the Kalb-Ramond field. Initially, we calculate the null geodesics and effective potential energy of ATSW, and investigate the variations in the photon sphere radius and critical impact parameter under different values of charge Q and Lorentz-violating parameter l. Based on these calculations, we determine the photon deflection angles and trajectories within this space-time structure. Specifically, depending on the photon impact parameters, the photon trajectories can be categorized into three types. By using a thin accretion disk as the sole background light source and incorporating two classical observational radiation models, we find that under conditions of equal mass M, charge quantity Q, and Lorentz-violation parameter l, the asymmetric thin-shell wormhole exhibits unique observational features such as additional lensing rings and photon ring clusters. Furthermore, distinct from black holes, as the charge quantity Q and the Lorentz-violation parameter l increase, the coverage area of the specific additional halo also expands correspondingly.

We search for gravitational-wave background signals produced by various early Universe processes in the Advanced LIGO O4a dataset, combined with the data from the earlier O1, O2, and O3 (LIGO-Virgo) runs. The absence of detectable signals enables powerful constraints on fundamental physics. We derive gravitational-wave background energy density upper limits from the O1-O4a data to constrain parameters associated with various possible processes in the early Universe: first-order phase transitions, cosmic strings, domain walls, stiff equation of state, axion inflation, second-order scalar perturbations, primordial black hole binaries, and parity violation. In our analyses, the presence of an astrophysical background produced by compact (black hole and neutron star) binary coalescences throughout the Universe is also considered. We address the implications for various cosmological and high energy physics models based on the obtained parameter constraints. We conclude that LIGO-Virgo data already yield significant constraints on numerous early Universe scenarios.

The f(Q,C) framework of gravity enables the depiction of an effective dark energy fluid that emerges from geometry itself, thus leading to modifications in the cosmological phenomenology of General Relativity. We pursue this approach to discover new and observationally supported (effective) evolving dark energy models. We propose a general f(Q,C) formulation that cannot be simply split into separate functions of Q and C, yet it still results in second-order field equations. By employing a particular type of connection, we derive guidelines for new cosmological models, including a variant of the DGP model that appears to be statistically favored over LambdaCDM. Notably, we also demonstrate how to translate solutions within this f(Q,C) framework to f(Q) counterparts at the background level.

At linear order we study perturbations to a Gödel background spacetime which includes expansion in addition to rotation. We investigate the transformation behaviour of these perturbations under gauge transformations and construct gauge invariant quantities. Using the perturbed energy conservation equation we find that there are conserved quantities in Expanding Gödel (EG) Cosmology, in particular a spatial metric trace perturbation, {zeta} SMTP , which is conserved on large scales for pressureless dust. We intend to extend our discussion to a perfect fluid matter content with a view to also obtaining conserved quantities in this context.

We consider detailed cosmological tests of dark energy models obtained from the general conformal transformation of the Kropina metric, representing an (alpha,beta)-type Finslerian geometry. In particular, we restrict our analysis to the osculating Barthel Kropina geometry. The Kropina metric function is defined as the ratio of the square of a Riemannian metric alpha and of the one-form beta. In this framework, we also consider the role of the conformal transformations of the metric, which allows us to introduce a family of conformal Barthel-Kropina theories in an osculating geometry. The models obtained in this way are described by second-order field equations, in the presence of an effective scalar field induced by the conformal factor. The generalized Friedmann equations of the model are obtained by adopting for the Riemannian metric alpha the Friedmann Lemaitre Robertson Walker representation. In order to close the cosmological field equations, we assume a specific relationship between the component of the one-form beta and the conformal factor. With this assumption, the cosmological evolution is determined by the initial conditions of the scalar field and a single free parameter gamma of the model. The conformal Barthel Kropina cosmological models are compared against several observational datasets, including Cosmic Chronometers, Type Ia Supernovae, and Baryon Acoustic Oscillations, using a Markov Chain Monte Carlo (MCMC) analysis, which allows the determination of gamma. A comparison with the predictions of standard LambdaCDM model is also performed. {Our results indicate that the conformal osculating Barthel Kropina model can be considered as a successful, and simple, alternative to standard cosmological models.

String geometry theory is one of the candidates of the non-perturbative formulation of string theory. In this paper, in the bosonic closed sector of string geometry theory, we completely identify the perturbative vacua, which include general string backgrounds in bosonic closed string theory. From fluctuations around these configurations, we derive the path-integrals of perturbative strings on the string backgrounds up to any order.

The spontaneous breaking of a discrete symmetry can lead to the formation of domain walls in the early Universe. In this work, we explore the impact of bias directions on the dynamics of Z_N domain walls, mainly focusing on the N = 3 model with a biased potential. Utilizing the Press-Ryden-Spergel method, we numerically investigate the dynamics of domain walls with lattice simulations. We find notable differences in the dynamics of domain walls due to bias directions. Our results indicate that the annihilation time depends not only on the vacuum energy difference delta V but also on bias directions described by the relative potential difference zeta .

We present general relativistic magnetohydrodynamic simulations of binary neutron star (BNS) mergers, where the collapse of the metastable massive neutron star (MNS) remnant leads to the production of an incipient jet having terminal Lorentz factor and Poynting-flux luminosity compatible with a short gamma-ray burst (GRB). We consider different MNS lifetimes of about 25 and 50 ms, long enough for massive polar outflows to emerge before black hole (BH) formation. The interaction of the following BH-driven jet with such polar outflows, responsible for shock heating and possible electromagnetic signatures, is self-consistently captured for the first time. Exploiting an unprecedentedly low numerical density floor scaling as r^-6, we explore the jet propagation up to distances of ~10^4 km. Comparing the outcome of different MNS lifetimes, we find that the latter, by strongly affecting the propagation environment, plays a major role in determining the final properties of the escaping jet. Finally, we consider a non-collapsing case, where the MNS-driven outflow is found too dense and slow to be compatible with a GRB jet, thus favoring a BH central engine scenario.

We consider a class of models in the framework of metric-affine gravity and establish their correspondence to the bosonic sector of a class of no-scale supergravity models. The excellent inflationary behavior of the gravitational models considered is carried over to the corresponding supergravity ones, thus, enriching the landscape of inflation-compatible models.

Pulsar Timing Array (PTA) projects have reported various lines of evidence suggesting the presence of a stochastic gravitational wave (GW) background in their data. One key line of evidence involves a detection statistic sensitive to inter-pulsar correlations, such as those induced by GWs. A p-value is then calculated to assess how unlikely it is for the observed signal to arise under the null hypothesis H_0, purely by chance. However, PTAs cannot empirically draw samples from H_0. As a workaround, various techniques are used in the literature to approximate p-values under H_0. One such technique, which has been heralded as a model-independent method, is the use of "scrambling" transformations that modify the data to cancel out pulsar correlations, thereby simulating realizations from H_0. In this work, scrambling methods and the detection statistic are investigated from first principles. The p-value methodology that is discussed is general, but the discussions regarding a specific detection statistic apply to the detection of a stochastic background of gravitational waves with PTAs. All methods in the literature to calculate p-values for such a detection statistic are rigorously analyzed, and many analytical expressions are derived. All this leads to the conclusion that scrambling methods are not model-independent and thus not completely empirical. Rigorous Bayesian and Frequentist p-value calculation methods are advocated, the evaluation of which depend on the generalized chi^2 distribution. This view is consistent with the posterior predictive p-value approach that is already in the literature. Efficient expressions are derived to evaluate the generalized chi^2 distribution of the detection statistic on real data. It is highlighted that no Frequentist p-values have been calculated correctly in the PTA literature to date.

We provide a concrete link between celestial amplitudes and cosmological correlators. We first construct a map from quantum field theories (QFTs) in (D+2)-dimensional Euclidean space to theories on the (D+1)-dimensional sphere, through a Weyl rescaling and a Fourier transformation. An analytic continuation extends this map to a relation between QFTs in Minkowski spacetime text{M}_{D+2} and in de Sitter spacetime text{dS}_{D+1} with the Bunch-Davies vacuum. Combining this relation with celestial holography, we show that the extrapolated operators in de Sitter space can be represented by operators on the celestial sphere S^{D}. Our framework offers a systematic route to transfer computational techniques and physical insights between celestial holography and the dS/CFT correspondence.

The Standard Model of particle physics is usually cast in symmetry-first terms. On this approach, one begins with a symmetry group and postulates matter fields as objects transforming under its representations, without requiring that the group be grounded in, or derived from, independent geometric structures. Recently, a geometry-first formulation has been proposed, in which the relevant symmetries are not fundamental. In this paper I extend this approach to two central mechanisms of the Standard Model: spontaneous symmetry breaking and the Yukawa coupling, both essential for particles to acquire mass. These reformulations offer alternative explanations cast in purely geometric terms. In particular, the quantisation of charge arises here as a purely geometric consequence of the tensorial construction of matter fields from the fundamental bundles -- a mechanism that is both more general and more transparent than the usual topological account based on the compactness of symmetry groups. More generally, I argue that a symmetry-first account in terms of principal and associated bundles admits a genuine geometry-first counterpart only under certain strict conditions.

Cosmological phase transitions are a frequent phenomenon in particle physics models beyond the Standard Model, and the corresponding gravitational wave signal offers a key probe of new physics in the early Universe. Depending on the underlying microphysics, the transition can exhibit either direct or inverse hydrodynamics, leading to a different phenomenology. Most studies to date have focused on direct transitions, where the cosmic fluid is pushed or dragged by the expanding vacuum bubbles. In contrast, inverse phase transitions are characterized by fluid profiles where the plasma is sucked in by the expanding bubbles. Using the sound shell model, we derive and compare the gravitational wave spectra from sound waves for direct and inverse phase transitions, providing new insights into the potential observable features and the possibility of discriminating among the various fluid solutions in gravitational wave experiments.

The dual formulations of an infinite tower of tree-level soft theorems in asymptotically flat spacetimes for scattering amplitudes in the standard energy-momentum basis and for correlators of a 2D celestial conformal field theory imply a correspondence between the celestial operator product expansion (OPE) and the higher spin charge bracket. We apply such correspondence to provide first a prescription to solve the double-soft limit ambiguity in the mixed-helicity sector of celestial OPEs. Furthermore, demanding the charge OPE/bracket correspondence to remain valid when operators are shadow transformed, we construct an algorithm to compute shadow celestial OPEs. We first test the algorithm by recovering results in the previous literature involving the celestial energy-momentum tensor; we then apply it to both gravity and Yang-Mills theory and generalize the OPE derivation to arbitrary spins.