We discuss the gravitational collapse of a spherically symmetric massive core in which the fluid component is suffering the influence of particle creation due to the temporal evolution of the system's gravity. The influence of the creation rate in the collapsing core is quantified by a phenomenological a-parameter as predicted by dimensional arguments. For all reasonable values of this free parameter, we find that the creation increases the collapsing time, but it cannot prevent the formation of a singular point. The condition for the generation of a black hole is closely related to the creation's rate whose parameter is constrained by α > −1 + 2/[3(1 + ω)], where ω is the barotropic parameter defining the equation of state.
Cosmological distances constrain dark-energy evolution through weighted integrals of the expansion history. This paper isolates a sign-change consequence for deformations around a flat [Formula: see text]CDM background. Let [Formula: see text], with [Formula: see text] continuous, and hold fixed [Formula: see text], the present matter and radiation sectors, flatness, the present dark-energy normalization, and standard early-universe physics. If [Formula: see text] is nonzero and one-signed, the exact comoving distance to last scattering [Formula: see text] is a strictly monotonic function of [Formula: see text] and cannot return to its [Formula: see text]CDM value at any nonzero amplitude. Hence an exactly CMB-distance-transparent nontrivial deformation must cross [Formula: see text]. Linearization gives the positive-kernel moment condition [Formula: see text], with [Formula: see text], which localizes crossings in smooth templates. For [Formula: see text], [Formula: see text] of the kernel mass lies below [Formula: see text], with quartiles [Formula: see text]; the smooth one-crossing templates considered here give [Formula: see text]. We perform a background posterior analysis using the public 13-component DESI DR2 BAO vector and full covariance together with the Planck 2018 distance-prior likelihood. In the CPL family, conditional on a positive crossing, [Formula: see text] at 68% credibility, with a 95% interval [Formula: see text]; [Formula: see text] of all retained draws cross in [Formula: see text]. The first-order transparent direction and an exact nuisance-conditioned transparent manifold remain inside the corresponding two-sided 95% posterior ranges. These posterior diagnostics quantify the transparency geometry without excluding either transparent diagnostic at two-sided 95% credibility. These are background-level results, not a full CMB/PPF likelihood or a microphysical phantom model.
Within the entropic gravity paradigm introduced by Verlinde, one assumes that the microscopic degrees of freedom residing on the holographic screen obey the equipartition law of energy. Nevertheless, implications of statistical mechanics suggest that this energy sharing can acquire corrections that depend on temperature. Taking such modifications into account leads to altered gravitational field equations when derived from thermodynamic considerations. We solve the resulting modified Einstein equations in the case of a static, spherically symmetric spacetime and determine the general structure of the metric components. Our findings indicate that if the temperature correction function behaves as f(T) ∝ T 2 , the corresponding spacetime geometry reduces to the flat Minkowski metric. Therefore, to obtain small deviations from flatness, it is necessary to consider slight departures from this purely quadratic temperature dependence. Such deviations can be interpreted as encoding additional gravitational effects, potentially associated with matter contributions that are not entirely described by the holographic screen. As a result of these effects, the effective gravitational potential acquires a logarithmic correction, which can give rise to deviations from Newtonian gravity. This aspect is especially significant in the regime of very weak gravitational fields, where these corrections may affect particle motion and could carry implications for both astrophysical and cosmological contexts.
Dark matter comprises the bulk of the matter in the universe, yet its fundamental particle nature remains mysterious. We propose a novel approach to constrain dark matter particle properties by simulating the total mass and size of the observable universe. The self-interaction fermionic model is extended to consider a relatively long-range interaction and is applied to describe dark matter. To reproduce the mass (1.28 × 10 23 M ⊙ ) and radius (4.4 × 10 23 km) of the observable universe, we find that the dark matter particle mass is approximately 10 −12 GeV in the short-range interaction model and about 10 ‒9 GeV in the long-range interaction model. This result indicates that dark matter particles might be remarkably light, significantly below the mass scale typically assumed for WIMPs. The cosmological-scale model presented here, as a highly idealized theoretical laboratory, can be broadly applied to constrain the properties of dark matter within different theoretical frameworks.
Abstract The Surface Water and Ocean Topography (SWOT) mission provides high‐resolution sea surface height (SSH) observations at 2‐km spacing, resolving submesoscale features typical of ageostrophic motions and associated with instabilities. Here, we demonstrate the assimilation of SWOT data into a regional configuration of the MITgcm for the California Current System using the four‐dimensional variational (4D‐Var) method with a 31‐day window. A generalized cost function was developed to support assimilation of the novel data set. The resulting state estimate fits SWOT data within the prescribed 2 cm uncertainty. It also shows improved agreement with non‐assimilated in situ data from SWOT post‐launch calibration/validation, indicating that SSH constraints propagate to subsurface dynamics and to other state variables. The results demonstrate that SWOT assimilation can accurately capture mesoscale variability while extending constraints into previously unconstrained spatial scales. Wavenumber spectra indicate effective constraint at scales as small as ∼70 km, highlighting the potential of high‐resolution SSH data to improve regional ocean state estimation.
Black hole X-ray binaries (BHXRBs) are variable sources in multiwavelength bands, especially behave as transient and bright X-ray and radio sources in the sky. In this review paper, we introduce the outbursts of BHXRBs, including multiwavelength observations, changes of accretion states and radiation models. We review the analyzed methods and present the status on measurements of black hole mass and spin, which are the fundamental parameters of black holes and are keys to understanding the birth, evolution and radiation properties of black hole systems. During the outbursts of BHXRBs, X-ray light curves also show a special timing feature, named as quasi-periodic oscillations (QPOs) with frequencies ranging from [Formula: see text] 0.1 Hz to 100[Formula: see text]Hz, which reflect the dynamics of accreting plasma near the black holes. We discuss the multiwavelength discoveries of QPOs in BHXRBs, specially including the new features in X-ray bands and radio QPOs, the possible physical models and implications for black hole physics.
Quantum entanglement is monogamous: a system cannot be maximally entangled with multiple partners. Yet thought experiments in gravitational contexts, notably the Hayden-Preskill protocol, appear to challenge this rule. We argue that the illusion of polygamy arises from neglecting the operational cost of decoding. Redistributing entanglement requires teleportation-like operations so computationally demanding that they have gravitational consequences. Entanglement can only "cheat" through complex operations whose backreaction is unavoidable: they may displace the event horizon, render a wormhole traversable, or induce more drastic changes in the spacetime geometry.
We provide a comprehensive overview of the recent developments of codimension-2 defect systems, in the context of AdS/CFT duality. The holographic description of the conformal defects is in terms of probe branes, while in the field theory picture the defect is realised through singular field configurations of a 2-dimensional surface embedded in [Formula: see text] SYM theory. Starting from the well-established classification of [Formula: see text]-BPS supersymmetric defects, we examine their realisation through probe branes and bubbling supergravity geometries. Special emphasis is placed on defect CFT’s involving nonsupersymmetric D3/D5 configurations and their holographic interpolations. We discuss the calculation of important physical observables, such as one-point functions of the stress–energy tensor and chiral primary operators, across both weak and strong coupling regimes. The agreement of the results in the two regimes exhibits the full power of the holographic principle. This is a proceedings contribution to the Athens Workshop in Theoretical Physics: 10th Anniversary, held at the National and Kapodistrian University of Athens on December 17–19 2025.
This paper demonstrates that the Schwarzschild and Kerr metrics can be derived fundamentally from the Einstein Equivalence Principle (EEP) without explicit reliance on the Einstein field equations. We show that the EEP implies a gravitational stretching of length in the direction of acceleration, a spatial counterpart to time dilation. In the static case, this effect yields the constraint [Formula: see text], from which the Schwarzschild solution is recovered by taking the Newtonian limit. For the stationary case, we employ oblate spheroidal coordinates and construct a physically motivated tetrad to implement the EP, introducing gravity through three unknown functions. By applying the EEP-induced constraints and showing that the principle requires the preservation of the spacetime volume determinant ([Formula: see text]), the system is reduced to a single unknown function that yields the Kerr metric. These results suggest that classical tests of General Relativity are, at their core, fundamental validations of the EEP. We further argue that the EEP leads naturally to Unimodular Gravity, implying that the existence of the cosmological constant is a direct consequence of this foundational postulate.
Modified Newtonian Dynamics (MOND) generally resolves the need for dark matter in galaxy rotation curves introducing a single new constant of acceleration a_0. It is well known that increasing a_0 by a factor of a few can alleviate the residual mass discrepancies that MOND leaves in galaxy clusters. Within a parameter-free Machian interpretation of MOND, in which a_0∼ GM_u/R_u^2 arises from the scalar sum of inverse-square distance gravitational mass contributions in the universe, we promote a_0 to a variable influenced by mass external to a locally enclosed region in the spherically symmetric case. Instead of a boost of a_0 in terms of gravitational potentials as in EMOND, we show that a boost in terms of this directionless inverse-square field roughly amounts to the boost needed to accommodate the mass discrepancies of MOND in galaxy clusters. We conclude by beginning to generalize the proposed formulation beyond spherical symmetry.
The goal of this paper is to compare the Wheeler-DeWitt approach, understood in a broad sense, with an alternative one, the so-called extended phase space approach to quantization of gravity. By "the Wheeler-DeWitt approach", I mean not only quantum geometrodynamics formulated by DeWitt in his seminal paper of 1967, but also any approach to quantization of gravity based on the Wheeler-DeWitt equation in some form. Since the Wheeler-DeWitt equation is a direct consequence of the Dirac formalism, its analysis requires examination of the latter and its application to gravity. In particular, I argue that there is a contradiction between canonical quantization and the idea put forward by founders of quantum gravity that, in this theory, all possible spacetime topologies should be taken into account. The path integral approach seems to be more adequate then the canonical approach. However, to derive the Wheeler-DeWitt equation from the path integral, most authors make the assumption about asymptotic states, that again contradicts the supposition of arbitrary spacetime topology. The extended phase space formalism is entirely based on the path integral approach. Note that if one refuses the assumption about asymptotic states, one cannot prove the gauge invariance of the path integral, and the Wheeler-DeWitt equation loses its sense. In the alternative approach, one derives the Schr & ouml;dinger equation instead. Thus, the extended phase space approach is really beyond the Wheeler-DeWitt approach. The features of this alternative approach are explored with special emphasis on conclusions that cannot be obtained by using the Wheeler-DeWitt quantum geometrodynamics.
The weak equivalence principle (WEP) has been tested with extraordinary precision over the past decades, but invariably with electrically neutral bodies. This leaves a fundamental blind spot: does gravity feel electric charge? A recent phenomenological analysis introduced a parameter [Formula: see text] to quantify this unexplored sector, but lacked a consistent field-theoretic origin. In this work we show that the simplest mechanism is kinetic mixing between the standard photon and a new massless vector field [Formula: see text] that mediates a long-range fifth force coupled to the ordinary mass current. The single gauge-invariant dimension-four operator [Formula: see text] preserves causality and unitarity. By adopting the action in which both fields couple to their respective currents with the same sign convention, the two-body potential is fully symmetric and exactly satisfies Newton’s third law, rendering the model free of any mathematical pathology. The fifth force between equal masses is repulsive, a known property of vector-mediated interactions, but its contribution to the neutral gravitational acceleration is absorbed into the measured Newtonian constant. For a static, spherically symmetric source, the mixing induces an electric field proportional to the Newtonian gravitational field, leading to a charge-dependent acceleration from which we identify [Formula: see text], where [Formula: see text] is the fifth-force coupling strength. The symmetric cross term [Formula: see text] in the two-body potential gives rise to dipole radiation from compact binaries, providing a direct link to pulsar-timing constraints. We derive explicit constraints from the Cassini bound on the PPN parameter [Formula: see text], from lunar laser ranging on [Formula: see text], and from the Hulse–Taylor pulsar, showing that the model is compatible with all existing data while providing a clear roadmap for future experimental tests.
We present the systematic description of quantum dynamics of horizons in spherically symmetric spacetime with the de Sitter center [Formula: see text] on the background of the de Sitter vacuum [Formula: see text] with [Formula: see text]. Spacetime configurations are described by regular solutions of the Kerr–Schild class, specified by the condition [Formula: see text] on stress–energy tensors, satisfying in addition the Weak Energy Condition. To this class belongs the most of solutions presented in the literature. Regular spacetime configurations can have at most three horizons, the internal Cauchy horizon [Formula: see text], the event horizon [Formula: see text] and the cosmological horizon [Formula: see text]. Generic behavior of the basic thermodynamical parameters and dynamics of horizons in the course of evaporation are determined by the spacetime symmetry and basic laws of thermodynamics. Dynamical evolution is governed by quantum evaporation of horizons and involves three basic types of the end products of the horizons evaporation: (i) the thermodynamically stable nonsingular cosmological black hole with the double horizon [Formula: see text], (ii) the thermodynamically unstable cosmology with the double horizon [Formula: see text] and (iii) the absolutely thermodynamically stable triple horizon configuration [Formula: see text], distinguished by the holographic principle.
The standard Λ Cold Dark Matter (ΛCDM) model has achieved remarkable success in explaining the formation and evolution of cosmic structures on large scales, supported by a wide range of observations, including the cosmic microwave background, large-scale structure surveys, and galaxy clusters. However, discrepancies between theoretical predictions and observations on small scales, have motivated the exploration of alternative dark matter models, including the self-interacting dark matter (SIDM) scenario. This review provides an overview of the theoretical foundations of CDM structure formation, the small-scale challenges, and the solutions proposed within the SIDM framework. We summarize recent theoretical developments in the SIDM framework and discuss current observational constraints on the dark matter self-interaction cross-section with particular emphasis on galaxy clusters.
In this paper, we present numerical quantum mechanical description of the formation of low-energy bound states between a dark atom OHe (a neutral atom-like composite of a heavy doubly charged particle O-- and a helium nucleus) and ordinary nuclei, with a focus on sodium and iodine. The total effective interaction potential is reconstructed by combining self-consistent nuclear, electromagnetic and centrifugal contributions. For the OHe-Na system, a single shallow bound state is found with binding energy in the 1-6keV range, which naturally accounts for the annual modulation signal observed by the DAMA/LIBRA experiment if the radiative capture proceeds via an E1 transition. For the OHe-I system, the potential well is much deeper, leading to bound states with energies of order hundreds of keV to MeV and to strongly suppressed capture cross-sections, in agreement with the absence of a modulation signal from iodine and with the upper limits on high-energy gamma rays from DAMA. The framework provides a consistent explanation for the target-material dependence in direct dark matter searches and highlights the importance of quantum effects of atomic and nuclear physics in the interpretation of underground detector signals.
Black holes are now tested not only as exact solutions of Einstein’s equations but as observationally constrained strong-gravity laboratories. Horizon-scale imaging, X-ray reflection spectroscopy, continuum-fitting analyses, and rapid X-ray timing have all matured into serious probes of the near-horizon region, yet they do not measure the same dynamical sector of the spacetime, and therefore cannot be interpreted as interchangeable tests. This review re-examines the electromagnetic program for testing black holes beyond Kerr, with emphasis on three observables that have become central to the field: shadows and photon rings, thin-disk and reflection-based spectroscopy and quasi-periodic oscillations (QPOs). Rather than presenting a catalogue of metrics, the discussion is organized by the structures that control the observables: null geodesics and the photon region, equatorial timelike circular orbits and the ISCO and the orbital and epicyclic frequencies of the innermost flow. This organization makes it possible to separate genuine geometric information from model-dependent astrophysical assumptions, and to identify where claimed beyond-Kerr signatures remain highly degenerate. Particular attention is given to parameterized metrics, exact modified-gravity black-hole solutions, regular and quantum-corrected geometries, and black holes affected by environmental matter such as plasma or perfect fluid dark matter. The central conclusion is methodological rather than conclusive: single-channel agreement is not decisive evidence for non-Kerr geometry, whereas internally consistent multi-observable analyses provide a much stronger route to testing strong gravity. For that reason, the review emphasizes literature-selection logic, minimal formalism, theory-versus-template distinctions, evidence grading, and best-practice directions for future joint inference frameworks.
Relativistic field equations are time-symmetric and permit both retarded and advanced contributions, yet cosmology typically enforces retarded behavior by boundary condition rather than dynamical necessity. This work develops a dual-sector framework in which advanced-looking effects arise as projections of sigma-ordered spacelike dynamics into the T-foliation, rather than from retrocausal signaling. Ordinary matter evolves along timelike trajectories parameterized by proper time T, while a sigma-sector evolves along spacelike worldlines under an invariant parameter sigma. The coexistence of distinct evolution parameters allows spacelike kinematic contributions to contribute to the effective cosmological background without requiring modifications to general relativity or the introduction of additional metric structure. A minimal stress-energy formulation shows that, once the sigma-sector approaches a statistically homogeneous state, it appears as a smooth, non-clustering background component with an effective equation-of-state parameter approaching-1, arising from Lorentz-invariant spacelike kinematics and projection effects rather than scalar potentials, negative masses, or exotic interactions. Dynamically negligible at early times and increasingly relevant as ordinary matter dilutes, the sigma-sector offers a structural explanation for late-time cosmic acceleration consistent with observed large-scale homogeneity. This framework suggests that cosmic acceleration may reflect the geometry of dual temporal foliations rather than a distinct dark-energy substance.
In this study, we construct exact higher-dimensional Lifshitz-type solutions in R2-corrected gravity at the critical point of the theory, where the field equations become degenerate because of the vanishing of the effective gravitational coupling. The analysis is performed on product manifolds of the form Lim & times;Omega(n-m), where Lim denotes an m-dimensional Lifshitz-type spacetime exhibiting anisotropic scaling with dynamical exponent z and Omega(n-m) represents an (n-m)-dimensional space of constant curvature. This geometric decomposition allows for a unified treatment of static, stationary (rotating), and hyperscaling-violating configurations within a purely gravitational framework. We show that the theory admits new broad families of exact Lifshitz black hole and black brane solutions, including extremal configurations, whose scaling exponents and horizon structures are constrained by higher-curvature terms. The stationary solutions are interpreted as rotating Lifshitz-type black holes with well-defined Killing horizons within the appropriate parameter ranges. Owing to the critical nature of the theory, these solutions exhibit vanishing entropy and zero conserved charges despite having nonzero temperature, reflecting the degenerate structure of the field equations. Our results extend the previously known Lifshitz constructions in Einstein and higher-derivative gravity and provide a systematic higher-dimensional framework for exploring anisotropic and hyperscaling-violating geometries supported by curvature-squared interactions.
Lichens are extraordinary organisms known for their ability to thrive in harsh climates, some of which are akin to the environmental conditions found on Mars. Despite extensive research, mysteries still shroud the mechanisms underlying lichen survival in such extreme environments. Numerous studies have investigated the resilience of various lichen species in space and simulated Martian conditions. Especially, evidence supporting their classification as extremophiles is multifaceted: (i) experiments conducted aboard the International Space Station (ISS) have further demonstrated their capacity to withstand extreme conditions akin to those found on Mars, (ii) scrutiny of Martian surface imagery has prompted speculation regarding the presence of structures resembling lichens, and (iii) observations in extreme terrestrial environments suggest the potential for lichens to inhabit Martian rocks, hinting at the existence of “endolithic lichens”. Under this scenario, this work encapsulates the multifaceted evidence based on the data analytic techniques supporting the extremophilic nature of lichens and their potential implications for understanding life beyond Earth. However, the results thus obtained do not claim a full-scale positive signature of the existence of lichen rather provide a statistical high probability and demand further experimental evidences.
A local vortex-structured aurora and a large-scale transpolar arc (TPA) were contemporaneously observed by the Polar ultraviolet imager (UVI) during the late recovery phase of a substorm, and the interplanetary magnetic field (IMF) B Y and B Z were negative and negative-to-positive. The TPA grew along the dawnside auroral oval from the nightside to the dayside, and an auroral spiral and several spots were located azimuthally near the poleward edge of the nightside auroral oval. Both auroras had tailward elongated source regions with scales of similar to 30 R E (spiral) and more than similar to 45 R E (TPA). To examine their magnetospheric/ionospheric field-aligned current (FAC) profiles, we performed global magnetohydrodynamic (MHD) simulations, using two different types of code: Block-Adaptive-Tree Solar-wind Roe Upwind Scheme (BATS-R-US) and improved REProduce Plasma Universe (REPPU). Both MHD simulations reproduced the tailward elongated TPA-associated FAC structures. The spiral-associated FAC intensity was, however, approximately three orders of magnitude weaker than the TPA-associated FAC intensity. Only improved REPPU simulations replicated faint but continuous poleward extending streak-like structures without evident FACs, instead of the auroral spiral. Geomagnetic field measurements showed that the spiral had upward (from the ionosphere to the magnetosphere) FACs, and its appearance might be accompanied by ultra-low-frequency Pc5 waves. Our results suggest that (a) a local-scale spiral might be formed with much weaker magnetotail FACs than global-scale TPA-associated FACs, although the spiral source region is elongated tailward, and (b) a solar wind-magnetosphere-ionosphere coupling system with minimal or no significant substorm effects is required to form the spiral with the weak magnetotail FACs.
