In this study, we introduce an extension of the quasi-dilaton massive gravity theory and derive the field equations by varying the action with respect to the metric. This extension elucidates the dynamics of the system and demonstrates how it can encompass and recover previous cosmological models through different parameter values. We present the cosmological background equations to analyze self-accelerating solutions that can explain the late-time accelerated expansion of the Universe, driven by an effective cosmological constant arising from massive gravity. Besides, we apply the quasi-dilaton massive gravity theory with non-minimal kinetic coupling to a Type Ia Supernovae (SNIa) data set to test its viability. Our findings indicate that the theory is able to account for the observed acceleration of the expansion of the universe without invoking dark energy. In addition, we carry out a comprehensive perturbation analysis examining tensor, vector, and scalar perturbations independently. We derive the dispersion relation of gravitational waves in a Friedman-Lemaitre-Robertson-Walker (FLRW) cosmology and determine the stability conditions of the system. Such an analysis results in a sharper quasi-dilaton massive gravity theory with non-minimal kinetic coupling by ensuring the stability conditions of the system are maintained and that strong constraints on theory parameters are provided.

China

Iran

Cosmology of quasi-dilaton massive gravity with non-minimal kinetic coupling

Membrane configurations in the Banks-Fischler-Shenker-Susskind matrix model are unstable due to the existence of flat directions in the potential and the decay process can be seen as a realization of chaotic scattering. In this note, we compute the lifetime of a membrane in a reduced model. The resulting lifetime exhibits scaling laws with respect to energy, coupling constant and a cut-off scale. We numerically evaluate the scaling exponents, which cannot be fixed by the dimensional analysis. Finally, some applications of the results are discussed.

Japan

Scaling law for membrane lifetime

Various studies show that the gravitational collapse of inhomogeneous matter clouds leads to naked singularity formation. We investigate here the spin precession frequency of a test gyroscope attached to a stationary observer in a rotating naked singularity spacetime. In the weak field limit, Lense-Thirring precession for rotating naked singularity and geodetic precession in asymptotic limit for null naked singularity are found to be equal to that of a Kerr black hole and a Schwarzschild black hole respectively. In addition, we can distinguish rotating naked singularity and Kerr naked singularity for an observer in the equatorial plane using spin precession. To this end, we have found the constraints on the parameters of rotating naked singularity by employing Monte Carlo Markov Chain simulation and using the observation from five quasi-periodic sources within the relativistic precession model. Our analysis shows that the measurement of spin parameter estimate for GRO J1655-40 is in disagreement with the value found from the continuum-fitting method, while for XTEJ1859+226 and GRS 1915+105, it is inconsistent with spectral analysis results.

Pakistan

Türkiye

Spin precession frequencies of a test gyroscope around a naked singularity and quasi-periodic oscillations

As in other partial differential equations, one ends up with some arbitrary constants or arbitrary functions when one integrates Einstein's equations, or more generally field equations of any other gravity. Interpretation of these arbitrary constants and functions as some physical quantities that can in principle be measured is a non-trivial matter. Concentrating on the case of constants, one usually identifies them as conserved mass, momentum, angular momentum, center of mass, or some other hairs of the solution. This can be done via the Arnowitt-Deser-Misner (ADM)-type construction based on pure geometry, and the solution is typically a black hole. Hence, one talks about the black hole mass and angular momentum etc. Here we show that there are several misunderstandings: First of all, the physical interpretation of the constants of a given geometry depends not only on pure geometry, i.e. the metric, but also on the theory under consideration. This becomes quite important, especially when there is a cosmological constant. Secondly, one usually assigns the maximally symmetric spacetime, say the flat or the (anti)-de Sitter spacetime, to have zero mass and angular momentum, and linear momentum. This declares the maximally symmetric spacetime to be the vacuum of the theory, but such an assignment depends on the coordinates in the ADM-type constructions and their extensions: in fact, one can introduce large gauge transformations (new coordinates) which map, say, the flat spacetime to flat spacetime but the resultant flat spacetime can have a nontrivial mass and angular momentum, if the new coordinates are such that the metric components do not decay properly. These issues, which are often overlooked, will be examined in detail, and a resolution, via the use of a divergence-free rank <math altimg='si1.svg'><mo stretchy='false'>(</mo><mn>0</mn><mo>,</mo><mn>4</mn><mo stretchy='false'>)</mo></math>-tensor, will be shown for the case of anti-de Sitter spacetimes.

Physical interpretations of integration constants and large gauge effects in flat and AdS spacetimes

This work presents a comprehensive analysis of the geodesic motion, scalar field perturbations, and thermodynamic behavior of a static, spherically symmetric black hole solution arising within the framework of non-commutative geometry and incorporating the presence of a global monopole. The study is motivated by the interplay between quantum gravity effects (modeled via non-commutative geometry) and topological defects such as global monopoles, both of which contribute to significant deviations from classical black hole solutions. We begin by formulating the effective potential governing the motion of test particles-both massive and massless-in the modified spacetime. The effects of the non-commutative parameter and the global monopole are systematically examined in the context of various key features: the photon sphere radius, light deflection angle, stability of circular orbits, and the innermost stable circular orbit (ISCO). Our results show that the presence of the non-commutative structure enlarges the photon sphere radius and increases the deflection of light, while simultaneously destabilizing circular orbits. Furthermore, we observe that the ISCO radius is highly sensitive to the global monopole parameter, with a noticeable increase for certain ranges of the non-commutative parameter. These findings suggest that non-commutative geometry and topological defects significantly affect the observable properties of black hole environments. In the second part of the study, we explore linear perturbations of a massless scalar field by solving the Klein-Gordon equation in the background spacetime. The resulting effective potential for scalar perturbations is shown to be deeply influenced by both the non-commutative geometry and the global monopole, modifying the scattering and decay characteristics of the scalar field. By employing the eikonal approximation, we compute the complex quasinormal mode (QNM) frequencies and analyze how their real and imaginary parts evolve with changes in the underlying parameters. The results provide insight into the stability of the black hole under perturbations and offer potential observational signatures of non-commutative and monopole-induced corrections. Finally, the thermodynamic properties of the black hole are investigated. We derive the expressions for the Hawking temperature, specific heat, and Gibbs free energy, showing how these quantities deviate from their counterparts in the standard Schwarzschild solution. Notably, we find that the introduction of a non-commutative structure and global monopole leads to non-trivial modifications in the thermodynamic phase structure. In particular, the black hole may exhibit modified thermal stability and phase transitions depending on the values of the non-commutative and monopole parameters.

India

Jordan

Exploring geodesics, quantum fields and thermodynamics of Schwarzschild-AdS black hole with a global monopole in non-commutative geometry

Deviations from unitarity of the CKM matrix in the quark sector are considered excellent windows to probe physics beyond the Standard Model. In its leptonic counterpart, the PMNS matrix, these searches are particularly motivated, as the new physics needed to generate neutrino masses often leads to non-unitary mixing among the standard neutrinos. It is then interesting to consider how neutrino oscillations are affected in such scenario. This simple question is, however, subject to several subtleties: What is the correct way to define oscillation probabilities for a non-unitary mixing matrix? Do these probabilities add up to one? Does a non-unitary mixing matrix lead to observable flavor transitions at zero distance? What is the interplay between unitarity constraints obtained from neutrino oscillations and from electroweak precision data? This work aims to shed light on these issues and to clarify the corresponding misconceptions commonly found in the literature. We also compile updated bounds from neutrino oscillation searches to compare with those from flavour and electroweak precision observables.

Sweden

Spain

France

Misconceptions in neutrino oscillations in presence of non-unitary mixing

The use of the vacuum projector <math altimg='si1.svg'><mo stretchy='false'>|</mo><mn>0</mn><mo stretchy='false'>&#9002;</mo><mo stretchy='false'>&#9001;</mo><mn>0</mn><mo stretchy='false'>|</mo></math> and of the unitary Weyl operators enables us to construct a set of Hermitian dichotomic operators in relativistic scalar Quantum Field Theory in Minkowski spacetime. Employing test functions supported in diamond regions, both Bell and Mermin inequalities are studied by means of a numerical setup. In addition to reporting expressive violations of both inequalities, the cluster property is also checked.

Brazil

Bell and Mermin inequalities in Quantum Field Theory from vacuum projectors and Weyl operators

We present a self-contained and consistent formulation of noncommutative (NC) gauge theory of gravity, focusing on spherically symmetric black hole geometries. Our construction starts from the gauge-theoretic viewpoint of Poincaré (or de Sitter) gravity and introduces noncommutativity through the Moyal star product and the Seiberg-Witten map, retaining NC gauge invariance at each order in the deformation parameter Θ. Working systematically to second order in Θ, we obtain explicit NC corrections to the spin connection, the vierbein, and various geometric objects such as the metric and curvature scalars. Using these results, we compute NC modifications of four-dimensional Schwarzschild and Reissner-Nordström solutions, including scenarios with a cosmological constant, as well as three-dimensional BTZ-type black holes (both uncharged and charged). For each black hole solution, we explore various possible Moyal twists, each of which generally breaks some symmetries and modifies the horizon structure, surface gravity, and curvature invariants. In particular, we show that while the radial location of horizons in Schwarzschild-like solutions remains unchanged for some twists, other twists introduce important but finite deformations in curvature scalars and can decouple the Killing horizon from the causal horizon. Similar patterns arise in the charged and lower-dimensional cases. Beyond constructing explicit examples, our approach provides a blueprint for systematically incorporating short-distance quantum corrections through noncommutativity in gravitational settings. The methods and expansions we present can be extended to more general geometries including rotating black holes and additional matter fields, offering a broad framework for future studies of NC effects in classical solutions of general relativity.

Croatia

Constructing noncommutative black holes

We present a theory of electroweak interactions in the lepton sector extended to six dimensions. This extension involves adding two additional time-like dimensions to the standard four-dimensional theory, resulting in a six-dimensional spacetime corresponding to the Clifford algebra C <italic>&#8467;</italic>(3,3). A matrix representation of C <italic>&#8467;</italic>(3,3) is used to construct Lorentz invariant quantities from its elements. We demonstrate that three specific elements of C <italic>&#8467;</italic>(3,3) are sufficient to describe the eight lepton states of a single generation. Using these elements, we define a representation of SU(2)&#215;U(1), leading to an electroweak theory that incorporates the chiral nature of the interactions. Our results show that components of the gauge bosons in the extra time-like dimensions form the graded superalgebra <math altimg='si1.svg'><mrow><mi mathvariant='fraktur'>su</mi></mrow><mo stretchy='false'>(</mo><mn>4</mn><mo stretchy='false'>|</mo><mn>4</mn><mo stretchy='false'>)</mo></math>, which generates quadratic and quartic terms for the extra-dimensional components of the <italic>W</italic> bosons. These terms facilitate a vacuum expectation value and spontaneous symmetry breaking, providing a novel interpretation of the Higgs potential. We predict a tree-level Higgs mass from this potential to be half the vacuum expectation value, or <math altimg='si2.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>H</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>123.11</mn></math> GeV. Furthermore, the model predicts a Weinberg angle of <math altimg='si156.svg'><msub><mrow><mi>&#952;</mi></mrow><mrow><mi>W</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><msup><mrow><mn>30</mn></mrow><mrow><mo>&#8728;</mo></mrow></msup></math>. This approach contrasts with previous models that utilized internal gauge symmetries and superalgebras like <math altimg='si4.svg'><mrow><mi mathvariant='fraktur'>su</mi></mrow><mo stretchy='false'>(</mo><mn>2</mn><mo stretchy='false'>|</mo><mn>1</mn><mo stretchy='false'>)</mo></math> or <math altimg='si5.svg'><mrow><mi mathvariant='fraktur'>su</mi></mrow><mo stretchy='false'>(</mo><mn>2</mn><mo stretchy='false'>|</mo><mn>2</mn><mo stretchy='false'>)</mo></math>, by deriving the superalgebra <math altimg='si1.svg'><mrow><mi mathvariant='fraktur'>su</mi></mrow><mo stretchy='false'>(</mo><mn>4</mn><mo stretchy='false'>|</mo><mn>4</mn><mo stretchy='false'>)</mo></math> directly from the spacetime geometry of C <italic>&#8467;</italic>(3,3).

An electroweak theory of leptons in six dimensions

This study examines the regular charged black hole solution by investigating particle dynamics and various related properties, such as the radius of the horizon, the effective force, the energy, the angular momentum, the ISCO radius, the photon radius, the red-blue shift, and the weak plasma lensing in uniform plasma fields, <italic>SIS</italic>, and <math altimg='si1.svg'><mi>N</mi><mi>S</mi><mi>I</mi><mi>S</mi></math>. Furthermore, we examine the image magnification for the black hole solution. A salient characteristic of this black hole solution is the incorporation of the magnetic charge parameter <italic>q</italic>, which differentiates it from the Schwarzschild black hole solution. By taking <math altimg='si2.svg'><mi>q</mi><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>0</mn></math>, we compare our results with the Schwarzschild black hole scenario and identify substantial variations. Our research elucidates the impact of the magnetic charge parameter <italic>q</italic> on multiple physical parameters of the black hole and offers a comprehensive comparison with the Schwarzschild black hole solution.