Testing the strong-field regime of gravity has become of great interest in the scientific community following the first gravitational wave detections and growing indications that General Relativity (GR) may require modifications in extreme environments.
In this context, black holes are remarkably simple yet powerful laboratories, providing a unique opportunity to search for deviations from GR and to constrain extensions of gravity in extreme conditions.
A natural framework to explore such deviations is through the inclusion of additional scalar degrees of freedom. Among them, scalar–Gauss–Bonnet (sGB) gravity offers a well-motivated framework, emerging naturally from fundamental high-energy theories such as string theory.
While most existing studies have focussed on massless versions of this theory, introducing a scalar field mass is crucial for a more realistic description and may uncover novel dynamical features.
Motivated by these considerations, I will present recent black hole simulations in massive sGB theory obtained using Numerical Relativity (NR), a rapidly expanding field that has only recently enabled fully non-linear, dynamical studies of compact objects beyond General Relativity and whose full potential is still being explored.
I will first focus on the role of the scalar mass in the spacetime dynamics, as well as the effects of massive scalar perturbations in isolated black holes in the context of quasinormal-mode spectroscopy.
Building on this groundwork, I will then introduce preliminary results from fully dynamical black hole merger simulations in massive sGB gravity, a regime that to date has seen only limited exploration and remains largely unexplored in the massive scalar case.

This work investigates the thermal deconfining phase transition from a hadronic gas composed of massive pions to a quark–gluon plasma (QGP) described by the Polyakov–Nambu–Jona-Lasinio (PNJL) model with two light quark flavors. The PNJL model extends the Nambu–Jona-Lasinio (NJL) approach, including chiral symmetry dynamics and the Polyakov loop, allowing a more realistic description of the QGP phase. A temperature-dependent switching function (SF) is used to ensure a smooth crossover between the hadronic and QGP phases. We analyze the temperature dependence of key thermodynamic quantities, including the pressure, energy density, entropy density, and the square of the speed of sound, and study the influence of pion and quark masses on the equation of state (EOS). This analysis provides a better understanding of the interplay between particle properties and the thermodynamic properties of the system, helping to provide a more comprehensive and accurate picture of the thermal behavior of hadronic matter at zero chemical potential, including its response to changes in temperature and energy. This is essential for interpreting heavy-ion collision results and simulating early-universe conditions. Our analysis results are systematically compared with available lattice QCD data, providing valuable insights and allowing us to assess the accuracy of the model and its ability to describe strongly interacting matter at high temperatures.

Experimental measurements of the large-scale anisotropy of Galactic cosmic rays (CRs) reveal a sharp change in the phase of the dipole component at energies around 100 TeV. Most traditional interpretations invoke macroscopic diffusion models of CR propagation or the influence of nearby sources; however, such approaches often do not adequately account for the local magnetic-field structure at the observer, which can substantially distort the measured anisotropy. Here we treat the anisotropy as a generic consequence of the local magnetic-field properties and the particle transport regime and therefore expect it to manifest at arbitrary locations in the Galaxy, rather than being specific to the vicinity of the Solar System. The source distribution and the local field parameters primarily determine the characteristic energy and the width of the dipole-phase transition region.

In this work, we model CR transport microscopically by numerically integrating particle trajectories in an interstellar magnetic field represented as the superposition of a uniform regular component and an isotropic turbulent component with a Kolmogorov spectrum. The turbulent field is generated using a harmonic method. The anisotropy is computed using the backtracking method to construct a sky map of the arrival intensity, followed by a multipole decomposition and extraction of the dipole term. We show that the sharp phase change is naturally associated with a crossover in the propagation regime. At energies where the Larmor radius is smaller than the turbulence correlation length, particles remain tied to local field lines and the dipole phase aligns with the local magnetic-field direction. Once the Larmor radius exceeds the correlation length, transport approaches standard three-dimensional diffusion, and the phase stabilizes along the direction set by the large-scale regular field. Our numerical results indicate that the position and the width of the phase-transition region are determined primarily by this transport-regime crossover.

Atomic nuclei are self-organized, many-body quantum systems bound by strong nuclear forces within femtometre-scale space. The shape and orientation of colliding nuclei play a crucial role in determining the initial conditions of the quark–gluon plasma (QGP), which influence key observables such as anisotropic and radial flow. In this talk, I will present an “imaging-by-smashing” approach using soft probes that are directly sensitive to the structure of colliding nuclei, from large to small systems, in high-energy nuclear collisions [1,2,3]. We present the measurements of , fluctuation, and − correlations in isobar-like²³⁸U+²³⁸U and ¹⁹⁷Au+¹⁹⁷Au collisions at 193 and 200 GeV, respectively. Our results reveal prominent differences in these observables between the two systems, particularly in the most central collisions. We also present the first measurements of in ¹⁶O+¹⁶O and d+¹⁹⁷Au collisions, providing insight into the impact of nucleon–nucleon correlations and further shedding light on the initial conditions of QGP droplets. Comparisons with the state-of-the-art hydrodynamic calculations enable us to extract the nuclear shape parameters across energy scales [4,5]. This interdisciplinary research enhances our understanding of the initial conditions in high-energy collisions and provides new insights into the evolution of nuclear structure across energy scales.

References:
[1] STAR Collaboration, Nature 635, 67-72 (2024)

[2] STAR Collaboration, Rep. Prog. Phys. 88, 108601 (2025)

[3] STAR Collaboration, arXiv:2510.19645

[4] C. Zhang, J. Jia, J. Chen, C. Shen, L. Liu, arXiv:2504.15245

[5] C. Zhang, J. Chen, G. Giacalone, S. Huang, J. Jia, Y.-G. Ma, PLB862, 139322(2025)

Schwarzschild solitons and Generalized Schwarzschild solitons have recently been studied on the new metrized smooth metric space of convex functions.

The completeness of geodesics and the Cauchy hypersurfaces are established for Schwarzschild spacetimes after correction [Hawking, S.W.: Commun. Math. Phys. 25, 152–166 (1972).].

The solitons are newly posed on the pseudo-spherical General Relativistic cylinder, with new equations set and uniquely solved.

The uniqueness of Schwarzschild solitons is newly investigated.

The concurrent vector field in General Relativity is newly defined, and the 4-position vector is newly indicated as a concurrent vector field in General Relativity; the Killing vector field is newly determined accordingly to the 4-velocity vector when the configuration of the observer frame is chosen as one solidal with the photon.

New isopoertimetric inequalities and new curvature estimates are provided to hold. New thoerems are proven about these geometries.

The Penrose tipping lightcones are newly written for these spacetimes; the cross-sections are determined accordingly. Accordingly, the Yamabe flow on the S2 sphere is newly proven to converge for submersion from a Schwarzschild spacetime: geodesics spheres are taken into account.

Submersions and immersions are newly discussed when these spacetimes are considered according to the pullback from the metric tensor.

The elements are also gathered for discussion of the Penrose 1965 Theorem.

We investigate finite-size effects on the density-driven deconfinement phase transition (DPT) in Quantum Chromodynamics (QCD) using a model of coexisting hadronic and quark–gluon plasma (QGP) phases in a finite volume. The QGP phase is modeled via the MIT bag approach, explicitly incorporating the color-singletness constraint to account for the color confinement. As a continuation of our previous work, in the present study, we will analyze the first and second chemical derivatives of the order parameter across a range of quark chemical potentials (μ), at fixed temperature (T) and for several volume (V) selections, to determine the effective transition point in a finite volume. Our results reveal that the effective transition chemical potential μ_c(V) shifts to higher values as the system size decreases, highlighting the pronounced influence of finite-volume effects. Moreover, the rapid variations in the order parameter and its chemical susceptibility at the transition in large volumes, are rounded off in small volumes, and the transition region is smeared out, acquiring a width δµ(V) which increases with decreasing volume. These findings provide a comprehensive understanding of how finite-volume constraints influence the QCD phase structure, offering important insights for interpreting results from heavy-ion collisions and other high-energy experiments where the system size is inherently limited.

General relativity (GR) lays the foundation for successfully explaining the current gravitational wave (GW) observations. We present a method for studying the orbital frequency evolution of GWs from binary black hole (BBH) systems based on their energy distribution on the time-frequency plane. The orbital frequency evolution of BBH systems is determined by the individual masses and spins of the component black holes and the governing gravity theory. If a beyond-GR theory of gravity governs the BBH orbital evolution for the same set of binary parameters, the time-frequency pixel energies will exhibit a different frequency evolution from what is predicted by GR. We develop a new consistency test to check whether GR explains the BBH orbital evolution. Through numerical simulation of beyond-GR theory of gravity, we demonstrate the efficiency of this new method in detecting any possible departure from GR in the framework of second-generation GW interferometers. Further, we discuss the utility of our method in probing missing physics in the GW waveform models. We apply our test to the GW190814 and GW190412 data from the LIGO-Livingston detector, assuming that the analyzing template waveform does not include higher-order modes. The lack of subdominant modes results in an incomplete representation of the GW signal, leading to systematic biases in the frequency evolution of the signal.

In this study, we investigated the Bianchi type-V model in the context of f(R, T) gravity.

Here, we have considered the f(R, T)=R+2f(T), where f(T)= λT where λ is an arbitrary constant, R is the Ricci scalar, and T is the trace of stress energy momentum tensor. We have made the assumption that the cosmic jerk parameter, j, is precisely proportional to the negative of the deceleration parameter, q, namely in order to solve the field equations. The conversation covers a variety of situations including the Hubble parameter (H), the spatial volume (V), the deceleration parameter (q), the energy density (ρ), the pressure of matter (p), and the cosmological constant (Λ). We investigate a bouncing point, the moment of transition between contraction to expansion. The value of the expansion scalar θ describes a smooth, bounded transition between contraction and expansion which represents a bouncing universe (avoiding Big Bang singularity) and also represents a bouncing universe with maximum expansion rate due to modified gravity. The model also diagnoses the de Sitter universe, where exponential expansion is driven by a cosmological constant or dark energy. The model looks at the physical and geometrical behavior of the model, as well as the exact solutions under the anticipated jerk parameter condition, and summarizes key findings.

Persistent tensions in the Hubble parameter ($H_0$) and the matter clustering amplitude ($S_8$) motivate theoretical extensions beyond the non-interacting dark sector assumed in $\Lambda$CDM. From an effective field theory perspective, a non-zero Yukawa interaction between a scalar dark-energy field $\phi$ and fermionic dark matter $\psi$ is technically natural unless prohibited by symmetry. In this work, we construct a microphysically consistent framework in which such an interaction emerges dynamically rather than being imposed phenomenologically.

We model the dark sector as an open quantum system, in which the scalar field constitutes a reduced subsystem that exchanges energy and information with an environmental dark-matter bath. Unlike a closed (unitary) quantum system, tracing over the fermionic degrees of freedom induces irreversible effects captured by a Lindblad evolution equation for the scalar density matrix. The scalar potential undergoes spontaneous symmetry breaking once the dark-matter density falls below a critical scale, inducing a time-dependent vacuum expectation value. The resulting interaction kernel $Q(a)$ and coupling profile $\beta(a)$ are therefore not free parameters, but consequences of the underlying symmetry-breaking and dissipative microphysics.

The resulting phenomenology is distinctive: the expansion history remains close to $\Lambda$CDM, while structure growth is modified through a late-time activation of the coupling. We test this against current large-scale structure and background probes using RSD, BAO, SN Ia, and CMB priors. A dynamical analysis—which self-consistently evolves both the background and perturbations—favours a late transition ($a_c \simeq 0.47$) and ($\beta_0 \simeq 0.52$). The model naturally suppresses the growth amplitude to $\sigma_{8,0} \simeq 0.59$ while simultaneously raising the inferred Hubble constant to $H_0 \simeq 69.6\ \mathrm{km\,s^{-1}\,Mpc^{-1}}$, suggesting that the $H_0$ and $S_8$ discrepancies may trace distinct physical origins. This places observationally inferred coupling strengths in direct correspondence with radiative stability constraints, making the next generation of spectroscopic surveys a decisive test of this class of models.

The Barrow holographic dark energy (BHDE) framework arises from Barrow entropy \cite{ Phys. Rev. D 102 (2020) 123525, Phys. Lett. B 808 (2020) 135643 }, which incorporates possible fractal deformations of the horizon surface and is characterized by the Barrow exponent Δ, thereby modifying the standard holographic dark energy scenario. We have studied the effect of dynamical radiation in the interacting Barrow holographic dark energy (BHDE) model for a non-flat universe. For both open and closed universes (“open” and “closed” refer explicitly to the sign of the spatial curvature parameter $k$ present in the FLRW metric ), we derived the evolution equations for the energy density parameters of dark energy, dark matter, and radiation by considering four different types of interactions among the other possible linear phenomenological forms. These coupled differential equations were then solved numerically to examine their behavior with respect to the redshift parameter. The variation of the dark energy equation-of-state parameter with redshift was also explored for the different interaction models. For all four interaction cases, we found that higher values of the Barrow exponent lead to a transition of the dark energy equation-of-state parameter from the quintessence region to the phantom region at late times, corresponding to lower redshift values. We also identified distinct epochs corresponding to dark energy–dark matter, dark energy–radiation, and dark matter–radiation crossings.

Using data from the Cosmic Chronometer, Baryon Acoustic Oscillation, and Pantheon+ samples, we constrained several cosmological parameters of the interacting BHDE model. The estimated Hubble parameter values in our model are higher than those predicted by the ΛCDM model. This result suggests that our framework could provide useful insights for future studies involving high-redshift data and may contribute to resolving the Hubble tension problem. A statistical comparison between our models and the ΛCDM model has also been carried out.