XIV Bolyai–Gauss–Lobachevsky Conference (BGL–2026): Non-Euclidean Geometry in Modern Physics and Mathematics

15 Sept 2026, 08:45 → 17 Sept 2026, 21:30 Europe/Bucharest

Main Building (Babeș-Bolyai University, Cluj-Napoca, Romania)

The 14^th Bolyai–Gauss–Lobachevsky (BGL-2026) Conference on Non-Euclidean Geometry in Modern Physics and Mathematics will take place in Cluj-Napoca, Romania at Babeș-Bolyai University's main building (adressed: Kogalniceanu Street no 1, 400084, Cluj-Napoca, Romania). The conference is organized by the Faculty of Mathematics and Computer Science and the Faculty of Physics. 

WARNING: There is an ongoing phising campaign from email addresses ending in @g-travelexperts.com, sending out scam emails. Please make sure to ignore these emails and not share any personal information.

This meeting follows the earlier ones held at Uzhgorod, Ukraine, (1997), Nyíregyháza, Hungary (1999), Targu Mures, Romania (2002), Nizhny Novgorod, Russia (2004), Minsk, Belarus (2006), Debrecen, Hungary (2008),  Cluj Napoca, Romania (2010), Uzhgorod, Ukraine (2012), Minsk, Belarus (2015), Gyönygyös, Hungary (2017), Kyiv, Ukraine (2019), Budapest, Hungary (2024) and Saïdia, Morocco (2025).

Topics

• Cosmology, gravitational waves, dark energy, and dark matter
• Modified theories of gravity
• The Hubble tension: observational results and possible solutions
• Recent developments in differential geometry
• Quantum gravity and quantum fields in curved spacetimes
• Mathematical modeling of physical systems, complex dynamical behaviors, instabilities and chaos
• Quantum groups and algebras, deformed quantum systems
• Quantum information
• The early Universe, inflation, reheating, and Big Bang Nucleosynthesis
• Non-Euclidean geometry in particle physics and quantum field theory
• Nuclear matter under extreme conditions
• History of science
• Outreach
  

 

Participation fee

The registration fee is 400 EUR/participant and 200 EUR/student (BSc, MSc or PhD) or accompanying person. For all participants we will provide a welcome banquet, coffee breaks, lunch and dinner, including the gala dinner. All fees have to be paid in advance by credit card clicking the following link https://plati.ubbcluj.ro/Event/Details/353.

The details for payment of fees by bank transfer are as follows:

 

Beneficiary: Universitatea Babes-Bolyai Cluj

Beneficiary address: Mihail Kogalniceanu nr.1, Cluj-Napoca, Cluj, Romania

Beneficiary's bank: Banca Comerciala Romana

Bank address: Calea Plevnei 159, Business Garden Bucharest, Bucuresti, Romania

Swift Code: RNCBROBU

IBAN: RO16RNCB0106026604700008 valuta EUR

 

For the payment details please specify: participation fee BGL2026.

 

Excursions

For the interested participants, we will organize an excursion on Friday to Turda City which is 35 minutes away from the conference location. There will be two destinations, namely:

Destination 1: Salina Turda (Turda Salt Mine), URL: https://www.salinaturda.eu/en/
Destination 2: Cheile Turzii (Turda Gorge), URL: https://turism-cheile-turzii.ro/?lang=en

Important dates

Deadline for online registration: September 1st

The payment of the participation fee must be completed by September 1st.

Arrival and registration: September 14, 2026  
Opening of the Conference: September 15, 2026, 9:00 am EEST (GMT+3)
Closing of the Conference: September 17, 2026, 7:00 pm EEST (GMT+3)

Excursion: September 18, 2026, leaving time to be announced.

 

 

 

 

   

Tuesday 15 September

08:45 → 09:30 Registration (Room 7/I)

09:30 → 10:00 Opening Ceremony (Room 2/I)

10:00 → 10:30 Sculpting geometric spacetimes

Traversable wormholes provide a rigorous laboratory for probing the interplay of geometry, matter, stability, and causality in general relativity. This talk centers on the thin-shell formalism: cut-and-paste construction via the Israel-Lanczos junction conditions, precise localization and quantification of exotic matter, and stability analysis. We then examine the geometric characterization of static and dynamical throats, chronology issues, generic thin shells and gravastars, and the black-bounce programme, in which regular black holes and traversable wormholes emerge from a single geometric framework.

Speaker: Francisco Lobo (University of Lisbon)

10:30 → 11:00 Little Sibling of the Big Rip: a new parameterization

We review the Little Sibling of the Big Rip (LSBR) model, by introducing a new parameterization designed to alleviate its negative energy behavior at early times. The main objective of this parameterization is to improve the theoretical and observational viability of the LSBR model and provide an alternative to the ΛCDM model. The proposed parameterization is constrained using cosmic microwave background (CMB) data, Dark Energy Spectroscopic Instrument (DESI DR2) measurements, and several Type Ia supernovae (SNIa) datasets, including PantheonPlus, Union3, and DES-Y5. The combination of CMB and DESI DR2 data indicates that, owing to the recent inferred value of the equation of state parameter, our model behaves like a phantom dark energy. However, when different SNIa datasets are included, the model shifts toward a quintessence like behavior. From a statistical perspective, using the combined CMB + DESI DR2 + DES-Y5 dataset, we find that the evidence in favor of the proposed parameterization is enhanced by approximately 2.1σ relative to the ΛCDM model. We further show that the inferred nature of this new LSBR parameterization has significant implications for the future evolution and ultimate fate of the Universe.

Speaker: Taoufik OUALI (Mohammed First University)

11:00 → 11:30 Peculiar signatures of Dark Universe

The modern Standard cosmological scenario involves inflation, baryosynthesis and dark matter/energy. Physics of all these elements of the cosmological paradigm lays Beyond the Standard model (BSM) of elementary particles and involves in its turn cosmological probes for its study. To specify this physics the idea of multi-messenger probes of new physics is proposed, involving the set of additional model dependent consequences of physical models for inflation, baryosynthesis and dark matter. We concentrate on probes for nonstandard features of BSM physics in primordial structures, from dark atoms to primordial nonlinear structures. In homogeneous and isotropic Universe strong primordial
nonhomogeneity is determined by specific model dependent choice of mechanisms of inflation and baryosynthesis and the BSM physics, underlying the modern cosmology. Positive evidence for dark atoms, Primordial Black Holes and their clustering, primordial inhomogeneity of dark matter or macroscopic antimatter existence leads beyond the standard paradigm of the cosmological scenario and specify with high precision the parameters of BSM physics.

Speaker: Maxim Khlopov (Virtual Institute of Astroparticle physics)

11:30 → 12:00 Coffee Break

12:00 → 12:30 Ultraviolet completion of Starobinsky inflation

I will present a supergravity action whose bosonic part contains the f(R) gravity. In a perturbative expansion in powers of $R$, one obtains a small deformation of the Starobinsky cosmological model that solves the problem of initial conditions within the validity of the effective field theory, below the scale of tower of states predicted by the swampland distance conjecture. Moreover, it allows better agreement with the recent cosmological data. A particular example of an underlying microscopic theory with such properties is provided by a four-dimensional heterotic string model containing the Standard Model of particle physics.

Speaker: Ignatios Antoniadis (LPTHE - CNRS, Sorbonne University and AEC University of Bern)

12:30 → 13:00 Black hole solutions in quadratic Weyl geometric gravity

We discuss exact and numerical black hole solutions obtained in the linear representation of the conformally invariant Weyl quadratic gravity. After adopting a static spherically symmetric geometry, the vacuum field equations for the gravitational, scalar, and Weyl fields can be reformulated in a dimensionless form, which can generally be solved numerically. We detect the formation of a black hole from the presence of a Killing horizon for the timelike Killing vector in the metric tensor components, indicating the existence of the singularity in the metric. An exact black hole model, corresponding to a Weyl vector having only a radial spacelike component, is also obtained. Constraints on the parameters of the exact black hole solution are obtained by using the Solar System tests. The thermodynamic properties of the Weyl geometric type black holes (horizon temperature, specific heat, entropy and evaporation time due to Hawking luminosity) are also analyzed in detail.

Speaker: Tiberiu Harko (Babes-Bolyai University)

13:00 → 13:30 General Relativity from Non-Riemannian Actions

The Einstein field equations are known to follow from a variational principle based on the Einstein-Hilbert action. In this work, we investigate actions formulated in non-Riemannian geometry that produce the metric dynamics of general relativity. We classify these actions into two broad categories: (i) those whose equivalence with general relativity holds only \textit{on-shell}, and (ii) those whose equivalence holds already \textit{off-shell}. We clarify that the standard teleparallel equivalents of general relativity belong to the first category, and we emphasise the role of the teleparallel constraint in establishing this equivalence. We then introduce an action belonging to the second category, differing from the Einstein-Hilbert action only by a boundary term. We show that imposing on-shell constraints can modify this pre-arranged, off-shell equivalence. We derive general conditions under which a connection constraint preserves the equivalence with general relativity and illustrate them with two explicit examples.

Speaker: Lehel Csillag (UBB, UniTBV)

13:30 → 15:00 Lunch

15:00 → 15:30 Noncommutativity, Noncanonicality and Observability in Quantum Mechanics

We explore the interplay among noncommutativity, noncanonicality, and observability
in quantum mechanics based on an extended Heisenberg algebra. Using the Bopp
shift technique, we develop a unified framework in which noncommutative and
noncanonical effects are systematically incorporated into quantum operators. The
resulting framework leads to modified Schrödinger, Klein–Gordon, Dirac, and Pauli
equations and provides a systematic description of their observable consequences. We
introduce an energy-dependent parameterization scheme to explore possible
connections between noncommutative quantum effects and several fundamental
problems, including dark energy and quantum decoherence. We also show that the
extended algebra can generate anomalous velocity and acceleration for free particles,
offering a possible quantum-mechanical perspective on the accelerated expansion of
the Universe. Furthermore, noncommutativity gives rise to novel quantum
fluctuations and generalized uncertainty relations beyond the standard Heisenberg
form. Our results indicate that noncommutativity and noncanonicality may have
genuine observable consequences, suggesting new connections between extended
quantum mechanics and cosmology.

Speaker: Shidong Liang (Sun Yat-sen University)

15:30 → 16:00 Quantum Spacetime Picture of Quantum Particle Dynamics and Black Hole Entropy

A simple picture of quantum spacetime is presented for quantum physics based on the original spirit of Heisenberg and Dirac and the modern perspective of noncommutative geometry. The quantum dynamics of a particle, with or without spin, in curved spacetime is analyzed from the Heisenberg picture, based on a covariant Hamiltonian formulation. For the spin-zero particle, quantum equations of geodesic motion are obtained. The formulation is not compatible with the usual approach based on the Schr\"odinger wavefunction representation, and has theoretically desirable features over the latter. It suggests an alternative path to quantum gravitation based on the observables. For an application to a black hole metric, an interesting special quantum effect is obtained as a positive contribution to the acceleration in the radial direction inside the black hole. Our analysis shows that it is the dominant contribution as we approach the singularity, indicating that quantum particles cannot fall into the singularity. A nontrivial quantum matter distribution is hence to be expected around the center of a black hole, giving the black hole entropy.

Speaker: Otto Kong (National Central University, Taiwan)

16:00 → 16:30 The differential geometry of constrained quantum states: Berry curvature and quantum metric

The geometric properties of quantum states are crucial for understanding many physical phenomena in quantum mechanics, condensed matter physics, and optics. The central object describing these properties is the quantum geometric tensor, which unifies the Berry curvature and the quantum metric. I will introduce the differential-geometric framework of vector bundles to analyze the properties of parameter-dependent quantum states and generalize the quantum geometric tensor to this setting. To illustrate these results, I will briefly discuss the sub-bundle geometry arising in the semiclassical treatment of Dirac fields propagating in curved spacetime and show how the quantum geometric tensor. This is based on https://doi.org/10.22331/q-2026-01-14-1965.

Speaker: Marius Oancea (University of Vienna)

16:30 → 17:00 How many degrees of freedom describe a quantum N-particle state?

In Newtonian spacetime, the canonical description of a classical $N$-particle system requires $3N$ degrees of freedom.
Not all of these are physical, however, since the conservation of the net momentum implies that only $3(N-1)$ accelerations are independent.
Hence, three constraints can be used to eliminate the unphysical centre-of-mass variables, at the level of the Lagrangian, leaving only the subset of observable displacements and momenta, which are relational.
Imposing the constraints does not change the dynamics of these variables, at the classical level, and is analogous to a gauge-fixing procedure, which removes redundancy in the description of the system.
In classical physics, therefore, the number of physical degrees of freedom equals the number of independent relational degrees of freedom.
Here, we show that this is not the case in quantum mechanics.
While an operator-analogue of the classical net momentum exists, it cannot be used to impose constraints that restrict the degrees of freedom in the theory, without a loss of physical information.
This means that all $3N$ canonical degrees of freedom are physical, even though only $3(N-1)$ of them are relational.
We explore the physical consequences of the non-relational variables and show that they give rise to generalised uncertainty relations (GURs), for the relational quantities that define the quantum reference frame (QRF).
Hence, it is shown that the non-relational degrees of freedom refer to the frame itself and that the non-Heisenberg terms in the GURs define its Galilean-invariant spreads, in both real space and momentum space.
The implications of this result for recent work on relational models, including the ``perspective neutral'' framework for QRFs, are discussed.
Its implications for the wider relational program, and, in particular, the relevance of the latter to quantum gravity research, are also critically assessed.

Speaker: Matthew J. Lake (Babes-Bolyai University)

17:00 → 17:30 Coffee Break

17:30 → 18:00 Eight Geometrical Representations of the Linear Constant of Hyperbolic Geometry

J. H. Lambert(1776) first suggested that the hypothesis of the acute angle, in which the sum of the angles of a triangle is always less than two right angles, implies the existence of an absolute measure of distance, which he implicitly defined as the linear constant relating the length of the side of a quadrilateral with three right angles to the magnitude of its acute angle (the first geometrical representation). He also interpreted the constant in terms of the radius of an imaginary sphere (the second representation). In his work on non-Euclidean hyperbolic geometry in the second decade of the nineteenth century, Carl Gauss denoted the constant by the letter k. He expressed its (unknown) magnitude in terms of the area of the triangle of maximum size whose vertices lie at infinitely distant points and whose angle sum is zero (the third representation). Ferdinand Schweikart (1818) expressed the constant in terms of the altitude of a right angled isosceles triangle whose acute angles approach zero as its sides are indefinitely extended (the fourth representation). In his log-spherical geometry based on a sphere with an imaginary radius Franz Taurinus (1826) deduced what has come to be known as the fundamental identity of hyperbolic geometry. Using this identity the constant can be defined as the length of the segment corresponding to an angle of parallelism of approximately 40 23’ 42’’ (the fifth representation). Nikolai Lobachevsky (1829) and János Bolyai (1832) both represented the constant in terms of horocycles, circles of infinite radius peculiar to hyperbolic geometry. They defined the constant as the radial distance separating two concentric horocyclic arcs between the same parallels where the ratio of the outer to the inner arc is e : 1 (the sixth representation). Karl Schwarzschild (1900) equated the linear constant to the radius of curvature of hyperbolic space (the seventh representation). Finally in numerous expository texts on hyperbolic geometry in the last century the constant was defined as the length of a horocycle whose tangent at one extremity is parallel to the axis through the other extremity (the eighth representation).

Speaker: Steven Rose (University College London)

18:00 → 18:30 An analytic approach to the Euler-Wildberger line in hyperbolic triangle geometry

In hyperbolic triangle geometry, the three classical triangle centers (the barycenter, the circumcenter and the orthocenter) do not align: there is no hyperbolic Euler line. It turns out, however, that there are two exotic analogues of the barycenter (the pseudobarycenter) and the circumcenter (the pseudocircumcenter) that lie on the same line as the orthocenter. This is the Euler-Wildberger line of the triangle. In this paper we provide an analytic proof of this fact, by using barycentric coordinates in the projective model of the hyperbolic plane. In the process, we find the barycentric coordinates of the two exotic centers and we, also, prove that when the curvature of the hyperbolic plane goes to zero, the exotic centers turn into their classical analogues, while the Euler-Wildberger line turns into the Euler line of the limit Euclidean triangle.

Speaker: Paul Blaga ("Babeș-Bolyai" University, Cluj-Napoca, Romania)

18:30 → 19:00 Statistical Physics of Planar Carroll Systems

We define and study the statistical physics of planar Carrollian systems. While it has been shown recently that, for general dimensions, the Carroll limit of Poincaré statistical physics typically does not converge, we show that thanks to the central extensions of the Carroll algebra in the plane, and by considering systems with angular momentum, there exists a well-defined notion of planar Carrollian statistical physics. Using Souriau’s geometric thermodynamics, we compute the partition function for particles on a uniformly rotating disc, and show that rotation is inevitable for thermal equilibrium of planar Carroll systems, with one of the central charges determining the direction of rotation. We derive thermodynamic quantities in particular entropy, which scales logarithmically with the disc area, and pressure, which follows the two-dimensional ideal gas law. Though all results are obtained from symmetry considerations, we also derive the corresponding effective Hamiltonian.

Speaker: Fei Huang (Sun Yat-sen University)

19:00 → 19:30 Equations of motion for dynamical systems with angular momentum on Finsler geometries

In this talk, I will present a derivation of the equations of motion for dynamical systems with angular momentum on Finsler geometries. While point-like particles without intrinsic angular momentum follow geodesics, angular momentum generally couples to spacetime curvature, causing deviations from geodesics. I will show how these equations can be derived from requiring diffeomorphism invariance of the energy-momentum distribution of the worldline of the particle, extending ideas of Mathisson and Souriau. This yields a Finsler generalization of the Mathisson-Papapetrou-Dixon (MPD) equations. I will discuss how spacetime symmetries lead to conserved quantities and explain that spin supplementary conditions are necessary to close the system, as in the standard MPD case. I will present common choices for these conditions and provide the final equations of motion in 3-dimensional space and 4-dimensional spacetime.

Speaker: Loïc Marsot (Sun Yat-sen University)

19:30 → 21:00 Dinner

Wednesday 16 September

09:00 → 09:30 Registration

09:30 → 10:00 Consistent energy-momentum trace couplings of fluids

Gravitational models with non-minimal couplings involving the trace of the energy-momentum tensor have become increasingly popular. The idea of coupling the trace of the matter tensor to the geometry can be applied to various matter models, including relativistic perfect fluids. However, it is well-known that the variational formulation of perfect fluids involves some technicalities. We carefully derive the field equations including the trace coupling of a perfect fluid using two different approaches, namely, that given by Brown using Lagrange multipliers, and that given by Schutz using velocity potentials. We show that previous results involving such trace couplings do not match the results presented here. We demonstrate that our fluid's equations of motion are consistent with the gravitational field equations. Moreover, we present a simple on-shell argument which further supports the correctness of our results.

Speaker: Christian Boehmer (University College London)

10:00 → 10:30 Approximate study of the Gravitational Memory Effect

The large-distance behaviour of a sandwich gravitational wave can be approximated by a continuous but not necessarily smooth profile, providing us with a simplified description of particle motion. Our approximate model is consistent with the Carroll symmetry. Our strategy is illustrated by the Poschl-Teller profile.

Speaker: Peter Horvathy

10:30 → 11:00 Plane-wave memory beyond the sandwich approximation

We investigate velocity and displacement memory in plane gravitational waves beyond the usual sandwich approximation. The central question is whether the memory observables remain meaningful when the wave profile is continuously decaying rather than compactly supported. We find that there is a critical inverse-square falloff: faster-decaying tails can preserve memory even when the outgoing motion is no longer strictly free, whereas inverse-square or slower tails destroy the standard memory observables. The inverse-cubic and inverse-square profiles illustrate these two possibilities, respectively.

Speaker: Qiliang Zhao

11:00 → 11:30 Compact objects in Modified gravity

This talk will present exact solutions of non-rotating black holes and neutron stars, with primary scalar hair, obtained in a subfamily of Degenerate Higher-Order Scalar-Tensor (DHOST) theories, which include and extend Horndeski theories. I will also discuss the perturbations of these black hole solutions.

Speaker: David Langlois (Universite Paris Cite, CNRS, Astroparticule et Cosmologie, 75013 Paris, France)

11:30 → 12:00 Coffee Break

12:00 → 12:30 Weyl-Dirac-Born-Infeld action for quantum gravity

Weyl conformal geometry is the natural underlying geometry of (local) gauge theories of the Weyl group (of dilatations and Poincare symmetry), such as Weyl quadratic gravity (WQG) and its generalisation, Weyl-Dirac-Born-Infeld
action (WDBI). The WDBI action is well-defined in arbitrary d dimensions with no need for a UV regulator scale/field. A series expansion of WDBI action (in dimensionless coupling) recovers in the leading order a Weyl gauge invariant (geometrically regularised!) version of d=4 Standard Model (SM) and WQG. Riemannian geometry, Einstein-Hilbert action and a cc>0 are recovered in the Stueckelberg broken phase of Weyl gauge symmetry. Interestingly, Weyl geometry can also be seen as
Riemannian geometry of a (Weyl gauge invariant) non-local dressed metric (by Wilson line of dilatations), at the ”cost” of UV non-commutativity in the vector space of observables, (due to Weyl flux). Quantum non-locality, in particular quantum entanglement, and UV non-commutativity are then artefacts of ”viewing” Weyl geometry from (our-world of) Riemannian geometry of Weyl gauge invariant fields/observables and are evidence of Weyl gauge symmetry. Based on e-Prints: 2508.10959 and 2606.08080.

Speaker: Dumitru Ghilencea (Horia Hulubei National Institute of Physics and Nuclear Engineering (RO))

12:30 → 13:00 Weyl gauge symmetry at LIGO-Virgo-KAGRA

With current advances in gravitational wave (GW) detection made by the worldwide LIGO-Virgo-KAGRA (LVK) network of detectors, ever-more sensitive tests of gravity in the strong-field regime are now possible. This enables one to test gauge theories beyond Einstein-Hilbert action, such as Weyl gauge theories of gravity. The only anomaly-free (quantum) gauge theory of a space-time symmetry beyond Poincaré is based on Weyl gauge group (of dilatations and Poincaré symmetry) with Weyl conformal geometry as its natural underlying geometry. This gauge theory has spontaneous breaking of Weyl gauge symmetry to Einstein-Hilbert and Proca actions, plus a positive cosmological constant. We investigate the GW polarisation modes of Weyl (quadratic) gauge theory of gravity in Weyl geometry and compare our findings to the most recent experimental data. We show how the geodesic deviation equation from Riemannian geometry translates to Weyl geometry, and explain why it is crucial to perform the analysis around de Sitter background, which is the correct low-energy limit of Weyl quadratic gravity, to not alter the GW content, and then compute the polarisation modes. In addition to the two transverse-traceless tensor modes predicted by Einstein-Hilbert action, we find two additional vector modes induced by the transverse fluctuations of the Weyl gauge field. If detected, these vectors modes would be important evidence for Weyl gauge symmetry.

Speaker: Vlad-Mihai Mandric (IFIN-HH Bucharest)

13:00 → 13:30 Quasinormal modes in theories beyond General Relativity

One important stage associated with the coalescence of two black holes is the ringdown phase, during which the remnant emits gravitational radiation described by black-hole perturbation theory. This gravitational radiation is characterized by quasinormal modes (QNMs) whose frequencies and damping times encode the properties of the spacetime and the underlying theory of gravity.

Notably, black hole QNMs may exhibit phenomena that can serve as a smoking gun to probe theories beyond General Relativity, where additional degrees of freedom arise. These phenomena include spectral instabilities, avoided crossings, and exceptional points. Motivated by this scenario, we develop a model-independent parameterization to obtain the excitation factors and the QNM frequency spectra of a general system of three coupled master equations on top of a Schwarzschild background. As an application, we consider a black hole in the Einstein-Maxwell-axion theory in the limit where quadratic contributions to the electric charge are neglected. We discuss our first results and the future steps.

Speaker: Miguel Pinto (Institute of Astrophysics and Space Sciences, University of Lisbon)

13:30 → 15:00 Lunch

15:00 → 15:30 Cosmological gravitational waves in f(G) gravity

From the action formalism, the Ricci scalar $R$ in the Einstein-Hilbert action can be decomposed into a bulk term $\mathcal{G}$ and a boundary term $\mathcal{B}$. A recent modified theory of gravity is the so-called $f(\mathcal{G})$ gravity, which arises from the action after removing its boundary term $\mathcal{B}$ from the Ricci scalar $R$ while mapping the remaining bulk term $\mathcal{G}$ to $f(\mathcal{G})$. We will begin by examining linearised $f(\mathcal{G})$ gravity and several of its interesting aspects.
Firstly, there is a correspondence between the linearised $\mathcal{G}$ and the Fierz-Pauli Lagrangian which governs the field theory of the (rank-2 tensor) graviton. Secondly, we discover that one can recover the field equations with the cosmological term from the linearised $f(\mathcal{G})$ field equations by a specific choice of $f(\mathcal{G})$. Third, we investigate a trial solution of the form $f(\mathcal{G}) = c_{1/2} \sqrt{\mathcal{G}} + \mathcal{G} + c_{3/2} \mathcal{G}^{3/2} + c_2 \mathcal{G}^2 \cdots$. We show that this solution is unphysical when expanding from the Minkowski background and discuss the numerical solution of its background Friedmann equation when expanding from the flat FLRW background.

Speaker: Neil Booker (University College London)

15:30 → 16:00 Thermodynamics and Information Recovery of Black Holes

In this work, we investigate the black hole information loss paradox and the thermodynamics of charged black holes using the island formula. We analyze the impact of the black hole's thermodynamic behavior on information recovery, focusing specifically on the calculation of the Page time. Our results demonstrate an explicit information-thermodynamics correspondence, revealing how the thermodynamic parameters of charged black holes directly influence the dynamics of information recovery during evaporation.

Speaker: Ladghami yahya (Mohammed I University)

16:00 → 16:30 Testing Running Vacuum Energy in (f(Q)) Gravity with DESI Data

n this work, we investigate the running vacuum energy model within f(Q) gravity, where the vacuum equation of state receives dynamical corrections involving (\dot{H}) and (\ddot{H}/H). We consider two models and constrain them using DESI BAO, Pantheon+, cosmic chronometer, and CMB data. Our results indicate mild deviations from (\Lambda)CDM, with Model I remaining statistically competitive, while Model II is disfavored due to its higher complexity and additional parameter.

Speaker: Ahmed Errahmani

16:30 → 17:00 Coffee Break

17:00 → 21:30 Artistic Event and Gala Dinner

Thursday 17 September

09:00 → 09:30 Registration

09:30 → 10:00 Dynamical Dark Energy or Dynamical Speed of Light? Recent evidence from SNe Ia (YONSEI) and z~14 H II Galaxies

The YONSEI group [MNRAS 544 (2025) 975] recently proposed a progenitor age-bias correction to Type Ia supernovae. Applying this empirical correction, we find that the Pantheon+ and DES5Y Hubble diagrams converge toward the special Kolb point $(w, \Omega_m)=(-1/3,0)$ in the flat $w$CDM parameter space. This point yields the compact zero-parameter logarithmic luminosity-distance relation $d_L=\frac{c}{H_0}(1+z)\ln(1+z)$, with $H_0$ calibrated to Cepheids. The $\ln(1+z)$ factor accounts for the excess moduli observed in high-redshift SNe Ia without requiring dark energy. Remarkably, the Kolb point is also consistent with the very recent H II-galaxy observations extending to $z∼14$ [Chávez et al., arXiv:2607.14254]. Within conventional GR, however, the Kolb point requires an unusual K-matter equation of state $w=-1/3$. Moreover, dynamical dark energy, such as the CPL parametrization, has become a default ad hoc model choice. To avoid both of these quandaries, we propose an alternative route in which the speed of light becomes dynamical during cosmic expansion rather than invoking a dynamical dark-energy sector. In the Dolgov–Barrow class of cosmologies, $a\propto t^\mu$ and $c\propto a^{-\zeta}$, the same logarithmic luminosity-distance relation arises naturally along the scale-invariant branch $(1+\zeta)\,\mu=1$, which leads to a non-trivial kinematic relation $c=\frac{c_0}{H_0}\dot a$ between the speed of light and the cosmic expansion rate. The 1998 discovery of cosmic acceleration therefore need not imply dark energy, but may instead point toward a scale-invariant Dolgov–Barrow cosmology in which the speed of light becomes dynamical on an expanding cosmic background.

Speaker: Hoang Nguyen (Babes-Bolyai University)

10:00 → 10:30 Constraining Early Dark Energy models with Big Bang Nucleosynthesis

Recent tensions in cosmological data strongly suggest that the standard $\Lambda$CDM picture may be incomplete. Many attempts have been made to alleviate the famous Hubble tension, out of which Early Dark Energy models offered one of the best theoretical frameworks so far that can possibly close the gap between early and late-time observations by dynamically modifying the spacetime background prior to recombination. However, the introduction of an additional accelerating component in the early Universe changes the Hubble expansion rate during the radiation-dominated epoch. This kinematic deviation from standard General Relativistic dynamics shifts the thermodynamics of the weak freeze-out, modifying the predictions of Big Bang Nucleosynthesis. To determine the viability of these modified expansion histories, we investigate the impact of four early dark energy formulations on the primordial element abundances: a baseline cosmological constant, alongside linear, polytropic, and temperature-dependent equations of state. We test these models against observational bounds to map the allowable parameter space, and we discuss which dynamical modifications are ruled out by nucleosynthesis constraints.

Speaker: Teodora Matei (Babes-Bolyai University, Cluj-Napoca)

10:30 → 11:00 Progenitor Age Bias in Type Ia Supernova Cosmology: Implications for Dark Energy and Cosmological Tensions

Supernova (SN) cosmology is based on the key assumption that the luminosity standardization process of Type Ia SNe remains invariant with progenitor age. However, direct and extensive age measurements of SN host galaxies reveal a significant ($5.5 \sigma$) correlation between standardized SN magnitude and progenitor age, which is expected to introduce a serious systematic bias with redshift in SN cosmology. This systematic bias is largely uncorrected by the commonly used mass-step correction, as progenitor age and host galaxy mass evolve very differently with redshift. After correcting for this age-bias as a function of redshift, the SN dataset aligns more closely with the $w_0w_a$CDM model recently suggested by the DESI BAO project from a combined analysis using only BAO and CMB data. This result is further supported by an evolution-free test that uses only SNe from young, coeval host galaxies across the full redshift range. When the three cosmological probes (SNe, BAO, CMB) are combined, we find a significantly stronger ($>9\sigma$) tension with the $\Lambda$CDM model than that reported in the DESI papers, suggesting a time-varying dark energy equation of state in a currently non-accelerating universe.

Speaker: Junhyuk Son (Yonsei University)

11:00 → 11:30 The trouble with $H_0$ after DESI DR2 and Age-Bias-Corrected Supernova Cosmology

The $\Lambda$CDM model has long been regarded as the standard paradigm of modern cosmology, providing an excellent description of the Universe across a wide range of observations, from the cosmic microwave background to large-scale structure and Type Ia supernovae. However, recent cosmological observations, particularly from DESI DR2, have renewed interest in possible departures from the cosmological constant. In this talk, I will first discuss the physics of progenitor age bias in Type Ia supernovae, motivated by the results of Project YONSEI, which provide evidence for a dependence of standardized SNe~Ia luminosities on progenitor age and host-galaxy properties. I will then discuss the cosmological implications of incorporating this progenitor age-bias correction into the analysis of different SNe~Ia samples combined with DESI DR2 and CMB observations. Without the age-bias correction, the preference for dynamical dark energy over $\Lambda$CDM ranges from approximately $2.0\sigma+$ to $3.0\sigma+$, depending on the SNe~Ia sample. Remarkably, after applying the age-bias correction, this preference increases substantially, reaching approximately $7\sigma+$--$11\sigma+$. The preferred dark energy evolution shows Quintom-B behavior, characterized by $w_0>-1$, $w_a<0$, and $w_0+w_a<-1$. I will also discuss the resulting evidence for phantom-crossing behavior in the evolution of dark energy. Finally, I will discuss the broader cosmological consequences of the age-bias correction, with particular emphasis on the $H_0$ tension. I will show how current cosmological observations constrain possible solutions to the $H_0$ tension and why the tension becomes increasingly difficult to resolve within the dark energy scenarios favored by DESI DR2 and the age-bias-corrected SNe~Ia data.

Speaker: Himanshu Chaudhary (Universitatea Babeș-Bolyai din Cluj-Napoca)

11:30 → 12:00 Coffee Break

12:00 → 12:30 Cosmological test of a length-preserving biconnection gravity

We investigate the cosmological implications of an extended gravitational framework based on biconnection gravity, constructed from the Schr\"odinger connection and its dual. In this approach, the difference between the two connections defines the mutual curvature, which encodes the non-Riemannian geometric degrees of freedom, while their symmetric combination reduces to the Levi-Civitae connection and hence reproduces general relativity at the background level. Within this setting, we derive the generalized Friedmann equations for a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker Universe. The resulting equations contain additional geometric contributions that may naturally encode an effective dark energy sector induced by the biconnection degrees of freedom. We explore this extra dark energy by adopting five commonly used parametrizations, namely B$\Lambda$CDM, $\omega$CDM, Chevallier-Polarski-Linder, Barboza-Alcaniz, and a logarithmic equations of state. These considerations are confronted with recent observational data, including DESI DR2, Pantheon$^+$, and CC observations. Our analysis shows that the four parameterizations enter the acceleration phase at almost the same redshifts and share the same current value of the Hubble rate.
Furthermore, the statistical comparison based on the Akaike, Bayesian, and Deviance Information Criterion shows that Barboza-Alcaniz, and logarithmic parameterizations have strong evidence and are competitive with $\Lambda$CDM. To classify this biconnection gravity in the plethora theoretical models describing the current cosmic acceleration, we examine its implications through cosmographic tools, including the deceleration, jerk, and snap
parameters, as well as through the Statefinder analysis and $Om(z)$ diagnostic. These diagnostics indicate that the geometric
sector generates a dynamical dark energy component that remains observationally close to $\Lambda$CDM at the background
level. Indeed, the effective evolution of the biconnection gravity begins initially from a quintessence-like regime and subsequently evolves toward a phantom-like regime. All parameterizations under consideration face this transition in the recent past, except for the wCDM model, which enters the phantom-like regime in the future.

Speaker: AMINE BOUALI (Mohammed First University)

12:30 → 13:00 Kinetic Schwinger pair production in the bare strange-star electrosphere

A bare strange star carries a thin electron layer, the electrosphere, whose overcritical electric field creates electron-positron pairs through the Schwinger mechanism. We revisit two quantities studied in earlier kinetic calculations, the surface pair-creation rate and the evolution of the electric field. We also extend the study by adding relaxation-time collisions. The calculation uses an independent kinetic solver in which the momentum resolution of the degenerate Fermi edge is an explicit parameter. The rate is controlled by that resolution and converges to the Usov value, about $2\times10^{53}\,\mathrm{s^{-1}}$ at $T=3\,m_{\mathrm e}$, roughly forty times below earlier dynamical estimates. The field is not dynamically enhanced and follows the electrostatic solution in the creation region, with the field energy growing by only $4$-$12\%$. Relaxation-time collisions reduce the pair output by $3$-$17\%$ across $T=3$-$9\,m_{\mathrm e}$, an upper bound that surface degeneracy lowers to the percent level.

Speaker: Tania Flavia Ciceo (Babeș-Bolyai University, Faculty of Physics)

13:00 → 13:30 Faint Satellite Streak Detection using YOLOv8 with a Feature-Enhancing Pipeline

Faint satellite streaks are difficult to detect in astronomical images. High sensor dynamic range, stellar clutter, and atmospheric noise can hide these weak linear features. Global normalization can remove useful information because background noise can dominate the normalized image. We explore different preprocessing stages, from adaptive normalization to methods that enhance geometric and directional features. We then combine selected intensity and structural information into three-channel images for YOLOv8. This approach helps us examine which features improve detection across different observation conditions and confidence thresholds.

Speaker: Cristian Croitoru (Technical University of Cluj Napoca)

13:30 → 15:00 Lunch

15:00 → 15:30 Thermodynamics of multiscalar-tensor gravity

We develop a covariant first-order thermodynamic description of Jordan-frame tensor–multi-scalar gravity by interpreting the additional gravitational degrees of freedom as an effective imperfect fluid. Unlike the single-field case, the presence of several scalar directions introduces independent thermal channels that cannot, in general, be reduced to a single effective temperature variable. We identify distinct contributions associated with the nonminimal coupling, the full time-like motion in scalar field space, and residual spatial scalar gradients, and derive their corresponding transport equations. This structure shows that freezing the effective gravitational coupling is generally insufficient to guarantee relaxation toward General Relativity: non-equilibrium degrees of freedom may remain in field-space directions orthogonal to the coupling. In homogeneous and isotropic cosmology, spatial contributions vanish by symmetry, while the intrinsically multi-field time-like dynamics survives. The resulting framework provides a covariant thermodynamic characterization of relaxation and GR-like behavior in multi-scalar extensions of gravity.

Speaker: David Pereira (Institute of Astrophysics and Space Sciences; Faculty of Sciences, University of Lisbon)

15:30 → 16:00 Cosmological constraints on f(Q) gravity

In this talk I will discuss cosmological tests of gravity theories, in particular I will focus on non-metric extensions. I will present two realizations within the f(Q) gravity framework: a minimal inverse non-metricity model, which introduces no additional free parameters relative to ΛCDM, and a self-accelerating logarithmic model, with a general branch (sLog) and a DGP-like limit (sDGP). Both are constrained using CMB data alone and in combination with BAO, RSD and SNIa (adding DES large-scale-structure data for the inverse model), considering fixed and varying neutrino mass. Finally, I will discuss the models in light of the H0 tension and provide a model selection analysis.

Speaker: Simão Marques Nunes

16:00 → 16:30 Searching for extra dimensions in neutron stars and black holes

In the Kaluza–Klein theory, one extra spatial dimension is added to usual spacetime. On one hand, particles moving in it gain excitations leading to a mass spectrum, which can be associated with hadronic states. Neutron stars are modelled using the equation of state of an extra-dimensional interacting zero-temperature Fermi gas. Macroscopic observables are calculated, compared to astronomical data, and a range of extra dimension sizes with the possibility of detection is given [1].

On the other hand, the extra dimension introduces a scalar field into the theory, which modifies the curvature of spacetime. On a curved background, the uncertainty relation of particles is modified, resulting in a modified dispersion relation and thermodynamics. Such effects become significant near the horizon of black holes, where even gravity induced particle decay is possible [2].

[1] A. Horváth, E. Forgács-Dajka, G.G. Barnaföldi: "Application of Kaluza-Klein Theory in Modeling Compact Stars: Exploring Extra Dimensions", MNRAS doi.org/10.1093/mnras/stae2637 (2024)
[2] A. Horváth, A. Wojnar, GG. Barnaföldi: "Modified Dispersion Relation in Kaluza–Klein Theory". Particles. 2026; 9(3):87. https://doi.org/10.3390/particles9030087

Speaker: Anna Horvath

16:30 → 17:00 Coffee Break

17:00 → 17:30 Effective thermodynamics from gravitational nonminimal couplings with relativistic perfect fluids

Modified gravity models with nonminimal coupling with matter have gained interest in the recent years due to their distinctive characteristics of breaking the stress-energy tensor conservation, entailing an additional force term. When applied to relativistic perfect fluids, such models can mimic late time acceleration and provide alternatives to dark sector interactions. The modified fluid equations of motion for these models result in effective thermodynamic terms. We show how different approaches to nonminimal couplings with relativistic perfect fluids exhibit different effective thermodynamics which can potentially provide a measure to differentiate between models and approaches.

Speaker: Eissa Alnasrallah (University College London)

17:30 → 18:00 Vortices in Bose-Einstein Condensates

The zero temperature dynamics of weakly interacting boson particles of confined and dilute BECs is described by a mean-field macroscopic wave function, ψ. To study the dynamical instabilities and relaxation to the equilibrium of the system, we perform numerical simulations using the time dependent GPE.
Inclusion of a damping term, γ into the GPE, we model the dissipative losses, which occur in the real environment. By using of the γ, the total energy is not conserved, it decreases. With dissipations fluctuations become damped and by inceasing γ, the path of vortices become shorter. With large γ fluctuations dissapear .
With vortex imprinting method, we imprint positive and negative vortices at location (x0, y0). We observe transformation of kinetic energy into sound energy: Esound = Ekinetic - Evortex
By generating a discontinuity in the phase, the system tries to smooth out this change and generate a soliton-like perturbation, which decays into vortices. Dark solitons have a local density minimum and a sharp phase gradient.

Speaker: Enikõ J. M. Madarassy (Department of Physics, Babes-Bolyai University, Cluj-Napoca, Romania)

18:00 → 18:30 Hermiticty as a Symmetry, Quasilocal Probability and Gravity

Hermiticity is usually treated in an axiomatic way in quantum mechanics, ensuring key properties of observables in a Hilbert space. In this talk I will discuss how Hermiticity may be something deeper; a symmetry law associated with the conservation of the inner product current in space-time. We will see that this symmetry breaks in the presence of gravitational fields, causal horizons, or space-time curvature for a restricted observer. I will motivate the background for such a symmetry showing what cosmology and near-horizon black hole thermodynamics tell us about it, namely that to preserve global flatness of the universe one needs effective Hermiticity to hold true, but local Hermiticity needs to break near black hole horizons to ensure the second law of thermodynamics. I then discuss how all of this leads to the notion that Probability itself, like energy in general relativity, becomes Quasilocal in curved spacetimes. We then conclude by discussing the implications of all this on quantum field theory in curved spacetimes and quantum gravity in general.

Speaker: Oem Trivedi (Department of Physics and Astronomy, Vanderbilt University, Nashville, USA)

18:30 → 19:30 Conference Concluding Remarks

19:30 → 21:00 Dinner