We investigate the existence of static, spherically symmetric compact objects within the framework of symmetric teleparallel scalar–tensor gravity. This theory extends the Brans-Dicke and scalar-tensor models within the symmetric teleparallel formalism. We consider a nontrivial connection that allows for genuinely nontrivial solutions in the limit of General Relativity. The field equations admit a minisuperspace description and by applying the method of variational symmetries we construct the corresponding conservation laws in vacuum. The application of these conservation laws enables the reconstruction of analytic black-hole solutions. Finally, we study the interior structure of compact objects matched to an extremal Reissner-Nordström exterior and show that the symmetric teleparallel scalar-tensor theory supports the existence of viable astrophysical objects.

Chile

Grigorios Panotopoulos

Andrés Lueiza-Colipí

Nikolaos Dimakis

South Africa

Andronikos Paliathanasis

Compact stars in symmetric teleparallel scalar-tensor gravity

We test the cosmological viability of the Viaggiu holographic dark energy (VHDE) model by using late-time observational data. In particular, we place constraints on the free parameters of the model using Type Ia supernovae from the PantheonPlus, Union3.0, and DES-Dovekie catalogues, the Cosmic Chronometers, and the Baryon Acoustic Oscillations from the DESI DR2. Our analysis suggests that the VHDE model fits the observational data better or similar to the ΛCDM for all dataset combinations considered. The value obtained for <italic>H</italic> <inf>0</inf> is similar to the &#923;CDM, while the current matter density parameter is constrained around &#937; <inf><italic>m</italic>0</inf>&#8239;&#8771;&#8239;0.24, smaller to that obtained by the &#923;CDM. Moreover, the parameter introduced by the VHDE is found to have a mean value within the range <math altimg='si42.svg'><mrow><mfrac><mi>&#960;</mi><mn>3</mn></mfrac><msup><mi>&#948;</mi><mn>2</mn></msup><mo>&#8764;</mo><mn>0.27</mn><mo linebreak='goodbreak'>&#8722;</mo><mn>0.33</mn></mrow></math>. Finally, we used Akaike&#8217;s Information Criterion (AIC) and Bayesian evidence to test the VHDE model against the &#923;CDM scenario. The AIC demonstrates that the two models are statistically indistinguishable, while Bayesian evidence reveals that the data have a mild preference for the &#923;CDM model for most of the dataset combinations considered. Nevertheless, the VHDE model remains consistent with current late-time cosmological observations and offers a feasible mechanism for describing the late-time accelerating scenario.

India

Amlan K. Halder

Italy

Stefano Viaggiu

Abdulla Al Mamon

Subhajit Saha

Viaggiu holographic dark energy in light of DESI DR2

We review the recent work by T.F. Dabash, M. Emam, and L. Schöppner (Eur. Phys. J. C 85, 1341, 2025) and identify an essential error in their analysis.

Comment on: “Symmetry and conserved quantities in $$f\left( R\right) $$ -gravity: Mei vs. Noether approaches”

We develop a general canonical quantization scheme for $\textit{k}$-essence cosmology in scalar–tensor theory. Utilizing the Dirac–Bergmann algorithm, we construct the Hamiltonian associated with the cosmological field equations and identify the first- and second-class constraints. The introduction of appropriate canonically conjugate variables with respect to Dirac brackets, allows for the canonical quantization of the model. In these new variables, the Hamiltonian constraint reduces to a quadratic function with no potential term. Its quantum realization leads to a Wheeler–DeWitt equation reminiscent of the massless Klein–Gordon case. As an illustrative example, we consider the action of a tachyonic field and investigate the conditions under which a phantom crossing can occur as a quantum tunneling effect. For the simplified constant potential case, we investigate the consequences of different boundary conditions on the singularity avoidance and to the mean expansion rate.

Andrés Lueiza Colipí

Dirac–Bergmann algorithm and canonical quantization of $\textit{k}$-essence cosmology

<list id='lst0002'><list-item id='lstitem0001'><label>&#8226;</label><para id='p0001'>The non-coincident branch introduces a genuine dynamical scalar degree of freedom from the connection, allowing a scalar-tensor representation of the theory and enabling nontrivial vacuum solutions.</para></list-item><list-item id='lstitem0002'><label>&#8226;</label><para id='p0002'>We obtain cosmologically significant analytic solutions including de Sitter, scaling regimes, &#923;CDM mimicking dynamics, standard and generalized Chaplygin gas, and CPL parameterizations, all reconstructed within the <italic>f</italic>(<italic>Q</italic>) framework.</para></list-item><list-item id='lstitem0003'><label>&#8226;</label><para id='p0003'>These analytic solutions show that the non-coincident formulation admits a richer solution space than both the coincident gauge and <italic>f</italic>(<italic>T</italic>) teleparallel limits, capturing both early and late accelerated phases without matter sources.</para></list-item><list-item id='lstitem0004'><label>&#8226;</label><para id='p0004'>The results demonstrate that non-coincident <italic>f</italic>(<italic>Q</italic>) gravity can unify inflation and late acceleration geometrically, offering a promising alternative to dark-energy driven &#923;CDM.</para></list-item></list>

Malaysia

Avik De

Exact cosmological solutions in vacuum non-coincidence <italic>f</italic>( <italic>Q</italic>)-theory

Kaniadakis holographic dark energy is a one-parameter extension of the standard HDE framework, in which the horizon entropy is reformulated using Kaniadakis entropy. At the cosmological level, it has been shown to give rise to modified Friedmann equations, leading to a richer phenomenology compared to $$\Lambda $$ CDM. In this work we test the Kanadiakis holography model against multiple late-time observational probes, including Type Ia supernovae from PantheonPlus and Union3, Cosmic Chronometer measurements of the Hubble parameter and Baryon Acoustic Oscillation from the Dark Energy Spectroscopic Instrument Data Release 2 (DESI DR2). Using a Bayesian inference approach with MCMC methods, we constrain the cosmological parameters of the model and evaluate its performance against $$\Lambda $$ CDM through the Akaike Information Criterion. We find that Kaniadakis holography can provide a better fit for some data combinations, although $$\Lambda $$ CDM remains slightly statistically favored overall. These results highlight Kaniadakis holography as a competitive alternative to the standard cosmological model, offering valuable insights into the role of generalized entropy in dark energy dynamics.

Spain

Giuseppe Luciano

Late-time cosmological constraints on Kaniadakis holographic dark energy

A generalized mass-to-horizon entropy has recently been proposed as an extension of the Bekenstein-Hawking area law, derived from a modified mass-horizon relation constructed to ensure consistency with the Clausius equation. Within the gravity-thermodynamics conjecture, this entropy formulation yields modified Friedmann equations, which recover the standard <math altimg='si45.svg'><mstyle mathvariant='normal'><mi>&#923;</mi></mstyle></math>CDM cosmology in the appropriate limit of the model&#8217;s two free parameters. In the present study, we constrain this framework using observations from Type Ia supernovae (SNIa), cosmic chronometers (CC) and baryon acoustic oscillations (BAO), including the Second Data Release of the Dark Energy Spectroscopic Instrument (DESI DR2) survey, together with the Supernovae <math altimg='si46.svg'><msub><mi>H</mi><mn>0</mn></msub></math> for the Equation of State (SH0ES) distance&#8211;ladder prior, across four combinations of data sets. Although the extended entropic scenario yields a slightly better, or statistically comparable, fit to the data, model selection via the Akaike Information Criterion (AIC) mildly favors the cosmological constant as the dark energy candidate. Moreover, the <math altimg='si45.svg'><mstyle mathvariant='normal'><mi>&#923;</mi></mstyle></math>CDM limit lies within <math altimg='si47.svg'><mrow><mo>&#8764;</mo><mspace width='-0.16em' /><mn>1</mn><mi>&#963;</mi></mrow></math> of our constraints, indicating no significant deviation from standard cosmology with current data.

Giuseppe Gaetano Luciano

Modified cosmology through generalized mass-to-horizon entropy: Observational constraints from DESI DR2 BAO data

Triggered by some recent studies within the framework of nonmetricity gravity, I discuss how the basic information regarding the equivalence of the gravitational theories forming the “Trinity of Gravity” has been lost in parts of the literature. As a result, several studies with focus in modified theories of gravity have research investigated the case of STEGR/GR, often without acknowledging this fundamental equivalence or recognizing that their results can be derived in GR.

The “Telephone Game” effect in modern gravity research

We show that <math altimg='si1.svg'><mi>f</mi><mo stretchy='false'>(</mo><mi>Q</mi><mo stretchy='false'>)</mo></math> cosmology with a non-trivial connection, namely the Connection II of the literature, is dynamically equivalent with a quintom-like model. In particular, we show that the scalar field arising from the non-linear <math altimg='si1.svg'><mi>f</mi><mo stretchy='false'>(</mo><mi>Q</mi><mo stretchy='false'>)</mo></math> form, and the scalar field associated to the non-trivial connection, are combined to provide one canonical and one phantom field in the minisuperspace Lagrangian. Hence, the combination of two well-behaved fields appears effectively as a phantom field. This is fundamentally different from the usual approach in phantom and quintom cosmology, in which the phantom field is introduced ad hoc, and thus it may open a new way to handle the phantom fields, namely as effective realizations of the connection-related canonical fields.

Greece

Cyprus

Spyros Basilakos

China

Emmanuel N. Saridakis

Equivalence of <italic>f</italic>( <italic>Q</italic>) cosmology with quintom-like scenario: The phantom field as effective realization of the non-trivial connection

We explore the introduction of the cosmological constant via canonical transformations in cosmology. We consider the Wheeler-DeWitt equation for the cold dark matter (CDM) universe and we construct the Hamilton-Jacobi action for the ΛCDM model, where Λ is the cosmological constant. We discuss how this approach allows us to relate different physical systems, providing insights into the role of the cosmological constant in cosmology. More precisely the de Sitter universe emerges from the vacuum solution of the spatially flat Friedmann–Lemaître–Robertson–Walker geometry under the application of canonical transformations within quantum cosmology.