In this article, we explore the Rényi law constraints on black hole merger in Gauß–Bonnet (GB) gravity. Specifically, we consider the case of static solutions in five-dimensional (5D) Anti-de-Sitter (AdS) spacetime and study the constraints on merger of two equal mass black holes. We calculate the general Rényi entropy expression and utilize it to study the bounds on the final black hole mass post-merger. We study its variation with the Rényi parameter. We also compare the results with those for black holes in General Relativity (GR). We find that the GB term has a significant impact on the bounds for black hole merger. The bounds for GB gravity become weaker for the zeroth order Rényi entropy and stronger for higher order Rényi entropies in comparison to GR.

Thailand

Neeraj Kumar

Germany

Ankur Srivastav

Phongpichit Channuie

Rényi law constraints on Gauß–Bonnet black hole merger

Recent observations from the Atacama Cosmology Telescope (ACT), especially when combined with DESI baryon acoustic oscillation data, indicate a scalar spectral index $$ n_s $$ higher than the value reported by $\textit{Planck}$ 2018, placing tension on universal inflationary attractor models. Motivated by this discrepancy, we investigate the inflationary predictions of the $$\beta $$ -exponential potential, $$ V(\phi )=V_0\left( 1-\lambda \beta \phi /M_p\right) ^{1/\beta }$$ considering both minimally and non-minimally coupled realizations. This potential generalizes standard exponential inflation and naturally arises in braneworld scenarios. We derive analytical expressions for the slow-roll parameters and inflationary observables using a perturbative expansion in the non-minimal coupling $$\xi $$ , and validate these results through numerical calculations. In the minimally coupled case, the model predicts $$ n_s \simeq 0.976 $$ and $$ r \simeq 0.035 $$ for $$N=50$$ and moderate values of $$\beta $$ , remaining compatible with ACT+DESI (P-ACT-LB) constraints at the $$1\sigma $$ level while yielding a spectral tilt larger than the universal attractor prediction. Introducing a small non-minimal coupling significantly improves agreement with observations by suppressing the tensor-to-scalar ratio while preserving the enhanced scalar tilt. For $$N=60 $$ , $$ \lambda \sim 0.3\!-\!0.5 $$ , and $$ \beta \sim \mathcal {O}(1\!-\!5) $$ , the non-minimally coupled model yields $$ n_s \simeq 0.974\!-\!0.976 $$ and $$ r \lesssim 0.03 $$ , comfortably consistent with ACT, DESI, and BICEP/Keck bounds. Our results show that the $$\beta $$ -exponential potential, especially when implemented with a non-minimal coupling, exhibits good agreement with the latest CMB observations. Our inflationary predictions of the non-minimal model of $$n_s$$ and $\textit{r}$ confirming the leading-order contributions in $$\xi $$ are sufficient to capture the essential features of both $\textit{r}$ and $$n_s$$ in observationally relevant regimes.

Jureeporn Yuennan

Uzbekistan

Azerbaijan

Farruh Atamurotov

Italy

Salvatore Capozziello

Constraining $$\beta $$ -exponential inflation with the latest ACT observations

In this work, we investigated the motion of spinning test particles in the spacetime of a Loop Quantum Black Hole. The dynamics of the spinning particles were formulated using the Mathisson–Papapetrou–Dixon equations, from which the effective potential was derived and analyzed with respect to the parameters of the LQBH metric. We examined how the particle’s spin influences the innermost stable circular orbit, as well as the corresponding specific angular momentum and energy, for various values of the black hole parameters. In addition, we determined the critical spin parameter <italic>s</italic>, beyond which time-like trajectories transition to non-physical, space-like motion, and analyzed its dependence on the deformation parameters <italic>&#945;</italic> and <italic>&#946;</italic>. Finally, we studied the center-of-mass energy resulting from the collision of two spinning particles near the event horizon, highlighting how spin and quantum corrections jointly affect high-energy processes in Loop Quantum Black Hole spacetimes.

Gulnisa Abdukayumova

China

Ahmadjon Abdujabbarov

G. Mustafa

Dynamics of spinning particles around the loop quantum black hole

In this article, we performed Simpson–Visser (SV)-regularization scheme to Anti-de Sitter (AdS) black holes and then studied thermal properties of the resulting spacetime geometry. We considered the validity of the first law of black hole thermodynamics in this case and derived an entropy formula consistent with this new regular geometry. Next, we carried out the free energy analysis and studied the phase structure of these black holes. We discovered non-trivial phase transition properties dependent on the SV-regularization parameter. We also considered the validity of the second law of black hole thermodynamics and analyzed a merger scenario of two equal mass SV-regular black holes. In particular, we investigated the impact of the SV-regularization parameter on the constraints on post-merger black hole mass. Interestingly, we found that the bounds initially increase and then fall sharply with increasing the SV-regularization parameter. All results are compared with standard black holes for vanishing SV-regularization parameter.

Simpson-visser-ads black holes: thermodynamics and binary merger

We study the dynamics of particles near a charged back hole (BH) in the $\textit{f}$($\textit{R}$, $\textit{T}$) theory of gravity coupled with nonlinear electrodynamics and analyze how the parameters of the BH affect the motion of test particles. We discuss the stability of the circular orbits by employing the effective potential technique. We derive the mathematical expressions for the particle energy and its angular momentum as a function of the BH parameters and study them graphically. We also study the innermost stable circular orbits and the effective force acting on the test particles. The epicyclic oscillations of test particles are examined, and the analytical expressions for the radial frequency, the vertical frequency, and the orbital frequency are obtained. We also discuss the frequency of the periastron precession of particles. We show that the BH parameters have a significant impact on the particle dynamics. We observe that the effective potential increases with increasing charge and angular momentum, and the orbits are more unstable compared with the smaller values of these parameters: as the BH charge or the particle’s angular momentum increases, the particle experiences a greater effective force. However, it is not affected by the BH parameters $\textit{a}$ and $\textit{b}$. We investigate the emission energy as a thermodynamic property of the BH and discuss the evaporation aspects of the BH.

Pakistan

Amna Saleem

Bushra Majeed

Zulfiqar Ali

Allah Ditta

Asalkhon Alimova

Impact of nonlinear electrodynamics on particle motion around a charged black hole with matter coupling

This work investigates a single-field inflationary model - a specific class of K-essence models - in which a coupling term exists between the canonical Lagrangian and the potential. This coupling term influences key inflationary parameters such as the power spectral index, the tensor-to-scalar ratio, the Hubble parameter, the equation of state parameter, and the slow-roll parameter. By solving the equations both numerically and analytically, we explore how this modification affects inflationary dynamics. Our results indicate that the coupling term, <italic>&#945;</italic>, reduces inflationary parameters such as the tensor-to-scalar ratio, <italic>r</italic>, and improves the consistency with observational constraints from Planck and BICEP / Keck at confidence levels 68&#8201;% and 95&#8201;%. These findings show that the studied model provides a promising alternative for describing early universe dynamics while aligning with recent cosmological observations.

Iran

Fereshteh Felegary

Thammarong Eadkhong

Single-field inflation with K-essence model: Predictions, constraints, and theoretical viewpoints

We investigate the inflationary phenomenology of a marginally deformed Starobinsky model, motivated by quantum corrections to the <italic>R</italic> 2 term, in light of the latest cosmological observations. In this framework, the inflationary potential acquires a small deformation parameter, <italic>&#947;</italic>, which shifts predictions away from the exact Starobinsky limit. Using the slow-roll formalism, we derive analytic expressions for the spectral index <italic>n<inf>s</inf></italic> and tensor-to-scalar ratio <italic>r</italic> and confront them with constraints from Planck, ACT, and DESI data. Our analysis shows that nonzero values of <italic>&#947;</italic> raise both <italic>n<inf>s</inf></italic> and <italic>r</italic>, thereby alleviating the &#8239;&#8819;&#8239;2 <italic>&#963;</italic> tension between the Starobinsky <italic>R</italic> 2 scenario and the ACT+DESI (P-ACT-LB) measurements, which favor <italic>n<inf>s</inf></italic>&#8239;&#8771;&#8239;0.9743&#8239;&#177;&#8239;0.0034. For <italic>N</italic>&#8239;&#8764;&#8239;60 <italic>e</italic>-foldings, the model consistently reproduces the observed amplitude of primordial perturbations while predicting tensor contributions within current observational bounds. We also demonstrate that the deformation softens the otherwise severe fine-tuning of the quartic self-coupling in minimally coupled inflation. The parameter range <math altimg='si7.svg'><mrow><mi>&#947;</mi><mo>&#8764;</mo><mi mathvariant='script'>O</mi><mo>(</mo><msup><mn>10</mn><mrow><mo>&#8722;</mo><mn>3</mn></mrow></msup><mo>)</mo></mrow></math>&#8211; <math altimg='si8.svg'><mrow><mi mathvariant='script'>O</mi><mo>(</mo><msup><mn>10</mn><mrow><mo>&#8722;</mo><mn>2</mn></mrow></msup><mo>)</mo></mrow></math> emerges as phenomenologically viable, providing a natural extension of Starobinsky inflation compatible with present data. Moreover, the model yields a small running of the spectral index that remains fully consistent with the ACT 95 % confidence constraints. We conclude that marginally deformed <italic>R</italic> 2 inflation remains a compelling and testable candidate for the primordial dynamics of the Universe, with future CMB and gravitational-wave observations expected to further probe its parameter space.

ACT constraints on marginally deformed starobinsky inflation

Recent measurements from the Atacama Cosmology Telescope (ACT), combined with Planck and DESI data, suggest a scalar spectral index $$n_s$$ higher than the Planck 2018 baseline, thereby placing conventional attractor-type inflationary models such as Starobinsky $$R^2$$ and Higgs inflation under increasing tension at the $$\gtrsim 2\sigma $$ level. In this work, we examine quantum-corrected $$\phi ^4$$ inflation with a non-minimal coupling to gravity. Introducing an anomalous scaling parameter $$\gamma $$ to capture quantum corrections to the effective potential, we derive analytic expressions for the inflationary observables $$n_s$$ and $\textit{r}$. Confronting these predictions with ACT, Planck, and BAO+lensing constraints, we demonstrate that modest values of $$\gamma $$ can raise $$n_s$$ into the ACT-preferred range while maintaining a strongly suppressed tensor-to-scalar ratio. For instance, with $$N=60$$ and $$\gamma \simeq 0.006$$ , the model predicts $$n_s\simeq 0.974$$ and $$r\simeq 0.007$$ , in excellent agreement with current bounds. We further investigate preheating dynamics, focusing on particle production via parametric resonance in quantum-corrected $$\phi ^4$$ inflation with a non-minimal coupling to gravity. In this scenario, the inflaton $$\phi $$ couples to an additional scalar $$\chi $$ through an interaction $$g^{2}\phi ^{2}\chi ^{2}$$ . In Minkowski spacetime, the resonance dynamics reduce to the Mathieu equation, and we find that broad resonance can be readily achieved, leading to efficient particle production.

Peeravit Koad

Quantum-corrected $$\phi ^{4}$$ inflation in light of ACT observations

In this work, we model static spherically symmetric compact stars within the framework of classical general relativity. In order to obtain exact solutions of the Einstein field equations with non-singular density profile we adopt the generalised Mak–Harko ansatz (Mak and Harko in Chin. J. Astron. Astrophys. 2:248, 2002) and demand that the complexity factor as defined by Herrera for static relativistic spheres (Herrera, Phys. Rev. D 97:044010, 2018) vanishes everywhere inside the self-gravitating object. Exact solutions of the Einstein field equations describing anisotropic fluid spheres are obtained via the gravitational decoupling method. We show that the decoupling constant, central and surface density values play a crucial role in dictating the stability of the stellar structure. The interplay between these factors accounts for mass–radius profiles associated with gravitational wave events such as GW190814 and further predicts stellar masses in the range 2.9 $$M_\odot $$ and 3.4 $$M_\odot $$ . Our models are excellent candidates for predicting compact objects such as neutron stars residing in the so-called $\textit{mass gap}$ associated with binary mergers without invoking exotic matter and modified gravity theories.

Oman

S. Maurya

Asifa Ashraf

Saudi Arabia

Akram Ali

South Africa

Megandhren Govender

Faisal Javed

Modeling anisotropic compact objects in the vanishing complexity regime through gravitational decoupling

Recent measurements from the Atacama Cosmology Telescope (ACT), particularly when combined with DESI baryon acoustic oscillation data, have reported a scalar spectral index <italic>n<inf>s</inf></italic> slightly higher than that inferred by <italic>Planck</italic> 2018, suggesting a mild tension with the predictions of standard inflationary attractor models. In this work, we revisit the quantum-corrected Higgs inflation scenario within the framework of a non-minimally coupled scalar field theory. Starting from the one-loop effective action, we incorporate radiative corrections through the anomalous scaling parameter <bold>A<inf>I</inf></bold> and derive analytic expressions for the inflationary observables <italic>n<inf>s</inf></italic> and <italic>r</italic> in the Einstein frame. Our analysis demonstrates that quantum corrections naturally shift <italic>n<inf>s</inf></italic> toward higher values while keeping the tensor-to-scalar ratio <italic>r</italic> suppressed. For <math altimg='si4.svg'><mrow><mi mathvariant='script'>N</mi><mo linebreak='goodbreak'>=</mo><mn>60</mn></mrow></math>, the model predicts <italic>n<inf>s</inf></italic>&#8239;&#8771;&#8239;0.9743 and <math altimg='si5.svg'><mrow><mi>r</mi><mo>&#8771;</mo><mn>5.4</mn><mo linebreak='goodbreak'>&#215;</mo><msup><mn>10</mn><mrow><mo>&#8722;</mo><mn>3</mn></mrow></msup></mrow></math>, in excellent agreement with the latest ACT+DESI (P-ACT-LB) data and fully consistent with the <italic>Planck</italic> 2018 limit <italic>r</italic>&#8239;&lt;&#8239;0.036. The derived constraint <math altimg='si6.svg'><mrow><mn>4.36</mn><mo linebreak='goodbreak'>&#215;</mo><msup><mn>10</mn><mrow><mo>&#8722;</mo><mn>10</mn></mrow></msup><mo linebreak='goodbreak'>&lt;</mo><mi>&#955;</mi><mo linebreak='goodbreak'>/</mo><msup><mi>&#958;</mi><mn>2</mn></msup><mo linebreak='goodbreak'>&lt;</mo><mn>10.77</mn><mo linebreak='goodbreak'>&#215;</mo><msup><mn>10</mn><mrow><mo>&#8722;</mo><mn>10</mn></mrow></msup></mrow></math> confirms the robustness of the quantum-corrected Higgs framework and indicates that near-future CMB polarization experiments such as CORE, AliCPT, LiteBIRD, and CMB-S4 will be able to probe the predicted parameter space with high precision.

Radiative-corrected Higgs inflation in light of the latest ACT observations