We show how a nonlocal gravitational interaction can circumvent the Weinberg no-go theorem on cosmological constant, which forbids the existence of any solution to the cosmological constant problem within the context of local field theories unless some fine-tuning is assumed. In particular, Infinite Derivative Gravity theories hint at a possible understanding of the cosmological constant as a nonlocal gravitational effect on very large scales. In this perspective, one can describe the observed cosmic acceleration in terms of an effective field theory without relying on the fine-tuning of parameters or additional matter fields.

Italy

Salvatore Capozziello

Netherlands

Anupam Mazumdar

Giuseppe Meluccio

The Weinberg no-go theorem for cosmological constant and nonlocal gravity

We study the thermodinamic features of static, spherically-symmetric Schwarzschild black holes adopting different types of Barrow entropy. Specifically, in addition to the standard Barrow entropy, we consider a logarithmic-corrected type of this entropy by taking into account some loop quantum gravity effects. Moreover, we investigate the black hole thermodynamics from the viewpoint of Barrow entropy in presence of non-extensivity effects coming from the Tsallis statistics. Finally, we compare the results obtained for different Barrow-based entropies.

Canada

Iran

Mehdi Shokri

Barrow entropies in black hole thermodynamics

We explore wormhole solutions sourced by Casimir energy density involving generalized uncertainty principle corrections within the framework of Rastall–Rainbow gravity. The questions of traversability and stability, as well as the presence of exotic matter, are carefully investigated. In particular, the stability issue is addressed via an approach that has not been previously employed in the context of wormholes. This method, which represents an improved version of the so-called Herrera cracking technique, has the potential to yield novel insights in the field of wormhole geometries.

Denmark

Emmanuele Battista

South Africa

Kazakhstan

Abdelghani Errehymy

Generalized uncertainty principle corrections in Rastall–Rainbow Casimir wormholes

The effects of cosmological expansion on quantum bosonic states are investigated, using quantum information theory. In particular, a generic Bogoliubov transformation of bosonic field modes is considered and the state change on a single mode is regarded as the effect of a quantum channel. Properties and capacities of this channel are thus explored in the framework of f(Q) non-metric gravity. The reason is that non-metric gravity can be considered under the standard of gauge theories with all the advantages of such a formulation. As immediate result, we obtain that the information on a single-mode state appears better preserved, whenever the number of particles produced by the cosmological expansion is small. Specifically, we investigate a power law f(Q) model, leaving unaltered the effective gravitational coupling, and minimise the corresponding particle production. We thus show how to optimise the preservation of classical and quantum information, stored in bosonic mode states in the remote past. Finally, we compare our findings with those obtained in General Relativity.

Alessio Lapponi

United States

Orlando Luongo

Stefano Mancini

Preserving quantum information in f(Q) non-metric gravity cosmology

We explore Noether symmetries of Horndeski gravity, extending the classification of general scalar–tensor theories. Starting from the minimally coupled scalar field and the first-generation scalar–tensor gravity, the discussion is generalised to kinetic gravity braiding and Horndeski gravity. We highlight the main findings by focusing on the non-minimally coupled Gauss–Bonnet term and the extended cuscuton model. Finally, we discuss how the presence of matter can influence Noether symmetries. It turns out that the selected Horndeski functions are unchanged with respect to the vacuum case.

Marcello Miranda

Daniele Vernieri

General analysis of Noether symmetries in Horndeski gravity

We consider non-local Integral Kernel Theories of Gravity in a homogeneous and isotropic universe background as a possible scenario to drive the cosmic history. In particular, we investigate the cosmological properties of a gravitational action containing the inverse d'Alembert operator of the Ricci scalar proposed to improve Einstein's gravity at both high and low-energy regimes. In particular, the dynamics of a physically motivated non-local exponential coupling is analyzed in detail by recasting the cosmological equations as an autonomous system of first-order differential equations with dimensionless variables. Consequently, we study the phase-space domain and its critical points, investigating their stability and main properties. In particular, saddle points and late-time cosmological attractors are discussed in terms of the free parameters of the model. Finally, we discuss the main physical consequences of our approach in view of dark energy behavior and the ΛCDM model.

Rocco D'Agostino

The phase-space view of non-local gravity cosmology

We discuss spherically symmetric dynamical systems in the framework of a general model of  $$f(\mathcal{R})$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> </math>   gravity, i.e.  $$f(\mathcal{R})=\mathcal{R}e^{\zeta \mathcal{R}}$$  <math> <mrow> <mi>f</mi> <mrow> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>R</mi> <msup> <mi>e</mi> <mrow> <mi>ζ</mi> <mi>R</mi> </mrow> </msup> </mrow> </math>  , where  $$\zeta $$  <math> <mi>ζ</mi> </math>   is a dimensional quantity in squared length units [L $$^2$$  <math> <msup> <mrow></mrow> <mn>2</mn> </msup> </math>  ]. We initially assume that the internal structure of such systems is governed by the Krori–Barua ansatz, alongside the presence of fluid anisotropy. By employing astrophysical observations obtained from the pulsar SAX J1748.9-2021, derived from bursting X-ray binaries located within globular clusters, we determine that  $$\zeta $$  <math> <mi>ζ</mi> </math>   is approximately equal to  $$\pm 5$$  <math> <mrow> <mo>±</mo> <mn>5</mn> </mrow> </math>   km $$^2$$  <math> <msup> <mrow></mrow> <mn>2</mn> </msup> </math>  . In particular, the model is capable of producing stable configurations for SAX J1748.9-2021, encompassing both its geometric and physical characteristics. We show that, within the framework of  $$f(\mathcal{R})$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> </math>   gravity, the Krori–Barua ansatz establishes semi-analytical connections between the radial ( $$p_r$$  <math> <msub> <mi>p</mi> <mi>r</mi> </msub> </math>  ) and tangential ( $$p_t$$  <math> <msub> <mi>p</mi> <mi>t</mi> </msub> </math>  ) pressures, and the density ( $$\rho $$  <math> <mi>ρ</mi> </math>  ). These relations are described as  $$p_r\approx v_r^2 (\rho -\rho _{I})$$  <math> <mrow> <msub> <mi>p</mi> <mi>r</mi> </msub> <mo>≈</mo> <msubsup> <mi>v</mi> <mi>r</mi> <mn>2</mn> </msubsup> <mrow> <mo>(</mo> <mi>ρ</mi> <mo>-</mo> <msub> <mi>ρ</mi> <mi>I</mi> </msub> <mo>)</mo> </mrow> </mrow> </math>   and  $$p_t\approx v_t^2 (\rho -\rho _{II})$$  <math> <mrow> <msub> <mi>p</mi> <mi>t</mi> </msub> <mo>≈</mo> <msubsup> <mi>v</mi> <mi>t</mi> <mn>2</mn> </msubsup> <mrow> <mo>(</mo> <mi>ρ</mi> <mo>-</mo> <msub> <mi>ρ</mi> <mrow> <mi>II</mi> </mrow> </msub> <mo>)</mo> </mrow> </mrow> </math>  . In this context,  $$v_r$$  <math> <msub> <mi>v</mi> <mi>r</mi> </msub> </math>   and  $$v_t$$  <math> <msub> <mi>v</mi> <mi>t</mi> </msub> </math>   denote the sound speeds in the radial and tangential directions, respectively. Meanwhile,  $$\rho _I$$  <math> <msub> <mi>ρ</mi> <mi>I</mi> </msub> </math>   pertains to the surface density, and  $$\rho _{II}$$  <math> <msub> <mi>ρ</mi> <mrow> <mi>II</mi> </mrow> </msub> </math>   is derived from the model parameters. These connections are consistent with the equations of state derived from the best-fit solutions identified in the ongoing investigation. Notably, within the framework of  $$f(\mathcal{R})$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> </math>   gravity where  $$\zeta $$  <math> <mi>ζ</mi> </math>   is negative, the maximum compactness, denoted as C, is inherently limited to values that do not exceed the Buchdahl limit. This contrasts with general relativity or  $$f(\mathcal{R})$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> </math>   gravity with  $$\zeta $$  <math> <mi>ζ</mi> </math>   positive, where the compactness has the potential to asymptotically reach the black hole threshold ( $$C\rightarrow 1$$  <math> <mrow> <mi>C</mi> <mo>→</mo> <mn>1</mn> </mrow> </math>  ). The model predictions suggest a central density that largely exceeds the saturation nuclear density, which is  $$\rho _{\text {nuc}} = 3\times 10^{14}$$  <math> <mrow> <msub> <mi>ρ</mi> <mtext>nuc</mtext> </msub> <mo>=</mo> <mn>3</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>14</mn> </msup> </mrow> </math>   g/cm $$^3$$  <math> <msup> <mrow></mrow> <mn>3</mn> </msup> </math>  . Also the surface density  $$\rho _I$$  <math> <msub> <mi>ρ</mi> <mi>I</mi> </msub> </math>   surpasses  $$\rho _{\text {nuc}}$$  <math> <msub> <mi>ρ</mi> <mtext>nuc</mtext> </msub> </math>  . We obtain a mass-radius diagram, corresponding to the boundary density, which is consistent with other observational data.

Egypt

Gamal Nashed

Constraining  $$f({\mathcal {R}})$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> </math>   gravity by Pulsar SAX J1748.9-2021 observations

We address the galaxy rotation curves through the Yukawa gravitational potential emerging as a correction of the Newtonian potential in extended theories of gravity. On the one hand, we consider the contribution of the galactic bulge, galactic disk, and the dark matter halo of the Navarro–Frenk–White profile, in the framework of the standard  $$\Lambda $$  <math> <mi>Λ</mi> </math>  CDM model. On the other hand, we use modified Yukawa gravity to show that the rotational velocity of galaxies can be addressed successfully without the need for dark matter. In Yukawa gravity, we recover MOND and show that dark matter might be seen as an apparent effect due to the modification of the law of gravitation in terms of two parameters: the coupling constant  $$\alpha $$  <math> <mi>α</mi> </math>   and the characteristic length  $$\lambda $$  <math> <mi>λ</mi> </math>  . We thus test our theoretical scenario using the Milky Way and M31 rotation velocity curves. In particular, we place observational constraints on the free parameters of Yukawa cosmology through the Monte Carlo method and then compare our results with the predictions of the  $$\Lambda $$  <math> <mi>Λ</mi> </math>  CDM paradigm by making use of Bayesian information criteria. Specifically, we find that  $$\lambda $$  <math> <mi>λ</mi> </math>   is constrained to be of the order of kpc, while cosmological data suggest  $$\lambda $$  <math> <mi>λ</mi> </math>   of the order of Gpc. To explain this discrepancy, we argue that there is a fundamental limitation in measuring  $$\lambda $$  <math> <mi>λ</mi> </math>   due to the role of quantum mechanics on cosmological scales.

Rocco D’Agostino

North Macedonia

Kimet Jusufi

Testing Yukawa cosmology at the Milky Way and M31 galactic scales

We explore the possibility of detecting resonant conversions of high frequency gravitational waves to radio-signals. We show how the graviton-photon transitions, from Gertsenshtein effect, can be resonantly amplified in a cosmological stochastic magnetic background. In particular, we found parametric resonances for both the monochromatic and scale invariant power spectrum models of magnetic field fluctuations. Our results substantially departs from those predicted in resonance-less magnetic domain-like models in a large region of magnetic and oscillation wavelength scales. Resonances can be tested by radio telescopes such as ARCADE 2 and EDGES as a probe of high frequency gravitational wave sources and new physics models for magnetogenesis. Analysis are performed with a robust perturbative approach in discrete scheme for including cosmological decoherence in graviton-photon oscillations.

China

Andrea Addazi

Qingyu Gan

Resonant graviton-photon transitions with cosmological stochastic magnetic field

Gravitational radiation-reaction phenomena occurring in the dynamics of inspiralling compact binary systems are investigated at the first post-Newtonian order beyond the quadrupole approximation in the context of Einstein–Cartan theory, where quantum spin effects are modeled via the Weyssenhoff fluid. We exploit balance equations for the energy and angular momentum to determine the binary orbital decay until the two bodies collide. Our framework deals with both quasi-elliptic and quasi-circular trajectories, which are then smoothly connected. Key observables like the laws of variation of the orbital phase and frequency characterizing the quasi-circular motion are derived analytically. We conclude our analysis with an estimation of the spin contributions at the merger, which are examined both in the time domain and the Fourier frequency space through the stationary wave approximation.

Vittorio Falco

Austria

Davide Usseglio

Radiative losses and radiation-reaction effects at the first post-Newtonian order in Einstein–Cartan theory