The recent observations of the Dark Energy Spectroscopic Instrument (DESI) indicated the possibility that the dark energy equation of state parameter $\textit{w}$ might change from $$w<-1$$ to $$w>-1$$ when the redshift $$z\sim 0.5$$ , which is called the inverse phantom crossing. In this paper, we investigate the possibility of the phantom crossing, and we construct realistic models realizing the crossing in the framework of the scalar–Einstein–Gauss-Bonnet gravity and ghost-free $$f(\mathcal {G})$$ gravity. We also investigate the scenario of the apparent phantom crossing, where dark matter energy density decreases more slowly than usually expected, which might explain the DESI observations. In the scenarios developed, the energy conditions are not violated by any component of the cosmic fluid. In the framework of the apparent phantom crossing, we also propose a new scenario, where the particle corresponding to the scalar field in the scalar–Einstein–Gauss-Bonnet gravity is dark matter. The mass of the particle might increase due to the coupling with the Gauss-Bonnet invariant, which makes the decrease of the dark matter energy density slower. This last scenario may suggest that the inverse phantom crossing might be related to the transition from the decelerating expansion of the Universe to the accelerating expansion.

Japan

Shin’ichi Nojiri

Spain

Sergei Odintsov

Greece

Azerbaijan

V. Oikonomou

Apparent phantom crossing in Gauss–Bonnet gravity

We confront the ghost-free non-local <italic>F</italic>( <italic>R</italic>) gravity theories with the latest Atacama Cosmology Telescope (ACT) constraints on the spectral index of the scalar perturbations and the updated constraints of Planck/BICEP on the tensor-to-scalar ratio. After reviewing how the ghost-free non-local version of <italic>F</italic>( <italic>R</italic>) gravity can be obtained, we show that the de Sitter solution can be obtained in this framework. Also, we show that the resulting theory can be cast in terms of an <italic>F</italic>( <italic>R, &#981;</italic>) theory of gravity. We analyze two models of non-local <italic>F</italic>( <italic>R</italic>) gravity, one power-law and the <italic>R</italic> <sup>2</sup> model, and we show that both models can be compatible with the ACT and updated Planck/BICEP constraints.

V.K. Oikonomou

Ghost-free non-local <italic>F</italic>( <italic>R</italic>) gravity compatible with ACT

Over the last decades, tests on the standard model of cosmology, the so-called $$\Lambda $$ CDM model, have been widely analysed and compared with many different models for describing dark energy. Modified gravities have played an important role in this sense as an alternative to $$\Lambda $$ CDM model. Previous observational data has been always favouring $$\Lambda $$ CDM model in comparison to any other model. While statistically speaking, alternative models have shown their power, fitting in some cases the observational data slightly better than $$\Lambda $$ CDM, the significance and goodness of the fits were not significantly relevant to exclude the standard model of cosmology. In this paper, a generalisation of exponential ( ) gravity is considered and compared with $$\Lambda $$ CDM model by using the latest observational data. Also some well-known model independent parameterisations for the equation of state (EoS) of dark energy are explored. These scenarios are confronted with the renewed observational data involving the Pantheon plus datasets of supernovae type Ia, the Hubble parameter estimations, data from the cosmic microwave background and baryon acoustic oscillations, where the latter includes the data provided by Dark Energy Spectroscopic Instrument Collaboration. Results of this analysis suggest that standard exponential ( ) models provide much better fits than $$\Lambda $$ CDM model, which is excluded at 4 $$\sigma $$ . Moreover, the parameterisations of the equation of state suggest a non-constant EoS parameter for dark energy, where $$\Lambda $$ CDM model is also excluded at 4 $$\sigma $$ .

Russia

Brazil

Diego Sáez-Chillón Gómez

German Sharov

Modified gravity/dynamical dark energy vs $$\Lambda $$ CDM: is the game over?

The  $$H_{0}$$  <math> <msub> <mi>H</mi> <mn>0</mn> </msub> </math>   tension problem is studied in the light of a matter creation mechanism (an effective approach to replacing dark energy), the way to define the matter creation rate being of pure phenomenological nature. Bayesian (probabilistic) Machine Learning is used to learn the constraints on the free parameters of the models, with the learning being based on the generated expansion rate, H(z). Taking advantage of the method, the constraints for three redshift ranges are learned. Namely, for the two redshift ranges:  $$z\in [0,2]$$  <math> <mrow> <mi>z</mi> <mo>∈</mo> <mo>[</mo> <mn>0</mn> <mo>,</mo> <mn>2</mn> <mo>]</mo> </mrow> </math>   (cosmic chronometers) and  $$z\in [0,2.5]$$  <math> <mrow> <mi>z</mi> <mo>∈</mo> <mo>[</mo> <mn>0</mn> <mo>,</mo> <mn>2.5</mn> <mo>]</mo> </mrow> </math>   (cosmic chronometers + BAO), covering already available H(z) data, to validate the learned results; and for a third redshift interval,  $$z\in [0,5]$$  <math> <mrow> <mi>z</mi> <mo>∈</mo> <mo>[</mo> <mn>0</mn> <mo>,</mo> <mn>5</mn> <mo>]</mo> </mrow> </math>  , for forecasting purposes. It is learned that the  $$3\alpha H_{0}$$  <math> <mrow> <mn>3</mn> <mi>α</mi> <msub> <mi>H</mi> <mn>0</mn> </msub> </mrow> </math>   term in the creation rate provides options that have the potential to solve the  $$H_{0}$$  <math> <msub> <mi>H</mi> <mn>0</mn> </msub> </math>   tension problem.

Emilio Elizalde

Martiros Khurshudyan

Can we learn from matter creation to solve the  $$H_{0}$$  <math> <msub> <mi>H</mi> <mn>0</mn> </msub> </math>   tension problem?

We discuss the emergence of black hole shadow and photon-sphere in the context of <math><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo></math> gravity. It is shown that the shadow is exponentially sensitive to linear instabilities of metric coming from some <math><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo></math> solutions. Thus, the instabilities of photon circular trajectories, delimiting the black hole photon-sphere, are double exponentialized. Specifically we individuate two Lyapunov exponents, rather than only one, related to two different sources of chaos in geodesic orbits as a sort of butterfly effect. Such a result violates the black hole chaos bound proposed by Maldacena, Shenker and Stanford for General Relativity. We also explore the impact of the black hole metric instabilities in <math><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo></math> gravity on the quasi-normal modes. In the framework of Extended Theories of Gravity, our analysis suggests a new paradigm to deal with black hole shadow and gravitational waves observations coming from black hole merging in the ringdown phase.

China

Italy

Andrea Addazi

Salvatore Capozziello

Chaotic solutions and black hole shadow in ( ) Gravity

We present a modified cosmological scenario that arises from the application of non-extensive thermodynamics with varying exponent. We extract the modified Friedmann equations, which contain new terms quantified by the non-extensive exponent, possessing standard $$\Lambda $$ <math><mi>Λ</mi></math> CDM cosmology as a subcase. Concerning the universe evolution at late times we obtain an effective dark energy sector, and we show that we can acquire the usual thermal history, with the successive sequence of matter and dark-energy epochs, with the effective dark-energy equation-of-state parameter being in the quintessence or in the phantom regime. The interesting feature of the scenario is that the above behaviors can be obtained even if the explicit cosmological constant is set to zero, namely they arise purely from the extra terms. Additionally, we confront the model with Supernovae type Ia and Hubble parameter observational data, and we show that the agreement is very good. Concerning the early-time universe we obtain inflationary de Sitter solutions, which are driven by an effective cosmological constant that includes the new terms of non-extensive thermodynamics. This effective screening can provide a description of both inflation and late-time acceleration with the same parameter choices, which is a significant advantage.

Emmanuel Saridakis

Modified cosmology from extended entropy with varying exponent

We discuss the possibility that a Born–Infeld condensate coupled to neutrinos can generate both neutrino masses and an effective cosmological constant. In particular, an effective field theory is provided capable of dynamically realizing the neutrino superfluid phase firstly suggested by Ginzburg and Zharkov. In such a case, neutrinos acquire a mass gap inside the Born–Infeld ether forming a long-range Cooper pair. Phenomenological implications of the approach are also discussed.