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In this paper, we investigate one of the established methods for reconstructing modified gravity models from a dark energy model, with the aim of discovering relationships between these theories. In this study, we focus on the f ( R ,  T ) modified gravity theory, where R denotes the Ricci scalar and T represents the trace of the energy–momentum tensor. We employ Barrow’s holographic dark energy model, derived from fractal surfaces of black holes, to investigate the reconstruction process. The numerical results are subsequently presented for various infrared cutoffs, such as the Hubble horizon, future event horizon, and Granda–Oliveros cutoff.

In this paper, Ricci-inverse gravity is investigated. It is an alternative theory of gravity that introduces into the Einstein–Hilbert action an anti-curvature scalar that is obtained from the anti-curvature tensor which is the inverse of the Ricci tensor. An axially symmetric spacetime with causality violation is studied. Two classes of the model are discussed. Different sources of matter are considered. Then a direct relation between the content of matter and causality violation is shown. Our results confirm that Ricci-inverse gravity allows the existence of Closed Time-like Curves (CTCs) that lead to the violation of causality. Furthermore, a comparison is made between the results of general relativity and Ricci-inverse gravity. Other spacetimes, such as Gödel and Gödel-type universes, which are exact solutions of general relativity and allow for causality violations, are also explored in Ricci-inverse gravity framework.

In the Rastall gravity a non-minimal coupling between geometry and matter fields is considered. Then the usual energy-momentum tensor conservation law is not valid. Here a Lagrangian formalism is proposed to the Rastall theory of gravity. The Gödel-type universe is studied in this gravitational model. Then it is studied whether this theory permits causality violation. The field equations do not exclude solutions with a breakdown of causality for a perfect fluid as matter content. In this case, an expression for the critical radius (beyond which the causality is violated) is determined. In addition, for a combination between perfect fluid and scalar field as matter content the theory accommodates causal Gödel-type solution.

In this paper, the cross section for the Compton scattering process at finite temperature is calculated. Temperature effects are introduced using the Thermofield Dynamics (TFD) formalism. It is a real-time finite temperature quantum field theory. Our result shows that thermal effects become relevant as the temperature increases. A comparison between the TFD and closed-time path results is presented.

In this paper, we investigate a causality-violating four-dimensional space-time, an extension of the two-dimensional Misner space within the framework of modified gravity theories, specifically in the context of Ricci-inverse gravity. To explore this, we focus on two classes of models denoted as f ( R , A μ ν A μ ν ) and f ( R , A , A μ ν A μ ν ) , and derive the modified field equations using the four-dimensional space-time under consideration. Here, A μ ν is the anti-curvature tensor defined by inverse of the Ricci tensor R μ ν , that is, A μ ν = R μ ν - 1 . Additionally, A denotes its scalar, defined as A = g μ ν A μ ν . Our analysis shows that in both models, the modified field equations can be solved by considering the matter energy content as a pure radiation field including the cosmological constant. Furthermore, we demonstrate that the energy density of the pure radiation field and the cosmological constant undergo modifications due to the coupling parameters, and these modifications converge to the results of general relativity when these parameters are set to zero. This observation confirms the possibility of causality violation in the form of closed time-like curves within the framework of Ricci-inverse gravity theory.

In this work, the behavior, evolution, and expansion of the universe are investigated within a Lorentz-violating framework driven by Tsallis holographic dark energy. The cosmological extension is implemented through a spontaneously symmetry-breaking Bumblebee field, which is assumed to play a fundamental role in the dynamics of the universe. Estimates for key Lorentz-violating quantities are obtained, and the evolution of the Hubble parameter is analyzed from the early universe era to the present epoch. This formulation provides an alternative perspective on the Hubble tension.

We investigate a family of axial symmetry solution constructed in general relativity (GR) within the framework of Ricci-inverse (RI) gravity theory. In GR, these solutions admitted closed time-like curves at an instant of time from an initial spacelike hypersurface in a causally well-behaved manner, thus, violates the causality condition. Our aim is to examine these axial symmetry solutions within the context of Ricci-inverse gravity theory to determine whether closed time-like curves still appear in this new gravity theory. We consider two Classes of RI-gravity models: (i) Class-II models defined by a function f = f ( R , A μ ν A μ ν ) gravity and (ii) Class-III models defined by f = f ( R , A , A μ ν A μ ν ) , where A μ ν is the anti-curvature tensor, A = g μ ν A μ ν as its scalar, and R μ ν is the Ricci tensor. We are able solved the modified field equations considering these axial symmetry solutions as background in RI-gravity with null radiation as the matter content and the cosmological constant. This confirms that the chosen family of axial symmetry solutions are valid solutions in RI-gravity theory and, consequently, closed time-like curves is still form, analogous to their formation in GR.

Temperature effects in a scalar field non-minimally coupled to gravity are investigated. The Thermo Field Dynamics formalism is used. This is a topological field theory that allows us to calculate different effects, such as the Stefan–Boltzmann law and the Casimir effect, on an equal footing. These phenomena are calculated assuming the Gödel space-time as a gravitational background. A possible implication of these results at the beginning of the universe is discussed.

During the necropsies of Iberian hares obtained in 2018/2019, along with signs of the nodular form of myxomatosis, other unexpected external lesions were also observed. Histopathology revealed nuclear inclusion bodies in stromal cells suggesting the additional presence of a nuclear replicating virus. Transmission electron microscopy further demonstrated the presence of herpesvirus particles in the tissues of affected hares. We confirmed the presence of herpesvirus in 13 MYXV-positive hares by PCR and sequencing analysis. Herpesvirus-DNA was also detected in seven healthy hares, suggesting its asymptomatic circulation. Phylogenetic analysis based on concatenated partial sequences of DNA polymerase gene and glycoprotein B gene enabled greater resolution than analysing the sequences individually. The hare' virus was classified close to herpesviruses from rodents within the Rhadinovirus genus of the gammaherpesvirus subfamily. We propose to name this new virus Leporid gammaherpesvirus 5 (LeHV-5), according to the International Committee on Taxonomy of Viruses standards. The impact of herpesvirus infection on the reproduction and mortality of the Iberian hare is yet unknown but may aggravate the decline of wild populations caused by the recently emerged natural recombinant myxoma virus.

The 13th International Foamy Virus (FV) Conference was held from 8 to 10 November 2023 at the BioParque/Zoological Garden in Rio de Janeiro, Brazil. This was the first conference on spumaretroviruses to be held in the Southern Hemisphere and in the unique environment of the rainforest. New developments and current perspectives in FV research were presented. Highlights of the conference included the structural biology of the envelope protein (Env) and insights into its function and evolution, epidemiologic identification of Amazonian indigenous people with a high prevalence of simian FV (SFV) infections, investigations of virus biology and genomics using synthetic FV DNAs, studies of humoral immune response, and development and applications of SFV vectors. The last day of the meeting was a special tour of the Centro de Primatologia do Rio de Janeiro, located northeast of Rio de Janeiro amidst the protected rainforest, where New World primate hosts of spumaretroviruses are rescued and studied. Our report summarizes the meeting highlights and outcomes for future discussions.