Gravitation and Cosmology最新文献

In a recent publications I proposed a new statistical theory of gravity [Riccardo Fantoni, Quantum Reports 6, 706 (2024)], which describes fluctuations of the space-time metric through a virial temperature. In a succeeding publication I discussed the foundations [Riccardo Fantoni, Stats 8, 23 (2025)] of such a theory. Here, I propose a possible way to render numerically accessible the path integral Monte Carlo computations required in the Statistical Gravity theory. This requires the use of the Arnowitt–Deser–Misner (ADM) splitting and of the Affine Quantization (AQ) method.

Our study focuses on the holographic dark energy (HDE) model where the Gauss–Bonnet and Ricci invariants jointly determine the infrared cutoff. We determine which possibilities are physically plausible by examining how well the model fits the data. We challenge the cosmological model using two cosmological datasets: the Pantheon sample of Supernovae (SNIa), in conjunction with the most recent cosmic chronometer dataset, by performing Monte Carlo Markov Chain (MCMC) analyses to constrains the model parameters. The evolution of the equation of state (EoS) parameter, the deceleration parameter (DP), and the energy density parameters are thoroughly examined based on the best fit values of the model parameters. These show the typical thermal history of the universe, including the epochs of matter, radiation, and dark energy, culminating in the eventual total dominance of dark energy (DE). During the cosmological evolution, the corresponding DE EoS parameter may experience phantom divide crossing, lie in the quintessence and phantom regimes, and have an asymptotic value that is exactly equal to the cosmological-constant value. These observational datasets show a good agreement between the model and observations. The dynamical behavior of the DP explains the transition of the Gauss–Bonnet universe from decelerated to accelerated expansion. Our tools include the ((omega_{D},omega_{D}^{{}^{prime}})) pair, and (Om(z)) diagnostic planes. Our results show that, across a range of model parameter values, the model displays both Chaplygin gas and quintessence behaviors in the ((omega_{D},omega^{prime}_{D})) planes. To set our model apart from existing DE models, we also use the (Om(z)) diagnostic analysis.

Quantization of unimodular gravity in minisuperspace leads to a time evolution of states generated by the Hamiltonian, as in usual quantum mechanics. We revisit the analysis made by Unruh, extending it to phantom scalar fields. It is argued that only in this case a nontrivial evolution for the scalar field can be obtained. The behavior of the scale factor presents a bounce followed by de Sitter expansion, reproducing the quantum cosmological scenario in General Relativity when the source is given by a cosmological term described by the Schutz variable. The analysis is extended to the Brans–Dicke scalar-tensor theory.

A mathematical model of the Universe evolution is constructed and investigated, based on an asymmetric doublet of classical and phantom scalar Higgs fields and a perfect fluid, both scalarly charged and neutral. The model is an extension of the scalarly charged perfect fluid model with a classical scalar Higgs singlet. The general properties of this model are studied, and its relation to the scalar neutral fluid model and the scalar vacuum model is established. A qualitative analysis of the cosmological model showed that the singular points of all three models, as well as their character, coincide. Examples of numerical integration of these models are given, confirming their theoretically established properties. It is shown that near unstable points of the model, generation of a phantom scalar field is possible.

The current article’s goal is to characterize generalized quasi-Einstein space-times. Two nontrivial examples have been provided to demonstrate the existence of such a space-time. It is shown that a generalized quasi-Einstein space-time represents a Ricci recurrent space-time, and the associated vectors are parallel. Moreover, we prove that a Ricci-symmetric generalized quasi-Einstein space-time is static. Next, we establish that a Ricci-semi-symmetric generalized quasi-Einstein space-time is a perfect fluid space-time under some restriction on the associated scalars. Moreover, we illustrate that a generalized quasi-Einstein twisted space-time becomes a perfect fluid space-time under certain condition, and also find the conditions under which this space-time represents a radiation era, a dust matter era and a dark matter era of the universe. Also, we see that if the space-time has a Riemann compatible (also, Weyl compatible) vector, and the space-time is purely electric. Finally, we acquire under what condition a generalized quasi-Einstein twisted space-time becomes a generalized Robertson–Walker space-time.

The equations of cosmological evolution are derived based on a new quasiclassical theory of gravity, which describes the effect of hidden mass or “dark matter.” The dynamic equations correspond to classical mechanics and are Eulerian equations for a dustlike medium. It is shown that taking into account the hidden mass effect leads to Friedmann’s equations for the scale factor of the medium, similar to the equations in general relativity. A comparative analysis of the evolution equations in general relativity and in the new theory is carried out. Based on a qualitative analysis of the Friedmann equations, some possible scenarios of cyclic cosmological evolution are considered, describing all its main stages, including the first and second inflationary stages with an intermediate stage of Friedman expansion.

An obvious but unknown fact is presented: the generally accepted conservation of the 4-momentum of matter together with the gravitational field means a violation of the conservation law for matter under gravitational interaction. It is shown that the local conservation law for matter, which is expressed by the equality to zero of the partial divergence of the energy-momentum tensor when using Galilean coordinates, is expressed by the equality to zero of the covariant divergence of the tensor density of the energy-momentum of matter when using curvilinear coordinates. And this local law ensures the validity of the integral conservation law. However, in the presence of a gravitational field, this local conservation law leads to a violation of the integral conservation law for matter. The article calculates the variable mass of an isolated body, which changes with a change in the gravitational field, and presents an expression for the conserved 4-momentum of matter together with the gravitational field.

We derive cosmological constraints for specific (f(R)) gravity models. We focus on two models: the Hu–Sawicki model and the Starobinsky model, introducing a distortion parameter to quantify deviations from the standard (Lambda)CDM cosmology. Data from the Big Bang nucleosynthesis (BBN), the Dark Energy Spectroscopic Instrument (DESI), baryon acoustic oscillations (BAO), and the most recent Pantheon Plus datasets—which include Cepheid host distances and covariance from SH0ES samples—are all employed in our investigation. A minor but nonzero divergence from (Lambda)CDM cosmology is slightly preferred, according to the results, which are corroborated by efficient values of the Bayesian Information Criterion (BIC) and the Akaike Information Criterion (AIC). This suggests that (f(R)) gravity aligns well with observational data and holds potential as a viable candidate for modified gravity. Additionally, the deceleration parameter for both models remains close to the corresponding value in the (Lambda)CDM model. The Statefinder diagnostics reveals distinct evolutionary differences between the two models, although their overall evolutionary trajectories are strikingly similar.

We study an anisotropic and homogeneous Bianchi type-V Universe with holographic dark energy (HDE) and interacting dark matter (DM). Solutions to the field equations have been obtained for a certain form of the deceleration parameter. As for (Lambda)CDM, we show that the coincidence problem disappears for a specific choice of the DM-HDE interaction. In this study, the observational data combination of OHD and JLA [Yu et al., Astrophys. J. 856, 3 (2018)], where (q_{0}=-0.52) and (H_{0}=69.2), have been taken into consideration. We also found that the anisotropy of the expansion tends to isotropy after some finite time. We have also explained the physical and geometric aspects of the model. The physical acceptability and stability of the model have been examined.

The problem of local (e.g., interplanetary) Hubble expansion is studied for a long time but remains a controversial subject till now; and of particular interest is a plausible value of the local Hubble parameter at the scale of the Solar system. Here, we tried to estimate the corresponding quantity by the analysis of surface temperatures on the Earth and Mars, which are formed by a competition between a variable luminosity of the Sun and increasing radii of the planetary orbits. Our work employs paleochemical and paleobiological data on the temperature of the ancient Earth, on the one hand, and geological data on the existence of an ocean of liquid water on the ancient Mars, on the other hand. As follows from our analysis, the martian data impose only a weak constraint on the admissible values of the Hubble parameter because of unknown salinity, and therefore the freezing point of the martian water. On the other hand, the terrestrial data turn out to be much more valuable, especially, for the Precambrian period, when temperature variation was sufficiently smooth and monotonic. For example, in the framework of the standard (Lambda)CDM model with 70(%) of dark energy, the contemporary value of the local Hubble parameter was found to be 70–90 km/s/Mpc under assumption that the Earth’s surface temperature at the end of Precambrian equaled (45^{circ})C. It is in reasonable agreement both with the intergalactic data and with an independent estimate of the local Hubble parameter from tidal evolution of the Earth–Moon system.