International Journal of Modern Physics D

In this paper, we extend an axion $F(R)$ gravity model, and apply the holographic principle to describe in a unifying manner the early and the late-time universe when the general equation-of-state (EoS) contains a bulk viscosity. We assume a spatially flat Friedmann–Robertson–Walker (FRW) universe model. We use a description based on the generalized infrared-cutoff holographic dark energy proposed by Nojiri and Odintsov [Eur. Phys. J. C 77 (2017) 528; Gen. Relativ. Gravit. 38 (2006) 1285], and explore the evolution of the universe when the EoS describes the asymptotic behavior between the dust in the early universe and the late universe. We explore various forms of the bulk viscosity, and calculate analytical expressions for the infrared cutoffs in terms of the particle horizon. In this way, we obtain a unifying description of the early and the late-time universe in the presence of axion matter, via a viscous holographic fluid model.

In this paper, we would like to examine whether a novel Starobinsky–Bel–Robinson (SBR) gravity model admits stable exponential inflationary solutions with or without spatial anisotropies. As a result, we are able to derive an exact de Sitter inflationary to this SBR model. Furthermore, we observe that an exact Bianchi type I inflationary solution does not exist in the SBR model. However, we find that a modified SBR model, in which the sign of coefficient of $R^2$ term is flipped from positive to negative, can admit the corresponding Bianchi type I inflationary solution. Unfortunately, stability analysis using the dynamical system approach indicates that both of these inflationary solutions turn out to be unstable. Interestingly, we show that a stable de Sitter inflationary solution can be obtained in the modified SBR gravity.

By considering Kehagias–Sfetsos black hole in the framework of the Hořava–Lifshitz gravity, we study the optical appearance of such black holes surrounded by spherical accretion flow. For the static/ infalling spherical accretion flow, we compute the observed specific intensity as a function of impact parameter. We also investigate the effect of the Hořava parameter and accreting matter on the luminosity of shadows and photon rings. It is found that an increase in the Hořava parameter decreases the shadow size, while the shadows and photon rings luminosities increase. Moreover, we constrain the Hořava parameter from the observational data reported by the Event Horizon Telescope for M87* and Sgr A*.

This paper aims to conduct an extensive examination of hyperbolically symmetrical static fluid distributions, wherein a precise analysis of their physical characteristics is carried out in the background of modified $f(G,T)$ gravity, where $G$ and $T$ stands for Gauss Bonnet invariant and energy-momentum trace, respectively. The outcomes reveal that the energy density exhibits negative value, thus implying that any utilization of such fluids necessitates extremely demanding circumstances where quantum effects would be significantly influential. Furthermore, it has been determined that these fluid distributions cannot take the vicinity surrounding their central point of symmetry and leave behind an empty vacuum cavity in its place. Additionally, an appropriate definition for mass function and the complexity factor is determined. Eventually, we exhibit a broad strategy for accomplishing particular solutions and showcase several instances of exact analytical solutions in the presence of $f(G,T)$ correction terms.

In this paper, we undertake a unified study of background dynamics and cosmological perturbations in the presence of the Chaplygin gas (CG). This is done by first constraining the background cosmological parameters of different Chaplygin gas models with SNeIa and $H(z)$ data for detailed statistical analysis of the CG models. Based on the statistical criteria we followed, none of the models has substantial observational support, but we show that the so-called “original” and “extended/generalised” Chaplygin gas models have some observational support and less observational support, respectively, whereas the “modified” and “modified generalised” Chaplygin gas models miss out on the category less observational support, but cannot be ruled out. The so-called “generalised” cosmic Chaplygin gas model, on the other hand, falls under the no observational support category of the statistical criterion and can be ruled out. The models which are statistically accepted are considered for perturbation level in both theoretical and observational aspects. We also apply the $1+3$ covariant formalism of perturbation theory and derive the evolution equations of the fluctuations in the matter density contrast of the matter–Chaplygin gas system for the models with some or less statistical support. The solutions to these coupled systems of equations are then computed in both short-wavelength and long-wavelength modes. Then we feed these observationally restricted parameters into the analysis of cosmological perturbations to address the growth of density contrast through redshift. Using the most recent linear growth of the data $f_{\sigma 8}$, CG models are considered to study the linear growth of the structure.

This paper advances the state-of-the-art by extending the study of the analogy between the fabric of spacetime and elasticity. As no prior work exists about a potential spacetime thermal expansion coefficient $\alpha$, we explore the analogy of general relativity with the theory of elasticity by considering the cosmological constant $\mathrm{\Lambda }$ as an additional space curvature of the structure of space due to a thermal gradient coming from the cosmic web and the cold vacuum and we propose ${(\frac{{\alpha }_S\mathrm{\Delta }T}{e})}^2={(\frac{1}{R_0})}^2=\mathrm{\Lambda }$ with $R_0$ being the curvature radius of the space fabric. It follows from this analogy and from the supposed space model consisting of thin sheets of Planck thickness $l_p$ curved by this thermal gradient $\mathrm{\Delta }$T a possible thermal expansion coefficient of the equivalent elastic medium modeling the space ${\alpha }_S=\frac{l_p\sqrt{\mathrm{\Lambda }}}{\mathrm{\Delta }T}$ of the order of ${\alpha }_{\mathrm{\text{space-QFT}}}=1.16\times 10^{-6}$K${}^{-1}$. As spacetime and not only space must be considered in general relativity, this paper also proposes an innovative approach which consists in introducing into the interval ds² of special relativity a temperature effect $T:d_s^2={(1\pm {\alpha }_{t}T)}^2{c}^2{dt}^2-{(1\pm {\alpha }_{s}T)}^2[{dx}^2+{dy}^2+{dz}^2]$ (entropy variations correlated with time laps, based on temperature variations affecting always physically the clocks) based on different thermal expansion coefficients for space and time with for the flow of time $t:\frac{ct}{cn\tau }=\frac{k_{B}t}{nh}\times \mathrm{\Delta }T={\alpha }_t\mathrm{\Delta }T$. With $T\approx$ 10⁶K, $n=1$, the associate time interval is ${4.8\times 10}^{-17}$s and ${\alpha }_t=1.0\times 10^{-6}{K}^{-1}$. The consequence of this hypothesis is that dark energy potentially becomes a thermal spacetime curvature ${(\frac{{\alpha }_{f}T}{l_p})}^2$ with $f$ equal to $s$ or $t$ depending of the temperature, the thermal entropy variation of the universe, the Planck thickness and time, that increases since the Big bang, depending on thermal expansion coefficients for spacetime ${\alpha }_s$ and ${\alpha }_t$ as a function, respectively, of $\mathrm{\Lambda }$, $\frac{k_B}{h}\times t$, in opposition to spacetime curvature gravity due to mass/energy density as described in general relativity.

In this paper, we consider a model of quintessential inflation based upon inverse hyperbolic potential. We employ curvaton mechanism for reheating which is more efficient than gravitational particle production; the mechanism complies with nucleosynthesis constraint due to relic gravity waves. We obtain a lower bound on the coupling constant $g$ that governs the interaction of curvaton with matter fields. We study curvaton decay before domination and decay after domination and plot the allowed region in the parameter space in both cases.

The ($1+1$)-dimensional dilatonic background geometry of [C. Callan, S. Giddings, J. Harvey and A. Strominger, Phys. Rev. D 45 (1992) R1005] admits an infinite class of colliding wave solutions. We give examples from real and ghost fields with bounded/ unbounded profiles. In particular, colliding ghost fields in the “lineland” geometry create particle-like wormholes. Such wormholes, however, collapse to a black hole whenever the supporting ghosts turn off. Lastly, it is shown that the vacuum collision of null fields yields a ghost wormhole which is unstable.