International Journal of Modern Physics D

In this work, we study quantum metrology protocol in the context of relativistic quantum fields in curved spacetimes. In particular, we restrict our attention to the isotropic and anisotropic models. We investigate how the oscillatory anisotropy affects a quantum metrology protocol. Here, the oscillatory anisotropy is incorporated as a small gravitational disturbance. We calculate the Fisher information for both cosmological parameters $𝜖$ (expansion volume) and $\rho$ (expansion rate). We find that, unlike the isotropic case, the Fisher information of both the parameters $𝜖$ and $\rho$ are oscillating functions of the momentum $k$. The oscillatory spectra of Fisher information can theoretically be understood as results of the dependence of the Bogoliubov coefficients on the anisotropic oscillations. In principle, this framework can be used to infer about the underlying spacetime structure and examine the effects of the universe scale factor oscillations.

Using the principle of least action, the motion equations for a singular hypersurface of arbitrary type in quadratic gravity are derived. Equations containing the “external pressure” and the “external flow” components of the surface energy–momentum tensor together with the Lichnerowicz conditions serve to find the hypersurface itself, while the remaining ones define arbitrary functions that arise due to the implicit presence of the delta function derivative. It turns out that neither double layers nor thin shells exist for the quadratic Gauss–Bonnet term. It is shown that there is no “external pressure” for null singular hypersurfaces. The Lichnerowicz conditions imply the continuity of the scalar curvature in the case of spherically symmetric null singular hypersurfaces. These hypersurfaces must be thin shells if the Lichnerowicz conditions are necessary. It is shown that for this particular case the Lichnerowicz conditions can be completely removed therefore a spherically symmetric null double layer exists. Spherically symmetric null singular hypersurfaces in conformal gravity are explored as application.

In this work, we study the cosmological implications of Kaniadakis holographic dark energy model in the context of Einstein and deformed Hořava-Lifshitz theories of gravity. We examine the various cosmological parameters to analyze the evolution of the universe. The Hubble parameter $(H)$ is discussed as it is evidence for accelerating universe and source to examine some other cosmological parameters. We probe coincidence parameter $(\tilde{r})$ to check the matter- and energy-dominated eras of the universe. The deceleration parameter $(q)$ is explored for the expansion behavior of the universe while the jerk parameter $(j)$ is examined to compare the different dark energy models with $\mathrm{\Lambda }$CDM $(j=1)$ model. We examine the equation of state parameter $({\omega }_d)$ to look into different phases of the universe. We calculate the perturbed parameter squared speed of sound $(C_s^2)$ to assess the stable behavior of the model. We also investigate the behavior of rate of change of entropy to discuss the validity of generalized second law of thermodynamics.

The slow-roll inflation scenario is studied in the frame of the scalar–tensor theory of gravity, where the scalar field has a nonminimal coupling to the geometric part. After deriving the main dynamical and observable parameter, the consistency of the model with data is considered and we could determine the constants of the model. Then, using the result, the swampland criteria are studied and it is revealed that they are perfectly satisfied. Finally, reheating phase is considered and it is found that the reheating phase lasts for a few e-fold of expansion, and the universe warms up to a high temperature.

The thick brane scenario built on the $f(T,B)$ teleparallel gravity theory was considered for the study of phase transitions, internal structures and new classes of solutions in a model. In this theory, $T$ denotes the torsion scalar and $B$ is a boundary term. An interesting result was observed when brane splitting occurs, i.e. internal structures in the model arise as a consequence of the appearance of new domain walls in the theory. In fact, this preliminary result influences the profile of the matter field (from kink to multikink) so that for appropriate values of the parameters $k_{1,2}$, multiple phase transitions are identified. To perform this analysis, the Differential Configurational Entropy (DCE) that has the ability to predict the existence of phase transitions through critical points was used. Furthermore, the DCE is able to select the most stable solutions since it gives us details about the informational content to the field settings.

In this paper, we eliminate the parameters originally present in models of inflation of the $\alpha$-attractor type in favor of the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$. We then write expressions for the number of $e$-folds during reheating $N_{re}$. By imposing reasonable conditions on $N_{re}$, we can restrict $n_s$ and $r$ and in turn, we use these constraints in order to find bounds for cosmological quantities of interest such as the number of $e$-folds during inflation and the radiation dominated eras, as well as for the reheating temperature and the running index. The minimum condition that $N_{re}$ must satisfy is $N_{re}\ge 0$ which we use to constrain the cosmological quantities mentioned above. In particular, we find that the tensor-to-scalar ratio $r$ (and as a consequence the energy scale of inflation) is bounded from below. We provide figures illustrating the behavior of these quantities as functions of $r$ for several values of $n_s$ and tables containing the bounds so obtained.

It was recently pointed out by Barrau et al. that the mass of the Schwarzschild black hole immersed in a heat bath tends to infinity in a finite time. We show that this singularity problem in the black hole mass may be avoided, if the stretched horizon of the black hole is constructed out of a fixed, finite number of discrete constituents, and an appropriate counting is applied to the energy eigenstates of the hole.

As in the case of Lanczos–Lovelock gravity, the main advantage of $F(T)$ gravity is said to be that it leads to second-order field equations, while $F(R)$ gravity theory leads to fourth-order equations. We show that it is rather a disadvantage, since it leads to the unresolved issue of ‘Branched Hamiltonian’. The problem is bypassed in $F(R,T)$ gravity theory.

According to a new tired-light cosmological model, where $H(z)=H_0(1+z)$, the number density of galaxies has been nearly constant over the last 10Gyr, at least, meaning that, as far as galaxy counts are concerned, the Universe has been stationary. In this context, an analysis of the luminosity distances of quasars and supernovae Ia shows that the Universe is far from being as transparent as assumed nowadays, the photon lifetime along the line-of-sight being one third of the Hubble time. Such a “low” value could mean that there are huge amounts of grey dust in the inter galactic medium, that have so far escaped detection. It could also be a signature of “new physics,” namely, a consequence of the decay of photons into lighter particles.

The tired-light model advocated in this study would be falsified if, for instance, the time-dilation of remote events was shown to have a general character, that is, if it were definitely observed for phenomena other than the light curves of supernovae Ia. On the other hand, further developments are needed in order to turn this model into an actual cosmology. In particular, the physical origin of the current stability of the Universe on galactic scales remains to be identified.