International Journal of Modern Physics D

We present a brief overview of attempts to construct de Sitter vacua in string theory and explain how the results of this 20-year endeavor could point to the fact that string theory harbors no de Sitter vacua at all. Making such a statement is often considered controversial and “bad news for string theory”. We discuss how perhaps the opposite can be true.

There is a conjecture that renormalizable higher-derivative gravity has a finite classical potential at the origin. In this work, we show clearly that the scale-invariant gravity (SIG) satisfies the conjecture. This gravity produces the better-behaved $1/k^4$ UV behavior as needed for renormalizability. It turns out that the SIG has the linear classical potential of $V\propto r$ and it is a UV complete theory.

In this work, we attempt to generalize the statistical mechanics of self-gravitating systems to systems consisting of two species gravitating particles. Under the nondegenerate condition, with the second-order approximation, and with virialization relationships as additional constraints, we obtain the entropy expression for the two species system. Then, by extremizing the constrained entropy with variational calculus, we obtain a series of equations to describe the equilibrium states of the system. Under the assumption that there is no difference of velocity distributions between the two species, from the series of equilibrium-state equations, we obtain the relative ratio of the spatial distributions for the two species. The two species may be mixed homogeneously in space, yet with different choices of the relevant parameters $h_1$ and $h_2$, two Lagrangian multipliers in the statistical equilibrium equations, their spatial distributions may also differ from each other. The deviation of the spatial distributions between the two species is usually explained as mass segregation, which is confirmed by recent observations in some Galactic stellar clusters. We emphasis, however, that the mass segregation here is not caused by kinetic-energy exchange between the two species, but a new feature of our statistical approach, which is different from Lynden-Bell’s statistics. We believe that these results may have general significance to the statistical mechanics for the general long-range interaction systems.

In this paper we present a complete and detailed analysis of the calculation of both the Wightman function and the vacuum expectation value of the energy–momentum tensor that arise from quantum vacuum fluctuations of massive and massless scalar fields in the cosmic dispiration spacetime, which is formed by the combination of two topological defects: a cosmic string and a screw dislocation. This spacetime is obtained in the framework of the Einstein–Cartan theory of gravity and is considered to be a chiral spacelike cosmic string. For completeness we perform the calculation in a high-dimensional spacetime, with flat extra dimensions. We found closed expressions for the energy–momentum tensor and, in particular, in $(3+1)$-dimensions, we compare our results with previous existing ones in the literature for the massless scalar field case.

Noncommutativity is an idea dating back to that early times of quantum mechanics and the string theory induced noncommutative (NC) geometry provided an effective framework to study the short distance spacetime dynamics. Also, string theory, a candidate for a consistent quantum theory of gravity, admits a variety of classical black hole solutions including black strings. In this paper, we study a NC geometry inspired rotating black string to cylindrical spacetime with a source given by a smeared distribution of mass. The resulting metric is a regular everywhere, i.e. curvature-singularity free rotating black string, that in large $r$ limit interpolates Lemos black string. Thermodynamical properties of the black strings are also investigated and exact expressions for the temperature, the entropy and the heat capacity are obtained. Owing to the NC correction in the solution, the thermodynamic quantities have also been modified and that the NC geometry inspired black string is always thermodynamically stable.

We investigate the Kerr–Newman–NUT black hole solution obtained from Plebański–Demiański solutions with several assumptions. The origin of the microscopic entropy of this black hole is investigated using the conjectured Kerr/CFT correspondence which is originally proposed for extremal Kerr black holes. The isometry of the near-horizon extremal Kerr–Newman–NUT black hole shows that the asymptotic symmetry group may be implemented to compute the central charge of the Virasoro algebra. Furthermore, by assuming the Frolov–Thorne vacuum, the conformal temperatures can be obtained. Then by using the Cardy formula, the microscopic entropy is gained which matches the Bekenstein–Hawking entropy. We also employ the Cardy prescription to find the logarithmic correction of the entropy. Then at limit $a\to 0$, the extremal Reissner–Nordström–NUT solution is recovered and by enhancing with the fibered coordinate we find the five-dimensional (5D) solution. The second dual CFT is applied to this black hole to gain the entropy. Finally, the microscopic entropy is still in agreement with the area law of 5D black hole solution. Hence, the extremal Reissner–Nordström–NUT solution is holographically dual to the CFT.

In this paper, we have investigated the gravitational lensing in a spherically symmetric spacetime with torsion in the generalized Einstein–Cartan–Kibble–Sciama (ECKS) theory of gravity by considering higher order terms. The torsion parameters change the spacetime structure, which affects the photon sphere, the deflection angle and the strong gravitational lensing. The condition of existence of horizons is not inconsistent with that of the photon sphere. Especially, there exists a novel case in which there is horizon but no photon sphere for the considered spacetime. In this special case, the deflection angle of the light ray near the event horizon also diverges logarithmically, but the coefficients in the strong-field limit are different from those in the cases with photon sphere. Moreover, in the far-field limit, we find that the deflection angle for certain torsion parameters approaches zero from the negative side, which is different from those in the usual spacetimes.

We consider the fluctuation modes around a hypersurface ${\mathrm{\Sigma }}_c$ in a $(d+2)$-dimensional product Einstein manifold, with ${\mathrm{\Sigma }}_c$ taken either near the horizon or at some finite cutoff from the horizon. By mapping the equations that govern the lowest nontrivial order of the fluctuation modes into a system of partial differential equations on a flat Newtonian spacetime, a system of compressible, forced viscous fluid is realized. This result generalizes the nonbulk/boundary holographic duality constructed by us recently to the case of a different background geometry.

The purpose of this study is to explain the super inflation scenario for the early universe with scalar field which is minimally coupled with theory of $F(R,T)$ gravity, where $R$ is the Ricci curvature scalar and $T$ is the trace of the energy–momentum tensor (EMT). We consider the model $F(R,T)=R+f_1(R)+f_2(T)$, where the curvature is minimally coupled with matter. All the modification terms are properly compacted in $f_2(T)$, by means of a unified solution. After a vacuum inflation, it is observed that the scalar field oscillations via the power-term in potential ${\varphi }^n$ naturally produce the quintessential type of dark energy. Then, we observe that the matter is created by this type of dark energy during inflation. Also, the spectral index parameter and the tensor-to-scalar ratio are examined for this inflation mechanism.

In this paper, we will deform the usual energy–momentum dispersion relation of a photon gas at an intermediate scale between the Planck and electroweak scales. We will demonstrate that such a deformation can have nontrivial effects on the physics of dumb holes. So, motivated by the physics of dumb holes, we will first analyze the effect of such a deformation on thermodynamics. Then, we observe that the velocity of sound also gets modified due to such a modification of the thermodynamics. This changes the position of the horizon of dumb holes, and the analogous Hawking radiation from a dumb hole. Therefore, dumb holes can be used to measure the deformation of the usual energy–momentum dispersion relation.

We study the photon’s motion around a black hole in the presence of a plasma whose density is a function of the radius coordinate by a renewed ray-tracing algorithm and investigate the influence of the plasma on the shadow of the black hole. The presence of plasma affects not only the size but also the shape of the black hole shadow. Furthermore, the influence of plasma on trajectories of photons depends on the frequency of the photons. For the high-frequency photons, the influence is negligible, on the contrary, the trajectories of low-frequency photons are affected significantly by the plasma. Interestingly, it is also found that the black hole image would take on a multi-ring structure due to the presence of plasma.

In this paper, we will discuss cosmological models using Bianchi type I for anisotropic fluid in $f(R,\varphi )$ theory of gravity which involves scalar potential. For this purpose, we consider power law assumptions of coupling function and scalar field along with the proportionality condition of expansion and shear scalars. We choose two $f(R,\varphi )$ models and obtain exact solutions of field equations in both cases. For these constructed models, the behavior of different physical quantities like EoS parameter, self-interacting potential as well as deceleration and skewness parameters are explored and illustrated graphically for the feasible ranges of free parameters. It is concluded that anisotropic fluid approaches isotropy in later cosmic times for both models.

Neutrino-induced reactions are a basic ingredient in astrophysical processes like star evolution. The existence of neutrino oscillations affects the rate of nuclear electroweak decays which participates in the chain of events that determines the fate of the star. Among the processes of interest, the production of heavy elements in core-collapse supernovae is strongly dependent upon neutrino properties, like the mixing between different species of neutrinos. In this work, we study the effects of neutrino oscillations upon the electron fraction as a function of the neutrino mixing parameters, for two schemes: the $1+1$-scheme (one active neutrino and one sterile neutrino) and the $2+1$-scheme (two active neutrinos and one sterile neutrino). We have performed this analysis considering a core-collapse supernovae and determined the physical conditions needed to activate the nuclear reaction chains involved in the r-process. We found that the interactions of the neutrinos with matter and among themselves and the initial amount of sterile neutrinos in the neutrino-sphere might change the electron fraction, therefore affecting the onset of the r-process. We have set constrains on the active-sterile neutrino mixing parameters. They are the square-mass-difference $\mathrm{\Delta }{m}_{14}^2$, the mixing angle ${sin}^{2}2{𝜃}_{14}$, and the hindrance factor ${\xi }_s$ for the occupation of sterile neutrinos. The calculations have been performed for different values of $X_{\alpha }$, which is the fraction of $\alpha$-particles. For $X_{\alpha }=0$ the r-process is taking place if $\mathrm{\Delta }{m}_{14}^2\ge 2{\mathrm{\text{eV}}}^2$, ${sin}^{2}2{𝜃}_{14}<0.8$ and ${\xi }_s<0.5$. For larger values of $X_{\alpha }$ the region of parameters is strongly reduced. The present results are compared to results available in the literature.

We have recently suggested a set of nonlocally extended field equations in which Newton’s constant $G$ was promoted to a covariant differential operator $G_{\mathrm{\Lambda }}({\square }_g)$ composed by two different contributions. These two terms independently operate in the infrared (IR) and ultraviolet (UV) energy regimes. Considering the recent spectacular direct gravitational radiation measurements, we aim to compute the nonlocally modified orbital dynamics of a two-body system. In a first attempt, we determine the effective Newtonian potential from the nonlocal UV-wave equation to work out the modified Kepler orbits. In addition, we use data obtained from the perihelion precession of Mercury in order to derive an upper bound for the dimensionless UV-parameter $\left|\sigma \right|\le 9.3\cdot 10^{-4}$. Finally, we determine the energy per unit time (power) released by a binary system in the form of gravitational waves. In this context, we observe that in the limit of vanishing UV-parameters, we eventually recover the result obtained in the context of the standard quadrupole formula.

It is known that almost all approaches to quantum gravity produce a logarithmic correction term to the entropy of a black hole, but the exact coefficient of such a term varies between the different approach to quantum gravity. Such logarithmic terms can also occur due to thermal fluctuations in both analogous and real black holes so that we will analyze the effects of logarithmic corrections term with variable coefficient on properties of analogous black hole. As these properties can be experimentally tested, they can be used to obtain the correct coefficient for such terms for an analogous black hole. We will argue that as even the real black holes can be considered as thermodynamical objects in Jacobson formalism, so such analogous black holes can be used to obtain the correct coefficient for the real black holes, and this in turn can be used to select the correct approach to quantum gravity. In that case, we use an adaptive model of graphene, which is still far from real graphene, to investigate some thermodynamics quantities of BTZ black hole.