International Journal of Modern Physics D

We hereby review a class of quantum gravitational theories based on weakly nonlocal analytic classical actions. The most general action is characterized by two nonpolynomial entire functions (form-factors) in terms quadratic in curvature. The form-factors avert the presence of poltergeists, that plague any local higher derivative theory of gravity and improve the high-energy behavior of loop amplitudes. For pedagogical purposes, it is proved that the theory is super-renormalizable in any dimension, i.e. only one-loop divergences survive, and is asymptotically free. Furthermore, due to dimensional reasons, in odd dimensions, there are no counterterms for pure gravity and the theory turns out to be finite. Moreover, we show that it is always possible to choose the additional terms in the action (higher in curvature) in such a way to make the full theory UV-finite and therefore, scale-invariant in quantum realm, also in even dimension.

Intermediate-mass black holes (IMBHs), with masses in the range $100$–$10^6$M${}_{\odot }$, are the link between stellar-mass BHs and supermassive BHs (SMBHs). They are thought to be the seeds from which SMBHs grow, which would explain the existence of quasars with BH masses of up to 10${}^{10}$M${}_{\odot }$ when the Universe was only 0.8 Gyr old. The detection and study of IMBHs has thus strong implications for understanding how SMBHs form and grow, which is ultimately linked to galaxy formation and growth, as well as for studies of the universality of BH accretion or the epoch of reionization. Proving the existence of seed BHs in the early Universe is not yet feasible with the current instrumentation; however, those seeds that did not grow into SMBHs can be found as IMBHs in the nearby Universe. In this review, I summarize the different scenarios proposed for the formation of IMBHs and gather all the observational evidence for the few hundreds of nearby IMBH candidates found in dwarf galaxies, globular clusters, and ultraluminous X-ray sources, as well as the possible discovery of a few seed BHs at high redshift. I discuss some of their properties, such as X-ray weakness and location in the BH mass scaling relations, and the possibility to discover IMBHs through high velocity clouds, tidal disruption events, gravitational waves, or accretion disks in active galactic nuclei. I finalize with the prospects for the detection of IMBHs with up-coming observatories.

We discuss here the geometry associated with the loop quantum gravity when it is considered to be generated from fermionic degrees of freedom. It is pointed out that a closed loop having the holonomy associated with the $SU(2)$ gauge group is realized from the rotation of the direction vector associated with the quantization of a fermion depicting the spin degrees of freedom. During the formation of a loop a noncyclic path with open ends can be mapped onto a closed loop when the holonomy involves $q$-deformed gauge group $S{U}_q(2)$. In this case, the spinorial variable attached to a node of a link is a quasispinor equipped with quasispin associated with the $S{U}_q(2)$ group. The quasispinors essentially correspond to the fermions attached to the end points of an open path in loop space. We can consider adiabatic iteration such that the quasispin associated with the $S{U}_q(2)$ group gradually evolves as the time dependent deformation parameter $q$ changes and we have the holonomy associated with the $SU(2)$ group in the limit $q=1$. In this way we can have a continuous geometry developed through a sequence of $q$-deformed holonomy-flux phase space variables which leads to a continuous gravitational field. Also it is pointed out that for a truncated general relativity given by loop quantum gravity on a fixed graph we can achieve twisted geometry and Regge geometry.

We study a family of noncommutative spacetimes constructed by one four-vector. The large set of coordinate commutation relations described in this way includes many cases that are widely studied in the literature. The Hopf-algebra symmetries of these noncommutative spacetimes, as well as the structures of star product and twist are introduced and considered at first order in the deformation, described by four parameters. We also study the deformations to relativistic kinematics implied by this framework, and calculate the most general expression for the momentum dependence of the Lorentz transformations on momenta, which is an effect that is required by consistency. At the end of the paper we analyse the phenomenological consequences of this large family of vectorlike deformations on particles propagation in spacetime. This leads to a set of characteristic phenomenological effects.

In this paper, we investigate ghost dark energy model in the presence of nonlinear interaction between dark energy and dark matter. We also extend the analysis to the so-called generalized ghost dark energy (GGDE) which ${\rho }_D=\alpha H+\beta H^2$. The model contains three free parameters as ${\mathrm{\Omega }}_D,\zeta (=\frac{8\pi G\beta }{3})$ and $b^2$ (the coupling coefficient of interactions). We propose three kinds of nonlinear interaction terms and discuss the behavior of equation of state, deceleration and dark energy density parameters of the model. We also find the squared sound speed and search for signs of stability of the model. To compare the interacting GGDE model with observational data sets, we use more recent observational outcomes, namely SNIa from JLA catalog, Hubble parameter, baryonic acoustic oscillation and the most relevant CMB parameters including, the position of acoustic peaks, shift parameters and redshift to recombination. For GGDE with the first nonlinear interaction, the joint analysis indicates that ${\mathrm{\Omega }}_D=0.7192\pm 0.0062$, $b^2=0.146_{-0.026}^{+0.030}$ and $\zeta =0.104\pm 0.047$ at 1 optimal variance error. For the second interaction, the best fit values at $1\sigma$ confidence are ${\mathrm{\Omega }}_D=0.72091\pm 0.0065$, $b^2=0.0395\pm 0.0080$ and $\zeta \le 0.0173$. According to combination of all observational data sets considered in this paper, the best fit values for third nonlinearly interacting model are ${\mathrm{\Omega }}_D=0.7287\pm 0.0062$, $b^2=0.0109\pm 0.0023$ and $\zeta \le 0.00764$ at $1\sigma$ confidence interval. Finally, we found that the presence of interaction is compatible in mentioned models via current observational datasets.

The 6D brane model is considered, where matter is trapped on the surface of a (2+4)-hyperboloid, as is suggested by the geometrical structure behind the 4D conformal group. The effective dimension of the bulk spacetime for matter fields is five, with the extra space-like and time-like domains. Using the embedding theory, the presence of the familiar factorizable 5D brane metrics in both domains is shown. These metrics with exponential warp factors are able to provide with the additional reduction of the effective spacetime dimensions down to four. It is demonstrated that the extra (1+1)-space is not simply connected and there is a gap in the range of the extra coordinates. This can explain the stability of the model in the domain with the time-like effective fifth dimension and the appearance of the cosmological constant due to the tachyon condensation. It is found that the model exhibits orbifold symmetry and thus is free from the fermion chirality problem.

We study a formulation of statistical mechanics in the context of symplectic structures of the IR and also UV & IR-deformed Snyder phase-spaces. We derive the corresponding invariant Liouville volume and by using it we obtain the deformed partition function. We then study the thermodynamical properties of the 3-dimensional harmonic oscillator in this set-up. By using the equipartition theorem, we show that two of the six degrees of freedom for a 3-dimensional harmonic oscillator will be frozen as the temperature increases. Also, at a constant temperature, whatever is the increase in oscillator length, this reduction of the number of degrees of freedom gets more and more appreciable and it offers an effective dimensional reduction of space from $3$ to $2$ when it is close to the IR-length scale.

We calculate moment of inertia of neutron star with different exotic constituents such as hyperons and antikaon condensates and study its variation with mass and spin frequency. The sets of equation-of-state (EoS), generated within the framework of relativistic mean field model with density-dependent couplings are adopted for the purpose. We follow the quasi-stationary evolution of rotating stars along the constant rest mass sequences, that varies considerably with different constituents in the EoS. We also explore the universal relations associated with some of the normalized properties, such as critical mass and moment of inertia for specific EoS or as a matter of fact constituents of the dense matter. Deviations in the universal relations for moment of inertia are observed at higher compactness. This study presents important results concerning the properties of neutron stars, that could be observationally verified in the near future using Square Kilometer Array telescope.

In this work, we have discussed the implications of shear-free condition on axially symmetric anisotropic gravitating objects in $f(R,T)$ theory. Restricted axial symmetry ignoring rotation and reflection entries containing three independent metric functions is taken into account for establishment of instability range. Implementation of linear perturbation on constitutive modified dynamical equations yield evolution equation. This equation associates adiabatic index $\mathrm{\Gamma }$ with material and dark source components of physical parameters defining stable and unstable regions in Newtonian (N) and post-Newtonian (pN) approximations. It is remarked that the axial system evolving under shear-free condition implicates high levels of stability in anisotropic environment.

Using comoving distance $d_c$ and angular diameter distance $d_A$, we recalculate parameters describing kinematical state of the universe, still combining the kinematical model of universe but not relying on dynamical equations for gravity. Comoving distance $d_c$ comes from Hubble data $H(z)$ and is more reliable. Angular diameter distance $d_A$ comes from SZE (Sunyaev–Zel’dovich Effect) and X-ray data, and needs calibration. In low redshift case, we use expansion of relation between luminosity distance and redshift about redshift $z$; in high redshift case, we take variable substitution $y=1/(1+z)$, and expand the relation between luminosity distance and redshift about variable $y$ in order to reduce computational errors. Finally, we get the more precise value of Hubble parameter $H_0=69.13\pm 0.24$ km ⋅ s${}^{-1}\cdot {\mathrm{\text{Mpc}}}^{-1}$, corresponding to $0.4\%$ uncertainty in $68.3\%$ confidence region, also deceleration factor $q_0=-0.57\pm 0.04$ and acceleration rate $j_0=1.28\pm 0.33$, and their statistical values and probability graph. We compare the values of $H_0$, $q_0$ and $j_0$ with those obtained from other observation data and model.

In this work, we investigate the dynamical evolution of spherical self-gravitating systems under their own gravity with $N$-body simulations. For this purpose, we study the evolution of the generalized virialization relations, and particularly focus on the time evolution of the coarse-grained entropy of dark matter halos under various perturbations. First, we construct six single perturbation models under four initial conditions to mimic typical disturbances that a realistic gravitating system may encounter. With the simulation results, we show the time evolution of the entropy for the six perturbation models. In all these models, at first the entropy increases rapidly for a short period of time, slowly evolves for a longer period of time and then remains nearly unchanged in the subsequent evolution. The main dynamical mechanisms behind these evolutions should be violent relaxation and phase mixing. However, under repeated perturbations to the system, the evolution of entropy of self-gravitating systems manifests complete differences from that of the usual thermodynamical systems. We see that the entropy of the end states of every single perturbation, according to different repeated perturbation modes, either decreases or increases. We argue that the increasing or decreasing of the end-state entropy should be the reflection of the complexity of the thermodynamical states of self-gravitating systems. These conclusions are independent of the initial conditions. Besides, we demonstrate that the generalized virialization relations can reveal whether or not, or in which radius interval, the collisionless Boltzmann equation is suitable for description of a self-gravitating system, and can be used as good stability criteria of the system.

The Immirzi parameter of loop quantum gravity is a one-parameter ambiguity of the theory whose precise interpretation is not universally agreed upon. It is an inherent characteristic of the quantum theory as it appears in the spectra of geometric operators, despite being irrelevant at the classical level. The parameter’s appearance in the area and volume spectra to the same power as the Planck area suggest that it plays a role in determining the fundamental length scale of space. In fact, a consistent interpretation is that it represents a constant rescaling of the kinematical spatial geometry. An interesting realization is that promoting the Immirzi parameter to be a general conformal transformation leads to a system which can be identified as analogous to the linking theory of shape dynamics. A three-dimensional gravitational gauge connection is then constructed within the linking theory in a manner analogous to loop quantum gravity, thereby facilitating the application of the established procedure of loop quantization.

We generalize the topological model recently proposed and investigate the cosmological perturbations of the model. The model has an exact de Sitter background solution associated with the Becchi–Rouet–Stora (BRS) quartet terms which are regarded as the Lagrangian density of the topological field theory. The de Sitter solution can be selected without spontaneously breaking the BRS symmetry, and be interpreted as a gauge fixing of de Sitter spacetime. The BRS symmetry is preserved for the perturbations around the de Sitter background before we solve the constraints of general relativity. We derive action to the second-order of the perturbations and confirm that even after solving the constraints, we have the BRS symmetry at least for the second-order action. We construct the cosmological perturbation theory involving the BRS sector, and obtain the two-point correlation functions for the curvature perturbation and the isocurvature perturbations which compose the BRS sector. Our result gives a new description for de Sitter spacetime and the quantum field theory in de Sitter spacetime.

The property difference between the neutron star PSR J0348$+$0432 and its proto neutron star is studied in the framework of the relativistic mean field theory considering neutrino trapping. We see that the central baryon number density of the proto neutron star PSR J0348+0432 is in the range ${\rho }_{c,\mathrm{\text{PNS}}}=0.539\text{–}0.698$fm${}^{-3}$, which is smaller than that of the neutron star PSR J0348$+$0432 ${\rho }_{c,\mathrm{\text{NS}}}=0.634\text{–}0.859$fm${}^{-3}$. Inside the neutron star PSR J0348$+$0432, only the neutrons, protons, $\mathrm{\Lambda }$ and ${\mathrm{\Xi }}^{-}$ produce, whereas the hyperons ${\mathrm{\Sigma }}^{-},{\mathrm{\Sigma }}^0,{\mathrm{\Sigma }}^{+}$ and ${\mathrm{\Xi }}^0$ all do not appear. But in the proto neutron star PSR J0348$+$0432, hyperons ${\mathrm{\Sigma }}^{-}$, ${\mathrm{\Sigma }}^0$, ${\mathrm{\Sigma }}^{+}$ and ${\mathrm{\Xi }}^0$ all will produce, though their relative particle number density is still very small, no more than 2%. This shows that higher temperature will be advantageous to the hyperon production.

We developed the nonlocal model recently published by Maggiore and Mancarella by introducing a new function $f$. This modification allows us to obtain a new analytical solution in the hyperbolic tangent form for the nonlocal distortion function. This model gives rise to an expansion history behaving exactly as $\mathrm{\Lambda }$CDM (Lambda Cold Dark Matter) with the same matter content, but without any need to neither cosmological constant nor dark energy. However, background evolution in our model and $\mathrm{\Lambda }$CDM are the same, but the results may be distinguishable in structure formation investigations or in light of new observations that probably contain additional information more than the background evolution.

In this paper, we study centrifugal acceleration of particles moving along a prescribed rotating curved trajectories. We consider the physical system embedded in an isotropic photon field and study the influence of the photon drag force on the acceleration process. For this purpose, we study three major configurations of the field lines: the straight line, the Archimede spiral and the dipolar field line configuration. By analyzing dynamics of particles sliding along the field lines in the equatorial plane, we have found several interesting features of motion. In particular, it has been shown that for rectilinear field lines, the particles reach the light cylinder (area where the linear velocity of rotation exactly equals the speed of light) zone relatively slowly for bigger drag forces. Considering the Archimedes’ spiral, we have found that in cases when the field lines lag behind the rotation, the particles achieve the force-free regime of dynamics regardless of the drag force. Unlike this scenario, when the spiral is oriented in an opposite direction, the particles do not reach the force free regime, but tend to stable equilibrium locations.

This work is the reconstruction of the interaction rate of holographic dark energy whose infrared cut-off scale is set by the Hubble length. We have reconstructed the interaction rate between dark matter and the holographic dark energy for a specific parameterization of the effective equation-of-state parameter. We have obtained observational constraints on the model parameters using the latest type Ia supernova (SNIa), baryon acoustic oscillations (BAO) and cosmic microwave background (CMB) radiation datasets. We have found that for the present model, the interaction rate increases with expansion and remains positive throughout the evolution. For a comprehensive analysis, we have also compared the reconstructed results of the interaction rate with other well-known holographic dark energy models. The nature of the deceleration parameter, the statefinder parameters and the dark energy equation-of-state parameter have also been studied for the present model. It has been found that the deceleration parameter favors the past decelerated and recent accelerated expansion phase of the universe. It has also been found that the dark energy equation-of-state parameter shows a phantom nature at the present epoch.

In the study of covariant wave equations, linear gravity manifests itself through the metric deviation ${\gamma }_{\mu \nu }$ and a two-point vector potential $K_{\lambda }$ itself constructed from ${\gamma }_{\mu \nu }$ and its derivatives. The simultaneous presence of the two gravitational potentials is noncontradictory. Particles also assume the character of quasiparticles and $K_{\lambda }$ carries information about the matter with which it interacts. We consider the influence of $K_{\lambda }$ on the dispersion relations of the particles involved, the particles’ motion, quantum tunneling through a horizon, radiation, energy–momentum dissipation and flux quantization.

In this paper, we take cosmological constant as a thermodynamical pressure and its conjugate quantity as a thermodynamical volume. This expression helps us to investigate the phase transition and holographic heat engine. So, in order to have Van der Waals fluid behavior in Horava–Lifshitz (HL) black hole, we modified the solution of such black hole with some cosmology ansatz. Also from holographic heat engine, we obtain Carnot efficiency for the HL black hole. The phase transition of the system lead us to investigate the stability condition for the corresponding black hole. In that case, we show that the stability exist only in special region of black hole.